In vivo moment arm calculations at the ankle using magnetic resonance imaging (MRI)

Muscle Moment Arm-Joint Angle Relations in the Hip, Knee, and Ankle: A Visualization of Datasets

Muscle moment arm is a property that associates muscle force with joint moment and is crucial to biomechanical analysis. In musculoskeletal simulations, the accuracy of moment arm is as important as that of muscle force, and calibrating moment arms in a musculoskeletal model requires data from anatomical measurements. Nonetheless, such data are elusive, and the complex relation between moment arm and joint angle can be unclear. Using common techniques in systematic review, we collected a total of 300 moment arm datasets from literature and visualized the muscle moment arm-joint angle relations in the human hip, knee, and ankle. The findings contribute to the analysis of musculoskeletal mechanics and providing reference regarding the experimental design for future moment arm measurements.

Foot tissue stress in chronic ankle instability during the stance phase of cutting

Lower limb biomechanics of chronic ankle instability (CAI) individuals has been widely investigated, but few have evaluated the internal foot mechanics in CAI. This study evaluated bone and soft tissue stress in CAI contrasted with copers and non-injured participants during a cutting task. Integrating scanned 3D foot shapes and free-form deformation, sixty-six personalized finite element foot models were developed. Computed Achilles tendon forces and measured regional plantar pressure were applied as boundary loading conditions for simulation. It was observed that the primary group differences in foot stress occurred during midstance and heel-off phases of the cutting task. Specifically, healthy individuals had significantly higher stress in the talus and soft tissue around the talus compared to CAI participants. In contrast, CAI participants had significantly higher stress in the cuneiforms and lateral forefoot bones during mid-stance and push-off phases. CAI participants appeared to adopt a protective strategy by transferring greater force to the lateral forefoot at the heel-off phase while lowering stress around the talus, which may be associated with pain relief near the ankle. These findings suggest further attention should be placed on internal stress in CAI at the push-off phase with implications for long-term foot adaptation.

Analysis of Quadriceps Fatigue Effects on Lower Extremity Injury Risks During Landing Phases in Badminton Scissor Jump

The scissor jump (SKJ) is vital in badminton, particularly for backcourt shots, but fatigue increases lower limb load and injury risk. This study investigates how quadriceps fatigue affects biomechanical characteristics and load during SKJ landing, aiming to understand its impact on injury risk. This study involved 27 amateur male badminton players from Ningbo University. Quadriceps fatigue was induced via knee exercises and footwork drills. Biomechanical data before (prior fatigue-PRF) and after fatigue (post fatigue-POF) were recorded using a force platform and motion capture system. Muscle activation was measured with EMG and analyzed through musculoskeletal modeling, with paired t-tests and SPM 1D (Statistical Parametric Mapping 1D) for statistical analysis. Under the POF condition, knee flexion angle increased, and power decreased (p < 0.001, p < 0.001, respectively); ankle plantarflexion angle increased, and power decreased (p < 0.001, p < 0.001, respectively). As fatigue progressed, joint reaction forces initially decreased but later increased. Joint energy dissipation decreased, with differences more pronounced in the coronal than sagittal plane. Achilles tendon force and anterior-posterior tibial shear force decreased, while coronal plane center-of-mass displacement increased. Findings show quadriceps fatigue harms limb stability, upping knee and ankle loads, disrupting the movement pattern, and risking coronal plane injuries. It is recommended that athletes enhance quadriceps endurance, improve neuromuscular control, and refine landing techniques to maintain stability and prevent injuries when fatigued.

Joint moment-angle/velocity relations in the hip, knee, and ankle: A meta-visualization of datasets

Joint moment is a prominent kinetic property in biomechanical investigations, whose pattern and magnitude reflect many characteristics of musculoskeletal motion and musculotendon biomechanics. Nonetheless, the relations of joint moment with joint angle and velocity are complicated, and it is often unclear how the kinetic capacity of each joint varies in different configurations. With common techniques in systematic review, we collected a total of 962 passive, isometric and isokinetic joint moment datasets based on human in vivo measurements from literature and visualized the major joint moment-angle and moment-velocity relations in the hip, knee, and ankle. The findings contribute to the analysis of musculoskeletal mechanics and providing reference regarding the experimental design for future moment measurement.

Effects of habitual foot strike patterns on patellofemoral joint and Achilles tendon loading in recreational runner

BACKGROUND

Most running biomechanics studies have focused on either the patellofemoral joint (PFJ) or Achilles tendon (AT) alone, generating fragmented understanding of how these structures interact as components of an integrated kinetic chain during running. This study was to investigate concurrent biomechanical changes in the PFJ and AT in recreational runners.

METHODS

The recreational runners who are accustomed to run with rearfoot strike (RFS, n = 15) and forefoot strike (FFS, n = 15) patterns were recruited. They were instructed to run at 10 km/h in cushion shoes with their habitual strike patterns on an instrumented split-belt treadmill. Kinematics of the ankle and knee joints in the sagittal plane and ground reaction forces were recorded simultaneously. The contact force and stress at the PFJ, as well as the force, loading rate, impulse, and stress of the AT, were calculated.

RESULTS

The habitual RFS runners had significantly higher peak extension moment (p = 0.019, ES = 0.906), peak quadriceps force (p = 0.010, ES = 1.008), PFJ contact force (p = 0.007, ES = 1.056) and stress (p = 0.042, ES = 0.958) than habitual FFS runners. The peak plantar flexion moment (p < 0.001, ES = 2.692), peak AT force (p < 0.001, ES = -1.788), average (p < 0.001, ES = -2.337) and peak AT loading rate (p < 0.001, ES =-1.996), AT impulse (p = 0.002, ES = -1.246) and stress (p = 0.006, ES = -1.082) of the habitual RFS runners were significantly lower than those of the habitual FFS runners.

CONCLUSION

The FFS pattern could decrease PFJ load but simultaneously increased the mechanical load on the AT. Conversely, the RFS pattern increased PFJ load, but imposed less load on the AT.

Immediate and long-term effects of zero-drop running shoes on lower extremity biomechanics

Objective

The purpose of the study was to investigate the immediate and long-term effects of zero-drop running shoes on lower extremity joint biomechanics.

Methods

Seven male runners participated in this study (height: 1.74 ± 0.03 m, weight: 62.5 ± 3.1 kg, body mass index: 20.6 ± 0.7 kg/m2). Kinematic and kinetic data were collected when the participants ran at a speed of 13 ± 0.65 km/h in running shoes with zero and 15 mm drop both immediately and after the 8-week intervention wearing zero-drop running shoes. Paired t-tests were used to analyze the biomechanical differences between the different drop shoes in the immediate test and the biomechanical changes during the intervention.

Results

The foot strike index increased (zero-drop: p = 0.021, 15 mm drop: p = 0.049), along with the negative work of ankle joint (15 mm drop: p = 0.018), and the hip joint (zero-drop: p = 0.004, 15 mm drop: p = 0.009), while metatarsophalangeal joint negative work decreased (zero-drop: p = 0.029, 15 mm drop: p = 0.028) in post-intervention test compared to the pre-intervention test.

Conclusion

Zero-drop running shoes promote a forefoot strike pattern, which affects the distribution of lower extremity joint work.

Does high-intensity running to fatigue influence lower limb injury risk?

OBJECTIVES

The aim of this study was to quantify changes in peak bending moments at the distal tibia, peak patellofemoral joint contact forces and peak Achilles tendon forces during a high-intensity run to fatigue at middle-distance speed.

DESIGN

Observational study.

METHODS

16 high-level runners (7 female) ran on a treadmill at the final speed achieved during a preceding maximum oxygen uptake test until failure (~3 min). Three-dimensional kinetics and kinematics were used to derive and compare tibial bending moments, patellofemoral joint contact forces and Achilles tendon forces at the start, 33 %, 67 % and the end of the run.

RESULTS

Average running speed was 5.7 (0.4) m·s-1. There was a decrease in peak tibial bending moments (-6.8 %, p = 0.004) from the start to the end of the run, driven by a decrease in peak bending moments due to muscular forces (-6.5 %, p = 0.001), whilst there was no difference in peak bending moments due to joint reaction forces. There was an increase in peak patellofemoral joint forces (+8.9 %, p = 0.026) from the start to the end of the run, but a decrease in peak Achilles tendon forces (-9.1 %, p < 0.001).

