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Grouping of the EMG response produced by quick stretches of contracting muscles has been thought to reflect 'long loop reflexes' through cerebral cortex adding to the segmental stretch reflex. Our recordings from human muscle spindle afferents responding to such stretches show that these discharges also tend to be grouped. EMG grouping may therefore be a consequence of successive segmental reflexes rather than of additional delays in long loop reflex arcs.

Prompted by previous reports on muscle thixotropy, we have investigated changes in inherent and reflex stiffness of the finger flexor muscles of human subjects at rest, following transient conditioning manoeuvres involving contractions and/or length changes of the finger flexors. The stiffness measurements were combined with electromyographic recordings from forearm and hand muscles and with microneurographic recordings of afferent stretch responses in finger flexor nerve fascicles. Finger flexor stiffness was evaluated by measuring (a) the flexion angle of the metacarpo-phalangeal joints at which the system during rest balanced the force of gravity and (b) the speed and amplitude of angular finger extensions induced by recurrent extension torque pulses of constant strength delivered by a torque motor. In the latter case, extension drifts in the resting position of the fingers were prevented by a weak flexion bias torque holding the fingers in a pre-determined, semiflexed position against a stop-bar. Stiffness changes following passive large amplitude finger flexions and extensions were studied in subjects with nerve blocks or nerve lesions preventing neurally mediated contractions in the forearm and hand muscles. Inherent stiffness was enhanced following transient finger flexions and reduced following transient finger extensions. The after-effects gradually declined during observation periods of several minutes. Similar results were obtained in subjects with intact innervation who succeeded during the pre- and post-conditioning periods in keeping the arm and hand muscles relaxed (i.e. showed no electromyographic activity). In these subjects it was also found that the after-effects were similar for active and passive finger movements and that isometric voluntary finger flexor contractions loosened the system in a way similar to finger extensions. In some subjects electromyographic reflex discharges appeared in the finger flexors in response to the extension test pulses. When elicited by small ramp stretch stimuli of constant amplitude, the stretch reflex responses were found to vary in strength in parallel with the changes in inherent stiffness following the various conditioning manoeuvres. The strength of the multi-unit afferent stretch discharges in the muscle nerve, used as index of muscle spindle stretch sensitivity, varied in parallel with the changes in inherent stiffness. Post-manoeuvre changes in muscle spindle stretch sensitivity were seen also when the spindles were de-efferented by a nerve block proximal to the recording site. The results can be explained in terms of thixotropic behaviour of extra- and intrafusal muscle fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

1. Different techniques were used to generate sudden ramp extension movements of the wrist while the subjects were either relaxed or maintaining a weak voluntary contraction in the wrist flexors. Afferent responses to the displacements were recorded with a tungsten micro-electrode inserted into a fascicle of the median nerve supplying one of the wrist flexor muscles, and e.m.g. responses were recorded with needle electrodes inserted into the same muscle.2. With the wrist flexors either relaxed or contracting, extensions at 100-200 degrees /sec for 60-70 msec (generated by either an hydraulic motor or a torque motor) produced segmented afferent responses with two to four afferent bursts, separated by intervals of 20-30 msec. The successive neural peaks, occuring during the stretch phase, were correlated to mechanical vibrations sensed by a strain gauge and sometimes also by a wrist goniometer. With the flexor muscles contracting, the successive peaks in the neurogram were followed by similar peaks in the e.m.g, the delay between neural and e.m.g. peaks being 20-25 msec.3. Small abrupt extension movements of 1-2 degrees lasting only 10-15 msec often produced segmented afferent responses with one neural burst occuring during the stretch phase and another 15-20 msec later, corresponding to a mechanical oscillatory event succeeding the stretch. The oscillation and the second neural burst were not present with small extension movements of smooth onset and halt. With the flexor muscles contracting, stimuli producing one afferent burst produced only one e.m.g. peak, whereas double-peaked afferent discharges produced double-peaked e.m.g. responses, the delay between individual neural e.m.g. peaks being 20-25 msec.4. Similar segmentation of the neural stretch responses was seen when abrupt displacements were produced by electrically induced muscle twitches, by manual pulls on a spring attached to the hand or by the subject making fast voluntary wrist extensions. This grouping of afferent discharges was seen in both multi-unit and in single-unit recordings from fibres identified as group Ia afferents.5. It is concluded that mechanical vibrations in the moving parts are initiated by abrupt joint movements and that these vibrations are sensed by the primary endings. With initial background contraction in the stretched muscles, synchronous volleys of spindle discharges produce, via segmental reflex arcs, modulation of the e.m.g. with the appearance of two or three e.m.g. peaks separated by intervals of 20-30 msec. Possible causes for the mechanical oscillations are discussed.6. For imposed movements with a duration of 60-70 msec the successive e.m.g. peaks caused a fused reflex contraction, appearing as a torque trace deflexion, starting at about the time when the movement ended and reaching its peak within about 40 msec. With longer-lasting movements the mechanical reflex response accompanying the successive e.m.g. bursts, appeared as a decelerative force, starting to oppose the ongoing movement about 60 msec after its start. Mechanical consequences of stretch reflex contractions starting after, rather than during, the stretch movement are discussed.