Mechanical properties of the triceps surae tendon and aponeurosis in relation to intensity of sport activity

Neuromuscular factors predicting lower limb explosive strength in male college sprinters

Purpose

This study aimed to explore the effects of neural and muscular factors on lower limb explosive strength in male college sprinters, and build models based on those factors to identify the key neuromuscular factors that predict the rate of force development (RFD) and 30 m sprint time.

Method

15 male college sprinters were recruited in this study, with 100 m personal best times under 10.93 s. The neuromuscular data were collected by H-reflex and V-wave, isokinetic muscle strength, vertical jumps, and 30 m sprint tests. Pearson correlation and multiple stepwise regression were used for data analysis. The level of statistical significance was set at p ≤ 0.05 for all analyses.

Results

30 m sprint time had a significant moderate positive correlation with Achilles tendon stiffness (r = 0.50, p = 0.05, 95%CI: 0.01-0.81) and a significant moderate negative correlation with the H-index (r = -0.54, p = 0.04, 95%CI: 0.82 to -0.03), V wave (V/MmaxA, r = -0.59, p = 0.02, 95%CI: 0.85 to -0.11) and the eccentric strength of Hamstring (HECC, r = -0.53, p = 0.04, 95%CI: 0.82 to -0.03). The rate of force development (RFD) had a significant positive correlation with H reflex (Hmax/Mmax, r = 0.57, p = 0.03.95%CI:0.08-0.84), and the eccentric strength of Quadriceps (QECC, r = 0.53, p = 0.04, 95%CI: 0.02-0.82). V/MmaxA and HECC were identified as predictors of 30 m sprint time, and the R 2 explained 57.5% of the variance. Vertical stiffness and QECC explained 82.7% of the variation in the RFD.

Conclusion

This study found that V/MmaxA and HECC were predictive factors of 30 m sprint time, vertical stiffness and QECC were the predictive factors of RFD. Neural factors such as the α-motoneurons excitability of the spinal and supraspinal centers, have a greater influence on lower limb explosive strength in male college sprinters. Therefore, training related to the neural function of sprinters should be emphasized. In addition, H reflex and V wave can be used widely to assess and monitor the neural function of sprinters in future research. The impact of neural drive on muscles in different levels and sexes of sprinters, and the neuromuscular modulation during muscle contractions can be further explored.

The Structure, Function, and Adaptation of Lower-Limb Aponeuroses: Implications for Myo-Aponeurotic Injury

The aponeurosis is a large fibrous connective tissue structure within and surrounding skeletal muscle and is a critical component of the muscle-tendon unit (MTU). Due to the lack of consensus on terminology and the heterogeneous nature of the aponeurosis between MTUs, there are several questions that remain unanswered. For example, the aponeurosis is often conflated with the free tendon rather than being considered an independent structure. This has subsequent implications when interpreting data regarding the structure, function, and adaptation of the aponeuroses from these studies. In recent years, a body of work has emerged to suggest that acute injury to the myo-aponeurotic complex may have an impact on return-to-sport timeframes and reinjury rates. Therefore, the purpose of this review is to provide a more detailed understanding of the morphology and mechanical behaviour common to all aponeuroses, as well as the unique characteristics of specific lower-limb aponeuroses that are commonly injured. This review provides the practitioner with a current understanding of the mechanical, material, and adaptive properties of lower limb aponeuroses and suggests directions for future research related to the myo-aponeurotic complex.

Optimizing Resistance Training for Sprint and Endurance Athletes: Balancing Positive and Negative Adaptations

Resistance training (RT) triggers diverse morphological and physiological adaptations that are broadly considered beneficial for performance enhancement as well as injury risk reduction. Some athletes and coaches therefore engage in, or prescribe, substantial amounts of RT under the assumption that continued increments in maximal strength capacity and/or muscle mass will lead to improved sports performance. In contrast, others employ minimal or no RT under the assumption that RT may impair endurance or sprint performances. However, the morphological and physiological adaptations by which RT might impair physical performance, the likelihood of these being evoked, and the training program specifications that might promote such impairments, remain largely undefined. Here, we discuss how selected adaptations to RT may enhance or impair speed and endurance performances while also addressing the RT program variables under which these adaptations are likely to occur. Specifically, we argue that while some myofibrillar (muscle) hypertrophy can be beneficial for increasing maximum strength, substantial hypertrophy can lead to macro- and microscopic adaptations such as increases in body (or limb) mass and internal moment arms that might, under some conditions, impair both sprint and endurance performances. Further, we discuss how changes in muscle architecture, fiber typology, microscopic muscle structure, and intra- and intermuscular coordination with RT may maximize speed at the expense of endurance, or maximize strength at the expense of speed. The beneficial effect of RT for sprint and endurance sports can be further improved by considering the adaptive trade-offs and practical implications discussed in this review.

Observation of the Achilles Tendon and Gastrocnemius Muscle's Passive Stiffness During an Incremental Running Protocol

OBJECTIVE

Passive stiffness, a biomechanical parameter, has a potential influence on running economy, thus playing a pivotal role in performance. This study aimed to quantify passive stiffness of the Achilles tendon and gastrocnemius muscle using myotonometry throughout an incremental running protocol.

METHODS

Twenty-one male participants underwent a multistage incremental test (Bordeaux test) on a treadmill until exhaustion. Passive stiffness of the gastrocnemius muscle and Achilles tendon was measured using a MyotonPRO device. Measurements were taken before, during, and after the incremental test.

RESULTS

Our findings indicated that passive stiffness of the medial gastrocnemius decreased at rest between prerun and postrun assessments (-20.12 N·m-1, P = .012). Furthermore, during the test, stiffness increased at low intensity (at 50% of maximal aerobic speed: +104.8 N·m-1, P = .042), returning to baseline values as intensity increased.

CONCLUSIONS

Throughout an incremental running protocol, both Achilles tendon and gastrocnemius muscle stiffness initially increased and then decreased with escalating percentages of maximal aerobic speed. A deeper understanding of stiffness in running can inform more effective recommendations for runners' strengthening and training.

The Use of Free Weight Squats in Sports: A Narrative Review-Squatting Movements, Adaptation, and Sports Performance: Physiological

ABSTRACT

Stone, MH, Hornsby, G, Mizuguchi, S, Sato, K, Gahreman, D, Duca, M, Carroll, K, Ramsey, MW, Stone, ME, and Haff, GG. The use of free weight squats in sports: a narrative review-squatting movements, adaptation, and sports performance: physiological. J Strength Cond Res 38(8): 1494-1508, 2024-The squat and its variants can provide numerous benefits including positively affecting sports performance and injury prevention, injury severity reduction, and rehabilitation. The positive benefits of squat are likely the result of training-induced neural alterations and mechanical and morphological adaptations in tendons, skeletal muscles, and bones, resulting in increased tissue stiffness and cross-sectional area (CSA). Although direct evidence is lacking, structural adaptations can also be expected to occur in ligaments. These adaptations are thought to beneficially increase force transmission and mechanical resistance (e.g., resistance to mechanical strain) and reduce the likelihood and severity of injuries. Adaptations such as these, also likely play an important role in rehabilitation, particularly for injuries that require restricted use or immobilization of body parts and thus lead to a consequential reduction in the CSA and alterations in the mechanical properties of tendons, skeletal muscles, and ligaments. Both volume and particularly intensity (e.g., levels of loading used) of training seem to be important for the mechanical and morphological adaptations for at least skeletal muscles, tendons, and bones. Therefore, the training intensity and volume used for the squat and its variations should progressively become greater while adhering to the concept of periodization and recognized training principles.

Jump Performance and Its Relationship with Lower Body Joint Kinetics and Kinematics in Children with Cerebral Palsy

PURPOSE

The aim was to quantify jump performance in children with cerebral palsy (CP) and determine if the expected deficit is related to their lower body joint kinetics and kinematics.

METHODS

Twenty-four ambulatory ( n = 17 level I and 7 level II in the Gross Motor Function Classification System) children with spastic CP ( n = 13 unilateral and 11 bilateral) and 24 age-, sex-, and race-matched typically developing controls were studied. Jump height and peak power and range of motion at the hip, knee, and ankle of the more affected limb in children with CP and the nondominant limb in controls were assessed during a countermovement jump using three-dimensional motion capture and a force platform.

RESULTS

Compared with controls, children with CP had lower jump height (33%, Cohen's d ( d ) = 1.217), peak power at the knee (39%, d = 1.013) and ankle (46%, d = 1.687), and range of motion at the hip (32%, d = 1.180), knee (39%, d = 2.067), and ankle (46%, d = 3.195; all P < 0.001). Jump height was positively related to hip, knee, and ankle power and range of motion in children with CP ( rs range = 0.474-0.613, P < 0.05), and hip and ankle power and knee and ankle range of motion in controls ( rs range = 0.458-0.630, P < 0.05). The group difference in jump height was no longer detected when ankle joint power, ankle range of motion, or knee range of motion was statistically controlled ( P > 0.15).

