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Tendon compliance facilitates power exertion during stretch‐shortening cycle exercises through muscle‐tendon interaction. Tendons stiffen in response to mechanical loadings, and their stiffness sometimes affects motor performance, but no consensus has been reached yet. We investigated the gastrocnemius muscle‐tendon properties of 10 male amateur high jumpers and 14 untrained males. Participants performed maximum voluntary contraction (MVC) of ankle plantar flexion. We measured the maximum joint torque and Achilles tendon stiffness using a torque dynamometer for force measurement, an ultrasound apparatus to track tendon elongation, and a motion capture system to correct joint rotation. High jumpers exerted significantly greater MVC torque than untrained individuals (152.8 ± 31.8 vs. 103.6 ± 18.9 Nm). Tendon stiffness did not significantly differ between groups (287.3 ± 90.9 vs. 258.4 ± 85.6 N/mm). This suggests that strengthening muscles and stiffening tendons may independently adapt through high jump training. In high jumpers, high jump personal best record significantly correlated with MVC torque (r = 0.73) but not significantly correlated with tendon stiffness (r = −0.07). Muscle force exertion ability enhanced by training should be important for improving high jump performance, while tendon stiffening is not necessary for performance. We suggest that humans may inherently have adequate tendon properties for jumping, even without specific training.

Ankle stiffness contributes to standing balance, counteracting the destabilizing effect of gravity. The ankle stiffness together with the compliance between the foot and the support surface make up the ankle-foot stiffness, which is relevant to quiet standing. The contribution of the intrinsic ankle-foot stiffness to balance, and the ankle-foot stiffness amplitude dependency remain a topic of debate in the literature. We therefore developed an experimental protocol to directly measure the bilateral intrinsic ankle-foot stiffness during standing balance, and determine its amplitude dependency. By applying fast (40ms) ramp-and-hold support surface rotations (0.005–0.08rad) during standing, reflexive contributions could be excluded, and the amplitude dependency of the intrinsic ankle-foot stiffness was investigated. Results showed that reflexive activity could not have biased the torque used for estimating the intrinsic stiffness. Furthermore, subjects required less recovery action to restore balance after bilateral rotations in opposite directions compared to rotations in the same direction. The intrinsic ankle-foot stiffness appears insufficient to ensure balance, ranging from 0.93±0.09 to 0.44±0.06 (normalized to critical stiffness ‘mgh’). This implies that changes in muscle activation are required to maintain balance. The non-linear stiffness decrease with increasing rotation amplitude supports the previous published research. With the proposed method reflexive effects can be ruled out from the measured torque without any model assumptions, allowing direct estimation of intrinsic stiffness during standing.

PurposeMuscle, tendon, and muscle–tendon unit (MTU) stiffness as well as passive peak torque (PPT) or delayed stretching pain sensation are typical explanatory approaches for stretching adaptations. However, in literature, differences in the study inclusion, as well as applying meta-analytical models without accounting for intrastudy dependency of multiple and heteroscedasticity of data bias the current evidence. Furthermore, most of the recent analyses neglected to investigate PPT adaptations and further moderators.MethodsThe presented review used the recommended meta-analytical calculation method to investigate the effects of stretching on stiffness as well as on passive torque parameters using subgroup analyses for stretching types, stretching duration, and supervision.ResultsChronic stretching reduced muscle stiffness ( −  0.38, p = 0.01) overall, and also for the supervised ( −  0.49, p = 0.004) and long static stretching interventions ( −  0.61, p < 0.001), while the unsupervised and short duration subgroups did not reach the level of significance (p = 0.21, 0.29). No effects were observed for tendon stiffness or for subgroups (e.g., long-stretching durations). Chronic PPT (0.55, p = 0.005) in end ROM increased. Only long-stretching durations sufficiently decreased muscle stiffness acutely. No effects could be observed for acute PPT.ConclusionWhile partially in accordance with previous literature, the results underline the relevance of long-stretching durations when inducing changes in passive properties. Only four acute PPT in end ROM studies were eligible, while a large number were excluded as they provided mathematical models and/or lacked control conditions, calling for further randomized controlled trials on acute PPT effects.

