Foot stiffening during the push-off phase of human walking is linked to active muscle contraction, and not the windlass mechanism

Joint synergy and muscle activity in the motion of the ankle-foot complex

The movement of the ankle-foot complex joints is coupled as a result of various physiological and physical constraints. This study introduces a novel approach to the analysis of joint synergies and their physiological basis by focusing on joint rotational directions and the types of muscle contractions. We developed a biomimetic model of the ankle-foot complex with seven degrees of freedom, considering the skeletal configuration and physiological axis directions. Motion capture experiments were conducted with eight participants performing dorsiflexion and plantarflexion in open-chain states, as well as various walking tasks in closed-chain states, across different ground inclinations (±10, ±5, 0 deg) and walking speeds (3 and 4 km h-1). Hierarchical cluster analysis identified joint synergy clusters and motion primitives, revealing that in open-chain movements, plantarflexion of the ankle, tarsometatarsal and metatarsophalangeal joints exhibited synergy with the inversion of the remaining joints in the complex; meanwhile, dorsiflexion was aligned with eversion. During closed-chain movements, the synergies grouping was exchanged in the subtalar, talonavicular and metatarsophalangeal joints. Further analysis showed that in open-chain movements, synergy patterns influenced by multi-joint muscles crossing oblique joint axes contribute to foot motion. In closed-chain movements, these changes in synergistic patterns enhance the propulsion of the center of mass towards the contralateral leg and improve foot arch compliance, facilitating human motion. Our work enhances the understanding of the physiological mechanisms underlying synergistic motion within the ankle-foot complex.

Role of insole material in treatment of plantar fasciitis: A randomized clinical trial

BACKGROUND

In this randomized clinical trial, we compared the early effects of polyethylene (PE), polyurethane (PU), and Carbon Fiber insoles in the treatment of PF using a set of patient-reported outcomes.

METHODS

Patients were randomly allocated one of the three prefabricated insoles - Carbon Fiber (n = 14), PU (n = 14), or PE (n = 17) for regular use. Their response was recorded using PROMIS 3a (for pain intensity), PROMIS 4a (for pain interference), FAOS (Foot and Ankle Outcome Score), and VAS for pain at baseline, two, six, and twelve weeks.

RESULTS

The PROMIS pain intensity scores improved in both the Carbon Fiber and the PE groups starting at the 6th week (p = 0.04) and 2nd week (p = 0.002), respectively. PROMIS pain interference scores also showed positive trends in these two groups (p = 0.02, p = 0.004, respectively).

CONCLUSION

Prefabricated Carbon Fiber and PE insoles showed significant pain-reducing effects in patients with PF.

LEVELS OF EVIDENCE

Level I, Randomized controlled trial.

First metatarsophalangeal joint dorsiflexion resistance in individuals with plantar fasciopathy

INTRODUCTION

Plantar fasciopathy is a common foot-related musculoskeletal disorder. It has been hypothesized that this disorder could be linked to a dysfunctional windlass mechanism. However, no study to date has quantified this link to validate or refute this hypothesis. The first metatarsophalangeal joint (1st MPJ) dorsiflexion resistance test is a reliable test to evaluate the force required to initiate the windlass mechanism. Comparing the results of this test in individuals with and without plantar fasciopathy will allow for a better understanding of the relationship between plantar fasciopathy and the windlass mechanism. Thus, this study aimed to compare 1st MPJ dorsiflexion resistance in individuals with plantar fasciopathy and healthy controls. Additionally, this study aimed to explore the correlations between 1st MPJ dorsiflexion resistance and other commonly used foot orthopedic tests, specifically the supination resistance test and the Foot Posture Index.

MATERIAL AND METHODS

Twenty participants with plantar fasciopathy and 20 healthy controls were recruited in this case-control study. First MPJ dorsiflexion resistance was quantified and compared between groups and between feet using a repeated measures ANOVA with one within-subject factor with two levels and one between-subject factor with two levels. It was also correlated with supination resistance and the Foot Posture Index.

RESULTS

There were no significant differences in 1st MPJ dorsiflexion resistance between injured and healthy feet as well as control and plantar fasciopathy groups. There was a moderate to strong correlation (r = 0.674 to 0.891) between 1st MPJ dorsiflexion resistance and supination resistance in both groups. There was no significant correlation between 1st MPJ dorsiflexion resistance and the Foot Posture Index.

CONCLUSIONS

The lack of alterations in 1st MPJ dorsiflexion resistance among individuals with plantar fasciopathy implies a potential need to reconsider the biomechanical model, proposing that a dysfunctional windlass mechanism is associated with the development of plantar fasciopathy, may need reconsideration.

Effects of Plantar Fascia Release and the Use of Foot Orthoses Affect Biomechanics of the Medial Longitudinal Arch of the Foot: A Cadaveric Study

OBJECTIVE

The aim of the study is to evaluate the effect of minimally invasive ultrasound-guided fascial release and a foot orthoses with first metatarsal head cutout on the biomechanics of the medial longitudinal arch of the foot in cadaveric specimens.

DESIGN

A cross-sectional study was designed (20 body donors). Anthropometric measurements of the foot, foot posture index, and the windlass test and force were measured in different conditions: unloaded, loaded position, with foot orthoses, after a 25% plantar fascia release and after a 50% release.

RESULTS

For the anthropometric measurements of the foot, differences were found in foot length ( P = 0.009), arch height ( P < 0.001), and midfoot width ( P = 0.019) when comparing the unloaded versus foot orthoses condition. When foot orthoses were compared with 25% plantar fascial release, differences were found in foot length ( P = 0.014) and arch height ( P < 0.001). In the comparison with 50% plantar fascial release, differences were found in the arch height ( P < 0.001). A significant interaction between foot orthoses condition and grades was found in the arch height during the windlass test ( P = 0.021).

CONCLUSIONS

The results indicate that the presence of foot orthoses leads to a significant increase in arch height compared with other conditions. Furthermore, when plantar fascia release is performed, the arch does not exhibit any signs of collapse.

The effect of including a mobile arch, toe joint, and joint coupling on predictive neuromuscular simulations of human walking

Predictive neuromuscular simulations are a powerful tool for studying the biomechanics of human walking, and deriving design criteria for technical devices like prostheses or biorobots. Good agreement between simulation and human data is essential for transferability to the real world. The human foot is often modeled with a single rigid element, but knowledge of how the foot model affects gait prediction is limited. Standardized procedures for selecting appropriate foot models are lacking. We performed 2D predictive neuromuscular simulations with six different foot models of increasing complexity to answer two questions: What is the effect of a mobile arch, a toe joint, and the coupling of toe and arch motion through the plantar fascia on gait prediction? and How much of the foot's anatomy do we need to model to predict sagittal plane walking kinematics and kinetics in good agreement with human data? We found that the foot model had a significant impact on ankle kinematics during terminal stance, push-off, and toe and arch kinematics. When focusing only on hip and knee kinematics, rigid foot models are sufficient. We hope our findings will help guide the community in modeling the human foot according to specific research goals and improve neuromuscular simulation accuracy.

Development of a novel technique to insert intramuscular electromyography electrodes into the deep intrinsic foot muscles via the dorsum of the foot

This study aimed to develop an insertion technique for intramuscular EMG recording of the oblique head of adductor hallucis (AddH) and first dorsal interosseous (FDI) muscles in humans via the dorsum of the foot, and report feasibility of intramuscular EMG data acquisition during walking in shoes. In eight individuals without musculoskeletal pain or injury (5 males; 32 ± 8 years), intramuscular electrodes were inserted into AddH (oblique head) and FDI through the right foot's dorsum (between metatarsals I-II) with ultrasound guidance. The ultrasound transducer was positioned on the plantar surface. Intramuscular EMG was also recorded from abductor hallucis, tibialis posterior, flexor digitorum longus and peroneus longus. Participants performed six overground walking trials wearing modified shoes, and rated pain associated with the intramuscular electrodes during walking (numerical rating scale, 0-10). High-quality EMG recordings were obtained from intrinsic and extrinsic foot muscles. Analyses of power spectral densities indicated that movement artefacts commonly observed during gait were removed by filtering. Pain associated with AddH/FDI electrodes during walking was low (median[IQR] 1[2]; range 0-4) and similar to other sites. Findings demonstrate that intramuscular EMG recording from AddH (oblique head) and FDI using this insertion technique is feasible and associated with minimal pain when walking in shoes.

