Background: The present article systematically reviews recent literature on the in vivo adaptation of asymptomatic human tendons following increased chronic mechanical loading, and meta-analyzes the loading conditions, intervention outcomes, as well as methodological aspects. Methods: The search was performed in the databases PubMed, Web of Knowledge, and Scopus as well as in the reference lists of the eligible articles. A study was included if it conducted (a) a longitudinal exercise intervention (≥8 weeks) on (b) healthy humans (18 to 50 years), (c) investigating the effects on mechanical (i.e., stiffness), material (i.e., Young’s modulus) and/or morphological properties (i.e., cross-sectional area (CSA)) of tendons in vivo, and was reported (d) in English language. Weighted average effect sizes (SMD, random-effects) and heterogeneity (Q and I ² statistics) of the intervention-induced changes of tendon stiffness, Young’s modulus, and CSA were calculated. A subgroup analysis was conducted regarding the applied loading intensity, muscle contraction type, and intervention duration. Further, the methodological study quality and the risk of bias were assessed. Results: The review process yielded 27 studies with 37 separate interventions on either the Achilles or patellar tendon (264 participants). SMD was 0.70 (confidence interval: 0.51, 0.88) for tendon stiffness (N=37), 0.69 (0.36, 1.03) for Young’s modulus (N=17), and 0.24 (0.07, 0.42) for CSA (N=33), with significant overall intervention effects (p<0.05). The heterogeneity analysis (stiffness: I ² =30%; Young’s modulus: I ² =57%; CSA: I ² =21%) indicated that differences in the loading conditions may affect the adaptive responses. The subgroup analysis confirmed that stiffness adaptation significantly (p<0.05) depends on loading intensity (I ² =0%), but not on muscle contraction type. Although not significantly different, SMD was higher for interventions with longer duration (≥12 weeks). The average score of 71±9% in methodological quality assessment indicated an appropriate quality of most studies. Conclusions: The present meta-analysis provides elaborate statistical evidence that tendons are highly responsive to diverse loading regimens. However, the data strongly suggests that loading magnitude in particular plays a key role for tendon adaptation in contrast to muscle contraction type. Furthermore, intervention-induced changes in tendon stiffness seem to be more attributed to adaptations of the material rather than morphological properties.

High strain magnitude and low strain frequency are important stimuli for tendon adaptation. Increasing the rate and duration of the applied strain may enhance the adaptive responses. Therefore, our purpose was to investigate the effect of strain rate and duration on Achilles tendon adaptation. The study included two experimental groups (N=14 and N=12) and a control group (N=13). The participants of the experimental groups exercised according to a reference protocol (14 weeks, four times a week), featuring a high strain magnitude (~6.5%) and a low strain frequency (0.17 Hz, 3 s loading/3 s relaxation) on one leg and with either a higher strain rate (one-legged jumps) or a longer strain duration (12 s loading) on the other leg. The strain magnitude and loading volume were similar in all protocols. Before and after the interventions, the tendon stiffness, Young9s modulus and cross-sectional area were examined using magnetic resonance imaging, ultrasound and dynamometry. The reference and long strain duration protocols induced significantly increased (P<0.05) tendon stiffness (57% and 25%), cross-sectional area (4.2% and 5.3%) and Young9s modulus (51% and 17%). The increases in tendon stiffness and Young9s modulus were higher in the reference protocol. Although region-specific tendon hypertrophy was also detected after the high strain rate training, there was only a tendency of increased stiffness (P=0.08) and cross-sectional area (P=0.09). The control group did not show any changes (P=0.86). The results provide evidence that a high strain magnitude, an appropriate strain duration and repetitive loading are essential components for an efficient adaptive stimulus for tendons.

According to the force–length–velocity relationships, the muscle force potential is determined by the operating length and velocity, which affects the energetic cost of contraction. During running, the human soleus muscle produces mechanical work through active shortening and provides the majority of propulsion. The trade-off between work production and alterations of the force–length and force–velocity potentials (i.e. fraction of maximum force according to the force–length–velocity curves) might mediate the energetic cost of running. By mapping the operating length and velocity of the soleus fascicles onto the experimentally assessed force–length and force–velocity curves, we investigated the association between the energetic cost and the force–length–velocity potentials during running. The fascicles operated close to optimal length (0.90 ± 0.10L₀) with moderate velocity (0.118 ± 0.039V_max[maximum shortening velocity]) and, thus, with a force–length potential of 0.92 ± 0.07 and a force–velocity potential of 0.63 ± 0.09. The overall force–length–velocity potential was inversely related (r= −0.52,p= 0.02) to the energetic cost, mainly determined by a reduced shortening velocity. Lower shortening velocity was largely explained (p< 0.001,R²= 0.928) by greater tendon gearing, shorter Achilles tendon lever arm, greater muscle belly gearing and smaller ankle angle velocity. Here, we provide the first experimental evidence that lower shortening velocities of the soleus muscle improve running economy.

