Evidence of different sensitivity of muscle and tendon to mechano-metabolic stimuli

Lambrianides Y, Epro G, Arampatzis A, Karamanidis K.

Scand J Med Sci Sports. 2024;34:e14638

This study aimed to examine the temporal dynamics of muscle–tendon adaptation  and whether  differences  between their  sensitivity  to  mechano-  metabolic  stimuli would lead to non- uniform changes within the triceps surae (TS) muscle–tendon unit (MTU).

Twelve young adults completed a 12-week training intervention of unilateral isometric cyclic plantarflexion contractions at 80% of maximal voluntary contraction until failure to induce a high TS activity and hence metabolic stress.

Each participant trained one limb at a short (plantarflexed position, 115°: PF) and the other at a long (dorsiflexed position, 85°: DF) MTU length to vary the mechanical load. MTU mechanical, morphological, and material properties  were  assessed  biweekly  via  simultaneous  ultrasonography–dynamometry  and  magnetic  resonance  imaging. 

Our  hypothesis  that  tendon  would  be  more  sensitive to the operating magnitude of tendon strain but less to metabolic stress exercise  was  confirmed  as  tendon  stiffness,  Young’s  modulus,  and  tendon  size  were only increased in the DF condition following the intervention.

The PF leg demonstrated a  continuous  increment  in  maximal  AT  strain  (i.e.,  higher  mechanical  demand)  over  time  along  with  lack  of  adaptation  in  its  biomechanical  properties.

The premise that skeletal muscle adapts at a higher rate than tendon and  does  not  require  high  mechanical  load  to  hypertrophy  or  increase  its  force  potential during exercise was verified as the adaptive changes in morphological and mechanical properties of the muscle did not differ between DF and PF.

Such differences in muscle–tendon sensitivity to mechano- metabolic stimuli may temporarily  increase  MTU  imbalances  that  could have  implications  for the  risk  of tendon overuse injury.