Human foot muscle strength and its association with sprint acceleration, cutting and jumping performance, and kinetics in high-level athletes

The primary objective of this study was to investigate the relationship between metatarsophalangeal joint (MTPj) flexion torque and sprint acceleration, cutting and jumping performance, and kinetics. A secondary aim was to explore this relationship when MTP flexion strength was associated with other foot and lower limb neuromuscular outputs. After an initial MTPj flexion torque assessment using a custom-built dynamometer, 52 high-level athletes performed the following tasks on a force platform system: maximal sprint acceleration, 90-degree cutting, vertical and horizontal jumps, and foot-ankle hops. Their foot posture, foot passive stiffness and foot-ankle reactive strength were assessed using the Foot Posture Index, the Arch Height Index Measurement System and the Foot-Ankle Rebound Jump Test. Ankle plantarflexion and knee extension isometric torque were assessed using an isokinetic dynamometer. During maximal speed sprinting, multiple linear regressions suggested a major contribution of MTPj flexion torque, foot passive stiffness and foot-ankle reactive strength to explain 28% and 35% of the total variance in the effective vertical impulse and contact time. Ankle plantarflexor and quadriceps isometric torques were aggregately contributors of acceleration performance and separate contributors of cutting and jumping performance. In conclusion, MTPj flexion torque was more strongly associated with sprinting performance kinetics especially at high-speed.

Variation in the origin of the plantar aponeurosis and its relationship to the origin of the abductor hallucis muscle

The plantar aponeurosis comprises medial, central, and lateral bands, which arise from the calcaneal tuberosity. Descriptions of the origin of the abductor hallucis vary among different textbooks. The central band and abductor hallucis muscles are related to the windlass mechanism. Given the uncertainties regarding the details of the origins of the central band and the abductor hallucis muscle, we examined those origins in 100 feet of 50 cadavers (25 males and 25 females) by dissection. There were three central band patterns, depending on the attachment sites of the origins of the central and lateral bands: Pattern Ia, the central band covers the lateral band completely; Pattern Ib, the central band covers part of the lateral band; Pattern II, the lateral band covers part of the central band. The origin of the abductor hallucis muscle was confirmed. It showed two types of variation: attachment type, originating from the central band; non-attachment type, not originating from the central band. Central band Patterns Ia, Ib, and II were found in 23 feet (17 males, 6 females), 24 feet (25 males, 28 females), and 24 feet (eight males, 16 females), respectively. Pattern Ia predominated in males and Pattern II in females. The attachment and non-attachment types of abductor hallucis muscle were observed in 28 feet (28%) and 72 feet (72%), respectively. The attachment type with Patterns Ia, Ib, and II was shown in 17 feet, 10 feet, and one foot, respectively. Thus, we revealed variation and sex differences in the central band, which could affect foot morphology and the efficacy of the windlass mechanism.

Analysis of windlass mechanism according to one walking cycle

[Purpose] This study aimed to calculate the windlass mechanism in one walking cycle (WC) using the medial longitudinal arch (MLA) height and compare its mechanism with joint moments, angles, and center of gravity movement. [Participants and Methods] The study analyzed the gait of 20 healthy adults (14 males, six females) using a three-dimensional motion analyzer to calculate several parameters. [Results] In the terminal stance, the MLA height reached 20.6 ± 6.0 mm (minimum value) at 49% WC. Simultaneously, the ankle dorsiflexion angle, ankle internal plantarflexion moment, and forward shift of the center of gravity reached the maximum values. At 62% WC, the MLA height was 26.8 ± 4.8 mm and reached maximum during the stance phase, indicating a windlass mechanism. Additionally, the MLA height was 61.7 ± 22.7 mm at 69% WC, indicating an MLA spiking phenomenon. [Conclusion] The MLA height was lowest at 49% WC due to reverse windlass mechanism. Although the windlass mechanism was activated at 62% WC, it was functionally equivalent to the swing phase. Push-off was impossible during the swing phase. At 69% WC, the swing phase showed a second windlass mechanism.

Plantar intrinsic foot muscle activity and its relationship with postural sway during tiptoe standing in ballet dancers and non-dancers

BACKGROUND

Minimizing postural sway during tiptoe standing is essential for ballet dancers. Investigation of the activity of the plantar intrinsic foot muscles (PIFMs) may provide insight into postural sway in dancers. Herein, we compared PIFM activity during tiptoe standing between dancers and non-dancers and examined its relationship with postural sway.

METHODS

We enrolled 14 female ballet dancers and 13 female non-dancers. Electromyography (EMG) amplitudes of 64 channels of PIFMs and center of pressure (COP) data were recorded during bipedal tiptoe standing tasks performed with ankle plantarflexion angles of 20°, 40°, and 60° (dancers only). The EMG amplitudes were normalized to those during the maximum voluntary contraction, and the muscle activity level and its coefficient of variation over time (EMG-CVtime) during the task were assessed. Standard deviations in the anteroposterior and mediolateral directions, velocity, and area were calculated from the COP data.

RESULTS

Most COP and EMG variables were significantly lower in dancers than in non-dancers in both the 20° and 40° tasks (p < 0.05). Significant correlations were found between most combinations of the COP and EMG variables in both the 20° and 40° tasks in the whole cohort (r = 0.468-0.807, p ≤ 0.014). In the 60° task in dancers, COP velocity was strongly correlated with EMG-CVtime (r = 0.700, p = 0.005).

CONCLUSION

These results provide novel evidence that the PIFMs do not require high activity, but rather that its low, steady activity is the key, to achieve less postural sway during bipedal tiptoe standing in dancers.

Lower-limb dominance does not explain subject-specific foot kinematic asymmetries observed during walking and running

Studies of human locomotion have observed asymmetries in lower-limb kinematics, especially at the more distal joints. However, it is unclear whether these asymmetries are related to functional differences between the dominant and non-dominant limb. This study aimed to determine the effect of lower-limb dominance on foot kinematics during human locomotion. Range of motion for the metatarsophalangeal joint (MPJ) and medial longitudinal arch (MLA), as well as time duration of windlass mechanism engagement, were recorded from healthy young adults (N = 12) across a range of treadmill walking and running speeds. On the group level, there were no differences in MPJ or MLA range of motion, or windlass engagement timing, between the dominant and non-dominant limb (p > 0.05). While not explained by limb dominance, between-limb differences in MPJ and MLA ranges of motion were observed for individual participants on the order of ∼2-6°, which could be clinically relevant or impact interpretation of research data.

Reliability of lower leg muscle elasticity using shear wave elastography in non-weight-bearing and weight-bearing

PURPOSE

Muscle elasticity can be quantified with shear wave elastography (SWE) and has been used as an estimate of muscle force but reliability has not been established for lower leg muscles. The purpose of this study was to examine the intra-rater and inter-rater reliability of elasticity measures in non-weight-bearing (NWB) and weight-bearing (WB) for the tibialis anterior (TA), tibialis posterior (TP), peroneal longus (PL), and peroneal brevis (PB) muscles using SWE.

METHODS

A total of 109 recreationally active healthy adults participated. The study employed a single-cohort, same-day repeated-measures test-retest design. Elasticity, measured in kilopascals as the Young's modulus, was converted to the shear modulus. All four muscles were measured in NWB and at 90% WB.

RESULTS

Intra-rater reliability estimates were good to excellent for NWB (ICC = 0.930-0.988) and WB (ICC = 0.877-0.978) measures. Inter-rater reliability estimates were moderate to good (ICC = 0.500-0.795) for NWB measures and poor to good (ICC = 0.346-0.910) for WB measures.

CONCLUSION

Despite the studies poor to good inter-rater variability, the intra-rater reproducibility represents the potential benefit of SWE in NWB and WB. Establishing the reliability of SWE with clinical and biomechanical approaches may aid in improved understanding of the mechanical properties of muscle.

Chasing footprints in time - reframing our understanding of human foot function in the context of current evidence and emerging insights

In this narrative review we evaluate foundational biomechanical theories of human foot function in light of new data acquired with technology that was not available to early researchers. The formulation and perpetuation of early theories about foot function largely involved scientists who were medically trained with an interest in palaeoanthropology, driven by a desire to understand human foot pathologies. Early observations of people with flat feet and foot pain were analogized to those of our primate ancestors, with the concept of flat feet being a primitive trait, which was a driving influence in early foot biomechanics research. We describe the early emergence of the mobile adaptor-rigid lever theory, which was central to most biomechanical theories of human foot function. Many of these theories attempt to explain how a presumed stiffening behaviour of the foot enables forward propulsion. Interestingly, none of the subsequent theories have been able to explain how the foot stiffens for propulsion. Within this review we highlight the key omission that the mobile adaptor-rigid lever paradigm was never experimentally tested. We show based on current evidence that foot (quasi-)stiffness does not actually increase prior to, nor during propulsion. Based on current evidence, it is clear that the mechanical function of the foot is highly versatile. This function is adaptively controlled by the central nervous system to allow the foot to meet the wide variety of demands necessary for human locomotion. Importantly, it seems that substantial joint mobility is essential for this function. We suggest refraining from using simple, mechanical analogies to explain holistic foot function. We urge the scientific community to abandon the long-held mobile adaptor-rigid lever paradigm, and instead to acknowledge the versatile and non-linear mechanical behaviour of a foot that is adapted to meet constantly varying locomotory demands.