According to the force-length-velocity relationships, the muscle force potential during locomotion is determined by the operating fibre length and velocity. We measured fascicle and muscle-tendon unit length and velocity as well as the activity of the human vastus lateralis muscle (VL) during walking and running. Furthermore, we determined the VL force-length relationship experimentally and calculated the force-length and force-velocity potentials (i.e. fraction of maximum force according to the force-length-velocity curves) for both gaits. During the active state of the stance phase, fascicles showed significantly (p < 0.05) smaller length changes (walking: 9.2 ± 4.7% of optimal length (L₀); running: 9.0 ± 8.4%L₀) and lower velocities (0.46 ± 0.36 L₀/s; 0.03 ± 0.83 L₀/s) compared to the muscle-tendon unit (walking: 19.7 ± 5.3%L₀, −0.94 ± 0.32 L₀/s; running: 34.5 ± 5.8%L₀, −2.59 ± 0.41 L₀/s). The VL fascicles operated close to optimum length (L₀ = 9.4 ± 0.11 cm) in both walking (8.6 ± 0.14 cm) and running (10.1 ± 0.19 cm), resulting in high force-length (walking: 0.92 ± 0.08; running: 0.91 ± 0.14) and force-velocity (0.91 ± 0.08; 0.97 ± 0.13) potentials. For the first time we demonstrated that, in contrast to the current general conception, the VL fascicles operate almost isometrically and close to L₀ during the active state of the stance phase of walking and running. The findings further verify an important contribution of the series-elastic element to VL fascicle dynamics.

Background Locomotor adaptability is based on the implementation of error-feedback information from previous perturbations to predictively adapt to expected perturbations (feedforward) and to facilitate reactive responses in recurring unexpected perturbations (‘savings’). The effect of aging on predictive and reactive adaptability is yet unclear. However, such understanding is fundamental for the design and application of effective interventions targeting fall prevention. Methods We systematically searched the Web of Science, MEDLINE, Embase and Science Direct databases as well as the reference lists of the eligible articles. A study was included if it addressed an investigation of the locomotor adaptability in response to repeated mechanical movement perturbations of healthy older adults (≥60 years). The weighted average effect size (WAES) of the general adaptability (adaptive motor responses to repeated perturbations) as well as predictive (after-effects) and reactive adaptation (feedback responses to a recurring unexpected perturbation) was calculated and tested for an overall effect. A subgroup analysis was performed regarding the factor age group [i.e., young (≤35 years) vs. older adults]. Furthermore, the methodological study quality was assessed. Results The review process yielded 18 studies [1009 participants, 613 older adults (70 ± 4 years)], which used various kinds of locomotor tasks and perturbations. The WAES for the general locomotor adaptability was 1.21 [95 % confidence interval (CI) 0.68–1.74, n = 11] for the older and 1.39 (95 % CI 0.90–1.89, n = 10) for the young adults with a significant (p < 0.05) overall effect for both age groups and no significant subgroup differences. Similar results were found for the predictive (older: WAES 1.10, 95 % CI 0.37–1.83, n = 8; young: WAES 1.54, 95 % CI 0.11–2.97, n = 7) and reactive (older: WAES 1.09, 95 % CI 0.22–1.96, n = 5; young: WAES 1.35, 95 % CI 0.60–2.09, n = 5) adaptation featuring significant (p < 0.05) overall effects without subgroup differences. The average score of the methodological quality was 67 ± 8 %. Conclusions The present meta-analysis provides elaborate statistical evidence that locomotor adaptability in general and predictive and reactive adaptation in particular remain highly effective in the elderly, showing only minor, not statistically significant age-related deficits. Consequently, interventions which use adaptation and learning paradigms including the application of the mechanisms responsible for an effective predictive and reactive dynamic stability control may progressively improve older adults’ recovery performance and, thus, reduce their risk of falling.

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.