CONCLUSIONS

Running at a fixed, high-intensity speed to failure led to reduced tibial bending moments and Achilles tendon forces, and increased patellofemoral joint forces. Thus, the altered neuromechanics of high-intensity running to fatigue may increase patellofemoral joint injury risk, but may not be a mechanism for tibial or Achilles tendon overuse injury development.

Effects of knee joint position on the triceps Suræ torque-size relationship during plantarflexion in healthy young adults

We determined the effects of knee joint position on the relationship between maximal voluntary contraction (MVC) isometric plantar flexor torque and architectural properties of the plantar flexors measured at rest in healthy young adults. We obtained 3-D reconstructed muscle architecture data of the right plantar flexor muscles of nine physically active males using T1 and DTI MRI sequences with the knee in ∼5° flexion and at rest. Muscle volume, fascicle length, pennation angle, and physiological cross-sectional area were estimated for the medial and lateral gastrocnemius and the soleus muscle. MVC isometric plantar flexor torque was assessed on a dynamometer with the knee flexed and extended. MVC isometric plantar flexor torque was 59 % lower when performed with the knee flexed (93.1 ± 22.3 N∙m) vs. extended (154.4 ± 37.8 N∙m). Medial (r = 0.70, p = 0.026) and lateral gastrocnemius (r = 0.49, p = 0.048), total soleus (r = 0.79, p = 0.01), and total triceps suræ muscle volume (r = 0.77, p = 0.012) correlated with MVC isometric plantarflexion torque produced with the knee extended. However, only total soleus (r = 0.64, p = 0.028) and triceps suræ volume (r = 0.64, p = 0.031) correlated with MVC isometric plantar flexor torque produced with the knee flexed. Only the total soleus (r = 0.66, p = 0.038) and triceps suræ physiological cross-sectional area (r = 0.55, p = 0.049) correlated with MVC isometric plantar flexor torque performed with knee extended. The data suggest that knee joint position affects torque-size relationship in the gastrocnemius muscles. Additionally, it appears that the total soleus and triceps suræ muscle volumes association with MVC isometric plantar flexor torque is larger than the total physiological cross-sectional area of the triceps suræ. In conclusion, the data suggest that knee joint position affects torque-size relationship in the gastrocnemii but not in the soleus muscle.

Structure and function of Achilles and patellar tendons following moderate slow resistance training in young and old men

The aim of this study was to investigate the effect of aging and resistance training with a moderate load on the size and mechanical properties of the patellar (PT) and Achilles tendon (AT) and their associated aponeuroses; medial gastrocnemius (MG) and vastus lateralis (VL). Young (Y55; 24.8 ± 3.8 yrs, n = 11) and old men (O55; 70.0 ± 4.6 yrs, n = 13) were assigned to undergo a training program (12 weeks; 3 times/week) of moderate slow resistance training [55% of one repetition maximum (RM)] of the triceps surae and quadriceps muscles. Tendon dimensions were assessed using 1.5 T magnetic resonance imaging before and after 12 weeks. AT and PT cross sectional area (CSA) were determined every 10% of tendon length. Mechanical properties of the free AT, MG aponeurosis, PT, and VL aponeurosis were assessed using ultrasonography (deformation) and tendon force measurements. CSA of the AT but not PT was greater in O55 compared with Y55. At baseline, mechanical properties were generally lower in O55 than Y55 for AT, MG aponeurosis and VL aponeurosis (Young's modulus) but not for PT. CSA of the AT and PT increased equally in both groups following training. Further, for a given force, stiffness and Young's modulus also increased equally for VL aponeurosis and AT, for boths groups. The present study highlights that except for the PT, older men have lower tendon (AT, MG aponeurosis, and VL aponeurosis) mechanical properties than young men and 12-weeks of moderate slow resistance training appears sufficient to improve tendon size and mechanical adaptations in both young and older men. New and Noteworthy: These novel findings suggest that short-term moderate slow resistance training induces equal improvements in tendon size and mechanics regardless of age.

Progression of partial to complete ruptures of the Achilles tendon during rehabilitation: A study using a finite element model

Substantial research on complete Achilles tendon ruptures is available, but guidance on partial ruptures is comparatively sparse. Conservative management is considered acceptable in partial tendon ruptures affecting less than 50% of the tendon's width, but supporting experimental evidence is currently lacking. Using a previously validated finite element model of the Achilles tendon, this study aimed to assess whether loading conditions simulating an early functional rehabilitation protocol could elicit progression to a complete rupture in partial ruptures of varying severity. In silico tendon rupture simulations were performed to locate the most likely rupture site for least, moderate, and extreme subtendon twist configurations. These three models were split at the corresponding rupture site and two sets of partial ruptures were created for each, starting from the medial and lateral sides, and ranging from 10% to 50% loss of continuity. Simulations were conducted with material parameters from healthy and tendinopathic tendons. Partial ruptures were considered to progress if the volume of elements showing a maximum principal strain above 10% exceeded 3 mm3. To assess whether the tendinopathic tendons typical geometric characteristics could compensate for the inferior material properties found in tendinopathy, an additional model with increased cross-sectional area in the free tendon region was developed. Progression to complete ruptures occurred even with less than a 50% loss of continuity, regardless of subtendon twisting, and material parameters. The tendinopathic tendon model with increased cross-sectional area showed similar results. These findings suggest the current criteria for surgical treatment of partial ruptures should be reconsidered. Statement of clinical significance: The clinical significance and most appropriate treatment of partial ruptures of the Achilles tendon is unclear. Despite the widespread use of the "50% rule" in treatment decisions of partial tendon ruptures, experimental evidence supporting it is missing. The present study provides new data, from a validated aponeurotic and free Achilles tendon finite element model, showing that partial ruptures may progress to complete ruptures under loading conditions elicited from functional rehabilitation protocols, even for partial ruptures affecting less than 50% of the tendon's width. Under these novel findings, the current criteria for surgical treatment of partial ruptures should be reconsidered.

Efficacy of heel lifts for mid-portion Achilles tendinopathy (the LIFT trial): study protocol for a randomised controlled trial

BACKGROUND

Mid-portion Achilles tendinopathy is a common condition, characterised by localised Achilles tendon load-related pain and dysfunction. Numerous non-surgical treatments have been proposed for the treatment of this condition, but many of these treatments have a poor or non-existent evidence base. Heel lifts have also been advocated as a treatment for Achilles tendinopathy, but the efficacy and mechanism of action of this intervention is unclear. This proposal describes a randomised controlled trial comparing the effectiveness of heel lifts versus sham heel lifts for reducing pain associated with mid-portion Achilles tendinopathy, with an embedded biomechanical analysis.

METHODS

One hundred and eight men and women aged 18 to 65 years with mid-portion Achilles tendinopathy (who satisfy the inclusion and exclusion criteria) will be recruited. Participants will be randomised, using the website Sealed Envelope, to either a control group (sham heel lifts) or an experimental group (heel lifts). Both groups will be provided with education regarding acceptable pain levels to ensure all participants receive some form of treatment. The participants will be instructed to use their allocated intervention for at least 8 h every day for 12 weeks. The primary outcome measure will be pain intensity (numerical rating scale) at its worst over the previous week. The secondary outcome measures will be additional measures of Achilles tendon pain and disability, participant-perceived global ratings of change, function, level of physical activity and health-related quality of life. Data will be collected at baseline and the primary endpoint (week 12). Data will be analysed using the intention-to-treat principle. In addition, the acute kinetic and kinematic effects of the interventions will be examined at baseline in a subpopulation of the participants (n = 40) while walking and running using three-dimensional motion analysis.

DISCUSSION

The LIFT trial (efficacy of heeL lIfts For mid-portion Achilles Tendinopathy) will be the first randomised trial to compare the efficacy of heel lifts to a sham intervention in reducing pain and disability in people with Achilles tendinopathy. The biomechanical analysis will provide useful insights into the mechanism of action of heel lifts.

TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry, ACTRN12623000627651 . Registered 7 June 2023.

