Habituation and conditioning of the human long latency stretch reflex

Abstract

The effects of stretch repetition rate, prior warning stimuli and self administered stretch were examined on the size of the short and long latency components of the stretch reflex electromyographic EMG response in flexor pollicis longus and the flexor muscles of the wrist and fingers. Stretches of constant velocity and extent were given every 10 s, 5 s, 2 s, or 1 s to either the wrist or thumb during a small background contraction of the flexor muscles. The size of the long latency component of the stretch reflex (measured as the area under the averaged rectified EMG responses) declined dramatically at faster repetition rates, especially in the wrist and finger flexors. The size of the short latency component was relatively unaffected. The size of the electrically elicited H-reflex in forearm muscles also failed to habituate under the same conditions. If each individual trial of a series was examined, the long latency component of the stretch reflex EMG could be seen to decrease in size over the first three to six stretches if stretches were given every 1 s, but not if stretches were given every 10 s. When stretches were given every 5 s to either wrist or thumb, an electrical stimulus applied to the digital nerves of the opposite hand 1 s before stretch reduced the size of the long latency component of the reflex EMG response. The short latency component was unaffected. Self triggering of wrist or thumb stretch by the subject pressing the stimulator button himself with his opposite hand, also decreased the size of the long latency component of the reflex EMG response without affecting the short latency component. It is concluded that factors other than stretch size or velocity can have marked effects on the size of the long latency component of the stretch reflex. These factors must be taken into account when comparing values of reflex size obtained with different stretching techniques and in different disease states in man.