Paired changes in electromechanical delay and musculo-tendinous stiffness after endurance or plyometric training
Résumé
When measured in vivo electromechanical
delay (EMD) depends mainly on the elastic properties of
the muscle–tendon unit. Recent studies have shown changes in stiffness of the triceps surae (TS) following a period
of training. To confirm the influence of musculo-tendinous
stiffness on EMD, this study investigates paired changes in
these two parameters after a training period. Two types of
training known to induce opposite changes in stiffness were
analysed. EMD and musculo-tendinous stiffness were
measured on adult subjects before and after 10 weeks of
endurance (n = 21) or plyometric (n = 9) trainings. EMD
was defined as the time lag between the TS M-wave
latency and the onset of muscle twitch evoked at rest by
supramaximal electrical stimulations of the posterior tibial
nerve. Quick release tests were used to evaluate the musculo-tendinous stiffness of the ankle plantar flexors. The
stiffness index was defined as the slope of the relationship
between angular stiffness and external torque values.
Endurance training, known to preferentially activate the
slow, stiffer muscle fibers, leads to a decrease in EMD and
to an increase in stiffness index. Following plyometric
training, which specifically recruits fast, more compliant
fibers, EMD and the stiffness index exhibited adaptations
directionally opposite to those seen with endurance training. When pooling the data for the two subject groups, a
correlation was found between changes in EMD and
changes in musculo-tendinous stiffness indexes. Thus,
changes in EMD values are proposed to indirectly link to
changes in musculo-tendinous stiffness for subjects
involved in muscle training.