Rightward shift of optimal fascicle length with decreasing voluntary activity level in the soleus and lateral gastrocnemius muscles.

Much of our understanding of in vivo skeletal muscle properties is based on studies performed under maximal activation, which is problematic because muscles are rarely activated maximally during movements such as walking. Currently, force-length properties of the human triceps surae at submaximal voluntary muscle activity levels are not characterized. We therefore evaluated plantar flexor torque- and force-ankle angle, and torque- and force-fascicle length properties of the soleus and lateral gastrocnemius muscles during voluntary contractions at three activity levels: 100, 30 and 22% of maximal voluntary contraction. Soleus activity levels were controlled by participants via real-time electromyography feedback and contractions were performed at ankle angles ranging from 10 deg plantar flexion to 35 deg dorsiflexion. Using dynamometry and ultrasound imaging, torque-fascicle length curves of the soleus and lateral gastrocnemius muscles were constructed. The results indicate that small muscle activity reductions shift the torque- and force-angle, and torque- and force-fascicle length curves of these muscles to more dorsiflexed ankle angles and longer fascicle lengths (from 3 to 20% optimal fascicle length, depending on ankle angle). The shift in the torque- and force-fascicle length curves during submaximal voluntary contraction have potential implications for human locomotion (e.g. walking) as the operating range of fascicles shifts to the ascending limb, where muscle force capacity is reduced by at least 15%. These data demonstrate the need to match activity levels during construction of the torque- and force-fascicle length curves to activity levels achieved during movement to better characterize the lengths that muscles operate at relative to their optimum during a specific task.