Differences in human muscle and tendon adaptation to mechano-metabolic stimuli

Abstract-Text (inkl. Referenzen und Bildunterschriften)

Tendon"s delayed response to rapid improvements in muscle strength during exercise could lead to an imbalanced adaptation within muscle-tendon unit (MTU) and place tendon under a higher mechanical demand (i.e. increased tendon strain).¹ The current study investigated the temporal dynamics of muscle-tendon adaptation and if differences in mechano-metabolic sensitivity can cause non-uniform adaptation within the triceps surae (TS) MTU. Twelve young male adults participated in a 12 week resistance exercise intervention 3x week using 3 sets of cyclic isometric plantarflexion contractions at 80% of MVC until failure failure to induce a high TS activity and hence metabolic stress. Mechanical load was varied by exercising one leg at short (plantarflexed, PF) and the other at long (dorsiflexed, DF) MTU length. Every 2 weeks TS MTU mechanical, material and morphological properties were examined using ultrasonography, dynamometry and MRI. We could confirm our hypothesis that tendon would show higher sensitivity towards the experienced tendon strain than to metabolic stress exercise as only for DF leg an increase (p<0.05) in tendon stiffness, Young"s modulus and tendon size were detected following the 12-week exercise intervention. The PF leg showed a continuous increment in maximal AT strain (i.e. higher mechanical demand) over the exercise period with no changes in its biomechanical properties. Further, we could verify a higher rate of muscle adaptation compared to tendon as well as its independence from the magnitude of mechanical load to hypertrophy or increase its strength, as the rate and magnitude adaptive changes in TS morphological and mechanical properties were not different between DF and PF condition. Hence, these differences in the sensitivity to mechano-metabolic stimuli between muscle and tendon may temporarily cause MTU imbalances that could have implications for the risk of tendinous tissue overuse injury. ¹Mersmann et al. (2017) Front Physiol