Identification of time-varying dynamics of reflex EMG in the ankle plantarflexors during time-varying, isometric contractions

The dynamic relationship between the joint position and reflex EMG in ankle muscles of healthy human subjects was studied for time-varying (TV) contractions. A linear parameter varying (LPV) identification algorithm was used to estimate the Hammerstein system relating ankle position to the reflex EMG response. The estimated Hammerstein system comprised a time-invariant (TI) linear element and a TV static nonlinearity that resembled a half-wave rectifier with a threshold and linear gain. The results demonstrated a systematic change in the reflex nonlinearity with the activation level. The gain of TV nonlinearity increased with activation level reaching its peak at 20-30% maximum voluntary contraction and then decreased. The threshold of the nonlinearity decreased with increasing activation level reaching it minimum at the same point where the gain was maximal. Using the LPV-Hammerstein method in this work, the underlying TV dynamics were extracted from small number of trials. Thus, this method can be used to study stretch reflexes in subjects with neuromuscular disorders.