Human achilles tendon plasticity in response to cyclic strain: effect of rate and duration

Abstract

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, Young's 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 Young's modulus (51% and 17%). The increases in tendon stiffness and Young's 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.

The Achilles tendon is mechanosensitive in older adults: adaptations following 14 weeks versus 1.5 years of cyclic strain exercise

“…These exercise-related increases in tendon stiffness and Young's modulus are comparable with earlier medium-term (12-14 weeks) exercise studies with younger adults using similar loading magnitudes on the AT (increases of 16-36% and 15-45% respectively; Arampatzis et al, 2007aArampatzis et al, , 2010Bohm et al, 2014;Fletcher et al, 2010;Foure et al, 2013;Kubo et al, 2007a). Moreover, we found a significant increase of 6% in average AT CSA after the medium-term (14 weeks) exercise intervention which is consistent with previous studies in younger adults (mean AT CSA increases of between 0.5 and 10%; Arampatzis et al, 2007aArampatzis et al, , 2010Bohm et al, 2014;Kongsgaard et al, 2007). When investigating the possible regionspecific effects, we did not find any significant group×time×interval interaction, indicating that the tendon hypertrophied uniformly along the whole tendon.…”

“…These findings show that the higher tendon stiffness for the stronger subjects is caused by a larger tendon CSA in combination with an intrinsic tendon material improvement, indicating that the elderly AT may still be mechanosensitive as shown in earlier studies with Table 1. Properties of the triceps surae muscle-tendon unit before (baseline), after a medium-term (14 weeks) and after a long-term (1.5 years) exercise intervention with high AT strain magnitudes for the experimental and the control group (N=13) younger adults (Arampatzis et al, 2007a;Bohm et al, 2014;Kongsgaard et al, 2007). However, one might argue that the above findings result from natural differences (genetic factors, diet and lifestyle etc.…”

“…Although not statistically significant, habitual exercise has been suggested to affect the Young's modulus (on average up to 16%) and may possibly trigger structural adaptive modifications in human tendons (Seynnes et al, 2013;Westh et al, 2008;Wiesinger et al, 2015). In addition, diverse exercise training interventions for 12-14 weeks in younger adults have led to increased PT and AT stiffness, Young's modulus and tendon CSA, concurrent with increments in muscle strength (Arampatzis et al, 2007a;Bohm et al, 2014;Kongsgaard et al, 2007;Kubo et al, 2001Kubo et al, , 2007aMalliaras et al, 2013). These study results therefore suggest that the PT and AT of younger adults are sensitive to mechanical loading and respond to changes in the mechanical environment by adapting their mechanical, material and morphological properties.…”

Finding the sweet spot via personalised Achilles tendon training: the future is within reach

“…Health of these load-deprived tendons, represented by increased expression of type I collagen, was restored following the same loading regime (6 days, 8 hours/day, magnitude 6%, 0.25 Hz). In vivo studies have also systematically modulated Achilles tendon training regime by varying strain magnitude,3 6 8 duration,6 rate6 and frequency 3 6. Training at a frequency of 0.17 Hz and strain magnitude of 6.5% resulted in superior material properties (ie, increased stiffness) when compared with training with lower strain magnitude (~3%),3 higher frequency (0.5 Hz),3 higher rates (ie, one-legged jump)6 or longer durations (12 s) 6.…”

Deducing a mechanism of all musculoskeletal injuries

“…We will refer to these non-MTU structures as having a contraction cycle notwithstanding the above significant difference to the MTU contraction cycle. LBU's demonstrate some ability to store elastic energy 22 and other musculoskeletal structures such as bones and ligaments demonstrate an ability to vary stiffness over longer time frames in response to training 23,24 and aging 25 . …”