Modulation of soleus fascicle stretch responses with changes in reflex-induced muscle contraction state

The aim of this study was to investigate the effects of reflex-induced muscle contraction state on muscle fascicle stretch responses in the human soleus. An ankle ergometer was used to induce double stretches with different inter-stretch intervals, and ultrasonography was used to record fascicle behaviour. When the 2^nd stretch occurred shortly after stretch reflex activation, force enhancement was very clear, and was associated with a stiff fascicle stretch response. Conversely, when the 2^nd stretch occurred much later, i.e. during muscle relaxation, fascicle stretch responses were more compliant, resulting in a decrease in force enhancement. These results suggest that fascicle stretch responses are dependent upon muscle contraction status, and are thus significantly influenced by variations in stretch reflex activation. Furthermore, these findings highlight the importance of the stretch reflex in fascicle stiffness regulation, and may have significant implications regarding the motor control and efficiency of human movement.