Evidence of different sensitivity of muscle and tendon to mechano-metabolic stimuli

This study aimed to examine the temporal dynamics of muscle-tendon adaptation and whether differences between their sensitivity to mechano-metabolic stimuli would lead to non-uniform changes within the triceps surae (TS) muscle-tendon unit (MTU). Twelve young adults completed a 12-week training intervention of unilateral isometric cyclic plantarflexion contractions at 80% of maximal voluntary contraction until failure to induce a high TS activity and hence metabolic stress. Each participant trained one limb at a short (plantarflexed position, 115°: PF) and the other at a long (dorsiflexed position, 85°: DF) MTU length to vary the mechanical load. MTU mechanical, morphological, and material properties were assessed biweekly via simultaneous ultrasonography-dynamometry and magnetic resonance imaging. Our hypothesis that tendon would be more sensitive to the operating magnitude of tendon strain but less to metabolic stress exercise was confirmed as tendon stiffness, Young's modulus, and tendon size were only increased in the DF condition following the intervention. The PF leg demonstrated a continuous increment in maximal AT strain (i.e., higher mechanical demand) over time along with lack of adaptation in its biomechanical properties. The premise that skeletal muscle adapts at a higher rate than tendon and does not require high mechanical load to hypertrophy or increase its force potential during exercise was verified as the adaptive changes in morphological and mechanical properties of the muscle did not differ between DF and PF. Such differences in muscle-tendon sensitivity to mechano-metabolic stimuli may temporarily increase MTU imbalances that could have implications for the risk of tendon overuse injury.

Effects of gait retraining using minimalist shoes on the medial gastrocnemius muscle-tendon unit behavior and dynamics during running

The effects of a 12-week gait retraining program on the adaptation of the medial gastrocnemius (MG) and muscle-tendon unit (MTU) were investigated. 26 runners with a rearfoot strike pattern (RFS) were randomly assigned to one of two groups: gait retraining (GR) or control group (CON). MG ultrasound images, marker positions, and ground reaction forces (GRF) were collected twice during 9 km/h of treadmill running before and after the intervention. Ankle kinetics and the MG and MTU behavior and dynamics were quantified. Runners in the GR performed gradual 12-week gait retraining transitioning to a forefoot strike pattern. After 12-week, (1) ten participants in each group completed the training; eight participants in GR transitioned to non-RFS with reduced foot strike angles; (2) MG fascicle contraction length and velocity significantly decreased after the intervention for both groups, whereas MG forces increased after intervention for both groups; (3) significant increases in MTU stretching length for GR and peak MTU recoiling velocity for both groups were observed after the intervention, respectively; (4) no significant difference was found for all parameters of the series elastic element. Gait retraining might potentially influence the MG to operate at lower fascicle contraction lengths and velocities and produce greater peak forces. The gait retraining had no effect on SEE behavior and dynamics but did impact MTU, suggesting that the training was insufficient to induce mechanical loading changes on SEE behavior and dynamics.

Effects of a 12-week gait retraining program on the Achilles tendon adaptation of habitually shod runners

PURPOSE

This study investigated the effects of a 12-week gait retraining program on the morphological and mechanical properties of the Achilles tendon (AT) during running on the basis of real-time dynamic ultrasound imaging.

METHODS

A total of 30 male recreational runners who were used to wearing cushioned shoes with a rearfoot strike (RFS) pattern were recruited. They were randomized into a retraining group (RG, n = 15) and a control group (CG, n = 15). The RG group was asked to run in five-fingered minimalist shoes with a forefoot strike (FFS) pattern, and the CG group was asked to keep their strike pattern. Three training sessions were performed per week. All the participants in RG uploaded running tracks obtained through a mobile application (.jpg) after each session for training supervision. The ground reaction force, kinematics, and kinetics of the ankle joint at 10 km/h were collected using an instrumented split-belt treadmill and a motion capture system. The morphological (length and cross-sectional area) and mechanical characteristics (force, stress, strain, etc.) of AT in vivo were recorded and calculated with a synchronous ultrasonic imaging instrument before and after the intervention. Repeated two-way ANOVA was used to compare the aforementioned parameters.

RESULTS

A total of 28 participants completed the training. The strike angle of RG after training was significantly smaller than that before training and significantly smaller than that of CG after training (F (1, 13)  = 23.068, p < 0.001, partial η2  = 0.640). The length (F (1, 13)  = 10.086, p = 0.007, partial η2  = 0.437) and CSA (F (1, 13)  = 7.475, p = 0.017, partial η2  = 0.365) of AT in RG increased after training. A significant main effect for time was observed for the time-to-peak AT force (F (1, 13)  = 5.225, p = 0.040, partial η2  = 0.287), average (F (1, 13)  = 7.228, p = 0.019, partial η2  = 0.357), and peak AT loading rate (F (1, 13)  = 11.687, p = 0.005, partial η2  = 0.473).

CONCLUSION

Preliminary evidence indicated that a 12-week gait retraining program could exert a beneficial effect on AT. 57% (8/14) runners in RG shifted from RFS to FFS pattern. Although not all runners were categorized as FFS pattern after the intervention, their foot strike angle was reduced. Retraining primarily positively promoted AT morphological properties (i.e., CSA and length) to strengthen AT capability for mechanical loading.

Hip and knee kinematics, center of pressure position, and ground reaction force are associated with Achilles tendon force during jump landing

PURPOSE

Jump-landing exercises are often performed during the rehabilitation of Achilles tendon (AT) injuries. However, the factors that affect the AT force (ATF) during landing are unclear. This study aimed to determine the kinematics and ground reaction force (GRF) variables associated with the peak ATF during a drop vertical jump (DVJ).

METHODS

The landing phase of DVJ was evaluated in 101 healthy participants (46 males, age: 21.2 ± 1.4 years old) using a three-dimensional motion analysis system with two force plates. ATF was estimated from the ankle flexion angle and moment. Univariate and multivariate regression analyses were performed with the peak ATF as the dependent variable. The vertical GRF (VGRF), center of pressure (COP), forward trunk leaning, hip/knee/ankle joint angles at peak ATF, and sex were used as independent variables.

RESULTS

In the univariate regression analysis, larger VGRF (β = 0.813), more anterior COP position (β = 0.214), smaller knee flexion (β = -0.251) and adduction (β = -0.252), smaller hip flexion (β = -0.407), smaller forward trunk lean (β = -0.492), and male sex (β = -0.282) were significantly associated with a larger peak ATF. Multivariate analysis revealed that larger VGRF (β = 1.018), more anterior COP position (β = 0.320), a larger knee (β = 0.442), and smaller hip flexion (β = -0.205) were associated with the larger peak ATF.

CONCLUSIONS

The VGRF, COP position, and knee and hip flexion were independently associated with ATF. Modifying these factors may be useful in managing tendon loading during jump-landing exercises.

Biomechanical Analysis of Latin Dancers' Lower Limb during Normal Walking

Latin dance involves fundamental walking steps, integral to the dance process. While resembling daily walking, Latin dance demands higher balance levels, necessitating body adjustments by dancers. These adaptations affect dancers' gait biomechanics, prompting our study on gait differences between Latin dancers (LDs) and non-dancers (NDs). We enlisted 21 female Latin dancers and 21 subjects based on specific criteria. Participants executed walking tasks, with an independent sample t-test for 1-dimensional statistical parameter mapping (SPM 1d) analyzing stance phase variations between LDs and NDs. Notably, significant differences in ankle and hip external rotation were evident during the 16.43-29.47% (p = 0.015) and 86.35-100% (p = 0.014) stance phase. Moreover, pronounced distinctions in rectus Achilles tendon force (ATF) (12.83-13.10%, p = 0.049; 15.89-80.19%, p < 0.001) and Patellofemoral joint contact force (PTF) (15.85-18.31%, p = 0.039; 21.14-24.71%, p = 0.030) during stance were noted between LDs (Latin dancers) and NDs (Non-dancers). The study revealed dancers' enhanced balance attributed to external ankle rotation for dance stability, coupled with augmented Achilles tendon and patellofemoral joint strength from prolonged practice. Moreover, integrating suitable Latin dance into rehabilitation may benefit those with internal rotation gait issues.

