Residual force enhancement after lengthening is present during submaximal plantar flexion and dorsiflexion actions in humans

Stretch of an activated muscle causes a transient increase in force during the stretch and a sustained, residual force enhancement ( RFE) after the stretch. The purpose of this study was to determine whether RFE is present in human muscles under physiologically relevant conditions ( i. e., when stretches were applied within the working range of large postural leg muscles and under submaximal voluntary activation). Submaximal voluntary plantar flexion ( PFv) and dorsiflexion ( DFv) activation was maintained by providing direct visual feedback of the EMG from soleus or tibialis anterior, respectively. RFE was also examined during electrical stimulation of the plantar flexion muscles ( PFs). Constant- velocity stretches ( 15 / s) were applied through a range of motion of 15 using a custom- built ankle torque motor. The muscles remained active throughout the stretch and for at least 10 s after the stretch. In all three activation conditions, the stable joint torque measured 9 - 10 s after the stretch was greater than the isometric joint torque at the final joint angle. When expressed as a percentage of the isometric torque, RFE values were 7, 13, and 12% for PFv, PFs, DFv, respectively. These findings indicate that RFE is a characteristic of human skeletal muscle and can be observed during submaximal ( 25%) voluntary activation when stretches are applied on the ascending limb of the force- length curve. Although the underlying mechanisms are unclear, it appears that sarcomere popping and passive force enhancement are insufficient to explain the presence of RFE in these experiments.