CONCLUSIONS

Jump performance is compromised in children with CP and is associated with low power generation and range of motion in the lower limb, especially at the ankle.

Effect of the temporal coordination and volume of cyclic mechanical loading on human Achilles tendon adaptation in men

Human tendons adapt to mechanical loading, yet there is little information on the effect of the temporal coordination of loading and recovery or the dose-response relationship. For this reason, we assigned adult men to either a control or intervention group. In the intervention group, the two legs were randomly assigned to one of five high-intensity Achilles tendon (AT) loading protocols (i.e., 90% maximum voluntary contraction and approximately 4.5 to 6.5% tendon strain) that were systematically modified in terms of loading frequency (i.e., sessions per week) and overall loading volume (i.e., total time under loading). Before, at mid-term (8 weeks) and after completion of the 16 weeks intervention, AT mechanical properties were determined using a combination of inverse dynamics and ultrasonography. The cross-sectional area (CSA) and length of the free AT were measured using magnetic resonance imaging pre- and post-intervention. The data analysis with a linear mixed model showed significant increases in muscle strength, rest length-normalized AT stiffness, and CSA of the free AT in the intervention group (p < 0.05), yet with no marked differences between protocols. No systematic effects were found considering the temporal coordination of loading and overall loading volume. In all protocols, the major changes in normalized AT stiffness occurred within the first 8 weeks and were mostly due to material rather than morphological changes. Our findings suggest that-in the range of 2.5-5 sessions per week and 180-300 s total high strain loading-the temporal coordination of loading and recovery and overall loading volume is rather secondary for tendon adaptation.

Achilles tendon and triceps surae muscle properties in athletes

PURPOSE

The aim of this study was to investigate internal Achilles tendon (AT) displacement, AT shear wave velocity (SWV), and triceps surae (TS) muscle shear modulus in athletes.

METHODS

Internal AT displacement was assessed using ultrasound during isometric contraction. Shear wave elastography was used to assess AT SWV (m × s-1) at rest and TS muscle shear modulus (kPa) during passive ankle dorsiflexion.

RESULTS

A total of 131 athletes participated in this study. Athletes who had not exercised within two days had greater AT non-uniformity and mean anterior tendon displacement, and lower SWV at the proximal AT measurement site (mean difference [95% CI]: 1.8 mm [0.6-2.9], p = 0.003; 1.6 mm [0.2-2.9], p = 0.021; - 0.9 m × s-1 [- 1.6 to - 0.2], p = 0.014, respectively). Male basketball players had a lower mean AT displacement compared to gymnasts (- 3.7 mm [- 6.9 to - 0.5], p = 0.042), with the difference localised in the anterior half of the tendon (- 5.1 mm [- 9.0 to - 1.1], p = 0.022). Male gymnasts had a smaller absolute difference in medial gastrocnemius-minus-soleus shear modulus than basketball players (59.6 kPa [29.0-90.2], p < 0.001) and track and field athletes (52.7 kPa [19.2-86.3], p = 0.004). Intraclass correlation coefficients of measurements ranged from 0.720 to 0.937 for internal AT displacement, from 0.696 to 0.936 for AT SWE, and from 0.570 to 0.890 for TS muscles.

CONCLUSION

This study provides a reliability assessment of muscle and tendon SWV. The relative differences in passive TS muscle shear modulus suggest sport-specific adaptation. Importantly, in healthy individuals, lower AT displacement after exercise may reflect the time required for tendon recovery.

Patellar Tendon Adaptations to Downhill Running Training and Their Relationships With Changes in Mechanical Stress and Loading History

ABSTRACT

Bontemps, B, Gruet, M, Louis, J, Owens, DJ, Miríc, S, Vercruyssen, F, and Erskine, RM. Patellar tendon adaptations to downhill running training and their relationships with changes in mechanical stress and loading history. J Strength Cond Res 38(1): 21-29, 2024-It is unclear whether human tendon adapts to moderate-intensity, high-volume long-term eccentric exercise, e.g., downhill running (DR) training. This study aimed to investigate the time course of patellar tendon (PT) adaptation to short-term DR training and to determine whether changes in PT properties were related to changes in mechanical stress or loading history. Twelve untrained, young, healthy adults (5 women and 7 men) took part in 4 weeks' DR training, comprising 10 sessions. Running speed was equivalent to 60-65% V̇O2max, and session duration increased gradually (15-30 minutes) throughout training. Isometric knee extensor maximal voluntary torque (MVT), vastus lateralis (VL) muscle physiological cross-sectional area (PCSA) and volume, and PT CSA, stiffness, and Young's modulus were assessed at weeks 0, 2, and 4 using ultrasound and isokinetic dynamometry. Patellar tendon stiffness (+6.4 ± 7.4%), Young's modulus (+6.9 ± 8.8%), isometric MVT (+7.5 ± 12.3%), VL volume (+6.6 ± 3.2%), and PCSA (+3.8 ± 3.3%) increased after 4 weeks' DR (p < 0.05), with no change in PT CSA. Changes in VL PCSA correlated with changes in PT stiffness (r = 0.70; p = 0.02) and Young's modulus (r = 0.63; p = 0.04) from 0 to 4 weeks, whereas changes in MVT did not correlate with changes in PT stiffness and Young's modulus at any time point (p > 0.05). To conclude, 4 weeks' DR training promoted substantial changes in PT stiffness and Young's modulus that are typically observed after high-intensity, low-volume resistance training. These tendon adaptations seemed to be driven primarily by loading history (represented by VL muscle hypertrophy), whereas increased mechanical stress throughout the training period did not seem to contribute to changes in PT stiffness or Young's modulus.

Neuromuscular Plantar Flexor Performance of Sprinters versus Physically Active Individuals

INTRODUCTION

Comparison of the neuromuscular performance of different athlete types may give insight into the in vivo variability of these measures and their underpinning mechanisms. The study aims to compare the neuromuscular function of the plantar flexors of sprinters and physically active individuals to assess any differences in explosive force performance.

METHODS

Neuromuscular performance of a group of sprinters (highly trained/national level, n = 12; elite/international level, n = 2) and physically active individuals ( n = 14) were assessed during involuntary, explosive, and maximum voluntary isometric plantar flexions, across different muscle-tendon unit (MTU) lengths (10° plantarflexion, 0° (anatomical zero/neutral), and 10° dorsiflexion). Plantarflexion rate of torque development (RTD) was measured in three 50-ms time windows from their onset. The synchronous activation of the plantar flexor agonist muscles was calculated as the time difference between 1) the first and last muscle onset and 2) the onsets of the two gastrocnemii muscles. Muscle size and MTU stiffness were assessed using sonograms of the medial gastrocnemius and myotendinous junction.

RESULTS

Sprinters exhibited greater involuntary RTD across time points (0-50 ms, 50-100 ms) and MTU lengths. In addition, sprinters demonstrated greater early phase voluntary RTD (0-50 ms, 50-100 ms) across MTU lengths. Sprinters also demonstrated greater late-phase RTD (100-150 ms), and relative maximal voluntary torque at the DF angle only. The sprinters demonstrated a more synchronous activation of the gastrocnemii muscles. There were no observable differences in muscle size and MTU stiffness between groups.

CONCLUSIONS

These findings suggest sprint-specific training could be a contributing factor toward improved explosive performance of the plantar flexors, particularly in the early phase of muscular contraction, evidenced by the greater explosive torque producing capabilities of sprinters.

Ankle and Plantar Flexor Muscle-Tendon Unit Function in Sprinters: A Narrative Review

Maximal sprinting in humans requires the contribution of various muscle-tendon units (MTUs) and joints to maximize performance. The plantar flexor MTU and ankle joint are of particular importance due to their role in applying force to the ground. This narrative review examines the contribution of the ankle joint and plantar flexor MTUs across the phases of sprinting (start, acceleration, and maximum velocity), alongside the musculotendinous properties that contribute to improved plantar flexor MTU performance. For the sprint start, the rear leg ankle joint appears to be a particularly important contributor to sprint start performance, alongside the stretch-shortening cycle (SSC) action of the plantar flexor MTU. Comparing elite and sub-elite sprinters revealed that elite sprinters had a higher rate of force development (RFD) and normalized average horizontal block power, which was transferred via the ankle joint to the block. For the acceleration phase, the ankle joint and plantar flexor MTU appear to be the most critical of the major lower limb joints/MTUs. The contribution of the ankle joint to power generation and positive work is minimal during the first stance, but an increased contribution is observed during the second stance, mid-acceleration, and late-acceleration. In terms of muscular contributions, the gastrocnemius and soleus have distinct roles. The soleus acts mainly as a supporter, generating large portions of the upward impulse, whereas the gastrocnemius acts as both an accelerator and a supporter, contributing significantly to propulsive and upward impulses. During maximum velocity sprinting the ankle joint is a net dissipater of energy, potentially due to the greater vertical loading placed on the plantar flexors. However, the ankle joint is critical for energy transfer from proximal joints to ground force application to maintain velocity. In terms of the contribution of musculoskeletal factors to ankle joint and plantar flexor performance, an optimal plantar flexor MTU profile potentially exists, which is possibly a combination of several musculoskeletal factors, alongside factors such as footwear and technique.