A classification of the plantar intrinsic foot muscles based on the physiological cross-sectional area and muscle fiber length in healthy young adult males

BACKGROUND

Plantar intrinsic foot muscles (PIFMs) are composed of 10 muscles and play an essential role in achieving functional diversity in the foot. Previous studies have identified that the morphological profiles of PIFMs vary between individuals. The morphological profiles of a muscle theoretically reflect its output potentials: the physiological cross-sectional area (PCSA) of a muscle is proportional to its maximum force generation, and the muscle fiber length (FL) is its shortening velocity. This implies that the PCSA and FL may be useful variables for characterizing the functional diversity of the individual PIFM. The purpose of this study was to examine how individual PIFMs can be classified based on their PCSA and FL.

METHODS

In 26 healthy young adult males, the muscle volume and muscle length of seven PIFMs (abductor hallucis, ABDH; abductor digiti minimi, ABDM; adductor hallucis oblique head, ADDH-OH; ADDH transverse head, ADDH-TH; flexor digitorum brevis, FDB; flexor hallucis brevis, FHB; quadratus plantae, QP) were measured using magnetic resonance imaging. The PCSA and FL of each of the seven PIFMs were then estimated by combining the data measured from the participants and those of muscle architectural parameters documented from cadavers in previous studies. A total of 182 data samples (26 participants × 7 muscles) were classified into clusters using k-means cluster analysis. The optimal number of clusters was evaluated using the elbow method.

RESULTS

The data samples of PIFMs were assigned to four clusters with different morphological profiles: ADDH-OH and FHB, characterised by large PCSA and short FL (high force generation and slow shortening velocity potentials); ABDM and FDB, moderate PCSA and moderate FL (moderate force generation and moderate shortening velocity potentials); QP, moderate PCSA and long FL (moderate force generation and rapid shortening velocity potentials); ADDH-TH, small PCSA and moderate FL (low force generation and moderate shortening velocity potentials). ABDH components were assigned equivalently to the first and second clusters.

CONCLUSIONS

The approach adopted in this study may provide a novel perspective for interpreting the PIFMs' function based on their maximal force generation and shortening velocity potentials.

Foot and Ankle Muscle Isometric Strength in Nonrearfoot Compared With Rearfoot Endurance Runners

Background

Transitioning to a forefoot strike pattern can be used to manage running-related knee injuries. However, adopting a nonrearfoot strike induces a higher load on foot and ankle structures than rearfoot strike. Sufficient foot muscle strength is also necessary to prevent excessive longitudinal arch (LA) deformation when running with nonrearfoot strike. The aim of this study was to investigate the potential differences in foot-ankle muscle strength between RF and NRF runners.

Methods

A cross-sectional study including 40 RF and 40 NRF runners was conducted. The foot posture and the maximal voluntary isometric strength (MVIS) of 6 foot-ankle muscles were measured. The footstrike pattern was determined using a 2-D camera during a self-paced run on a treadmill.

Results

NRF had higher MVIS for ankle plantar flexor (+12.5%, P = .015), ankle dorsiflexor (+17.7%, P = .01), hallux flexor (+11%, P = .04), and lesser toe flexor (+20.8%, P = .0031). We found a small positive correlation between MVIS of ankle plantar flexor with MVIS of hallux flexor (r = 0.26; P = .01) and lesser toe flexor (r = 0.28; P = .01).

Conclusion

In this cross-sectional study, we found that NRF runners on average have a higher MVIS of hallux and lesser toe flexor compared with RF runners. NRF runners also have a higher MVIS of ankle plantar flexor and dorsiflexor than RF runners. We found only a small correlation between ankle plantar flexor and foot muscle strength.

Level of Evidence

Level III, case-control study.

Effects of intrinsic foot muscle strengthening on the medial longitudinal arch mobility and function: A systematic review

OBJECTIVE

To Investigate the effects of intrinsic foot muscle (IFM) strengthening on foot's medial longitudinal arch (MLA) mobility and function in healthy individuals. We also identified exercise type and resistance training characteristics (series and repetitions).

METHODS

Eight databases were searched, between October 2020 and February 2021 and updated in May 2021. We included randomized controlled trials involving IFM strengthening exercises compared with controls (no exercise or exercises not involving isolated intrinsic foot muscle strengthening). Methodological quality of the studies was assessed using PEDro scale and Cochrane Risk of Bias tool. Quality of evidence was evaluated using GRADE model (Grading of Recommendations, Assessment, Development and Evaluations).

RESULTS

Four randomized controlled trials (RCT) were included. IFM strengthening did not change MLA mobility in the short-term (4 weeks); however, it promoted medium-term effects (8 weeks - low quality of evidence). IFM exercises improved function in the short and medium-term (low quality of evidence). Most studies used the short-foot exercise and the toe-towel curl exercise with contractions of 5 s and load progression from sitting to standing.

CONCLUSION

IFM strengthening exercises change MLA mobility in the medium-term (8 weeks) and improve the dynamic balance of healthy individuals in short- (4 weeks) and medium-terms.

A Bio-Inspired Arched Foot with Individual Toe Joints and Plantar Fascia

This paper presents the design and testing of an arched foot with several biomimetic features, including five individual MTP (toe) joints, four individual midfoot joints, and plantar fascia. The creation of a triple-arched foot represents a step further in bio-inspired design compared to other published designs. The arched structure creates flexibility that is similar to human feet with a vertical deflection of up to 12 mm. The individual toe joints enable abduction-adduction in the forefoot and therefore a natural pronation motion. Adult female bone data was obtained and converted into a CAD model to accurately identify the location of bones, joints, and arches. An analytical model is presented that gives the relationship between the vertical stiffness and horizontal stiffness of the longitudinal arches and therefore allows the optimization of stiffness elements. Experimental tests have demonstrated a vertical arch stiffness of 76 N/mm which is similar to adult human feet. The range of movement of the foot is similar to human feet with the following values: dorsi-plantarflexion (28°/37°), inversion-eversion (30°/15°), and abduction-adduction (30°/39°). Tests have also demonstrated a three-point contact with the ground that is similar to human feet.

Mechanics of the human foot during walking on different slopes

When humans walk on slopes, the ankle, knee, and hip joints modulate their mechanical work to accommodate the mechanical demands. Yet, it is unclear if the foot modulates its work output during uphill and downhill walking. Therefore, we quantified the mechanical work performed by the foot and its subsections of twelve adults walked on five randomized slopes (-10°, -5°, 0°, +5°, +10°). We estimated the work of distal-to-hindfoot and distal-to-forefoot structures using unified deformable segment analysis and the work of the midtarsal, ankle, knee, and hip joints using a six-degree-of-freedom model. Further, using a geometric model, we estimated the length of the plantar structures crossing the longitudinal arch while accounting for the first metatarsophalangeal wrapping length. We hypothesized that compared to level walking, downhill walking would increase negative and net-negative work magnitude, particularly at the early stance phase, and uphill walking would increase the positive work, particularly at the mid-to-late stance phase. We found that downhill walking increased the magnitude of the foot's negative and net-negative work, especially during early stance, highlighting its capacity to absorb impacts when locomotion demands excessive energy dissipation. Notably, the foot maintained its net dissipative behavior between slopes; however, the ankle, knee, and hip shifted from net energy dissipation to net energy generation when changing from downhill to uphill. Such results indicate that humans rely more on joints proximal to the foot to modulate the body's total mechanical energy. Uphill walking increased midtarsal's positive and distal-to-forefoot negative work in near-equal amounts. That coincided with the prolonged lengthening and delayed shortening of the plantar structures, resembling a spring-like function that possibly assists the energetic demands of locomotion during mid-to-late stance. These results broaden our understanding of the foot's mechanical function relative to the leg's joints and could inspire the design of wearable assistive devices that improve walking capacity.

Examining the intrinsic foot muscles' capacity to modulate plantar flexor gearing and ankle joint contributions to propulsion in vertical jumping

BACKGROUND

During human locomotion, a sufficiently stiff foot allows the ankle plantar flexors to generate large propulsive powers. Increasing foot stiffness (e.g., via a carbon plate) increases the ankle's external moment arm in relation to the internal moment arm (i.e., increasing gear ratio), reduces plantar flexor muscles' shortening velocity, and enhances muscle force production. In contrast, when activation of the foot's intrinsic muscles is impaired, there is a reduction in foot and ankle work and metatarsophalangeal joint stiffness. We speculated that the reduced capacity to actively control metatarsophalangeal joint stiffness may impair the gearing function of the foot at the ankle.

