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 with 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 human triceps surae (TS) muscle-tendon unit (MTU) and extracellular matrix turnover-related biomarkers to repetitive high tendon strain cyclic loading. Eleven young adult males 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) after 8 consecutive exercise days, along with a corresponding decrease in AT stiffness. These effects were maintained even after a 48 h rest period. The AT mechanical properties for LegT1 were unaltered. Biomarker analysis revealed no sign of inflammation but there was altered collagen turnover and a delay in collagen type I synthesis. Accordingly, we suggest that tendon is vulnerable to frequent high magnitude cyclic mechanical loading as accumulation of micro-damage can potentially exceed the rate of biological repair, leading to increased maximal tendon strain.