Vertical Jumping Performance: Recording the Effects of Proprioceptive Neuromuscular Facilitation Stretching at Different Plantar Flexor Lengths

INTRODUCTION

Flexibility seems to be an essential part of both the training and rehabilitation processes. Several stretching techniques have been used to improve the range of motion (ROM) of the joints with the proprioceptive neuromuscular facilitation (PNF) method being the most effective one. Although plantar flexors are ideal to compare the acute effects of synergistic muscle groups on performance, it is not clear whether the PNF stretch at different muscle lengths could result in different alterations.

MATERIAL AND METHODS

Sixteen male students randomly performed 2 levels of stretching (PNF with bended knees, or PNFshort, and with extended knee, or PNFlong) and 3 types of jumps, separated by 48 hours (7 sessions in total). Jumping parameters were recorded by a force plate, and the final jumping height (H) and ground reaction forces (Fz) were analyzed. Furthermore, the ROM of the ankle joint was recorded before, right after, and 15 minutes after the stretches.  Results: The ankle joint's ROM joint was increased after both interventions. No significant changes were found in the jumping height of all jumps. The Fz, during the squat jump (SJ) and countermovement jump (CMJ), were increased after PNFshort. Similarly, a significant increase was found in Fz in drop jumps (DJ) right after the PNFshort.

CONCLUSION

Our findings demonstrated that PNF stretches of different lengths could potentially alter the stretch-shortening cycle's performance, possibly leading to a non-optimal muscle-tendon interaction.

PIEZO1 gain-of-function gene variant is associated with elevated tendon stiffness in humans

Prolonged periods of increased physical demands can elicit anabolic tendon adaptations that increase stiffness and mechanical resilience or conversely can lead to pathological processes that deteriorate tendon structural quality with ensuing pain and potential rupture. Although the mechanisms by which tendon mechanical loads regulate tissue adaptation are largely unknown, the ion channel PIEZO1 has been implicated in tendon mechanotransduction, with human carriers of the PIEZO1 gain-of-function variant E756del displaying improved dynamic vertical jump performance compared with noncarriers. Here, we sought to examine whether increased tendon stiffness in humans could explain this increased performance. We assessed tendon morphological and mechanical properties with ultrasound-based techniques in 77 participants of Middle- and West-African descent, and we measured their vertical jumping performance to assess potential functional consequences in the context of high tendon strain-rate loading. Carrying the E756del gene variant (n = 30) was associated with 46.3 ± 68.3% (P = 0.002) and 45.6 ± 69.2% (P < 0.001) higher patellar tendon stiffness and Young's modulus compared with noncarrying controls, respectively. Although these tissue level measures strongly corroborate the initial postulate that PIEZO1 plays an integral part in regulating tendon material properties and stiffness in humans, we found no detectable correlation between tendon stiffness and jumping performance in the tested population that comprised individuals of highly diverse physical fitness level, dexterity, and jumping ability.NEW & NOTEWORTHY The E756del gene variant causes overactivity of the mechanosensitive membrane channel PIEZO1 and is suspected to upregulate tendon collagen cross linking. In human carriers of E756del, we found increased patellar tendon stiffness but similar tendon lengths and cross-sectional areas, directly supporting the premise that PIEZO1 regulates human tendon stiffness at the level of tissue material properties.

Reliability of mechanical properties of the plantar flexor muscle tendon unit with consideration to joint angle and sex

The reliability of mechanical measures can be impacted by the protocol used, including factors such as joint angle and the sex of participants. This study aimed to determine the inter-day reliability of plantar flexor mechanical measures across ankle joint angles and contraction types and consider potential sex-specific effects. 14 physically-active individuals participated in two identical measurement sessions involving involuntary and voluntary plantar flexor contractions, at three ankle angles (10° plantarflexion (PF), 0° (anatomical zero (AZ)), and 10° dorsiflexion (DF)), while torque and surface EMG were recorded. The reliability of mechanical parameters of maximal voluntary torque (MVT), rate of torque development (RTD), electromechanical delay, and tendon stiffness were assessed using absolute and relative reliability measures. MVT measures were reliable across ankle angles. RTD measures showed good group level reliability and moderate reliability for an individual during the early phase of contraction across ankle angles. Explosive voluntary torque measures tended to be less reliable from 50 ms onward, with varied reliability across angles for late-phase RTD. Tendon stiffness demonstrated the best reliability at the DF angle. Sex-based differences in the reliability of tendon measures found that females had significantly different initial tendon length between testing sessions. Despite this, tendon excursion, force, and stiffness measures demonstrated similar reliability compared to males. Ankle angle changes influence the reliability of plantar flexor mechanical measurements across contraction types, particularly for voluntary contractions. These results highlight the importance of establishing potential protocol effects on measurement reliability prior to quantifying plantar flexor mechanical measures.

Muscle-tendon unit design and tuning for power enhancement, power attenuation, and reduction of metabolic cost

The contractile elements in skeletal muscle fibers operate in series with elastic elements, tendons and potentially aponeuroses, in muscle-tendon units (MTUs). Elastic strain energy (ESE), arising from either work done by muscle fibers or the energy of the body, can be stored in these series elastic elements (SEEs). MTUs vary considerably in their design in terms of the relative lengths and stiffnesses of the muscle fibers and SEEs, and the force and work generating capacities of the muscle fibers. However, within an MTU it is thought that contractile and series elastic elements can be matched or tuned to maximize ESE storage. The use of ESE is thought to improve locomotor performance by enhancing contractile element power during activities such as jumping, attenuating contractile element power during activities such as landing, and reducing the metabolic cost of movement during steady-state activities such as walking and running. The effectiveness of MTUs in these potential roles is contingent on factors such as the source of mechanical energy, the control of the flow of energy, and characteristics of SEE recoil. Hence, we suggest that MTUs specialized for ESE storage may vary considerably in the structural, mechanical, and physiological properties of their components depending on their functional role and required versatility.

Exploration of muscle-tendon biomechanics one year after Achilles tendon rupture and the compensatory role of flexor hallucis longus

Achilles tendon (AT) rupture leads to long-term structural and functional impairments. Currently, the predictors of good recovery after rupture are poorly known. Thus, we aimed to explore the interconnections between structural, mechanical, and neuromuscular parameters and their associations with factors that could explain good recovery in patients with non-surgically treated AT rupture. A total of 35 patients with unilateral rupture (6 females) participated in this study. Muscle-tendon structural, mechanical, and neuromuscular parameters were measured 1-year after rupture. Interconnections between the inter-limb differences (Δ) were explored using partial correlations, followed by multivariable linear regression to find associations between the measured factors and the following markers that indicate good recovery: 1) tendon length, 2) tendon non-uniform displacement, and 3) flexor hallucis longus (FHL) normalized EMG amplitude difference between limbs. Δmedial gastrocnemius (MG) (β = -0.12, p = 0.007) and Δlateral gastrocnemius (β = -0.086, p = 0.030) subtendon lengths were associated with MG tendon Δstiffness. MG (β = 11.56, p = 0.003) and soleus (β = 2.18, p = 0.040) Δsubtendon lengths explained 48 % of variance in FHL EMG amplitude. Regression models for tendon length and non-uniform displacement were not significant. Smaller inter-limb differences in Achilles subtendon lengths were associated with smaller differences in the AT stiffness between limbs, and a smaller contribution of FHL muscle to the plantarflexion torque. In the injured limb, the increased contribution of FHL appears to partially counteract a smaller contribution from MG due to the elongated tendon, however the role of FHL should not be emphasized during rehabilitation to allow recovery of the TS muscles.

Towards modern understanding of the Achilles tendon properties in human movement research

The Achilles tendon (AT) is the strongest tendon in humans, yet it often suffers from injury. The mechanical properties of the AT afford efficient movement, power amplification and power attenuation during locomotor tasks. The properties and the unique structure of the AT as a common tendon for three muscles have been studied frequently in humans using in vivo methods since 1990's. As a part of the celebration of 50 years history of the International Society of Biomechanics, this paper reviews the history of the AT research focusing on its mechanical properties in humans. The questions addressed are: What are the most important mechanical properties of the Achilles tendon, how are they studied, what is their significance to human movement, and how do they adapt? We foresee that the ongoing developments in experimental methods and modeling can provide ways to advance knowledge of the complex three-dimensional structure and properties of the Achilles tendon in vivo, and to enable monitoring of the loading and recovery for optimizing individual adaptations.