METHODS

We used a tibial nerve block to examine the direct effects of the intrinsic foot muscles on ankle joint kinetics, in vivo medial gastrocnemius' musculotendinous dynamics, and ankle gear ratio on 14 participants during maximal vertical jumping.

RESULTS

Under the nerve block, the internal ankle plantar flexion moment decreased (p = 0.004) alongside a reduction in external moment arm length (p = 0.021) and ankle joint gear ratio (p = 0.049) when compared to the non-blocked condition. Although medial gastrocnemius muscle-tendon unit and fascicle velocity were not different between conditions, the Achilles tendon was shorter during propulsion in the nerve block condition (p < 0.001).

CONCLUSION

In addition to their known role of regulating the energetic function of the foot, our data indicate that the intrinsic foot muscles also act to optimize ankle joint torque production and leverage during the propulsion phase of vertical jumping.

The experimental investigation of foot slip-turning motion of the musculoskeletal robot on toe joints

Owing to their complex structural design and control system, musculoskeletal robots struggle to execute complicated tasks such as turning with their limited range of motion. This study investigates the utilization of passive toe joints in the foot slip-turning motion of a musculoskeletal robot to turn on its toes with minimum movements to reach the desired angle while increasing the turning angle and its range of mobility. The different conditions of plantar intrinsic muscles (PIM) were also studied in the experiment to investigate the effect of actively controlling the stiffness of toe joints. The results show that the usage of toe joints reduced frictional torque and improved rotational angle. Meanwhile, the results of the toe-lifting angle show that the usage of PIM could contribute to preventing over-dorsiflexion of toes and possibly improving postural stability. Lastly, the results of ground reaction force show that the foot with different stiffness can affect the curve pattern. These findings contribute to the implementations of biological features and utilize them in bipedal robots to simplify their motions, and improve adaptability, regardless of their complex structure.

Kinetic coupling in distal foot joints during walking

BACKGROUND

Kinematic coupling between the first metatarsophalangeal (MTP) and midtarsal joints is evident during gait and other movement tasks, however kinetic foot coupling during walking has not been examined. Furthermore, contributing factors to foot coupling are still unclear. Therefore, the purpose of this study was to investigate kinematic and kinetic coupling within the foot by restricting MTP motion during overground walking. We hypothesized that when the MTP joint was prevented from fully extending, the midtarsal joint would achieve less peak motion and generate less positive work compared to walking with normal MTP motion.

METHODS

Twenty-six individuals participated in this randomized cross-over study. Using motion capture to track motion, participants walked at 1.3 m/s while wearing a brace that restricted MTP motion in a neutral (BR_NT) or extended (BR_EX) position. Additionally, participants walked while wearing the brace in a freely moveable setting (BR_UN) and with no brace (CON). A pressure/shear sensing device was used to capture forces under each foot segment. During stance, peak joint motion and work were calculated for the MTP and midtarsal joints using inverse dynamics. A series of ANOVAs and Holm post hoc tests were performed for all metrics (alpha = 0.05).

RESULTS

The brace successfully decreased peak MTP motion by 19% compared to BR_UN and CON. This was coupled with 9.8% less midtarsal motion. Kinetically, the work absorbed by the MTP joint (26-51%) and generated by the midtarsal joint (30-38%) were both less in BR_EX and BR_NT compared to BR_UN.

CONCLUSION

Implications and sources of coupling between the MTP and midtarsal joints are discussed within the context of center of pressure shifts and changes to segmental foot forces. Our results suggest that interventions aimed at modulating MTP negative work (such as footwear or assistive device design) should not ignore the midtarsal joint.

Immediate effects of elastic tape application on the foot sole: a randomized controlled trial

[Purpose] In this study, we investigated the effectiveness of elastic tape in increasing the dorsiflexion angle and plantar flexor strength in healthy individuals. [Participants and Methods] This randomized controlled trial included 24 healthy university students who were categorized into the following groups (12 participants in each group): the intervention group (elastic tape was applied to the dominant foot) and the control group (no intervention was performed). We performed intergroup comparison of the pre- and post-intervention dorsiflexion angles and plantar flexor strength. Additionally, we performed subgroup analyses based on a straight-leg raise angle of 70°. [Results] We observed no significant intergroup differences in the dorsiflexion angle or plantar flexor strength. However, the post-intervention dorsiflexion angle was significantly greater than the pre-intervention angle in the subgroup with a straight-leg raise angle of <70° among participants in the elastic tape group. [Conclusion] Elastic tape application may effectively increase the dorsiflexion angle in individuals without hamstring extensibility.

Effects of short foot exercise combined with lower extremity training on dynamic foot function in individuals with flexible flatfoot: A randomized controlled trial

BACKGROUND

Flexible flatfoot has demonstrated biomechanical linkages between distal and proximal lower extremities. However, supporting evidence is required to investigate the benefits of short foot exercise (SF) and short foot exercise in combination with lower extremity training (SFLE) on dynamic foot function.

RESEARCH QUESTION

This study aimed to determine the effects of a 6-week SF, 6-week SFLE, or control condition (no intervention) on dynamic foot function during gait in individuals with flexible flatfoot.

METHODS

Forty-five individuals with flexible flatfoot were randomly assigned into three conditions: (1) SF, (2) SFLE, and (3) control conditions. Participants in two intervention programs performed daily training via telerehabilitation and a home-based exercise program. Foot kinematics and center of pressure excursion index (CPEI) during the gait, intrinsic foot muscle test, and navicular drop test were assessed at baseline and after the 6-week intervention program.

RESULTS

Post-intervention participants in the SF and SFLE conditions showed a shorter time to the lowest medial longitudinal arch (MLA) and improved MLA motion during the stance phase compared with the baseline. In addition, participants in the SFLE conditions showed greater changes in CPEI than in the SF and control conditions. Improvements in intrinsic foot muscle and navicular drop tests were also observed in participants in both intervention programs post-intervention.

SIGNIFICANCE

A major finding of the study was the improvement in dynamic foot function during gait in individuals with flexible flatfoot after the six weeks of the SF and SFLE intervention programs. Both intervention programs appear to have the potential for inclusion in a corrective program for individuals with flexible flatfoot.

Comparison of intrinsic foot muscle morphology and isometric strength among runners with different strike patterns

This study aimed to compare the intrinsic foot muscle (IFM) morphology and isometric strength among runners with habitual rearfoot strike (RFS) and non-rearfoot strike (NRFS) patterns. A total of 70 recreational male runners were included in this study (32 RFS and 38 NRFS), an ultrasound device and hand-held dynamometry were used to measure IFM morphology and isometric strength. Results indicated that the RFS runners had significantly thicker tibialis anterior (P = 0.01, ES = 0.64, 95% CI [0.01-0.07]) in IFMs morphology and higher Toe2345 flexion strength in IFMs strength (P = 0.04, ES = 0.50, 95% CI [0.01-0.27]) than NRFS runners, the cross-sectional area of flexor digitorum brevis was positively correlated with T2345 flexion strength (r = 0.33, p = 0.04), T12345 (r = 0.37, p = 0.02) and Doming (r = 0.36, p = 0.03) for runners with NRFS. IFMs morphology and isometric strength were associated with foot strike pattern, preliminary findings provide new perspectives for NRFS runners through the simple measurement of IFMs morphological characteristics predicting IFMs strength, future studies could adopt IFMs training to compensate the muscle strength defects and prevent foot-related injuries.

Three-Dimensional Analysis of the Windlass Mechanism Using Weightbearing Computed Tomography in Healthy Volunteers

BACKGROUND

The windlass mechanism (WM) increases the longitudinal arch of the foot via tension of the plantar aponeurosis during dorsiflexion of the metatarsophalangeal (MTP) joint. The purpose of this study was to perform a 3-dimensional evaluation of the displacement of each joint and the height of the navicular during dorsiflexion of the first MTP joint by using weightbearing computed tomography (CT).

METHODS

Participants were 6 men and 8 women with 23 healthy feet. CT of the foot with a load equivalent to the participant's body weight was performed. The first MTP joint was in the neutral position and dorsiflexed 30 degrees. Between the conditions, we measured the (1) rotation of each bone, (2) rotation of the distal bone with respect to the proximal bone at each joint, and (3) height of the navicular.