Mechanics of The Medial Gastrocnemius-Tendon Unit in Behaving more Efficiently in Habitual Non-Rearfoot Strikers than in Rearfoot Strikers during Running

This study aims to quantify how habitual foot strike patterns would affect ankle kinetics and the behavior and mechanics of the medial gastrocnemius-tendon unit (MTU) during running. A total of 14 runners with non-rearfoot strike patterns (NRFS) and 15 runners with rearfoot strike patterns (RFS) ran on an instrumented treadmill at a speed of 9 km/h. An ultrasound system and a motion capture system were synchronously triggered to collect the ultrasound images of the medial gastrocnemius (MG) and marker positions along with ground reaction forces (GRF) during running. Ankle kinetics (moment and power) and MG/MTU behavior and mechanical properties (MG shortening length, velocity, force, power, MTU shortening/lengthening length, velocity, and power) were calculated. Independent t-tests were performed to compare the two groups of runners. Pearson correlation was conducted to detect the relationship between foot strike angle and the MTU behavior and mechanics. Compared with RFS runners, NRFS runners had 1) lower foot strike angles and greater peak ankle moments; 2) lower shortening/change length and contraction velocity and greater MG peak force; 3) greater MTU lengthening, MTU shortening length and MTU lengthening velocity and power; 4) the foot strike angle was positively related to the change of fascicle length, fascicle contraction length, and MTU shortening length during the stance phase. The foot strike angle was negatively related to the MG force and MTU lengthening power. The MG in NRFS runners appears to contract with greater force in relatively isometric behavior and at a slower shortening velocity. Moreover, the lengthening length, the lengthening velocity of MTU, and the MG force were greater in habitual NRFS runners, leading to a stronger stretch reflex response potentially.

Tendon compliance and preload must be considered when determining the in vivo force-velocity relationship from the torque-angular velocity relation

In vivo, the force-velocity relation (F-v-r) is typically derived from the torque-angular velocity relation (T-ω-r), which is subject to two factors that may influence resulting measurements: tendon compliance and preload prior to contraction. The in vivo plantar flexors' T-ω-r was determined during preloaded maximum voluntary shortening contractions at 0-200°/s. Additionally, we used a two factor block simulation study design to independently analyze the effects of preload and tendon compliance on the resulting T-ω-r. Therefore, we replicated the in vivo experiment using a Hill-type muscle model of the gastrocnemius medialis. The simulation results matched a key pattern observed in our recorded in vivo experimental data: during preloaded contractions, torque output of the muscle was increased when compared with non-preloaded contractions from literature. This effect increased with increasing contraction velocity and can be explained by a rapidly recoiling tendon, allowing the contractile element to contract more slowly, thus developing higher forces compared with non-preloaded contractions. Our simulation results also indicate that a more compliant tendon results in increased ankle joint torques. The simulation and the experimental data clearly show that the deduction of the in vivo F-v-r from the T-ω-r is compromised due to the two factors preloading and tendon compliance.

Design and validation of a finite element model of the aponeurotic and free Achilles tendon

The Achilles tendon (AT) is a common injury site. Ruptures are usually located in the free tendon but may cross the myotendinous junction into the aponeurotic region. Considering the possibility of aponeurotic region involvement in AT ruptures, a novel three dimensional (3D) finite element (FE) model that includes both the aponeurotic and free AT regions and features subtendon twisting and sliding was developed. It was hypothesized that the model would be able to predict in vivo data collected from the literature, thus being considered valid, and that model outputs would be most sensitive to subtendon twist configurations. The 3D model was constructed using magnetic resonance images. The model was divided into soleus and gastrocnemius subtendons. In addition to a frictionless contact condition, the interaction between subtendons was modeled using two contact formulations: sliding with anisotropic friction and no sliding. Loads were applied on the tendon's most proximal cross-section and anterior surface, with magnitudes estimated from in vivo studies. Model outputs were compared with experimental data regarding 3D deformation, transverse plane rotation, and nodal displacements in the free tendon. The FE model adequately simulated the free tendon behavior regarding longitudinal strain, cross-section area variation, transverse plane rotation, and sagittal nodal displacements, provided that subtendon sliding was allowed. The frictionless model exhibited noticeable medial transverse sliding of the soleus subtendon, which was present to a much lesser degree in the anisotropic friction model. Model outputs were most sensitive to variations in subtendon twist and dispersion of the collagen fiber orientations. Clinical Significance: This Achilles tendon finite element model, validated using in vivo experimental data, may be used to study its mechanical behavior, injury mechanisms, and rupture risk factors.

The load borne by the Achilles tendon during exercise: A systematic review of normative values

The Achilles tendon (AT) can be exposed to considerable stress during athletic activities and is often subject to pathologies such as tendinopathies. When designing a prevention or rehabilitation protocol, mechanical loading is a key factor to consider. This implies being able to accurately determine the load applied to the AT when performing exercises that stress this tendon. A systematic review was performed to synthesize the load borne by the AT during exercises/activities. Three databases (Pubmed, Embase and Cochrane) were searched for articles up to May 2021, and only the studies assessing the AT load in newtons relative to body-weight (BW) on humans during activities or exercises were included. Most of the 11 included studies assessed AT load when running or walking (N = 10), and only three tested exercises were usually performed during rehabilitation. The load on the tendon ranged from 2.7 to 3.95 BW when walking, from 4.15 to 7.71 BW when running, and from 0.41 to 7.3 BW according to the strengthening exercise performed. From the collected data, a progression of exercises progressively loading the Achilles tendon, as well as the possible connections with walking and running activities, could be defined. However, the trends highlighted in the relationship between tendon loading and walking or running speeds present some inconsistencies. Further research is still needed to clarify them, but also to complete the data set in healthy and injured people.

Faster triceps surae muscle cyclic contractions alter muscle activity and whole body metabolic rate

Hundred years ago, Fenn demonstrated that when a muscle shortens faster, its energy liberation increases. Fenn's results were the first of many that led to the general understanding that isometric muscle contractions are energetically cheaper than concentric contractions. However, this evidence is still primarily based on single fiber or isolated (ex vivo) muscle studies and it remains unknown whether this translates to whole body metabolic rate. In this study, we specifically changed the contraction velocity of the ankle plantar flexors and quantified the effects on triceps surae muscle activity and whole body metabolic rate during cyclic plantar flexion (PF) contractions. Fifteen participants performed submaximal ankle plantar flexions (∼1/3 s activation and ∼2/3 s relaxation) on a dynamometer at three different ankle angular velocities: isometric (10° PF), isokinetic at 30°/s (5-15° PF), and isokinetic at 60°/s (0-20° PF) while target torque (25% MVC) and cycle frequency were kept constant. In addition, to directly determine the effect of ankle angular velocity on muscle kinematics we collected gastrocnemius medialis muscle fascicle ultrasound data. As expected, increasing ankle angular velocity increased gastrocnemius medialis muscle fascicle contraction velocity and positive mechanical work (P < 0.01), increased mean and peak triceps surae muscle activity (P < 0.01), and considerably increased net whole body metabolic rate (P < 0.01). Interestingly, the increase in triceps surae muscle activity with fast ankle angular velocities was most pronounced in the gastrocnemius lateralis (P < 0.05). Overall, our results support the original findings from Fenn in 1923 and we demonstrated that greater triceps surae muscle contraction velocities translate to increased whole body metabolic rate.NEW & NOTEWORTHY Single muscle fiber studies or research on isolated (ex vivo) muscles demonstrated that faster concentric muscle contractions yield increased energy consumption. Here we translated this knowledge to muscle activation and whole body metabolic rate. Increasing ankle angular velocity increased triceps surae contraction velocity and mechanical work, increasing triceps surae muscle activity and substantially elevating whole body metabolic rate. Additionally, we demonstrated that triceps surae muscle activation strategy depends on the mechanical demands of the task.

Mechanical Linkage between Achilles Tendon and Plantar Fascia Accounts for Range of Motion of Human Ankle-Foot Complex

PURPOSE

The human ankle-foot complex possesses a passive range of motion (ROM) through changes in tibiocalcaneal ( θcal ) and foot arch ( θarch ) angles. Based on the anatomical linkage between the Achilles tendon (AT) and plantar fascia (PF), we hypothesized that AT and PF with different mechanical properties conjointly modulate the passive ROM of the human ankle-foot complex. We examined the association of AT and PF stiffness with passive ankle-foot ROM and further addressed differences between sexes.

