Short and long latency reflexes in human muscles following electrical and mechanical stimulation.

Electromyographic (EMG) recordings were made during slight voluntary contraction or during rest in various human muscles following electrical and mechanical stimulation. The electrical stimuli used in most of these experiments were square wave pulses with the duration of 100 microseconds given at a frequency of 3 Hz on cutaneous and mixed nerves. The mechanical stimuli were constant stretches of the muscle induced by taps of the electromechanical reflex hammer. The aims of the series of studies were to investigate the characteristic features of electrically and mechanically elicited muscular responses, the correspondence between electrically and mechanically elicited responses, the short latency response following muscle afferent stimulation and the modifications of the electrically elicited responses. The electrical stimulation during slight voluntary activity caused short and long latency excitatory phases in averaged and rectified EMG with the onset latencies much shorter than the voluntary reaction time. The responses were considered reflexive. The most constant short and long latency reflexes following electrical stimulation were found in the first dorsal interosseus muscle in the hand and in the extensor digitorum brevis in the foot. The mean onset latency for the short latency reflex was 30.9 +/- 1.7 ms (SD) and for the long latency reflex 50.9 +/- 2.8 ms in the hand muscle. In the foot muscle the mean short latency reflex latency was 54.9 +/- 4.1 ms and the long latency reflex latency was 80.5 +/- 5.5 ms. The onset latencies of the short and long latency reflexes during slight voluntary activity following stretching and following cutaneous nerve stimulation corresponded in the first dorsal interosseus muscle. It was concluded that they are generated at least partly via mutual pathways. The stretches given to the triceps brachii muscle during various head positions, different elbow angles and during the Jendrassik maneuver revealed short latency reflex facititations. The lesser degree of the facilitation was caused by the tonic neck reflex and the Jendrassik maneuver caused the greater degree of the facilitation. The short latency reflex following muscle afferent stimulation (the H-reflex) was elicited in the flexor carpi radialis muscle with a mean latency of 15.4 +/- 0.5 ms. The amplitude of the 50% H-reflex was modified by contracting or by stretching of the muscle. Voluntary contraction increased the 50% H-reflex amplitude about 30% and the passive stretching decreased it to about 80% of the control value. The patients with different central nervous system disorders showed abnormal electrically elicited short and long latency reflexes. The s