Influence of isometric training at short and long muscle-tendon unit lengths on the history dependence of force.

Abstract

The history dependence of force is an intrinsic property of muscle whereby a muscle actively shortened or lengthened to an isometric steady-state produces less (residual force depression; rFD) or more force (residual force enhancement; rFE), respectively, than a purely isometric contraction at the same muscle length and level of activation. Previous studies on the modifiability of the history dependence of force have been inconclusive, and none have attempted to modify rFD and rFE through isometric resistance training biased to short vs long muscle-tendon unit (MTU) lengths. We tested maximal voluntary rFD and rFE in seven males and six females before and after 8weeks of maximal isometric dorsiflexion training 3days/wk. Participants trained one leg at 0° of plantar flexion (short-MTU training) and one at 40° of plantar flexion (long-MTU training). Ultrasonography of the tibialis anterior assessed resting muscle architecture. Tibialis anterior fascicle length decreased by ~3% following short-MTU training (P=.03) and increased by ~4% following long-MTU training (P=.01). rFD did not change following training at either MTU length (absolute rFD: P=.53; percent rFD: P=.51), nor did rFE (absolute rFE: P=.78; percent rFE: P=.80), with no relationships between the change in fascicle length and the change in percent rFD (R2 =.01, P=.62) nor rFE (R2 =.001, P=.88). Our data indicate that voluntary rFD and rFE were not modified by isometric training and not related to the fascicle length adaptations we observed.