Statistical Considerations When Assessing Short Latency Stretch Reflexes in the Human Soleus Muscle

The aim was to investigate trial-by-trial response characteristics in the short-latency stretch reflex (SSR). Fourteen dorsiflexion stretches were applied to the ankle joint with a pre-contracted soleus muscle on two days. The magnitude and variability of trial-by-trial responses of the SSR were assessed. The SSR was log-normally distributed and variance heterogeneous between subjects. For some subjects, the magnitude and variance differed between days and stretches. As velocity increased, variance heterogeneity tended to decrease and response magnitude increased. The current study demonstrates the need to assess trial-by-trial response characteristics and not averaged curves. Moreover, it provides an analysis of SSR characteristics accounting for log-normally distributed and variance heterogeneous trial-by-trial responses.