RESULTS

With respect to the tibia, the calcaneus was at 0.8 ± 0.7 degrees dorsiflexion and 1.4 ± 0.9 degrees inversion, while the talus was at 2.0 ± 1.2 degrees dorsiflexion and 0.1 ± 0.8 degrees eversion. The navicular was at 1.3 ± 1.2 degrees dorsiflexion and 3.2 ± 2.1 degrees inversion, whereas the medial cuneiform was at 0.3 ± 0.6 degrees plantarflexion and 1.3 ± 1.1 degrees inversion. At the talonavicular joint, the navicular was at 0.7 ± 1.3 degrees plantarflexion, whereas at the cuneonavicular joint, the medial cuneiform bone was at 1.4 ± 1.4 degrees plantarflexion. The height of the navicular increased by 1.1 ± 0.6 mm.

CONCLUSION

We 3-dimensionally confirmed the dynamics of WM and found that the calcaneus, navicular, and medial cuneiform moved in all 3 planes. The results suggest that the cuneonavicular joint has the greatest movement among the joints. We believe that these findings will help to elucidate the pathogenesis of WM-related diseases and lead to advances in treatments for pathologies involving the longitudinal arch.

LEVEL OF EVIDENCE

Level IV, case series.

Plantar intrinsic foot muscle activation during functional exercises compared to isolated foot exercises in younger adults

BACKGROUND

Training the plantar intrinsic foot muscles (PIFMs) has the potential to benefit patients with lower extremity musculoskeletal conditions as well as the aged population. Isolated foot exercises, often standard in clinical practice, are difficult to perform, whereas functional exercises are much easier to accomplish. However, it is unclear whether functional exercises are comparable to isolated foot exercises in activating the PIFMs.

OBJECTIVE

This study aims to compare the activation of PIFMs between functional exercises versus isolated foot exercises.

METHODS

Using surface electromyography (EMG), muscle activation of three PIFMs was measured in four functional exercises (i.e. normal/unstable toe stance, toe walking, and hopping) versus a muscle-specific isolated foot exercise in 29 younger adults, resulting in 12 comparisons.

RESULTS

Functional exercises showed larger mean EMG amplitudes than the isolated foot exercises in 25% of the 12 comparisons, while there was no difference in the remaining 75%.

CONCLUSION

Functional exercises provoked comparable or even more activation of the PIFMs than isolated foot exercises. Given that functional exercises are easier to perform, this finding indicates the need to further investigate the effectiveness of functional exercises in physical therapy to improve muscle function and functional task performance in populations that suffer from PIFM weakness or dysfunction.

Immediate effects of forefoot wedges on multi-segment foot kinematics during jogging in recreational runners with a symptomatic pronated foot

Background: Foot orthoses (FOs) have been used to alter lower limb kinematics and kinetics in pronated feet. A clear relationship between FOs' features, e.g., the amount of wedging and support, and the corresponding biomechanical responses is vital for the design and prescription of FOs. In this study, we sought to determine if changing the level of the forefoot wedge would cause a linear response in the multi-segment foot kinematics during jogging, and if this effect would be enhanced by an arch support. Methods: Ten pairs of 3D printed FOs with five levels of forefoot wedges and two levels of arch supports were tested on 12 recreational runners with a symptomatic pronated foot. Multi-segment foot kinematic data during jogging was measured using the Oxford Foot Model. Two-way ANOVAs were performed to examine the main effect of the forefoot wedge and arch support, as well as their interaction on peak joint angles. Statistical parametric mapping and paired-t tests were used to identify differences in the foot kinematic traces and the joint range of motion (ROM) between each FO and the control, respectively. Results: Linear main effects for the forefoot wedge level were found in the forefoot peak dorsiflexion, eversion and rearfoot peak dorsiflexion of jogging. FOs with a medial forefoot wedge caused an average of 2.5° reduction of the forefoot peak abduction during jogging. Furthermore, forefoot wedges showed an opposite effect on the sagittal ROM of the forefoot and rearfoot. Adding an arch support did not improve the kinematic performance of a forefoot wedge during jogging. Conclusion: This study highlights a linear dose-response effect of a forefoot wedge on forefoot kinematics during jogging, and suggests using a medial forefoot wedge as an anti-pronator component for controlling forefoot motion of a pronated foot.

Effects of a Single Electrical Stimulation Session on Foot Force Production, Foot Dome Stability, and Dynamic Postural Control

CONTEXT

Mounting evidence suggests neuromuscular electrical stimulation (NMES) as a promising modality for enhancing lower limb muscle strength, yet the functional effects of a single electrical stimulation session for improving the function of the intrinsic foot muscles (IFM) has not been evaluated.

OBJECTIVE

To investigate the immediate effects of an NMES session compared with a sham stimulation session on foot force production, foot dome stability, and dynamic postural control in participants with static foot pronation.

DESIGN

Randomized controlled clinical trial.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 46 participants (23 males, 23 females) with static foot pronation according to their Foot Posture Index (score ≥ 6) were randomly assigned to an NMES (n = 23) or control (n = 23) group.

INTERVENTION(S)

The NMES group received a single 15-minute NMES session on the dominant foot across the IFM. The control group received a 15-minute sham electrical stimulation session.

MAIN OUTCOME MEASURE(S)

All outcome measurements were assessed before and after the intervention and consisted of foot force production on a pressure platform, foot dome stability, and dynamic postural control. Statistical analysis was based on the responsiveness of the outcome measures and responder analysis using the minimum detectable change scores for each outcome measure.

RESULTS

In the NMES group, 78% of participants were classified as responders for at least 2 of the 3 outcomes, compared with only 22% in the control group. The relative risk of being a responder in the NMES group compared with the control group was 3.6 (95% CI = 1.6, 8.1]. Interestingly, we found that all participants who concomitantly responded to foot strength and navicular drop (n = 8) were also responders in dynamic postural control.

CONCLUSIONS

Compared with a sham stimulation session, a single NMES session was effective in immediately improving foot function and dynamic postural control in participants with static foot pronation. These findings support the role of NMES for improving IFM function in this population.

Foot shape is related to load-induced shape deformations, but neither are good predictors of plantar soft tissue stiffness

Modern human feet are considered unique among primates in their capacity to transmit propulsive forces and re-use elastic energy. Considered central to both these capabilities are their arched configuration and the plantar aponeurosis (PA). However, recent evidence has shown that their interactions are not as simple as proposed by the theoretical and mechanical models that established their significance. Using three-dimensional foot scans and statistical shape and deformation modelling, we show that the shape of the longitudinal and transverse arches varies widely among the healthy adult population, and that the former is subject to load-induced arch flattening, whereas the latter is not. However, longitudinal arch shape and flattening are only one of the various foot shape-deformation relationships. PA stiffness was also found to vary widely. Yet only a small amount of this variability (approx. 10-18%) was explained by variations in foot shape, deformation and their combination. These findings add to the mounting evidence showing that foot mechanics are complex and cannot be accurately represented by simple models. Especially the interactions between longitudinal arch and PA appear to be far less constrained than originally proposed, most likely due to the many degrees of freedom provided by the structural complexity of our feet.

Evaluation of assumptions in foot and ankle biomechanical models

BACKGROUND

A variety of biomechanical models have been used in studies of foot and ankle disorders. Assumptions about the element types, material properties, and loading and boundary conditions are inherent in every model. It was hypothesized that the choice of these modeling assumptions could have a significant impact on the findings of the model.

METHODS

We investigated the assumptions made in a number of biomechanical models of the foot and ankle and evaluated their effects on the results of the studies. Specifically, we focused on: (1) element choice for simulation of ligaments and tendons, (2) material properties of ligaments, cortical and trabecular bones, and encapsulating soft tissue, (3) loading and boundary conditions of the tibia, fibula, tendons, and ground support.

FINDINGS

Our principal findings are: (1) the use of isotropic solid elements to model ligaments and tendons is not appropriate because it allows them to transmit unrealistic bending and twisting moments and compressive forces; (2) ignoring the difference in elastic modulus between cortical and trabecular bones creates non-physiological stress distribution in the bones; (3) over-constraining tibial motion prevents anticipated deformity within the foot when simulating foot deformities, such as progressive collapsing foot deformity; (4) neglecting the Achilles tendon force affects almost all kinetic and kinematic parameters through the foot; (5) the axial force applied to the tibia and fibula is not equal to the ground reaction force due to the presence of tendon forces.

INTERPRETATION

The predicted outcomes of a foot model are highly sensitive to the model assumptions.

