Asbjørn Klomp, Erwin de Vlugt, Carel G.M. Meskers, Jurriaan H. de Groot, J. Hans Arendzen and Frans C.T. van der Helm
Volume: 60, Issue: 6, Page:1721-1727
In addition to voluntary control from the brain, the central nervous system (CNS) uses proprioceptive information about joint displacement for the correct actuation of muscles. This study focussed on the sensitivity, or gain, of the proprioceptive reflex system responsible for muscle stretch-rate.
A decrease in reflex gain with joint displacement amplitude was observed in studies using linear models relating EMG to perturbation velocity. The origin of the reflex gain reduction can however be explained by two different causes. First, the reduction of reflex sensitivity via neural control (gamma drive), and second a methodological error, being the mismatch between a linear model and the nonlinear reflex system.
Here we show that the inability of a simple and commonly used linear model to describe the nonlinear reflex response results in a reduction of the reflex gain. The mismatch increases with amplitude of wrist rotation resulting in a further decrease of the linear reflex gain. The mismatch was explained from discontinuity in muscle activation, as typically seen in response to joint rotations and likely originates from refractoriness of the motoneurons.
Displacements that last longer than the M1 response (approximately 22ms in this study) will result in a model output that covers part of the refractory period and explains a lower reflex gain with rotation amplitude, or strictly speaking, with rotation duration. The design of measurement protocols that assess reflexive activity using EMG therefore requires the knowledge of which part of the EMG one is interested in, and will require alignment of the displacement duration to the start of the refractory period of the reflex system.