Evidence-Based High-Loading Tendon Exercise for 12 Weeks Leads to Increased Tendon Stiffness and Cross-Sectional Area in Achilles Tendinopathy: A Controlled Clinical Trial

BACKGROUND

Assuming that the mechanisms inducing adaptation in healthy tendons yield similar responses in tendinopathic tendons, we hypothesized that a high-loading exercise protocol that increases tendon stiffness and cross-sectional area in male healthy Achilles tendons may also induce comparable beneficial adaptations in male tendinopathic Achilles tendons in addition to improving pain and function.

OBJECTIVES

We investigated the effectiveness of high-loading exercise in Achilles tendinopathy in terms of inducing mechanical (tendon stiffness, maximum strain), material (Young's modulus), morphological (tendon cross-sectional area (CSA)), maximum voluntary isometric plantar flexor strength (MVC) as well as clinical adaptations (Victorian Institute of Sports Assessment-Achilles (VISA-A) score and pain (numerical rating scale (NRS))) as the primary outcomes. As secondary outcomes, drop (DJ) and counter-movement jump (CMJ) height and intratendinous vascularity were assessed.

METHODS

We conducted a controlled clinical trial with a 3-month intervention phase. Eligibility criteria were assessed by researchers and medical doctors. Inclusion criteria were male sex, aged between 20 and 55 years, chronic Achilles tendinopathy confirmed by a medical doctor via ultrasound-assisted assessment, and a severity level of less than 80 points on the VISA-A score. Thirty-nine patients were assigned by sequential allocation to one of three parallel arms: a high-loading intervention (training at ~ 90% of the MVC) (n = 15), eccentric exercise (according to the Alfredson protocol) as the standard therapy (n = 15) and passive therapy (n = 14). Parameters were assessed pre- and-post-intervention. Data analysis was blinded.

RESULTS

Primary outcomes: Plantar flexor MVC, tendon stiffness, mean CSA and maximum tendon strain improved only in the high-loading intervention group by 7.2 ± 9.9% (p = 0.045), 20.1 ± 20.5% (p = 0.049), 8.98 ± 5.8% (p < 0.001) and -12.4 ± 10.3% (p = 0.001), respectively. Stiffness decreased in the passive therapy group (-7.7 ± 21.2%; p = 0.042). There was no change in Young's modulus in either group (p > 0.05). The VISA-A score increased in all groups on average by 19.8 ± 15.3 points (p < 0.001), while pain (NRS) dropped by -0.55 ± 0.9 points (p < 0.001).

SECONDARY OUTCOMES

CMJ height decreased for all groups (-0.63 ± 4.07 cm; p = 0.005). There was no change in DJ height and vascularity (p > 0.05) in either group.

CONCLUSION

Despite an overall clinical improvement, it was exclusively the high-loading intervention that induced significant mechanical and morphological adaptations of the plantar flexor muscle-tendon unit. This might contribute to protecting the tendon from strain-induced injury. Thus, we recommend the high-loading intervention as an effective (alternative) therapeutic protocol in Achilles tendinopathy rehabilitation management in males.

CLINICAL TRIALS REGISTRATION NUMBER

NCT02732782.

Relationship between Muscle-Tendon Stiffness and Drop Jump Performance in Young Male Basketball Players during Developmental Stages

BACKGROUND

The relationship between stiffness and drop jump performance in athletes in various stages of development has yet to be fully investigated. The first aim of this study was to investigate the association between the stiffness of the patellar and quadriceps tendon (PT, QT), gastrocnemius-Achilles tendon unit (GAT), and rectus femoris (RF) using drop jump (DJ) performance in young basketball players. The second aim was to investigate possible variations in the stiffness levels of those tissues in different developmental stages.

METHODS

The stiffness levels of the GAT, PT, QT, and RF were measured in both limbs in 73 male basketball players aged 12 to 18 years. The reactive strength index (RSI), contact time (CT) and jump height (JH) during 30 and 40 cm DJs were also measured.

RESULTS

Pearson correlation coefficients showed a significant association between DJ performance and PT, QT, GAT, and RF dynamic stiffness. Moreover, the youngest subjects were found to have lower stiffness values than the older ones.

CONCLUSIONS

Tissue stiffness can affect athletic performance by modifying the stretch-shortening cycle in young basketball players. Stiffness of muscles and tendons increases during the maturation process. Further investigations could shed light on the effect of training on the stiffness of muscles and tendons.

Patellar Tendon Shear Wave Velocity Is Higher and has Different Regional Patterns in Elite Competitive Alpine Skiers than in Healthy Controls

Competitive alpine skiers are exposed to enormous forces acting on their bodies–particularly on the knee joint and hence the patellar tendon - during both the off-season preparation and in-season competition phases. However, factors influencing patellar tendon adaptation and regional pattern differences between alpine skiers and healthy controls are not yet fully understood, but are essential for deriving effective screening approaches and preventative countermeasures. Thirty elite competitive alpine skiers, all members of the Swiss Alpine Ski Team, and 38 healthy age-matched controls were recruited. A set of two-dimensional shear wave elastography measurements of the PT was acquired and projected into three-dimensional space yielding a volumetric representation of the shear wave velocity profile of the patellar tendon. Multivariate linear models served to quantify differences between the two cohorts and effects of other confounding variables with respect to regional shear wave velocity. A significant (p < 0.001) intergroup difference was found between skiers (mean ± SD = 10.4 ± 1.32 m/s) and controls (mean ± SD = 8.9 ± 1.59 m/s). A significant sex difference was found within skiers (p = 0.024), but no such difference was found in the control group (p = 0.842). Regional SWV pattern alterations between skiers and controls were found for the distal region when compared to the mid-portion (p = 0.023). Competitive alpine skiers exhibit higher SWV in all PT regions than healthy controls, potentially caused by long-term adaptations to heavy tendon loading. The presence of sex-specific differences in PT SWV in skiers but not in controls indicates that sex effects have load-dependent dimensions. Alterations in regional SWV patterns between skiers and controls suggest that patellar tendon adaptation is region specific. In addition to the implementation of 3D SWE, deeper insights into long-term tendon adaptation and normative values for the purpose of preventative screening are provided.

Reversing the Mismatch With Forefoot Striking to Reduce Running Injuries

Recent studies have suggested that 95% of modern runners land with a rearfoot strike (RFS) pattern. However, we hypothesize that running with an RFS pattern is indicative of an evolutionary mismatch that can lead to musculoskeletal injury. This perspective is predicated on the notion that our ancestors evolved to run barefoot and primarily with a forefoot strike (FFS) pattern. We contend that structures of the foot and ankle are optimized for forefoot striking which likely led to this pattern in our barefoot state. We propose that the evolutionary mismatch today has been driven by modern footwear that has altered our footstrike pattern. In this paper, we review the differences in foot and ankle function during both a RFS and FFS running pattern. This is followed by a discussion of the interaction of footstrike and footwear on running mechanics. We present evidence supporting the benefits of forefoot striking with respect to common running injuries such as anterior compartment syndrome and patellofemoral pain syndrome. We review the importance of a gradual shift to FFS running to reduce transition-related injuries. In sum, we will make an evidence-based argument for the use of minimal footwear with a FFS pattern to optimize foot strength and function, minimize ground reaction force impacts and reduce injury risk.

No Association between Jump Parameters and Tissue Stiffness in the Quadriceps and Triceps Surae Muscles in Recreationally Active Young Adult Males

While the main contributor for drop jump (DJ) performance is the calf muscle–tendon unit (MTU), for countermovement jump (CMJ) performance, it is the quadriceps MTU. However, to date, it is not clear if the muscle and/or tendon stiffness of the respective MTUs can be related to DJ or CMJ performance. Therefore, the purpose of this study was to investigate the relationships between DJ and CMJ performance parameters and tissue stiffness (i.e., muscle stiffness, tendon stiffness) of the calf MTU and quadriceps MTU, respectively. Consequently, with 16 healthy volunteers, the tissue stiffness of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL) Achilles tendon (AT), vastus medialis (VM), vastus lateralis (VL), rectus femoris (RF), and patellar tendon (PT) were recorded with a Myoton device. Moreover, DJ and CMJ performances were assessed with a force plate. The alpha level was set to 0.05. Pearson correlation coefficients revealed no significant association between DJ performance and GM, GL, or AT stiffness (−0.07 to 0.24; p > 0.05). Similarly, no association was found between CMJ performance parameters and VM, VL, RF, or PT stiffness (−0.13–0.36; p > 0.05). According to our results, other variables, such as jump technique, body weight, or strength, were likely play a more important role in DJ and CMJ performance.

Muscle-tendon morphomechanical properties of non-surgically treated Achilles tendon 1-year post-rupture

BACKGROUND

Achilles tendon rupture appears to alter stiffness and length of the tendon. These alterations may affect the function of tendon in force transmission and in energy storage and recovery. We studied the mechanical properties of the Achilles' tendon post-rupture and their association with function.

METHODS

Twenty-four (20 males, 4 females) participants (mean age: 43 y, 176 cm, 81 kg) were recruited. Ultrasonography and dynamometry were used to assess the muscle-tendon unit morphological and mechanical properties of non-surgically treated patients 1-year post rupture.