Mechanics and energetics of human feet: a contemporary perspective for understanding mobility impairments in older adults

Much of our current understanding of age-related declines in mobility has been aided by decades of investigations on the role of muscle-tendon units spanning major lower extremity joints (e.g., hip, knee and ankle) for powering locomotion. Yet, mechanical contributions from foot structures are often neglected. This is despite the emerging evidence for their critical importance in youthful locomotion. With rapid growth in the field of human foot biomechanics over the last decade, our theoretical knowledge of young asymptomatic feet has transformed, from long-held views of a stiff lever and a shock-absorber to a versatile system that can modulate mechanical power and energy output to accommodate various locomotor task demands. In this perspective review, we predict that the next set of impactful discoveries related to locomotion in older adults will emerge by integrating the novel tools and approaches that are currently transforming the field of human foot biomechanics. By illuminating the functions of feet in older adults, we envision that future investigations will refine our mechanistic understanding of mobility deficits affecting our aging population, which may ultimately inspire targeted interventions to rejuvenate the mechanics and energetics of locomotion.

Neuromechanical adaptations of foot function when hopping on a damped surface

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces.NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

Foot arch rigidity in walking: In vivo evidence for the contribution of metatarsophalangeal joint dorsiflexion

Human foot rigidity is thought to provide a more effective lever with which to push against the ground. Tension of the plantar aponeurosis (PA) with increased metatarsophalangeal (MTP) joint dorsiflexion (i.e., the windlass mechanism) has been credited with providing some of this rigidity. However, there is growing debate on whether MTP joint dorsiflexion indeed increases arch rigidity. Further, the arch can be made more rigid independent of additional MTP joint dorsiflexion (e.g., when walking with added mass). The purpose of the present study was therefore to compare the influence of increased MTP joint dorsiflexion with the influence of added mass on the quasi-stiffness of the midtarsal joint in walking. Participants walked with a rounded wedge under their toes to increase MTP joint dorsiflexion in the toe-wedge condition, and wore a weighted vest with 15% of their body mass in the added mass condition. Plantar aponeurosis behavior, foot joint energetics, and midtarsal joint quasi-stiffness were compared between conditions to analyze the mechanisms and effects of arch rigidity differences. Midtarsal joint quasi-stiffness was increased in the toe-wedge and added mass conditions compared with the control condition (both p < 0.001). In the toe-wedge condition, the time-series profiles of MTP joint dorsiflexion and PA strain and force were increased throughout mid-stance (p < 0.001). When walking with added mass, the time-series profile of force in the PA did not increase compared with the control condition although quasi-stiffness did, supporting previous evidence that the rigidity of the foot can be actively modulated. Finally, more mechanical power was absorbed (p = 0.006) and negative work was performed (p < 0.001) by structures distal to the rearfoot in the toe-wedge condition, a condition which displayed increased midtarsal joint quasi-stiffness. This indicates that a more rigid foot may not necessarily transfer power to the ground more efficiently.

Novel Multi-Segment Foot Model Incorporating Plantar Aponeurosis for Detailed Kinematic and Kinetic Analyses of the Foot With Application to Gait Studies

Kinetic multi-segment foot models have been proposed to evaluate the forces and moments generated in the foot during walking based on inverse dynamics calculations. However, these models did not consider the plantar aponeurosis (PA) despite its potential importance in generation of the ground reaction forces and storage and release of mechanical energy. This study aimed to develop a novel multi-segment foot model incorporating the PA to better elucidate foot kinetics. The foot model comprised three segments: the phalanx, forefoot, and hindfoot. The PA was modeled using five linear springs connecting the origins and the insertions via intermediate points. To demonstrate the efficacy of the foot model, an inverse dynamic analysis of human gait was performed and how the inclusion of the PA model altered the estimated joint moments was examined. Ten healthy men walked along a walkway with two force plates placed in series close together. The attempts in which the participant placed his fore- and hindfoot on the front and rear force plates, respectively, were selected for inverse dynamic analysis. The stiffness and the natural length of each PA spring remain largely uncertain. Therefore, a sensitivity analysis was conducted to evaluate how the estimated joint moments were altered by the changes in the two parameters within a range reported by previous studies. The present model incorporating the PA predicted that 13%–45% of plantarflexion in the metatarsophalangeal (MTP) joint and 8%–29% of plantarflexion in the midtarsal joints were generated by the PA at the time of push-off during walking. The midtarsal joint generated positive work, whereas the MTP joint generated negative work in the late stance phase. The positive and negative work done by the two joints decreased, indicating that the PA contributed towards transfer of the energy absorbed at the MTP joint to generate positive work at the midtarsal joint during walking. Although validation is limited due to the difficulty associated with direct measurement of the PA force in vivo, the proposed novel foot model may serve as a useful tool to clarify the function and mechanical effects of the PA and the foot during dynamic movements.

Improving the measurement of intrinsic foot muscle morphology and composition from high-field (7T) magnetic resonance imaging

Magnetic resonance imaging (MRI) can be used to quantify intrinsic foot muscle morphology and composition. Due to the high spatial resolution required to adequately capture the architecturally complex anatomy, manual segmentation is time consuming and not clinically feasible. The aim of this study was to evaluate if a reduced number of MRI slices can be used to accurately estimate intrinsic foot muscle volume and composition. A three-dimensional 2-point Dixon sequence of the whole foot was acquired at 7-Tesla for thirteen asymptomatic individuals and twenty individuals with plantar heel pain. Slice intervals of 2, 3, 5, 10, 15 and 30 were used to calculate alternative muscle volume and composition, and were compared to reference values calculated from every available slice. Agreement between methods was assessed by calculating mean differences and 95% limits of agreement, and inspection of Bland -Altman plots. In both groups, slice intervals of 2, 3 and 5 provided excellent precision for all muscles (measurement error < 1%). Larger slice intervals of 10, 15 and 30 provided excellent precision for some muscles, but for other muscles (e.g. small forefoot muscles), error was up to 7.3%. Bland-Altman plots showed no systematic measurement bias. This study provides a quantitative basis for selecting a reduced number of slices to measure intrinsic foot muscle volume and composition from MRI. A slice interval of 10 may provide a balance between efficiency (36 mins vs. 6 h) and accuracy (error < 2.4%) across all intrinsic foot muscles in asymptomatic individuals and those with plantar heel pain.

Contributions of Intrinsic and Extrinsic Foot Muscles during Functional Standing Postures

Purpose. Maintaining balance during static standing postures requires the coordination of many neuromuscular mechanisms. The role of the intrinsic and extrinsic foot muscles in this paradigm has yet to be clearly defined. The purpose of this study was to explore foot muscle activation during static phases on common weight-bearing tasks of varying loads and balance demands. Methods. Twenty healthy young adults performed 6 standing postures (single-limb and double-limb stand, squat, and heel raise) with one foot on a force plate. Muscle activity was recorded from the abductor hallucis, flexor hallucis longus and brevis, and tibialis posterior using intramuscular electrodes; surface electrodes were used to record activity from the peroneus longus and tibialis anterior. Two-way repeated measures ANOVA (2 loading conditions × 3 postures) were run to compare muscle activation and center of pressure velocity. Results. Intrinsic foot muscle activity increased as loading and postural demand increased; however, the specific effects varied for each of the extrinsic foot muscles. Conclusions. These results suggest that the intrinsic foot muscles play an important role in maintaining static balance. Strengthening intrinsic and extrinsic foot muscles may help increase stability in people who have weak toe flexors or who suffer from a variety of foot pathologies.

Inter-strides variability affects internal foot tissue loadings during running

Running overuse injuries result from an imbalance between repetitive loadings on the anatomical structures and their ability to adapt to these loadings. Unfortunately, the measure of these in-vivo loadings is not easily accessible. An optimal amount of movement variability is thought to decrease the running overuse injury risk, but the influence of movement variability on local tissue loading is still not known. A 3D dynamic finite element foot model driven by extrinsic muscle forces was developed to estimate the stress undergone by the different internal foot structures during the stance phase. The boundary conditions of different trials with similar running speed were used as input. Variability in bone stress (10%) and cartilage pressure (16%) can be expected while keeping the overall running speed constant. Bone and cartilage stress were mainly influenced by the muscle force profiles rather than by ground reaction force. These findings suggest, first, that the analysis of a single trial only is not representative of the internal tissue loadings distribution in the foot and second, that muscle forces must be considered when estimating bone and cartilage loadings at the foot level. This model could be applied to an optimal clinical management of the overuse injury.

Determination of relationship between foot arch, hindfoot, and hallux motion using Oxford foot model: Comparison between walking and running

BACKGROUND

The foot arch plays an important role in propulsion and shock absorption during walking and running; however, the relationship among the foot arch, metatarsal locking theory, and nature of the windlass mechanism (WM) remain unclear.

RESEARCH QUESTION

What are the differences in the kinematic relationship between the foot arch, hindfoot, and hallux during walking and running?

