The spinal reflex circuitry of the rectus abdominis (RA)
muscle in man was investigated by the application of a
mechanical tap to the muscle. Electromyographic
recordings were made in ten healthy subjects,
performing a series of manoeuvres, using pairs of
surface electrodes placed bilaterally. The reflex
responses elicited largely depended on the amount of tonic (postural)
activity of the trunk. When standing in the upright position, no reflex
activity was recorded in response to the tap. Reflex activity due to
mechanical tap was readily recorded when the muscle became tonically
active. Moderate, backward trunk extension introduced short-latency
reflexes at 18.8 ± 1.9 ms (mean ± S.D.) ipsilaterally and 20.8 ± 1.8 ms
contralaterally. Excitatory reflex activity of longer latency was also
recorded contralaterally in all subjects (latency 45.1 ± 4.3 ms) and
ipsilaterally in five of the ten subjects (latency 47.2 ± 2.6 ms). Vibration
of the tapped muscle produced a reduction in the amplitude of the early
reflex responses, whilst increasing the amplitude of the late responses.
Moreover, the early reflexes were facilitated by the Jendrassik
manoeuvre. Such observations are consistent with the early responses
being mediated, at least partly, monosynaptically, and the late responses
being of polysynaptic nature. This implies that muscle spindle afferents
from rectus abdominis monosynaptically activate motoneurones
contralaterally.