The male Maasai of East Africa perform a ritual of repetitive jumping ceremonies and are known anecdotally for outstanding jumping performance. The aim of the present study was to assess vertical jumping ability and anatomical/biomechanical characteristics of Maasai jumpers. Twenty-two Maasai men performed maximal vertical countermovement jumps (CMJ), and repetitive jumps (RJ) on an instrumented force plate. Twelve age-matched Norwegian men served as controls (CON). Anthropometrics, kinematic/kinetic and electromyographic data were recorded during jumping. Resting and dynamic plantarflexor muscle architecture and Achilles tendon stiffness were measured using ultrasonography. Maximal jump height (CMJ and RJ) was similar between groups, however the Maasai jumped faster, with less vertical displacement, and greater ground-reaction peak forces and power in CMJ. In contrast, greater vertical displacement in RJ was seen for the Maasai compared to CON. The Maasai demonstrated longer relative leg length, and tendon structures, but lower fascicle pennation angles, shorter fascicle lengths and more compliant tendons. Although maximal jump height was similar, jump kinetics and kinematics differed between groups, which may relate to jumping tradition or to anthropometrical and/or muscle–tendon morphological/mechanical characteristics. Overall, the observed muscle–tendon characteristics of the Maasai may favor economy of movement during walking and perhaps during RJ.

During human running, the soleus, as the main plantar flexor muscle, generates the majority of the mechanical work through active shortening. The fraction of chemical energy that is converted into muscular work (enthalpy efficiency) depends on the muscle shortening velocity. Here, we investigated the soleus muscle fascicle behaviour during running with respect to the enthalpy efficiency as a mechanism that could contribute to improvements in running economy after exercise-induced increases of plantar flexor strength and Achilles tendon (AT) stiffness. Using a controlled longitudinal study design ( n = 23) featuring a specific 14-week muscle–tendon training, increases in muscle strength (10%) and tendon stiffness (31%) and reduced metabolic cost of running (4%) were found only in the intervention group ( n = 13, p < 0.05). Following training, the soleus fascicles operated at higher enthalpy efficiency during the phase of muscle–tendon unit (MTU) lengthening (15%) and in average over stance (7%, p < 0.05). Thus, improvements in energetic cost following increases in plantar flexor strength and AT stiffness seem attributed to increased enthalpy efficiency of the operating soleus muscle. The results further imply that the soleus energy production in the first part of stance, when the MTU is lengthening, may be crucial for the overall metabolic energy cost of running.

This study assessed the effects of a 12-week strength training (ST) intervention on Achilles and patellar tendon stiffness and tendon-matrix blood-borne markers in well-trained triathletes performing concomitant high-volume aerobic training. Eighteen triathletes (VO 2 max: 64.43 ± 4.65 ml kg −1 min −1 ) were randomized into a combined endurance and ST group (3-times/week; intervention) or endurance-only group (control). Pre- and post-intervention assessments included ultrasound-based force-elongation measurements of the Achilles and patellar tendons, blood analysis of selected markers before and after an experimental ST session in weeks 1 and 12, and dynamic strength and endurance testing. Achilles (39.1 ± 31.8%) and patellar tendon stiffness (15.8 ± 8.5%), isometric maximal knee extensor strength (8.9 ± 5.7%), and squat one-repetition maximum (20.0 ± 9.7%) significantly increased in the intervention group (all p  ≤ 0.002). Additionally, serum concentrations of matrix metalloproteinase (MMP) I (46.30 ± 20%; p  = 0.027) and decorin (30.50 ± 19.94%; p  = 0.015) increased after the first experimental ST session, while after the second session MMP-III (40.77 ± 20.13%; p  = 0.040) and decorin (36.59 ± 37.81%; p  = 0.007) were increased. Baseline tenascin-c concentrations decreased significantly only in the control group (-19.49 ± 20.55%; p  = 0.029). Our data clearly show improved muscle-tendon properties of the plantar flexor muscles and quadriceps femoris, indicating Achilles and patellar tendon stiffness to be similarly responsive to heavy ST despite concomitantly performing high volumes of endurance training.