FINDINGS

Injured tendons were longer with difference of 1.8 cm (95%CI: 0.5-1.9 cm; P < 0.001), and thicker by 0.2 mm (0.2-0.3 mm; P < 0.01). Medial gastrocnemius cross-sectional area was 1.0 cm² smaller (0.8-1.1 cm²; P < 0.001), fascicles were 0.6 cm shorter (0.5-0.7 cm; P < 0.001) and pennation angle was 2.5° higher (1.3-3.6°; P < 0.001) when compared to the uninjured limb. We found no differences between injured and uninjured tendon stiffness 1-year post-rupture (mean difference: 29.8 N/mm, -7.7-67.3 N/mm; P = 0.170). The injured tendon showed 1.8 mm (1.2-2.4 mm; P < 0.01) lower elongation during maximal voluntary isometric contractions. Patient-reported functional outcome was related to the tendon resting length (β = 0.68, r(10) = 4.079, P = 0.002). Inter-limb differences in the medial gastrocnemius fascicle length were related to inter-limb differences in maximum contractions (β = 1.17, r(14) = 2.808, P = 0.014).

INTERPRETATION

Longer Achilles tendon resting length was associated with poorer self-evaluated functional outcome. Although the stiffness of non-surgically treated and uninjured tendons was similar 1-year post rupture, plantar flexion strength deficit was still present, possibly due to shorter medial gastrocnemius fascicle length.

An Identical Twin Study on Human Achilles Tendon Adaptation: Regular Recreational Exercise at Comparatively Low Intensities Can Increase Tendon Stiffness

Achilles tendon adaptation is a key aspect of exercise performance and injury risk prevention. However, much debate exists about the adaptation of the Achilles tendon in response to exercise activities. Most published research is currently limited to elite athletes and selected exercise activities. Also, existing studies on tendon adaptation do not control for genetic variation. Our explorative cross-sectional study investigated the effects of regular recreational exercise activities on Achilles tendon mechanical properties in 40 identical twin pairs. Using a handheld oscillation device to determine Achilles tendon mechanical properties, we found that the Achilles tendon appears to adapt to regular recreational exercise at comparatively low intensities by increasing its stiffness. Active twins showed a 28% greater Achilles tendon stiffness than their inactive twin (p < 0.05). Further, our research extends existing ideas on sport-specific adaptation by showing that tendon stiffness seemed to respond more to exercise activities that included an aerial phase such as running and jumping. Interestingly, the comparison of twin pairs revealed a high variation of Achilles tendon stiffness (305.4–889.8 N/m), and tendon adaptation was only revealed when we controlled for genetic variance. Those results offer new insights into the impact of genetic variation on individual Achilles tendon stiffness, which should be addressed more closely in future studies.

Exercise Effects on the Biomechanical Properties of the Achilles Tendon—A Narrative Review

Simple Summary

The Achilles tendon influences the running economy because of its ability to store and release strain energy, and it remains one of the most vulnerable tendons among athletes and recreational runners. Exercised-related mechanical loading appears to induce changes in the Achilles tendon morphology and mechanical material properties. Both acute and relatively long-term exercise induces tendon adaptation, although biomechanical changes, e.g., cross-sectional area, plantarflexion moment, Young’s modulus, and stiffness, in response to exercise duration, type, and loading-regimes differ widely. Furthermore, a strong Achilles tendon can be developed by chronic exposure to habitual mechanical loading from daily exercise, which is associated with greater energy storage, release and overall health.

Abstract

The morphological and mechanical properties (e.g., stiffness, stress, and force) of the Achilles tendon (AT) are generally associated with its tendinosis and ruptures, particularly amongst runners. Interest in potential approaches to reduce or prevent the risk of AT injuries has grown exponentially as tendon mechanics have been efficiently improving. The following review aims to discuss the effect of different types of exercise on the AT properties. In this review article, we review literature showing the possibility to influence the mechanical properties of the AT from the perspective of acute exercise and long-term training interventions, and we discuss the reasons for inconsistent results. Finally, we review the role of the habitual state in the AT properties. The findings of the included studies suggest that physical exercise could efficiently improve the AT mechanical properties. In particular, relatively long-term and low-intensity eccentric training may be a useful adjunct to enhance the mechanical loading of the AT.

Shear-stress sensing by PIEZO1 regulates tendon stiffness in rodents and influences jumping performance in humans

Athletic performance relies on tendons, which enable movement by transferring forces from muscles to the skeleton. Yet, how load-bearing structures in tendons sense and adapt to physical demands is not understood. Here, by performing calcium (Ca²⁺) imaging in mechanically loaded tendon explants from rats and in primary tendon cells from rats and humans, we show that tenocytes detect mechanical forces through the mechanosensitive ion channel PIEZO1, which senses shear stresses induced by collagen-fibre sliding. Through tenocyte-targeted loss-of-function and gain-of-function experiments in rodents, we show that reduced PIEZO1 activity decreased tendon stiffness and that elevated PIEZO1 mechanosignalling increased tendon stiffness and strength, seemingly through upregulated collagen cross-linking. We also show that humans carrying the PIEZO1 E756del gain-of-function mutation display a 13.2% average increase in normalized jumping height, presumably due to a higher rate of force generation or to the release of a larger amount of stored elastic energy. Further understanding of the PIEZO1-mediated mechanoregulation of tendon stiffness should aid research on musculoskeletal medicine and on sports performance.

A modified surface EMG biomarker for gait assessment in spastic cerebral palsy

OBJECTIVE

Muscle clinical metrics are crucial for spastic cocontraction management in children with Cerebral Palsy (CP). We investigated whether the ankle plantar flexors cocontraction index (CCI) normalized with respect to the bipedal heel rise (BHR) approach provides more robust spastic cocontraction estimates during gait than those obtained through the widely accepted standard maximal isometric plantar flexion (IPF).

METHODS

Ten control and 10 CP children with equinus gait pattern performed the BHR and IPF testing and walked barefoot 10-m distance. We compared agonist medial gastrocnemius EMG during both testing and CCIs obtained as the ratios of antagonist EMG during swing phase of gait and either BHR or IPF agonist EMG.

RESULTS

Agonist EMG values from the BHR were: (i) internally reliable (Cronbach's α = 0.993), (ii) ~50 ± 0.4% larger than IPF, (iii) and positively correlated. Derived CCIs were significantly smaller (p < 0.05) in both populations.

CONCLUSION

The bipedal heel rise approach may be accurate enough to reveal greater agonist activity of plantar flexors than the maximal isometric plantar flexion and seems to be more appropriate to obtain cocontraction estimates during swing of gait.

SIGNIFICANCE

This modified biomarker may represent a step forward towards improved accuracy of spastic gait management in pediatric.

A Comparison of the Effects of Foam Rolling and Stretching on Physical Performance. A Systematic Review and Meta-Analysis

Foam rolling and stretching with its various techniques are frequently used as a warm-up routine to increase the range of motion of a joint. While the magnitude of the changes in range of motion between foam rolling and stretching (static and dynamic techniques) is similar, it is not clear if this also holds true for performance parameters (e.g., strength, jump height). The purpose of this meta-analysis was to compare the effects of an acute bout of foam rolling (with and without vibration) with an acute bout of stretching (with all techniques included) on performance parameters in healthy participants. We assessed the results from 13 studies and 35 effect sizes by applying a random-effect meta-analysis. Moreover, by applying a mixed-effect model, we performed subgroup analyses with the stretching technique, type of foam rolling, tested muscle, treatment duration, and type of task. We found no significant overall effect, and the analysis revealed only a trend of the performance parameters in favor of foam rolling when compared to stretching (when considering all techniques). Significantly favorable effects of foam rolling on performance were detected with subgroup analyses when compared to static stretching, when applied to some muscles (e.g., quadriceps) or some tasks (e.g., strength), when applied for longer than 60 s, or when the foam rolling included vibration. When foam rolling was compared to dynamic stretching or applied in the non-vibration mode, the same magnitude of effect was observed. While the present meta-analysis revealed no significantly different effect between foam rolling and stretching (including all techniques) prior to exercise, differences could be observed under specific conditions.

Multijoint Musculoarticular Stiffness Derived From a Perturbation Is Highly Variable

ABSTRACT

Schofield, M, Tinwala, F, Cronin, J, Hébert-Losier, K, and Uthoff, A. Multijoint musculoarticular stiffness derived from a perturbation is highly variable. J Strength Cond Res 35(9): 2498-2503, 2021-Testing musculoarticular stiffness may provide insights into multijoint elastic properties. Yet, most research has focused on quantifying stiffness, or elastic potential, at a single joint. The purpose of this study was to quantify the test-retest reliability of musculoarticular stiffness derived from the perturbation technique across the bench pull, bench press, and squat movements. Eight resistance-trained men performed bench pull, bench press, and squat repetition maximums, after which a perturbation protocol was tested over multiple days. During the 3 movements, a brief perturbation was applied to the bar. The resulting sinusoidal wave was measured by an underbench force plate and a linear position transducer attached to the bar. From the sinusoidal wave, stiffness was derived and found to be unreliable across movements and days (change in mean: -35.1 to 15.8%; coefficient of variation: 7.1-111%; intraclass correlation: -0.58 to -0.89). Squat data were removed from the analysis entirely because of the inability to consistently determine the perturbations on the force plate. Practitioners need to be aware that musculoarticular stiffness as measured using the perturbation technique on the movements performed in this study has considerable limitations in terms of reproducibility.