METHODS

A series of sagittal magnetic resonance images of the foot and passive ankle plantar flexion torque were obtained for 20 men and 20 women with their ankle-foot passively rotated from 30° of plantar flexion to 20° of dorsiflexion. Based on the measured changes in AT and PF lengths, θcal , θarch , and passive torque, AT and PF stiffness were determined.

RESULTS

Upon passive ankle dorsiflexion, AT and PF were lengthened; their length changes were inversely correlated. Men showed a stiffer AT, more compliant PF, less calcaneal rotation, and greater foot arch deformation compared with women. Furthermore, we found inverse correlations between AT stiffness and ROM of θcal , and between PF stiffness and ROM of θarch in men and women.

CONCLUSIONS

Passive AT and PF extensibility counter each other. AT and PF stiffness and passive ROM of ankle-foot components were countered between sexes; however, associations between stiffness and passive ROM of the ankle-foot complex were consistent between sexes. Our findings support the notion that the balanced mechanical interaction between the AT and PF can account for the passive ROM of the human ankle-foot complex in vivo , and the differences between sexes.

Effects of 12-week transition training with minimalist shoes on Achilles tendon loading in habitual rearfoot strike runners

Minimalist shod runners have reported greater material and mechanical properties of the Achilles tendon (AT) due to increased loading than runners who wear more cushioned running shoes. This study aimed to investigate the effects of 12-week transition training from conventional shoes to minimalist shoes on AT loading in habitual rearfoot strike runners. Seventeen healthy male habitual rearfoot strikers completed 12-week transition training. They were instructed either to run in minimalist shoes with a forefoot strike pattern (MIN + FFS, n = 9) or run in minimalist shoes but were free to develop their strike pattern (MIN, n = 8). Ultrasound images were captured to determine the cross-sectional area of the AT. Sagittal plane ankle kinematics and ground reaction forces were recorded simultaneously to quantify ankle joint mechanics and AT loading. The strike angle significantly decreased in MIN + FFS after the transition training, indicating a flatter foot at initial contact, whereas no changes were observed in MIN. After training, a significant increase in peak plantarflexion moment was observed for MIN + FFS (15.4%) and MIN (7.6%). Significantly increased peak AT force, peak loading rate and peak stress were observed after training in both groups. Specifically, MIN + FFS had a greater increase in peak AT force (20.3% versus 10.1%), peak loading rate (37.2% versus 25.4%) and peak AT stress (13.7% versus 8.1%) than MIN. Furthermore, for both groups, there were no significant differences in the moment arm and cross-sectional area of the AT observed before and after 12 weeks of training. The results of this study suggested that it was insufficient to promote the morphological adaptation of the AT, but the mechanical loading of the AT was adapted during running after 12-week transition training with minimalist shoes in MIN + FFS and MIN. Preliminary evidence showed that a gradual transition to minimalist shoes with a forefoot strike pattern may be beneficial to the mechanical loading of the AT.

Internal Tibial Forces and Moments During Graded Running

The stress experienced by the tibia has contributions from the forces and moments acting on the tibia. We sought to quantify the influence of running grade on internal tibial forces and moments. Seventeen participants ran at 3.33 m/s on an instrumented treadmill at 0 deg, ±5 deg, and ±10 deg while motion data were captured. Ankle joint contact force was estimated from an anthropometrically-scaled musculoskeletal model using inverse dynamics-based static optimization. Internal tibial forces and moments were quantified at the distal 1/3rd of the tibia, by ensuring static equilibrium with all applied forces and moments. Downhill running conditions resulted in lower peak internal axial force (range of mean differences: -9% to -16%, p < 0.001), lower peak internal anteroposterior force (-14% to -21%, p < 0.001), and lower peak internal mediolateral force (-14% to -15%, p < 0.001), compared to 0 deg and +5 deg. Furthermore, downhill conditions resulted in lower peak internal mediolateral moment (-11%to -21%, p < 0.001), lower peak internal anteroposterior moment (-13% to -14%, p < 0.001), and lower peak internal torsional moment (-9% to -21%, p < 0.001), compared to 0 deg, +5 deg, and +10 deg. The +10 deg condition resulted in lower peak internal axial force (-7% to -9%, p < 0.001) and lower peak internal mediolateral force (-9%, p = 0.004), compared to 0 deg and +5 deg. These findings suggest that downhill running may be associated with lower tibial stresses than either level or uphill running.

The effect of speed on Achilles tendon forces and patellofemoral joint stresses in high-performing endurance runners

Achilles tendinopathy and patellofemoral pain are common running injuries associated with increased Achilles tendon (AT) forces and patellofemoral joint (PFJ) stresses. This study examined AT forces and PFJ stresses at different running speeds in high-performing endurance runners. Twenty runners ran overground at four running speeds (3.3, 3.9, 4.8, and 5.6 m/s). AT forces and PFJ stresses were estimated from kinematic and kinetic data. Repeated measures ANOVA with partial eta squared effect sizes was conducted to assess differences between running speeds. Increased peak AT forces (19.5%; p < 0.001) and loading rates (57.3%; p < 0.001) from 3.3 m/s to 5.6 m/s were observed. Cumulative AT loading was greater in the faster speeds compared to the slower speeds. Faster running speeds resulted in increased peak plantar flexor moments, increased peak plantar flexion angles, and a more flexed knee and an anterior center of pressure position at touchdown. Peak PFJ stress was lower in the slowest speed (3.3 m/s) compared to the faster running speeds (3.9-5.6 m/s; p = 0.005). PFJ stress loading rate significantly increased (43.6%; p < 0.001). Greater AT loading observed could be associated with strategies such as increased plantar flexor moments and altered lower body position at touchdown which are commonly employed to generate greater ground contact forces. Greater AT and PFJ loading rates were likely due to shorter ground contact times and therefore less time available to reach the peak. Running at faster speeds could increase the risk of developing Achilles tendinopathy and patellofemoral pain or limit recovery from these injuries without sufficient recovery.

Shorter heels are linked with greater elastic energy storage in the Achilles tendon

Previous research suggests that the moment arm of the m. triceps surae tendon (i.e., Achilles tendon), is positively correlated with the energetic cost of running. This relationship is derived from a model which predicts that shorter ankle moment arms place larger loads on the Achilles tendon, which should result in a greater amount of elastic energy storage and return. However, previous research has not empirically tested this assumed relationship. We test this hypothesis using an inverse dynamics approach in human subjects (n = 24) at speeds ranging from walking to sprinting. The spring function of the Achilles tendon was evaluated using specific net work, a metric of mechanical energy production versus absorption at a limb joint. We also combined kinematic and morphological data to directly estimate tendon stress and elastic energy storage. We find that moment arm length significantly determines the spring-like behavior of the Achilles tendon, as well as estimates of mass-specific tendon stress and elastic energy storage at running and sprinting speeds. Our results provide support for the relationship between short Achilles tendon moment arms and increased elastic energy storage, providing an empirical mechanical rationale for previous studies demonstrating a relationship between calcaneal length and running economy. We also demonstrate that speed and kinematics moderate tendon performance, suggesting a complex relationship between lower limb geometry and foot strike pattern.

Foot Contact Dynamics and Fall Risk among Children Diagnosed with Idiopathic Toe Walking

Children that are diagnosed with Idiopathic Toe walking (cITW) are characterized by persistent toe-to-toe contacts. The objective of this study was to explore whether typical foot contact dynamics during walking predisposes cITW to a higher risk of falling. Twenty cITW and age-matched controls performed typical and toe walking trials. The gait parameters related to foot contact dynamics, vertical force impulses during stance, slip, and trip risk were compared for both groups. We found that cITW manifest less stable gait and produced significantly higher force impulses during push-off. Additionally, we found that cITW had a higher slip-initiation risk that was associated with higher foot contact horizontal and vertical velocities in addition to lower transitional acceleration of center of mass. We found that cITW exhibited a higher trip risk with toe clearance being significantly lower when compared to healthy counterparts. This study allowed for a quantitative description of foot contact dynamics and delineated typical from toe walking among cITW. Overall, the results indicate that cITW are less stable during typical walking and are prone to a higher risk of slip and trip-like falls.

Fit to Burst: Toward Noninvasive Estimation of Achilles Tendon Load Using Burst Vibrations

OBJECTIVE

Tendons are essential components of the musculoskeletal system and, as with any mechanical structure, can fail under load. Tendon injuries are common and can be debilitating, and research suggests that a better understanding of their loading conditions could help mitigate injury risk and improve rehabilitation. To that end, we present a novel method of noninvasively assessing parameters related to mechanical load in the Achilles tendon using burst vibrations.