PurposeTo document the magnitude and time course of human Achilles tendon adaptations (i.e. changes in tendon morphological and mechanical properties) during a 12-week high-load plantar flexion training program.MethodsUltrasound was used to determine Achilles tendon cross-sectional area (CSA), length and elongation as a function of plantar flexion torque during voluntary plantar flexion. Tendon force–elongation and stress–strain relationships were determined before the start of training (pre-training) and after 4 (post-4), 8 (post-8) and 12 (post-12) training weeks.ResultsAt the end of the training program, maximum isometric force had increased by 49% and tendon CSA by 17%, but tendon length, maximal tendon elongation and maximal strain were unchanged. Hence, tendon stiffness had increased by 82%, and so had Young’s modulus, by 86%. Significant changes were first detected at post-4 in stiffness (51% increase) and Young’s modulus (87% increase), and at post-8 in CSA (15% increase).ConclusionsAchilles tendon material properties already improved after 4 weeks of high-load training: stiffness increased while CSA remained unchanged. Tendon hypertrophy (increased CSA) was observed after 8 training weeks and contributed to a further increase in Achilles tendon stiffness, but tendon stiffness increases were mostly caused by adaptations in tissue properties.

Patellar complications frequently limit the success of total knee arthroplasty. In addition to the musculoskeletal forces themselves, patellar tendon elastic properties are essential for driving patellar loading. Elastic properties reported in the literature exhibit high variability and appear to differ according to the methodologies used. Specifically in total knee arthroplasty patients, only limited knowledge exists on in vivo elastic properties and their corresponding loads. For the first time, we report stiffness, Young’s modulus, and forces of the patellar tendon, derived from four patients with telemetric total knee arthroplasties using a combined imaging and measurement approach. To achieve this, synchronous in vivo telemetric assessment of tibio-femoral contact forces and fluoroscopic assessment of knee kinematics, along with full body motion capture and ground reaction forces, fed musculoskeletal multi-body models to quantify patellar tendon loading and elongation. Mechanical patellar tendon properties were calculated during a squat and a sit-stand-sit activity, with resulting tendon stiffness and Young’s modulus ranging from 511 to 1166 N/mm and 259 to 504 MPa, respectively. During these activities, the patellar tendon force reached peak values between 1.31 and 2.79 bodyweight, reaching levels of just ∼0.5 bodyweight below the tibio-femoral forces. The results of this study provide valuable input data for mechanical simulations of the patellar tendon and the whole resurfaced knee.

Aim we investigated the effects of a 10 week training program (i.e., minute oscillatory stretching; MOS) on the mechanical responses and walking capability in people with type 2 diabetes (T2D). Methods seventeen T2D patients performed maximum voluntary contractions of the plantar flexor muscles during which Achilles tendon stiffness (k T ) and muscle–tendon stiffness (k M ) were evaluated at different percentages of the maximum voluntary force (MVC). In addition, each participant was requested to walk at different walking speeds (i.e. 2, 3, 4, 5, and 6 kmh −1 ) while their net energy cost of walking (C net ), cumulative EMG activity per distance travelled (CMAPD) and kinematic parameters (step length, step frequency, the ankle/knee range of motion) were evaluated. Results maximum tendon elongation increased after MOS training, and k T significantly decreased (between 0 and 20% of MVC). No differences were observed for muscle elongation or k M after training. C net decreased after training (at the slowest tested speeds) while no changes in CMAPD were observed. Step length and ankle ROM during walking increased after training at the slowest tested speeds, while step frequency decreased; no significant effects were observed for knee ROM. Conclusion these results indicate the effectiveness of 10 weeks of MOS training in reducing tendon stiffness and the energy cost during walking in people with T2D. This training protocol requires no specific instrumentation, can be easily performed at home, and has a high adherence (92 ± 9%). It could, thus, be useful to mitigate mechanical tendon deterioration and improve physical behaviour in this population.

PurposeThe goal of this paper was to determine if sports participation influences torque control differently for adolescent boys and young men during a slow ramp task.MethodsTwenty-one adolescent boys (11 athletes) and 31 young men (16 athletes) performed a slow ramp increase in plantar flexion torque from 0 to maximum. We quantified torque control as the coefficient of variation (CV) of torque during the ramp and quantified the Achilles tendon mechanical properties using ultrasonography.ResultsRelative to adolescent boys, young men were taller, heavier, stronger, and had a longer and stiffer Achilles tendon. However, these characteristics were not different between athletes and non-athletes in adolescent boys. For the CV of torque, there was a significant interaction with sports participation, indicating that only adolescent boys who were non-athletes had greater variability than young men. The CV of torque of all participants was predicted from the maximum torque and torque oscillations from 1 to 2 Hz, whereas the CV of torque for adolescent boys was predicted only from torque oscillations from 1 to 2 Hz.ConclusionThese findings suggested that adolescent boys who participate in sports exhibited lower torque variability during a slow ramp task, which was not explained by differences in Achilles tendon properties or strength.