A series of electromyographic (EMG) discharges can be recorded following the sudden stretch of a muscle that is either isometrically contracting or in motion. The first discharge (M1) is generally agreed to reflect activity of the monosynaptic stretch reflex. The nature and physiological basis of the later EMG activity is controversial (Chan, 1983; Marsden, Rothwell & Day, 1983), but there appears to be a reflex component (the so-called M2 response) followed by an overlapping component (M3) that is also of reflex origin but is influenced to a greater degree by voluntary mechanisms in some circumstances (Hammond, 1956; Lee & Tatton, 1978; Rothwell, Traub & Marsden, 1980; Jaeger, Gottlieb & Agarwal, 1982). Late EMG activity of similar appearance can be elicited either by interrupting a limb in motion or by perturbing a limb held isometrically against a constant force.

To account for this late EMG activity, some authors have favoured purely segmental mechanisms such as (1) grouped discharges in group la primary afferents (Hagbarth et al., 1980, 1981); (2) separate discharges in fast- and slow-conducting afferent fibres following the stretch stimulus (Matthews, 1984); or (3) slowly conducting polysynaptic pathways within the cord (Hultborn & Wigstrom, 1980). Others, by contrast, have postulated that a long-loop, possibly transcortical, reflex pathway is involved (Lee & Tatton, 1978; Rothwell et al., 1980; Marsden et al., 1983; Abbruzzese et al., 1985; Matthews, Farmer & Ingram, 1990).