METHODS

These vibrations, produced by a small vibration motor on the skin superficial to the tendon, are sensed by a skin-mounted accelerometer, which measures the tendon's response to burst excitation under varying tensile load. In this study, twelve healthy subjects performed a variety of everyday tasks designed to expose the Achilles tendon to a range of loading conditions. To approximate the vibration motor-tendon system and provide an explanation for observed changes in tendon response, a 2-degree-of-freedom mechanical systems model was developed.

RESULTS

Reliable, characteristic changes in the burst response profile as a function of Achilles tendon tension were observed during all loading tasks. Using a machine learning-based approach, we developed a regression model capable of accurately estimating net ankle moment-which captures general trends in tendon tension-across a range of walking speeds and across subjects (R² = 0.85). Simulated results of the mechanical model accurately recreated behaviors observed in vivo. Finally, preliminary, proof-of-concept results from a fully wearable system demonstrated trends similar to those observed in experiments conducted using benchtop equipment.

CONCLUSION

These findings suggest that an untethered, unobtrusive system can effectively assess tendon loading during activities of daily life.

SIGNIFICANCE

Access to such a system would have broad implications for injury recovery and prevention, athletic training, and the study of human movement.

Longer Achilles tendon moment arm results in better running economy

Based on the current literature, the link between Achilles tendon moment arm length and running economy is not well understood. Therefore, the aim of this study was to further investigate the connection between Achilles tendon moment arm and running economy and the influence of Achilles tendon moment arm on the function of the plantarflexor muscle-tendon unit during running.Ten male competitive marathon runners volunteered for this study. The participants ran on a treadmill at two running speeds: 3 and 3.5 m s-1. During running the oxygen consumption, lower leg kinematics, electrical activity of plantar flexor muscles, and fascicle behavior of the lateral gastrocnemius were measured simultaneously. On the second occasion, an MRI scan of the right leg was taken and used to estimate the Achilles tendon moment arm length.There was a negative correlation between running economy and the body height normalized moment arm length at both selected speeds (r = -0.68, P = 0.014 and r = -0.70, P = 0.01). In addition, Achilles tendon moment arm length correlated with the amplitude of the ankle flexion at both speeds (r = -0.59, P = 0.03 and r = -0.60, P = 0.03) and with the electrical activity of the medial gastrocnemius muscle at 3 m s-1 speed (r = -0.62, P = 0.02). Our finding supports the concept that a longer moment arm could be beneficial for distance runners.

The validity and reliability of the Achilles tendon moment arm assessed with dual-energy X-ray absorptiometry, relative to MRI and ultrasound assessments

Dual-energy X-ray absorptiometry (DXA) in single energy mode has been shown to permit the visualisation of bone and soft tissue, such as the patellar tendon through two-dimensional sagittal imaging. However, there is no validated DXA-based measurement of the Achilles tendon moment arm (d_AT). The aims of this study were: 1) to compare in vivo DXA derived measurements of the d_AT at rest against two previously validated methods: tendon excursion (TE) and magnetic resonance imaging (MRI) at three ankle angles (-5°, 0° and +10°). 2) analyse the intra-day reliability of the DXA method at all ankle angles and compare between methods. Twelve healthy adults (mean ± SD: 31.4 ± 9.5 years; 174.0 ± 9.5 cm; 76.2 ± 16.6 kg) participated in this study, involving test-retest DXA scans, ultrasound scans and one MRI scan. The d_AT was defined as the distance from the centre of the calcaneal-tibial joint axis to the Achilles tendon (AT) muscle-tendon line of action. DXA derived d_AT measures were significantly greater than MRI measurements (19.7-24.9%) and were 45.2% significantly larger than the TE method. The test-retest reliability of the DXA technique at 0° was high [CV = 1.38%; ICC = 0.96] and despite the consistently larger d_AT lengths obtained using DXA, MRI and DEXA data were strongly correlated (r = 0.878, p < 0.001). In conclusion, the DXA technique allowed for highly reproducible in vivo d_AT measurement at rest, which has implications for the calculation of AT forces in vivo and the ability to predict the measurement from one tool to the other, thereby providing a novel basis to contrast existing and future studies.

Considerations on the human Achilles tendon moment arm for in vivo triceps surae muscle-tendon unit force estimates

Moment arm-angle functions (MA-a-functions) are commonly used to estimate in vivo muscle forces in humans. However, different MA-a-functions might not only influence the magnitude of the estimated muscle forces but also change the shape of the muscle’s estimated force-angle relationship (F-a-r). Therefore, we investigated the influence of different literature based Achilles tendon MA-a-functions on the triceps surae muscle–tendon unit F-a-r. The individual in vivo triceps torque–angle relationship was determined in 14 participants performing maximum voluntary fixed-end plantarflexion contractions from 18.3° ± 3.2° plantarflexion to 24.2° ± 5.1° dorsiflexion on a dynamometer. The resulting F-a-r were calculated using 15 literature-based in vivo Achilles tendon MA-a-functions. MA-a-functions affected the F-a-r shape and magnitude of estimated peak active triceps muscle–tendon unit force. Depending on the MA-a-function used, the triceps was solely operating on the ascending limb (n = 2), on the ascending limb and plateau region (n = 12), or on the ascending limb, plateau region and descending limb of the F-a-r (n = 1). According to our findings, the estimated triceps muscle–tendon unit forces and the shape of the F-a-r are highly dependent on the MA-a-function used. As these functions are affected by many variables, we recommend using individual Achilles tendon MA-a-functions, ideally accounting for contraction intensity-related changes in moment arm magnitude.

Changes of the Patellar Tendon Moment rm Length in Different Knee Angles: A Biomechanical in Vivo Study

Patellar tendon moment arm length (PTma) changes at different knee flexion angles have not been determined in in vivo studies. We aimed to determine PTma in four different knee angles using Magnetic Resonance Imaging (MRI) to predict in vivo changes in the moment arm length from different knee angles during running. PTma was measured as the perpendicular distance from muscle-tendon line of action to the knee joint axis of rotation at 0° (full extension), 20°, 40°, and 60° flexion of knee by using MRI method. Repeated measure ANOVA method was applied to compare the moment arm length among four degrees of knee flexion (P<0.05). A regression analysis was used to predict the PTma during different knee joint angles. The PTma in the four angles at 0°, 20°, 40°, and 60° of knee flexion were 42.55±4.20, 39.91±2.98, 37.73±2.87, and 36.18±2.90 mm, respectively (P<0.05). The regression analysis provided an equation to predict the PTma from different knee joint angles during running. PTma values decreased from knee extension to flexion in a linear manner. These findings have important implications for estimating PTma using a regression equation model from different knee joint angles.

Minute oscillation stretching: A novel modality for reducing musculo-tendinous stiffness and maintaining muscle strength

A novel stretching modality was developed to provide repetitive small length changes to the plantar flexors undergoing passive stretch defined as "minute oscillation stretching" (MOS). This study investigated the effects of MOS on neuromuscular activity during force production, the rate of torque development (RTD), and the elastic properties of the plantar flexors and Achilles tendon. Ten healthy males participated in this study. The neuromuscular activity of the triceps surae and tibialis anterior muscles during maximal voluntary plantar flexion torque [MVT], RTD of plantar flexion, Achilles tendon stiffness, and muscle stiffness were measured before and after two types of interventions for a total of 5 minutes: static stretching (SS) and MOS at 15 Hz and without intervention (control). Achilles tendon stiffness was calculated from the tendon elongation measuring by ultrasonography. Muscle stiffness was determined for the medial gastrocnemius [MG] using shear wave elastography. The MVT, mean electromyographic amplitudes [mEMG] of MG and lateral gastrocnemius [LG], and RTD were significantly decreased following SS (MVT: -7.2 ± 7.9%; mEMG of MG: -8.7 ± 10.2%; mEMG of LG: -12.4 ± 10.5%; RTD: -6.6 ± 6.8%), but not after MOS. Achilles tendon stiffness significantly decreased after SS (-13.4 ± 12.3%) and MOS (-9.7 ± 11.5%), with no significant differences between them. Muscle stiffness significantly decreased in SS and MOS, with relative changes being significantly greater for MOS (-7.9 ± 8.3%) than SS (-2.3 ± 2.9%) interventions. All variables remained unchanged in the controls. In conclusion, MOS changed muscle-tendon compliance without loss of muscle function.

