Interaction between short- and long-latency components of the human stretch reflex during sinusoidal stretching

1. Using wrist muscles, the subdivision of the human stretch reflex into separate components was examined with small amplitude sinusoidal stretching of relatively high frequency (10-40 Hz). The reflex was evoked by angular rotation of the wrist (below 1 deg amplitude), applied via the hand during maintained voluntary contraction of the muscle studied; both flexors and extensors were tested. 2. The reflex response was recorded electromyographically rather than mechanically. For each condition, the surface EMG was rectified and averaged to give a cycle average showing the mean response evoked by a cycle of stretching. The cycle average was fitted with a sinusoid, the amplitude and phase of which were used to assess the reflex; their value reflects the combined action of its various sub-components. Fourier analysis gave similar results (the EMG was then rectified but not averaged). 3. The amplitude of the reflex response typically fell to a minimum in the region of 20-25 Hz. The phase lag of the response in relation to the stimulus increased approximately linearly with frequency, except in the region of the amplitude minimum. Here the lag tended to remain constant, or to decrease slightly; this created a discontinuity between the upper and lower limbs of the phase plot. 4. Such effects are attributed to an interaction between two components of the reflex that differ in latency by 20-25 ms. These would progressively change their relative phase as the frequency increased. At first they would come to interfere with each other, but then the more delayed reflex produced by a given cycle of stretching would begin to sum with the shorter latency reflex evoked by the next stretch. 5. At high and low frequencies the cycle averages were normally well fitted by a single sinusoid. Around 20-25 Hz, however, they typically showed appreciable harmonic distortion, with the second harmonic larger than the fundamental. The cycle average then showed two separate responses per cycle of stretching. These were considered to represent the uncancelled non-linear residua of separate components of the reflex response. Their relative timing shifted appropriately with change of frequency. 6. These 'double responses' are unlikely to be due to mechanical resonance. First, the relative sizes of the two components could be altered by the reflex action of cutaneous afferents. Second, the same pattern of behaviour was found when the mechanical stimuli were applied directly to the tendon of flexor carpi radialis while the hand remained fixed.(ABSTRACT TRUNCATED AT 400 WORDS)