Anticipatory changes of long-latency stretch responses during preparation for directional hand movements

We have investigated, in 9 normal subjects, the time course of amplitude changes in the automatic long latency stretch reflex of wrist flexors during the preparatory period as a function of the precued direction of hand movement. Subjects maintained right hand position against a weak force and learned to compensate precisely for brief ramp stretch (250 deg./s, 50 ms) which imposed wrist extension. This probe stretch was applied randomly at different times during the 1 s preparatory period of a reaction time task. The warning signal gave directional advance information (DAI) about the voluntary movement that had to be performed. The EMG activity, recorded between 20 and 80 ms after the stretch started, was measured in terms of 3 successive components, M1, M2 and M3, identified on the basis of their respective latency. There was no significant change in the M1 component. Following a warning signal which precued an extension movement, M2 was depressed prior to the response signal. The time course of M3 was clearly different according to DAI: it increased following a warning signal which precued a flexion movement and decreased in the alternative case. This difference reached statistical significance 400 ms before the response signal. In fast-performing subjects the differential development of M3 was more marked than for slow-performing subjects. This underlines its preparatory significance. These results suggest that the neuronal pathways involved in the M3 response to stretch: i, are partly different from those conveying the earlier components, and ii, include structures which take part in the pre-programming of rapid movements.