Factors Associated With Achilles Tendon Rupture in Women's Collegiate Gymnastics

BACKGROUND

Achilles injury risk in women's collegiate gymnastics is 10-fold higher than in all other collegiate sports. This study aims to identify risk factors for Achilles tendon ruptures in collegiate female gymnasts.

HYPOTHESIS

Gymnasts with Achilles tendon ruptures will be more likely to report early gymnastics specialization, elite-level training before college, and performance of high-difficulty skills on floor and vault.

STUDY DESIGN

Cross-sectional study.

LEVEL OF EVIDENCE

Level 4.

METHODS

Anonymous surveys were distributed to current and former collegiate female gymnasts, aged 18 to 30 years, via coaches, athletic trainers, Twitter, and ResearchMatch. Information about Achilles tendon ruptures, gymnastics-related injuries, sport specialization, event/skills participation, and medication use were collected.

RESULTS

A total of 581 gymnasts were included. One hundred gymnasts (17.2%; 95% CI: 14.1%-20.3%) reported Achilles tendon ruptures during collegiate training or competition. Most ruptures (91%) occurred on floor exercise; 85.7% of these occurred during back tumbling-take-off. Compared with gymnasts without ruptures during college, a greater percentage of gymnasts with ruptures competed at a Division I program, trained elite, competed difficult vaults and floor passes before and during college, competed in all 4 events during college, identified as Black/African American, and used retinoid medications.

CONCLUSION

Achilles tendon ruptures are more common in women's collegiate gymnastics than other sports. Competing at the elite level, performing difficult floor and vault skills, and competing in all 4 events may increase the risk for an Achilles tendon rupture. Potential nontraining risk factors include retinoid exposure and Black/African American race. Future studies regarding the mechanisms of Achilles tendon ruptures in female collegiate gymnasts are warranted.

CLINICAL RELEVANCE

Collegiate gymnasts who compete at the elite level, perform high levels of difficulty on floor and vault, and compete in all 4 events may be at increased risk for Achilles tendon ruptures.

Implementing Ultrasound Imaging for the Assessment of Muscle and Tendon Properties in Elite Sports: Practical Aspects, Methodological Considerations and Future Directions

Ultrasound (US) imaging has been widely used in both research and clinical settings to evaluate the morphological and mechanical properties of muscle and tendon. In elite sports scenarios, a regular assessment of such properties has great potential, namely for testing the response to training, detecting athletes at higher risks of injury, screening athletes for structural abnormalities related to current or future musculoskeletal complaints, and monitoring their return to sport after a musculoskeletal injury. However, several practical and methodological aspects of US techniques should be considered when applying this technology in the elite sports context. Therefore, this narrative review aims to (1) present the principal US measures and field of applications in the context of elite sports; (2) to discuss, from a methodological perspective, the strengths and shortcomings of US imaging for the assessment of muscle and tendon properties; and (3) to provide future directions for research and application.

Effect of Achilles Tendon Mechanics on the Running Economy of Elite Endurance Athletes

The Achilles tendon stores and releases strain energy, influencing running economy. The present study aims to verify the influence of the Achilles tendon tangent modulus, as a material property, on running economy by comparing two groups of elite endurance-performance athletes undergoing different running training volumes. Twelve athletes, six long-distance runners and six pentathletes, were studied. Long-distance runners had a higher weekly running training volume (116.7±13.7 vs. 58.3±20.4 km, p<0.05) and a better running economy (204.3±12.0 vs. 222.0±8.7 O₂ mL ∙ kg^-1 ∙ km^-1, p<0.05) evaluated in a treadmill at 16 km·h^-1, 1% inclination. Both groups presented similar VO_2max (68.5±3.8 vs. 65.7±5.0 mL ∙ min^-1 ∙ kg^-1, p>0.05). Achilles tendon tangent modulus was estimated from ultrasound-measured deformations, with the ankle passively mobilized by a dynamometer. True stress was calculated from the measured torque. The long-distance runners had a higher maximum tangent modulus (380.6±92.2 vs. 236.2±82.6 MPa, p<0.05) and maximum true stress than pentathletes (24.2±5.1 vs. 16.0±3.5 MPa, p<0.05). The correlation coefficient between tangent modulus at larger deformations was R=-0.7447 (p<0.05). Quantifying tendon tissue adaptations associated with different running training volumes will support subject and modality-specific workouts prescription of elite endurance athletes.

Inhomogeneous and anisotropic mechanical properties of the triceps surae muscles and aponeuroses in vivo during submaximal muscle contraction

This study aimed to identify characteristics of the triceps surae muscles and aponeuroses stiffness in vivo, during graded isometric submaximal plantarflexion efforts. A total of twelve healthy male subjects (age: 27 ± 4 years) participated and were required to stay at rest (0% MVC) and perform isometric submaximal plantar flexion contractions (20%, 40%, 60% of MVC) on a dynamometer. Young's modulus of triceps surae muscles and the adjoining aponeuroses between gastrocnemii and soleus at the proximal and distal sites were obtained in the longitudinal direction (along muscle's line of action) during at rest and submaximal plantar flexions. Additionally, Young's modulus of adjoining aponeuroses in the transverse direction at the distal sites was also calculated. Young's modulus of LG (lateral gastrocnemius), SOL-lat (lateral part of soleus) and LPS (superficial aponeurosis of LG) at the proximal site showed significant (p < 0.001) graded increase response to the submaximal contraction levels. Besides, in the lateral side, significant differences in the Young's modulus of aponeuroses were observed between longitudinal and transverse directions at rest and during contractions (p < 0.002). Changes of aponeuroses length were significantly correlated with Young's modulus changes of the proximal gastrocnemii muscle bellies (r = 0.43-0.45, p = 0.006-0.008) and superficial aponeuroses (r = 0.49-0.60, p < 0.002). The results further indicate that the triceps surae muscles and aponeuroses showed inhomogeneous and anisotropic mechanical properties during submaximal muscle contractions, and the stiffening effect of muscle belly possibly make influence on the mechanical properties of aponeuroses during muscle contractions, especially for the lateral gastrocnemius.

Quantifying mechanical loading and elastic strain energy of the human Achilles tendon during walking and running

The purpose of the current study was to assess in vivo Achilles tendon (AT) mechanical loading and strain energy during locomotion. We measured AT length considering its curve-path shape. Eleven participants walked at 1.4 m/s and ran at 2.5 m/s and 3.5 m/s on a treadmill. The AT length was defined as the distance between its origin at the gastrocnemius medialis myotendinous junction (MTJ) and the calcaneal insertion. The MTJ was tracked using ultrasonography and projected to the reconstructed skin surface to account for its misalignment. Skin-to-bone displacements were assessed during a passive rotation (5°/s) of the ankle joint. Force and strain energy of the AT during locomotion were calculated by fitting a quadratic function to the experimentally measured tendon force-length curve obtained from maximum voluntary isometric contractions. The maximum AT strain and force were affected by speed (p < 0.05, ranging from 4.0 to 4.9% strain and 1.989 to 2.556 kN), yet insufficient in magnitude to be considered as an effective stimulus for tendon adaptation. Besides the important tendon energy recoil during the propulsion phase (7.8 to 11.3 J), we found a recoil of elastic strain energy at the beginning of the stance phase of running (70-77 ms after touch down) between 1.7 ± 0.6 and 1.9 ± 1.1 J, which might be functionally relevant for running efficiency.

Evidence that ageing does not influence the uniformity of the muscle-tendon unit adaptation in master sprinters

Differences in the adaptation processes between muscle and tendon in response to mechanical loading can lead to non-uniform mechanical properties within the muscle-tendon unit (MTU), potentially increasing injury risk. The current study analysed the mechanical properties of the triceps surae (TS) MTU in 10 young (YS; 22 ± 3 yrs) and 10 older (OS; age 65 ± 8 yrs; i.e. master) (inter)national level sprinters and 11 young recreationally active adults (YC; 23 ± 3 yrs) to detect possible non-uniformities in muscle and tendon adaptation due to habitual mechanical loading and ageing. Triceps surae muscle strength, tendon stiffness and maximal tendon strain were assessed in both legs during maximal voluntary isometric plantarflexion contractions via dynamometry and ultrasonography. Irrespective of the leg, OS and YC in comparison to YS demonstrated significantly (P < 0.05) lower TS muscle strength and tendon stiffness, with no differences between OS and YC. Furthermore, no group differences were detected in the maximal tendon strain (average of both legs: OS 3.7 ± 0.8%, YC 4.4 ± 0.8% and YS 4.3 ± 0.9%) as well as in the inter-limb symmetry indexes in muscle strength, tendon stiffness and maximal tendon strain (range across groups: -5.8 to 4.9%; negative value reflects higher value for the non-preferred leg). Thus, the findings provide no clear evidence for a disruption in the TS MTU uniformity in master sprinters, demonstrating that ageing tendons can maintain their integrity to meet the increased functional demand due to elite sports.

