Effect of different joint angle on stretch reflex sensitivity in lower leg muscles

This study examined the effect of different joint angle on the sensitivity of short- and long-latency stretch reflex responses in ankle flexor and ankle extensor muscles. The B-mode echo image of each muscle in individual subject was used to estimate the change of muscle fiber length. The stretch reflex sensitivity of short- and long-latency components was evaluated by the multiple linear regression analysis with two independent variables, muscle fiber stretching velocity (MSV) and background EMG activity (BGA). The reflex sensitivity of short-latency component (M1) in soleus (SOL) muscle is dependent on the ankle joint angle and initial muscle fiber length. However, the reflex sensitivity of short- and long-latency components (M1 and M2) in gastrocnemius (GAS) muscle is not always dependent on initial muscle fiber length. Moreover, the reflex sensitivity of M1 in SOL is dependent on knee joint angle. On the other hand, in all conditions the reflex sensitivity of ankle flexor muscle: tibialis anterior (TA) was significantly lower than that of ankle extensor muscles. These results indicated that the joint angle dependency of the standardized sensitivity of short- and long-latency stretch reflex responses was different in different muscles.