METHODS

Relative angles within the foot were measured in 18 healthy men using the Oxford foot model (OFM). Data for barefoot walking at a comfortable speed and rearfoot running at 2.0 m/s were collected. Angles of the forefoot relative to the hindfoot (OFM-arch), hallux relative to the forefoot (Hallux) on the sagittal plane, and hindfoot relative to the shank (Hindfoot) on three anatomical planes were obtained. The medial longitudinal arch (MLA) angle was calculated to verify that OFM-arch can substitute the MLA angle. Each parameter was subjected to cross-correlation analysis and Wilcoxon signed-rank tests to examine the relationship with OFM-arch and compare them during walking and running.

RESULT

OFM-arch was similar to the conventional MLA projection angle in both trials (gait: 0.79, running: 0.96 p < 0.01). Synchronization of the OFM-arch and Hallux angles was higher in running than in walking (gait: -0.09, running: -0.75 p < 0.01). Hindfoot supination was unrelated to OFM-arch. Hindfoot angle on the transverse plane exhibited a moderate relationship with OFM-arch, indicating different correlations in walking and running (gait: 0.63, running: -0.68 p < 0.01).

SIGNIFICANCE

The elevation of the foot arch due to hallux dorsiflexion differed during walking and running; hence, other factors besides WM (such as intrinsic muscles) may affect the foot arch elevation during running. The hindfoot in the frontal plane does not contribute to arch raising and foot stability during running; it features different relationships with OFM-arch during walking and running.

Intra-assessor reliability and measurement error of ultrasound measures for foot muscle morphology in older adults using a tablet-based ultrasound machine

Background

To gain insight into the role of plantar intrinsic foot muscles in fall-related gait parameters in older adults, it is fundamental to assess foot muscles separately. Ultrasonography is considered a promising instrument to quantify the strength capacity of individual muscles by assessing their morphology. The main goal of this study was to investigate the intra-assessor reliability and measurement error for ultrasound measures for the morphology of selected foot muscles and the plantar fascia in older adults using a tablet-based device. The secondary aim was to compare the measurement error between older and younger adults and between two different ultrasound machines.

Methods

Ultrasound images of selected foot muscles and the plantar fascia were collected in younger and older adults by a single operator, intensively trained in scanning the foot muscles, on two occasions, 1–8 days apart, using a tablet-based and a mainframe system. The intra-assessor reliability and standard error of measurement for the cross-sectional area and/or thickness were assessed by analysis of variance. The error variance was statistically compared across age groups and machines.

Results

Eighteen physically active older adults (mean age 73.8 (SD: 4.9) years) and ten younger adults (mean age 21.9 (SD: 1.8) years) participated in the study. In older adults, the standard error of measurement ranged from 2.8 to 11.9%. The ICC ranged from 0.57 to 0.97, but was excellent in most cases. The error variance for six morphology measures was statistically smaller in younger adults, but was small in older adults as well. When different error variances were observed across machines, overall, the tablet-based device showed superior repeatability.

Conclusions

This intra-assessor reliability study showed that a tablet-based ultrasound machine can be reliably used to assess the morphology of selected foot muscles in older adults, with the exception of plantar fascia thickness. Although the measurement errors were sometimes smaller in younger adults, they seem adequate in older adults to detect group mean hypertrophy as a response to training. A tablet-based ultrasound device seems to be a reliable alternative to a mainframe system. This advocates its use when foot muscle morphology in older adults is of interest.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13047-022-00510-1.

Stiffening the human foot with a biomimetic exotendon

Shoes are generally designed protect the feet against repetitive collisions with the ground, often using thick viscoelastic midsoles to add in-series compliance under the human. Recent footwear design developments have shown that this approach may also produce metabolic energy savings. Here we test an alternative approach to modify the foot-ground interface by adding additional stiffness in parallel to the plantar aponeurosis, targeting the windlass mechanism. Stiffening the windlass mechanism by about 9% led to decreases in peak activation of the ankle plantarflexors soleus (~ 5%, p < 0.001) and medial gastrocnemius (~ 4%, p < 0.001), as well as a ~ 6% decrease in positive ankle work (p < 0.001) during fixed-frequency bilateral hopping (2.33 Hz). These results suggest that stiffening the foot may reduce cost in dynamic tasks primarily by reducing the effort required to plantarflex the ankle, since peak activation of the intrinsic foot muscle abductor hallucis was unchanged (p = 0.31). Because the novel exotendon design does not operate via the compression or bending of a bulky midsole, the device is light (55 g) and its profile is low enough that it can be worn within an existing shoe.

Effects of footwear cushioning on leg and longitudinal arch stiffness during running

During running, humans increase leg stiffness on more compliant surfaces through an in-series spring relationship to maintain constant support mechanics. Following this notion, the compliant midsole material of standard footwear may cause individuals to increase leg stiffness while running, especially in footwear with very thick midsoles. Recently, researchers have also proposed that footwear stiffness can affect the stiffness of the foot's longitudinal arch (LA) via a similar mechanism. To test these ideas, we used 3D motion capture to record 20 participants running on a forceplate-instrumented treadmill while barefoot, and while wearing three types of sandals composed of materials ranging an order of magnitude in Young's modulus: ethylene vinyl acetate (EVA), and two varieties of polyurethane rubber (R30 and R60). We calculated leg stiffness using standard methods, and measured LA stiffness based on medial midfoot kinematics. While there was an overall significant effect of footwear on leg stiffness (P = 0.047), post-hoc tests revealed no significant differences among individual pairs of conditions, and there was no effect of footwear on LA stiffness. However, participants exhibited significantly greater LA compression when barefoot than when running in EVA (P = 0.004) or R30 (P = 0.036) sandals. These results indicate that standard footwear midsole materials are too stiff to appreciably affect leg stiffness during running, meaning that increasing midsole thickness is unlikely to cause individuals to alter their leg stiffness. However, use of footwear does cause individuals to restrict LA compression when compared to running barefoot, and further research is needed to understand why.

Comparative radiographic analysis of three-dimensional innate mobility of the foot bones under axial loading of humans and African great apes

The human foot is considered to be morphologically adapted for habitual bipedal locomotion. However, how the mobility and mechanical interaction of the human foot with the ground under a weight-bearing condition differ from those of African great apes is not well understood. We compared three-dimensional (3D) bone kinematics of cadaver feet under axial loading of humans and African great apes using a biplanar X-ray fluoroscopy system. The calcaneus was everted and the talus and tibia were internally rotated in the human foot, but such coupling motion was much smaller in the feet of African great apes, possibly due to the difference in morphology of the foot bones and articular surfaces. This study also found that the changes in the length of the longitudinal arch were larger in the human foot than in the feet of chimpanzees and gorillas, indicating that the human foot is more deformable, possibly to allow storage and release of the elastic energy during locomotion. The coupling motion of the calcaneus and the tibia, and the larger capacity to be flattened due to axial loading observed in the human foot are possibly morphological adaptations for habitual bipedal locomotion that has evolved in the human lineage.

Daily activity in minimal footwear increases foot strength

The human foot is uniquely adapted to bipedal locomotion and has a deformable arch of variable stiffness. Intrinsic foot muscles regulate arch deformation, making them important for foot function. In this study we explore the hypothesis that normal daily activity in minimal footwear, which provides little or no support, increases foot muscle strength. Western adults wore minimal footwear for a six-month period (the “intervention” group). Foot strength, i.e., maximum isometric plantarflexion strength at the metatarsophalangeal joints, and foot biometrics were measured before and after the intervention. An additional group was investigated to add further insight on the long-term effects of footwear, consisting of Western adults with an average 2.5 years of experience in minimal footwear (the “experienced” group). This study shows that foot strength increases by, on average, 57.4% (p < 0.001) after six months of daily activity in minimal footwear. The experienced group had similar foot strength as the post intervention group, suggesting that six months of regular minimal footwear use is sufficient to gain full strength, which may aid healthy balance and gait.

