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

¹Muscle Contraction Group, Department of Physiology, School of Medical Sciences, University of Bristol, Bristol, United Kingdom, and School of Biomedical and Chemical Sciences, The University of Western Australia, Crawley, West Australia, Australia; and ²School of Human Movement Studies and Division of Physiotherapy, The University of Queensland, St. Lucia, Queensland, Australia, and Department of Neuroscience, Karolinska Institute, Stockholm, Sweden

Submitted 18 May 2006 ; accepted in final form 14 August 2006

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 (PF_v) and dorsiflexion (DF_v) 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 (PF_s). 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 PF_v, PF_s, DF_v, 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.

eccentric; cross bridge; skeletal muscle; passive force enhancement; triceps surae

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