Human muscle-tendon unit mechanobiological responses to consecutive high strain cyclic loading

In response to a mechanical stimulus, tendons have a slower tissue renewal rate compared to muscles. This could over time lead to a higher mechanical demand (experienced strain) for the tendon, especially when a high strain magnitude exercise is repeated without sufficient recovery. The current study investigated the adaptive responses of the triceps surae (TS) muscle-tendon unit (MTU) and extracellular matrix turnover-related biomarkers to repetitive high tendon strain cyclic loading. Eleven young male adults performed a progressive resistance exercise over 12 consecutive days, consisting of high Achilles tendon (AT) strain cyclic loading (90% MVC) with one leg once a day (LegT1) and the alternate leg three times a day (LegT3). Exercise-related changes in TS MTU mechanical properties and serum concentrations of extracellular matrix turnover-related biomarkers were analysed over the intervention period. Both legs demonstrated similar increases in maximal AT force (∼10%) over the 12-day period of exercise. A ∼20% increase in maximal AT strain was found for LegT3 (p<0.05) already after 8 consecutive exercise days, along with a corresponding decrease in AT stiffness. These effects were maintained even after a 48h rest period. The AT mechanical properties for LegT1 were unaltered. Biomarker analysis revealed no sign of inflammation, but altered collagen turnover and delayed increase in the collagen type I synthesis rate. Accordingly, we suggest that tendon is vulnerable to frequent high-magnitude and volume of cyclic mechanical loading, as accumulation of micro-damage can potentially exceed the rate of biological repair, leading to increased maximal tendon strain