Foot anatomy, walking energetics, and the evolution of human bipedalism

Interspecies differences in locomotor efficiency have been extensively researched, but within-species variation in the metabolic cost of walking and its underlying causes have received much less attention. This is somewhat surprising given the importance of walking energetics to natural selection, and the fact that the mechanical efficiency of striding bipedalism in modern humans is thought to be related in some part to the unique morphology of the human foot. Previous studies of human running have linked specific anatomical traits in the foot to variations in locomotor energetics to provide insight into form-function relationships in human evolution. However, such studies are relatively rare, particularly for walking. In this study, relationships between a range of functional musculoskeletal traits in the human lower limb and the energetics of walking over compliant and noncompliant substrates are examined, with particular focus on the lower limb and foot. Twenty-nine young, healthy individuals walked across three surfaces-a noncompliant laboratory floor, and compliant 6 cm and 13 cm thick foams-at self-selected speeds while oxygen consumption was measured, from which the metabolic cost of transport was calculated. Lower limb lengths, calcaneus lengths, foot shape indices, and maximum isometric plantarflexion torques were also measured and subsequently tested for relationships with metabolic cost over these surfaces using linear regression. It was found that metabolic cost varied considerably between individuals within and across substrate types, but this variation was not statistically related to or explained by variations in musculoskeletal parameters considered to be adaptively important to efficient bipedal locomotion. This therefore provides no supportive evidence that variations in these gross anatomical parameters confer significant advantages to the efficiency of walking, and therefore suggest caution in the use of similar metrics to infer differences in walking energetics in closely related fossil species.

Effects of age and locomotor demand on foot mechanics during walking

Older adults exhibit reductions in push-off power that are often attributed to deficits in plantarflexor force-generating capacity. However, growing evidence suggests that the foot may also contribute to push-off power during walking. Thus, age-related changes in foot structure and function may contribute to altered foot mechanics and ultimately reduced push-off power. The purpose of this paper was to quantify age-related differences in foot mechanical work during walking across a range of speeds and at a single fixed speed with varied demands for push-off power. 9 young and 10 older adults walked at 1.0, 1.2, and 1.4 m/s, and at 1.2 m/s with an aiding or impeding horizontal pulling force equal to 5% BW. We calculated foot work in Visual3D using a unified deformable foot model, accounting for contributions of structures distal to the hindfoot's center-of-mass. Older adults walked while performing less positive foot work and more negative net foot work (p < 0.05). Further, we found that the effect of age on mechanical work performed by the foot and the ankle-foot complex increased with increased locomotor demand (p < 0.05). Our findings suggest that during walking, age-related differences in foot mechanics may contribute to reduced push-off intensity via greater energy loss from distal foot structures, particularly during walking tasks with a greater demand for foot power generation. These findings are the first step in understanding the role of the foot in push-off power deficits in older adults and may serve as a roadmap for developing future low-cost mobility interventions.

The rise of the longitudinal arch when sitting, standing, and walking: Contributions of the windlass mechanism

The original windlass mechanism describes a one-to-one coupling between metatarsal joint dorsiflexion and medial longitudinal arch rise. The description assumes a sufficiently stiff plantar aponeurosis and absence of foot muscle activity. However, recent research calls for a broader interpretation of the windlass mechanism that accounts for an extensible plantar aponeurosis and active foot muscles. In this study, we investigate the rise of the arch in response to toe dorsiflexion when sitting, standing, and walking to discuss the windlass mechanism’s contributions in static and dynamic load scenarios. 3D motion analysis allowed a kinematic investigation of the rise and drop of the arch relative to the extent of toe dorsiflexion. The results suggest that static windlass effects poorly predict the relationship between arch dynamics and metatarsophalangeal joint motion during dynamic load scenarios, such as walking. We were able to show that toe dorsiflexion resulted in an immediate rise of the longitudinal arch during sitting and standing. In contrast, a decrease in arch height was observed during walking, despite toe dorsiflexion at the beginning of the push-off phase. Further, the longitudinal arch rose almost linearly with toe dorsiflexion in the static loading scenarios, while the dynamic load scenario revealed an exponential rise of the arch. In addition to that, the rate of change in arch height relative to toe motion was significantly lower when sitting and standing compared to walking. Finally, and most surprisingly, arch rise was found to correlate with toe dorsiflexion only in the dynamic loading scenario. These results challenge the traditional perspective of the windlass mechanism as the dominating source of foot rigidity for push-off against the ground during bipedal walking. It seems plausible that other mechanisms besides the windlass act to raise the foot arch.

Heel Rise and Non-Weight-Bearing Ankle Plantar Flexion Tasks to Assess Foot and Ankle Function in People With Diabetes Mellitus and Peripheral Neuropathy

OBJECTIVE

The objective of this study was to examine the effects of diabetes mellitus and peripheral neuropathy (DMPN), limited joint mobility, and weight-bearing on foot and ankle sagittal movements and characterize the foot and ankle position during heel rise.

METHODS

Sixty people with DMPN and 22 controls participated. Primary outcomes were foot (forefoot on hindfoot) and ankle (hindfoot on shank) plantar-flexion/dorsiflexion angle during 3 tasks: unilateral heel rise, bilateral heel rise, and non-weight-bearing ankle plantar flexion. A repeated-measures analysis of variance and Fisher exact test were used.

RESULTS

Main effects of task and group were significant, but not the interaction in both foot and ankle plantar flexion. Foot and ankle plantar flexion were less in people with DMPN compared with controls in all tasks. Both DMPN and control groups had significantly less foot and ankle plantar flexion with greater weight-bearing; however, the linear trend across tasks was similar between groups. The DMPN group had a greater percentage of individuals in foot and/or ankle dorsiflexion at peak unilateral heel rise compared with controls, but the foot and ankle position was similar at peak bilateral heel rise between DMPN and control groups.

CONCLUSION

Foot and ankle plantar flexion is less in people with DMPN. Less plantar flexion in non-weight-bearing suggests that people with DMPN have limited joint mobility. However, peak unilateral and bilateral heel rise is less than the available plantar flexion range of motion measured in non-weight-bearing, indicating that limited joint mobility does not limit heel rise performance. A higher frequency of people with DMPN are in foot and ankle dorsiflexion at peak unilateral heel rise compared with controls, but the position improved with lower weight-bearing.

IMPACT

Proper resistance should be considered with physical therapist interventions utilizing heel rise because foot and ankle plantar flexion position could be improved by reducing the amount of weight-bearing.

Cutaneous and muscular afferents from the foot and sensory fusion processing: Physiology and pathology in neuropathies

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy.

The extensibility of the plantar fascia influences the windlass mechanism during human running

The arch of the human foot is unique among hominins as it is compliant at ground contact but sufficiently stiff to enable push-off. These behaviours are partly facilitated by the ligamentous plantar fascia whose role is central to two mechanisms. The ideal windlass mechanism assumes that the plantar fascia has a nearly constant length to directly couple toe dorsiflexion with a change in arch shape. However, the plantar fascia also stretches and then shortens throughout gait as the arch-spring stores and releases elastic energy. We aimed to understand how the extensible plantar fascia could behave as an ideal windlass when it has been shown to strain throughout gait, potentially compromising the one-to-one coupling between toe arc length and arch length. We measured foot bone motion and plantar fascia elongation using high-speed X-ray during running. We discovered that toe plantarflexion delays plantar fascia stretching at foot strike, which probably modifies the distribution of the load through other arch tissues. Through a pure windlass effect in propulsion, a quasi-isometric plantar fascia's shortening is delayed to later in stance. The plantar fascia then shortens concurrently to the windlass mechanism, likely enhancing arch recoil at push-off.

Association between foot thermal responses and shear forces during turning gait in young adults

BACKGROUND

The human foot typically changes temperature between pre and post-locomotion activities. However, the mechanisms responsible for temperature changes within the foot are currently unclear. Prior studies indicate that shear forces may increase foot temperature during locomotion. Here, we examined the shear-temperature relationship using turning gait with varying radii to manipulate magnitudes of shear onto the foot.

METHODS

Healthy adult participants (N = 18) walked barefoot on their toes for 5 minutes at a speed of 1.0 m s-1 at three different radii (1.0, 1.5, and 2.0 m). Toe-walking was utilized so that a standard force plate could measure shear localized to the forefoot. A thermal imaging camera was used to quantify the temperature changes from pre to post toe-walking (ΔT), including the entire foot and forefoot regions on the external limb (limb farther from the center of the curved path) and internal limb.

RESULTS

We found that shear impulse was positively associated with ΔT within the entire foot (P < 0.001) and forefoot (P < 0.001): specifically, for every unit increase in shear, the temperature of the entire foot and forefoot increased by 0.11 and 0.17 °C, respectively. While ΔT, on average, decreased following the toe-walking trials (i.e., became colder), a significant change in ΔT was observed between radii conditions and between external versus internal limbs. In particular, ΔT was greater (i.e., less negative) when walking at smaller radii (P < 0.01) and was greater on the external limb (P < 0.01) in both the entire foot and forefoot regions, which were likely explained by greater shear forces with smaller radii (P < 0.0001) and on the external limb (P < 0.0001). Altogether, our results support the relationship between shear and foot temperature responses. These findings may motivate studying turning gait in the future to quantify the relationship between shear and foot temperature in individuals who are susceptible to abnormal thermoregulation.