Effect of contraction level and magnitude of stretch on tonic stretch reflex transmission characteristics

Electromyogram tonic stretch reflex responses were recorded from biceps brachii muscles in normal and cerebral palsied subjects sustaining either 10% or 20% of maximum voluntary contraction and attempting to keep the elbow stiff in a fixed position. The muscle was stretch by a sinusoidal perturbation applied by the experimenter to the elbow angle. Five different amplitudes of stretch were employed ranging 1.67 to 10.0 degrees peak to peak variation of elbow angle. Spectral analysis of the rectified and filtered electromyogram revealed "noisy" sinusoidal reflex responses with negligible harmonic distortion but the amplitude of the reflex responses did not increase linearly with the amplitude of stretch. An analysis of variance showed that for both groups of subjects the gain of the tonic stretch reflex increased significantly (p less than 0.001) with contraction level and decreased significantly (p less than 0.001) with magnitude of stretch. This finding illustrates that both magnitude of stretch and level of contraction need to be carefully controlled when measures of tonic stretch reflex responses are used to assess changes of muscle tone.