Increasing the midsole bending stiffness of shoes alters gastrocnemius medialis muscle function during running

In recent years, increasing the midsole bending stiffness (MBS) of running shoes by embedding carbon fibre plates in the midsole resulted in many world records set during long-distance running competitions. Although several theories were introduced to unravel the mechanisms behind these performance benefits, no definitive explanation was provided so far. This study aimed to investigate how the function of the gastrocnemius medialis (GM) muscle and Achilles tendon is altered when running in shoes with increased MBS. Here, we provide the first direct evidence that the amount and velocity of GM muscle fascicle shortening is reduced when running with increased MBS. Compared to control, running in the stiffest condition at 90% of speed at lactate threshold resulted in less muscle fascicle shortening (p = 0.006, d = 0.87), slower average shortening velocity (p = 0.002, d = 0.93) and greater estimated Achilles tendon energy return (p ≤ 0.001, d = 0.96), without a significant change in GM fascicle work (p = 0.335, d = 0.40) or GM energy cost (p = 0.569, d = 0.30). The findings of this study suggest that running in stiff shoes allows the ankle plantarflexor muscle-tendon unit to continue to operate on a more favourable position of the muscle's force-length-velocity relationship by lowering muscle shortening velocity and increasing tendon energy return.

Achilles Tendon Morphology Is Related to Triceps Surae Muscle Size and Peak Plantarflexion Torques During Walking in Young but Not Older Adults

The interaction of the triceps surae muscles and the Achilles tendon is critical in producing the ankle plantarflexion torque required for human walking. Deficits in plantarflexor output are a hallmark of reduced mobility in older adults and are likely associated with changes in the triceps surae muscles that occur with age. Structural differences between young and older adults have been observed in the Achilles tendon and in the triceps surae muscles. However, less is known about how age-related differences in muscle and tendon morphology correspond with each other and, furthermore, how those morphology differences correlate with age-related deficits in function. The goal of this work was to investigate whether there is a correlation between age-related differences in triceps surae muscle size and Achilles tendon cross-sectional area (CSA) and whether either is predictive of ankle plantarflexion torque during walking. We used magnetic resonance imaging (MRI) to measure triceps surae muscle volumes and tendon CSAs in young (n = 14, age: 26 ± 4 years) and older (n = 7, age: 66 ± 5 years) adults, and we determined peak plantarflexion torques during treadmill walking. We found that individual muscle volumes as a percentage of the total triceps surae volume did not differ between young and older adults, though muscle volumes per body size (normalized by the product of height and mass) were smaller in older adults. Achilles tendon CSA was correlated with body size and muscle volumes in young adults but not in older adults. The ratio of tendon CSA to total triceps surae muscle volume was significantly greater in older adults. Peak ankle plantarflexion torque during walking correlated with body size and triceps surae volume in young and older adults but was correlated with tendon CSA only in the young adults. Structure–function relationships that seem to exist between the Achilles tendon and the triceps surae muscles in young adults are no longer evident in all older adults. Understanding mechanisms that determine altered Achilles tendon CSA in older adults may provide insight into age-related changes in function.

Monitoring Muscle-Tendon Adaptation Over Several Years of Athletic Training and Competition in Elite Track and Field Jumpers

Differences in muscle and tendon responsiveness to mechanical stimuli and time courses of adaptive changes may disrupt the interaction of the musculotendinous unit (MTU), increasing the risk for overuse injuries. We monitored training-induced alterations in muscle and tendon biomechanical properties in elite jumpers over 4 years of athletic training to detect potential non-synchronized adaptations within the triceps surae MTU. A combined cross-sectional and longitudinal investigation over 4 years was conducted by analyzing triceps surae MTU mechanical properties in both legs via dynamometry and ultrasonography in 67 elite track and field jumpers and 24 age-matched controls. Fluctuations in muscle and tendon adaptive changes over time were quantified by calculating individual residuals. The cosine similarity of the relative changes of muscle strength and tendon stiffness between sessions served as a measure of uniformity of adaptive changes. Our cross-sectional study was unable to detect clear non-concurrent differences in muscle strength and tendon stiffness in elite jumpers. However, when considering the longitudinal data over several years of training most of the jumpers demonstrated greater fluctuations in muscle strength and tendon stiffness and hence tendon strain compared to controls, irrespective of training period (preparation vs. competition). Moreover, two monitored athletes with chronic Achilles tendinopathy showed in their affected limb lower uniformity in MTU adaptation as well as higher fluctuations in tendon strain over time. Habitual mechanical loading can affect the MTU uniformity in elite jumpers, leading to increased mechanical demand on the tendon over an athletic season and potentially increased risk for overuse injuries.

Mechanical properties of muscle and tendon at high strain rate in sprinters

The aim of the present study was to compare the mechanical properties of muscles and tendons at high strain rates between sprinters and untrained men. Fifteen sprinters and 18 untrained men participated in this study. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast stretching at five different angular velocities (100, 200, 300, 500, and 600 deg·s-1 ) after submaximal isometric contractions. Stiffness and hysteresis of tendon structures were measured during ramp and ballistic contractions. Active muscle stiffness at 500 deg·s-1 (p = .070) and 600 deg·s-1 (p = .041) was greater in sprinters than untrained men, whereas no differences in those at 100, 200, and 300 deg·s-1 were found between the two groups. There were no differences in stiffness or hysteresis of tendon structures measured during ramp and ballistic contractions between the two groups. These results suggest that, for sprinters, greater active muscle stiffness at a high angular velocity is caused by exercising with a high angular velocity that is typical of their training.

Effects of long-term athletic training on muscle morphology and tendon stiffness in preadolescence: association with jump performance

Purpose

Evidence on training-induced muscle hypertrophy during preadolescence is limited and inconsistent. Possible associations of muscle strength and tendon stiffness with jumping performance are also not investigated. We investigated the thickness and pennation angle of the gastrocnemius medialis muscle (GM), as indicators for potential muscle hypertrophy in preadolescent athletes. Further, we examined the association of triceps surae muscle–tendon properties with jumping performance.

Methods

Eleven untrained children (9 years) and 21 similar-aged artistic gymnastic athletes participated in the study. Muscle thickness and pennation angle of the GM were measured at rest and muscle strength of the plantar flexors and Achilles tendon stiffness during maximum isometric contractions. Jumping height in squat (SJ) and countermovement jumps (CMJ) was examined using a force plate. We evaluated the influence of normalised muscle strength and tendon stiffness on jumping performance with a linear regression model.

Results

Muscle thickness and pennation angle did not differ significantly between athletes and non-athletes. In athletes, muscle strength was greater by 25% and jumping heights by 36% (SJ) and 43% (CMJ), but Achilles tendon stiffness did not differ between the two groups. The significant predictor for both jump heights was tendon stiffness in athletes and normalised muscle strength for the CMJ height in non-athletes.

Conclusion

Long-term artistic gymnastics training during preadolescence seems to be associated with increased muscle strength and jumping performance but not with training-induced muscle hypertrophy or altered tendon stiffness in the plantar flexors. Athletes benefit more from tendon stiffness and non-athletes more from muscle strength for increased jumping performance.

Individualized Muscle-Tendon Assessment and Training

The interaction of muscle and tendon is of major importance for movement performance and a balanced development of muscle strength and tendon stiffness could protect athletes from overuse injury. However, muscle and tendon do not necessarily adapt in a uniform manner during a training process. The development of a diagnostic routine to assess both the strength capacity of muscle and the mechanical properties of tendons would enable the detection of muscle-tendon imbalances, indicate if the training should target muscle strength or tendon stiffness development and allow for the precise prescription of training loads to optimize tendon adaptation. This perspective article discusses a framework of individualized muscle-tendon assessment and training and outlines a methodological approach for the patellar tendon.