Acute shoe effects on Achilles tendon loading in runners with habitual rearfoot strike pattern

OBJECTIVES

Although the overuse injury rate of the Achilles tendon (AT) for running is high, the effect of shoe conditions on AT loading remains unclear. Hence, this study aims to determine the mechanical properties of AT during running under different shoe conditions (minimalist vs. conventional shoes).

DESIGN

This work is a controlled laboratory study.

METHODS

Sixteen healthy male rearfoot strike runners were recruited to complete over ground running trials at 3.33 m/s (±5%) under two shoe conditions (minimalist shoes: INOV-8 Bare-XF 210; conventional shoes: NIKE AIR ZOOM PEGASUS 34). Sagittal plane ankle kinematics and ground reaction forces were simultaneously recorded. Ankle joint mechanics (ankle joint angle and moment) and the mechanical properties (peak force, impulse, stress, strain, and their corresponding peak rate) of the AT were calculated.

RESULTS

In comparison with conventional shoes, wearing minimalist shoes showed significant changes (p < 0.05): (1) decreased strike angle (48.92 ± 9.01 vs. 41.04 ± 8.69°); (2) increased ankle moment (2.34 ± 0.44 vs. 2.55 ± 0.46 Nm/kg); (3) increased peak AT force (5.85 ± 1.22 vs. 6.24 ± 1.13 BW), AT force impulse (0.65 ± 0.13 vs. 0.70 ± 0.13 BW·s), peak AT loading rate (109.94 ± 9.33 vs. 118.84 ± 26.62 BW/s), and average loading rate (48.42 ± 15.64 vs. 54.90 ± 17.47 BW/s); (4) decreased time to peak AT force (126.31 ± 20.68 vs. 117.77 ± 17.62 ms); (5) increased AT stress (66.96 ± 14.59 vs. 71.89 ± 14.74 MPa), strain (8.19 ± 1.77 vs. 8.78 ± 1.80 %), peak AT stress rate (66.96 ± 14.59 vs. 71.89 ± 14.74 MPa/s), and strain rate (148.71 ± 48.52 vs. 167.28 ± 42.82 %/s).

CONCLUSION

Increased AT force, loading rate, and stress were observed in runners who habitually wear conventional shoes with rearfoot strike patterns when they wore minimalist shoes. Hence, we recommend a gradual transition to minimalist shoes for runners who habitually wear conventional shoes with rearfoot strike patterns.

Estimation of the force-velocity properties of individual muscles from measurement of the combined plantarflexor properties

The force-velocity (F-V) properties of isolated muscles or muscle fibers have been well studied in humans and other animals. However, determining properties of individual muscles in vivo remains a challenge because muscles usually function within a synergistic group. Modeling has been used to estimate the properties of an individual muscle from the experimental measurement of the muscle group properties. While this approach can be valuable, the models and the associated predictions are difficult to validate. In this study, we measured the in situ F-V properties of the maximally activated kangaroo rat plantarflexor group and used two different assumptions and associated models to estimate the properties of the individual plantarflexors. The first model (Mdl1) assumed that the percent contributions of individual muscles to group force and power were based upon the muscles' cross-sectional area and were constant across the different isotonic loads applied to the muscle group. The second model (Mdl2) assumed that the F-V properties of the fibers within each muscle were identical, but because of differences in muscle architecture, the muscles' contributions to the group properties changed with isotonic load. We compared the two model predictions with independent estimates of the muscles' contributions based upon sonomicrometry measurements of muscle length. We found that predictions from Mdl2 were not significantly different from sonomicrometry-based estimates while those from Mdl1 were significantly different. The results of this study show that incorporating appropriate fiber properties and muscle architecture is necessary to parse the individual muscles' contributions to the group F-V properties.

Achilles tendon moment arms are similar when computed using a single fixed axis versus a moving instantaneous helical axis

Accurate location of the axis of ankle rotation is critical to in vivo estimates of Achilles tendon moment arm. Here we investigated how the plantarflexion moment arm of the Achilles tendon is affected by using an instantaneous helical axis that moves with ankle motion as opposed to a single fixed joint axis that approximates the average axis of rotation. Twenty young healthy adults performed a series of weightbearing cyclical plantar- and dorsi-flexion motions. Motion analysis tracked the motions of markers placed on the foot and shank and also tracked an ultrasound probe imaging the Achilles tendon. Differences in ATma between the methods were investigated using a two-way repeated-measures ANOVA with factors of joint angle (+5°, 0°, -5°, -10°, -15°) and method (instantaneous helical axes, fixed axis). Moment arms computed between the two methods were moderately to strongly correlated, especially in the mid-range of motion (for 0° to 10° plantarflexion, all r² > 0.619 and all p < 0.004). The two methods produced Achilles tendon moment arms that were comparable and not significantly different except in the most dorsiflexed position, when they differed on average by 9.35 ± 3.23 mm (p = 0.001). Our results suggest that either approach for locating the axis of ankle rotation would be appropriate for the purpose of estimating ATma, but that a fixed axis may be preferable because it is applicable over a greater range of ankle motion.

Biarticular muscles in light of template models, experiments and robotics: a review

Leg morphology is an important outcome of evolution. A remarkable morphological leg feature is the existence of biarticular muscles that span adjacent joints. Diverse studies from different fields of research suggest a less coherent understanding of the muscles' functionality in cyclic, sagittal plane locomotion. We structured this review of biarticular muscle function by reflecting biomechanical template models, human experiments and robotic system designs. Within these approaches, we surveyed the contribution of biarticular muscles to the locomotor subfunctions (stance, balance and swing). While mono- and biarticular muscles do not show physiological differences, the reviewed studies provide evidence for complementary and locomotor subfunction-specific contributions of mono- and biarticular muscles. In stance, biarticular muscles coordinate joint movements, improve economy (e.g. by transferring energy) and secure the zig-zag configuration of the leg against joint overextension. These commonly known functions are extended by an explicit role of biarticular muscles in controlling the angular momentum for balance and swing. Human-like leg arrangement and intrinsic (compliant) properties of biarticular structures improve the controllability and energy efficiency of legged robots and assistive devices. Future interdisciplinary research on biarticular muscles should address their role for sensing and control as well as non-cyclic and/or non-sagittal motions, and non-static moment arms.

Controlled trial to compare the Achilles tendon load during running in flatfeet participants using a customized arch support orthoses vs an orthotic heel lift

Background

Achilles tendinopathy is one of the most common overuse injuries in running, and forefoot pronation, seen in flatfeet participants, has been proposed to cause additional loading across the Achilles tendon. Foot orthoses are one of the common and effective conservative treatment prescribed for Achilles tendinopathy, it works by correcting the biomechanical malalignment and reducing tendon load. Previous studies have shown reduction of Achilles Tendon load (ATL) during running by using customized arch support orthosis (CASO) or an orthotic heel lift (HL). However, there are still little biomechanical evidence and comparative studies to guide orthotic prescriptions for Achilles tendinopathy management. Therefore, this study seeks to investigate the two currently employed orthotic treatment options for Achilles tendinopathy: CASO and HL for the reduction of ATL and Achilles tendon loading rate (ATLR) in recreational runners with flatfeet.

Methods

Twelve participants were recruited and run along the runway in the laboratory for three conditions: (1) without orthoses, (2) with CASO (3) with HL. Kinematic and kinetic data were recorded by 3D motion capturing system and force platform. Ankle joint moments and ATL were computed and compared within the three conditions.

Results

Participants who ran with CASO (p = 0.001, d = 0.43) or HL (p = 0.001, d = 0.48) associated with a significant reduction in ATL when compared to without orthotics while there was no significant difference between the two types of orthoses, the mean peak ATL of CASO was slightly lower than HL. Regarding the ATLR, both orthoses, CASO (p = 0.003, d = 0.93) and HL (p = 0.004, d = 0.78), exhibited significant lower value than the control but similarly, no significant difference was noted between them in which the use of CASO yielded a slightly lower loading rate than that of HL.