Influence of Sport Type on Metatarsophalangeal and Ankle Joint Stiffness and Hopping Performance

While individual ankle and metatarsophalangeal joint stiffness is related to training intensity and sport performances, sport athletes may develop specific passive joint stiffness among the spectrum from endurance to powerful types of sports. The objective of this study examined whether marathon runners, basketball players, and other sports athletes would demonstrate distinct passive ankle and metatarsophalangeal joint stiffness as well as vertical stiffness. Fifteen marathon runners, nineteen basketball players, and seventeen other sports athletes performed both joint stiffness measurement and single-leg hopping tests. We used a computerized dynamometer to control foot alignment and speed for passive ankle and metatarsophalangeal joint stiffness measurements. We calculated vertical stiffness by body deceleration and body mass displacement during hopping on the force platform. One-way ANOVA was performed to identify the group differences. Bivariate correlation test was also performed among ankle, metatarsophalangeal, and vertical stiffness. The basketball group displayed 13% higher ankle passive stiffness than the other sports players group (P = 0.03). Metatarsophalangeal joint passive stiffness in sitting and standing positions was 23% higher in the basketball group than the runner and other sports athlete groups (P < 0.01). However, there was no significant group differences in metatarsophalangeal joint passive stiffness and vertical stiffness. Significant correlations among all stiffness variables were determined (P < 0.05). These findings indicate that ankle and metatarsophalangeal joint passive stiffness, rather than vertical leg stiffness, would be in relation to types of sports participation. Ankle and toe strengthening exercises could improve basketball players' performance and prevent injury.

Muscle-tendon morphology and function following long-term exposure to repeated and strenuous mechanical loading

We mapped structural and functional characteristics of muscle-tendon units in a population exposed to very long-term routine overloading. Twenty-eight military academy cadets (age = 21.00 ± 1.1 years; height = 176.1 ± 4.8 cm; mass = 73.8 ± 7.0 kg) exposed for over 24 months to repetitive overloading were profiled via ultrasonography with a senior subgroup of them (n = 11; age = 21.4 ± 1.0 years; height = 176.5 ± 4.8 cm; mass = 71.4 ± 6.6 kg) also tested while walking and marching on a treadmill. A group of eleven ethnicity- and age-matched civilians (age = 21.6 ± 0.7 years; height = 176.8 ± 4.3 cm; mass = 74.6 ± 5.6 kg) was also profiled and tested. Cadets and civilians exhibited similar morphology (muscle and tendon thickness and cross-sectional area, pennation angle, fascicle length) in 26 out of 29 sites including the Achilles tendon. However, patellar tendon thickness along the entire tendon was greater (P < .05) by a mean of 16% for the senior cadets compared with civilians. Dynamically, cadets showed significantly smaller ranges of fascicle length change and lower shortening velocity in medial gastrocnemius during walking (44.0% and 47.6%, P < .05-.01) and marching (27.5% and 34.3%, P < .05-.01) than civilians. Furthermore, cadets showed lower normalized soleus electrical activity during walking (22.7%, P < .05) and marching (27.0%, P < .05). Therefore, 24-36 months of continuous overloading, primarily occurring under aerobic conditions, leads to more efficient neural and mechanical behavior in the triceps surae complex, without any major macroscopic alterations in key anatomical structures.

Towards the Exploitation of Physical Compliance in Segmented and Electrically Actuated Robotic Legs: A Review Focused on Elastic Mechanisms

Physical compliance has been increasingly used in robotic legs, due to its advantages in terms of the mechanical regulation of leg mechanics and energetics and the passive response to abrupt external disturbances during locomotion. This article presents a review of the exploitation of physical compliance in robotic legs. Particular attention has been paid to the segmented, electrically actuated robotic legs, such that a comparable analysis can be provided. The utilization of physical compliance is divided into three main categories, depending on the setting locations and configurations, namely, (1) joint series compliance, (2) joint parallel compliance, and (3) leg distal compliance. With an overview of the representative work related to each category, the corresponding working principles and implementation processes of various physical compliances are explained. After that, we analyze in detail some of the structural characteristics and performance influences of the existing designs, including the realization method, compliance profile, damping design, and quantitative changes in terms of mechanics and energetics. In parallel, the design challenges and possible future works associated with physical compliance in robotic legs are also identified and proposed. This article is expected to provide useful paradigmatic implementations and design guidance for physical compliance for researchers in the construction of novel physically compliant robotic legs.

Do habitual foot-strike patterns in running influence functional Achilles tendon properties during gait?

The capacity of foot-strike running patterns to influence the functional properties of the Achilles tendon is controversial. This study used transmission-mode ultrasound to investigate the influence of habitual running foot-strike pattern on Achilles tendon properties during barefoot walking and running. Fifteen runners with rearfoot (RFS) and 10 with a forefoot (FFS) foot-strike running pattern had ultrasound transmission velocity measured in the right Achilles tendon during barefoot walking (≈1.1 ms^-1) and running (≈2.0 ms^-1). Temporospatial gait parameters, ankle kinematics and vertical ground reaction force were simultaneously recorded. Statistical comparisons between foot-strike patterns were made using repeated measure ANOVAs. FFS was characterised by a significantly shorter stance duration (-4%), greater ankle dorsiflexion (+2°), and higher peak vertical ground reaction force (+20% bodyweight) than RFS running (P < .05). Both groups adopted a RFS pattern during walking, with only the relative timing of peak dorsiflexion (3%), ground reaction force (1-2%) and peak vertical force loading rates (22-23%) differing between groups (P < .05). Peak ultrasound transmission velocity in the Achilles tendon was significantly higher in FFS during walking (≈100 ms^-1) and running (≈130 ms^-1) than RFS (P < .05). Functional Achilles tendon properties differ with habitual footfall patterns in recreational runners.

Morphological and Mechanical Properties of the Quadriceps Femoris Muscle-Tendon Unit From Adolescence to Adulthood: Effects of Age and Athletic Training

The combined effects of mechanical loading and maturation during adolescence are still not well understood. The purpose of the study was to investigate the development of the quadriceps femoris muscle-tendon unit from early adolescence (EA), late adolescence (LA) to young adulthood (YA), and examine how it is influenced by athletic training in a cross-sectional design. Forty-one male athletes and forty male non-athletes from three different age groups (EA: 12-14 years, n = 29; LA: 16-18 years, n = 27; and YA: 20-35 years, n = 25) participated in the present study. Maximum strength of the knee extensor muscles, architecture of the vastus lateralis (VL) muscle and patellar tendon stiffness were examined using dynamometry, motion capture, electromyography, and ultrasonography. Muscle strength and tendon stiffness significantly increased (p < 0.001) from EA to LA without any further alterations (p > 0.05) from LA to YA. Athletes compared to non-athletes showed significantly greater (p < 0.001) absolute muscle strength (EA: 3.52 ± 0.75 vs. 3.20 ± 0.42 Nm/kg; LA: 4.47 ± 0.61 vs. 3.83 ± 0.56 Nm/kg; and YA: 4.61 ± 0.55 vs. 3.60 ± 0.53), tendon stiffness (EA: 990 ± 317 vs. 814 ± 299 N/mm; LA: 1266 ± 275 vs. 1110 ± 255 N/mm; and YA: 1487 ± 354 vs. 1257 ± 328), and VL thickness (EA: 19.7 ± 3.2 vs. 16.2 ± 3.4 mm; LA: 23.0 ± 4.2 vs. 20.1 ± 3.3 mm; and YA: 25.5 ± 4.2 vs. 23.9 ± 3.9 mm). Athletes were more likely to reach strain magnitudes higher than 9% strain compared to non-athlete controls (EA: 28 vs. 15%; LA: 46 vs. 16%; and YA: 66 vs. 33%) indicating an increased mechanical demand for the tendon. Although the properties of the quadriceps femoris muscle-tendon unit are enhanced by athletic training, their development from early-adolescence to adulthood remain similar in athletes and non-athletes with the major alterations between early and LA. However, both age and athletic training was associated with a higher prevalence of imbalances within the muscle-tendon unit and a resultant increased mechanical demand for the patellar tendon.

Patellar Tendon Strain Associates to Tendon Structural Abnormalities in Adolescent Athletes

High mechanical strain is thought to be one of the main factors for the risk of tendon injury, as it determines the mechanical demand placed upon the tendon by the working muscle. The present study investigates the association of tendon mechanical properties including force, stress and strain, and measures of tendon micromorphology and neovascularization, which are thought to be indicative of tendinopathy in an adolescent high-risk group for overuse injury. In 16 adolescent elite basketball athletes (14–15 years of age) we determined the mechanical properties of the patellar tendon by combining inverse dynamics with magnetic resonance and ultrasound imaging. Tendon micromorphology was determined based on a spatial frequency analysis of sagittal plane ultrasound images and neovascularization was quantified as color Doppler area. There was a significant inverse relationship between tendon strain and peak spatial frequency (PSF) in the proximal tendon region (r = −0.652, p = 0.006), indicating locally disorganized collagen fascicles in tendons that are subjected to high strain. No such associations were present at the distal tendon site and no significant correlations were observed between tendon force or stress and tendon PSF as well as between tendon loading and vascularity. Our results suggest that high levels of tendon strain might associate to a micromorphological deterioration of the collagenous network in the proximal patellar tendon, which is also the most frequent site affected by tendinopathy. Neovascularization of the tendon on the other hand seems not to be directly related to the magnitude of tendon loading and might be a physiological response to a high frequency of training in this group. Those findings have important implications for our understanding of the etiology of tendinopathy and for the development of diagnostical tools for the assessment of injury risk.