Conclusions

Both CASO and HL were able to cause a significant reduction in peak ATL and ATLR comparing to without orthotics condition. There were subtle but no statistically significant differences in the biomechanical effects between the two types of orthoses. The findings help to quantify the effect of CASO and HL on load reduction of Achilles tendon and suggests that foot orthoses may serve to prevent the incidence of Achilles tendon pathologies.

Trial registration

NCT04003870 on clinicaltrials.gov 1 July 2019.

Architectural and functional specifics of the human triceps surae muscle in vivo and its adaptation to microgravity

Long-term exposure to microgravity (μG) is known to reduce the strength of a skeletal muscle contraction and the level of general physical performance in humans, while little is known about its effect on muscle architecture. Architectural and contractile properties of the triceps surae (TS) muscle were determined in vivo for male cosmonauts in response ( n = 8) to a spaceflight (213.0 ± 30.5 days). The maximal voluntary contraction (MVC), tetanic tension ( Ро), and voluntary and electrically evoked contraction times and force deficiency (Pd) were determined. The ankle was positioned at 15° dorsiflexion (−15°) and 0, 15, and 30° plantar flexion, with the knee set at 90°. At each position, longitudinal ultrasonic images of the medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles were obtained while the subject was relaxed. After a spaceflight, MVC and Pо decreased by 42 and 26%, respectively, and Pd increased by 50%. The rate of tension of a voluntary contraction substantially reduced but evoked contractions remained unchanged. In the passive condition, fiber length ( Lf) changed from 43, 57, and 35 mm (knee, 0°; ankle, −15°) to 34, 38, and 25 mm (knee, 0°; ankle, 30°) for MG, LG, and SOL, respectively, and Θf changed from 27, 21, and 23° (knee, 0°; ankle, −15°) to 43, 29, and 34° (knee, 0°; ankle, 30°) for MG, LG, and SOL, respectively. Different Lf and Θf, and their changes after spaceflight, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses.

NEW & NOTEWORTHY The present work was the first to combine measuring the fiber length and pennation angle (ultrasound imaging) as main determinants of mechanical force production and evaluating the muscle function after a long-duration spaceflight. The results demonstrate that muscles with different functional roles may differently respond to unloading, and this circumstance is important to consider when planning rehabilitation after unloading of any kind, paying particular attention to postural muscles.

The effect of foot type on the Achilles tendon moment arm and biomechanics

BACKGROUND

The aim was to calculate the Achilles tendon moment arm in different degrees of plantarflexion for pes planus, pes cavus and normal arched feet.

METHODS

99 patients (99 radiographs; 40 males, 59 females; mean age 49 years, SD 15) with a healthy ankle joint and a preoperative weightbearing lateral radiograph of the foot were included. Three groups (pes planus, pes cavus and normal-arched feet) with equal sample sizes (n=33) were formed. On radiographs, the angle formed between a horizontal line and the line connecting the insertion of the Achilles tendon with the center of rotation of the ankle, was measured. The interrater reliabilities (ICC) of the angle alpha were compared on radiographs and on MRIs. Using the angle alpha, the Achilles tendon moment arm was calculated in different plantarflexion positions.

RESULTS

The ICC of alpha was higher on radiographs (0.84, [0.73-0.91]) than on MRIs (0.61, [0.27-0.81]). The average alpha was statistically significantly different (normal arched foot 31 degrees (°), pes planus 24°, pes cavus 36°, p=0.021), resulting in a significant shorter Achilles tendon moment arm for pes cavus than for pes planus (p<0.0001) and normal arched feet (p=0.006) in neutral position.

CONCLUSION

The data suggests that it is feasible to use radiographs to measure the Achilles tendon moment arm. The maximum Achilles tendon moment arm is reached at different angles of ankle flexion for pes cavus, pes planus and normal-arched feet. This has to be taken into consideration when planning surgeries.

Estimates of Achilles Tendon Moment Arm Length at Different Ankle Joint Angles: Effect of Passive Moment

The length of a muscle's moment arm can be estimated noninvasively using ultrasound and the tendon excursion method. The main assumption with the tendon excursion method is that the force acting on the tendon during passive rotation is constant. However, passive force changes through the range of motion, and thus moment arm is underestimated. The authors attempted to account for passive force on the measurement of Achilles tendon moment arm using the tendon excursion method in 8 male and female runners. Tendon excursion was measured using ultrasound while the ankle was passively rotated at 0.17 rad·s-1. Moment arm was calculated at 5° intervals as the ratio of tendon displacement to joint rotation from 70° to 115°. Passive moment (MP) was measured using a dynamometer. The displacement attributable to MP was calculated by monitoring tendon displacement during a ramp isometric maximum contraction. MP was 5.7 (2.1) N·m at 70° and decreased exponentially from 70° to 90°. This resulted in MP-corrected moment arms that were significantly larger than uncorrected moment arms at joint angles where MP was present. Furthermore, MP-corrected moment arms did not change with ankle angle, which was not the case for uncorrected moment arms.

Wrist tendon moment arms: Quantification by imaging and experimental techniques

Subject-specific musculoskeletal models require accurate values of muscle moment arms. The aim of this study was to compare moment arms of wrist tendons obtained from non-invasive magnetic resonance imaging (MRI) to those obtained from an in vitro experimental approach. MRI was performed on ten upper limb cadaveric specimens to obtain the centrelines for the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), and abductor pollicis longus (APL) tendons. From these, the anatomical moment arms about each of the flexion-extension (FE) and radioulnar deviation (RUD) axes of the wrist were calculated. Specimens were mounted on a physiologic wrist simulator to obtain functional measurements of the moment arms using the tendon excursion method. No differences were observed between anatomical and functional values of the FE and RUD moment arms of FCR, ECRL and ECRB, and the RUD moment arm of ECU (p > .075). Scaling the anatomical moment arms relative to ECRB in FE and ECU in RUD reduced differences in the FE moment arm of FCU and the RUD moment arm of APL to less than 15% (p > .139). However, differences persisted in moment arms of FCU in RUD, and ECU and APL in FE (p < .008). This study shows that while measurements of moment arms of wrist tendons using imaging do not always conform to values obtained using in vitro experimental approaches, a stricter protocol could result in the acquisition of subject-specific moment arms to personalise musculoskeletal models.

Altered Achilles tendon function during walking in people with diabetic neuropathy: implications for metabolic energy saving

The Achilles tendon (AT) has the capacity to store and release elastic energy during walking, contributing to metabolic energy savings. In diabetes patients, it is hypothesized that a stiffer Achilles tendon may reduce the capacity for energy saving through this mechanism, thereby contributing to an increased metabolic cost of walking (CoW). The aim of this study was to investigate the effects of diabetes and diabetic peripheral neuropathy (DPN) on the Achilles tendon and plantarflexor muscle-tendon unit behavior during walking. Twenty-three nondiabetic controls (Ctrl); 20 diabetic patients without peripheral neuropathy (DM), and 13 patients with moderate/severe DPN underwent gait analysis using a motion analysis system, force plates, and ultrasound measurements of the gastrocnemius muscle, using a muscle model to determine Achilles tendon and muscle-tendon length changes. During walking, the DM and particularly the DPN group displayed significantly less Achilles tendon elongation (Ctrl: 1.81; DM: 1.66; and DPN: 1.54 cm), higher tendon stiffness (Ctrl: 210; DM: 231; and DPN: 240 N/mm), and higher tendon hysteresis (Ctrl: 18; DM: 21; and DPN: 24%) compared with controls. The muscle fascicles of the gastrocnemius underwent very small length changes in all groups during walking (~0.43 cm), with the smallest length changes in the DPN group. Achilles tendon forces were significantly lower in the diabetes groups compared with controls (Ctrl: 2666; DM: 2609; and DPN: 2150 N). The results strongly point toward the reduced energy saving capacity of the Achilles tendon during walking in diabetes patients as an important factor contributing to the increased metabolic CoW in these patients. NEW & NOTEWORTHY From measurements taken during walking we observed that the Achilles tendon in people with diabetes and particularly people with diabetic peripheral neuropathy was stiffer, was less elongated, and was subject to lower forces compared with controls without diabetes. These altered properties of the Achilles tendon in people with diabetes reduce the tendon's energy saving capacity and contribute toward the higher metabolic energy cost of walking in these patients.