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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.

1. Experiments were performed to check the validity of previous claims concerning the ;long-loop' aetiology of ;late' reflex electromyogram (e.m.g.) responses to muscle stretch in man. The primary aim was to investigate whether observations previously presented in favour of the ;long-loop hypothesis' are explicable also in terms of the ;resonance hypothesis', according to which the ;late' reflex components represent spinal, short-latency responses to intramuscular oscillations initiated by the impact.2. The contracting wrist flexors of healthy subjects were exposed to trains of recurrent 25-50 Hz stretch stimuli (wrist torque pulses). Each of the initial two or three pulses in the train was followed by e.m.g. peaks with a latency of 20-25 msec. The e.m.g. peaks driven in this way had the following characteristics in common with the successive two or three e.m.g. peaks which were induced by single ramp stretches or tendon taps. (a) Changes in stimulus parameters which altered the strength of the initial e.m.g. peak often had an opposite effect on the strength of the succeeding peak(s). Muscle vibration which attenuated the initial peak often enchanced the succeeding one(s). (b) The initial e.m.g. peak was less affected than the succeeding peak(s) by the subjects' attempts to respond with rapid ;resist' or ;let go' reactions.3. Intramuscular oscillations (monitored by a needle accelerometer) and e.m.g. responses evoked by single ramp stretches and/or tendon taps were also studied in the long thumb flexor, the calf muscles and the masseter muscle. In the thumb flexor, the initial accelerometer deflexion was only rarely succeeded by a short latency e.m.g. peak, but the succeeding wave in the needle accelerogram was followed by such a peak, appearing about 40 msec after stimulus application. By contrast, the calf muscles and the jaw elevators exhibited a high amplitude, short-latency e.m.g. response to the first but only rarely to the second intramuscular oscillation wave.4. The interval between initial and second e.m.g. peaks following tendon taps was longer for calf muscles than for wrist flexors and longer for wrist flexors than for jaw elevators. Similar differences were observed with respect to the intervals between the damped intramuscular oscillations initiated by the impacts.5. Without denying the existence of ;long-loop reflexes' it is concluded that the characteristics of the ;late' reflex responses to muscle stretch in man are explicable also in terms of the ;resonance hypothesis'.

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. Brisk joint displacements and tendon taps were applied to voluntarily contracting wrist flexor muscles in subjects who did not attempt to react to them. Different types of mechanotransducers, some of them attached to an intramuscular needle, were used to detect mechanical oscillations in the wrist flexors, resulting from the imposed impacts. The transducer responses to the perturbations were compared with simultaneously recorded reflex electromyogram (e.m.g.) responses. Experiments were also carried out on a rubber band model, exposed to similar mechanical stimuli.2. During brisk ramp wrist extensions the transducers signalled damped muscular oscillations at 30-50 Hz. The oscillations grew in amplitude with increasing speed of onset of the stretch movement and at angular accelerations exceeding about 2 x 10(4) deg/sec(2) the e.m.g. response changed from a non-segmented to a progressively more pronounced segmented pattern. Peak intervals in the segmented reflex e.m.g. responses were similar to those of the mechanical oscillations and did not change significantly with small or moderate variations in background contraction force. Latencies from successive deflexions in the accelerometer records to corresponding deflexions in the e.m.g. were 20-25 msec.3. Damped muscular oscillations in the 30-50 Hz range were also initiated by sudden halts of voluntary wrist movements, by electrically induced twitches, and by voluntary brisk contractions. In these instances too, the mechanical oscillations were reflected in the shape of the succeeding e.m.g. response.4. The interval between the two initial peaks in the accelerometer records was always shorter with tendon taps than in ramp stretch experiments. A corresponding difference was noted in the intervals between the following two peaks in the reflex e.m.g. response.5. The initial peak in the accelerometer records could be ascribed to a wave propagated at about 40 m/sec in the wrist flexor muscles. Inconclusive results were obtained in attempts to determine whether the subsequent oscillations represented reflexions of the propagated wave at the ends of the muscle.6. The muscles were also exposed to recurrent stretch stimuli (torque pulses) with a repetition rate varying between 15 and 100 Hz. At 30-50 Hz the intramuscular oscillations reached their maximal amplitude, and such repetition rates were also most efficient in producing synchronized e.m.g. bursts, time-locked to the oscillations. The mechanical responses of the wrist flexors to single or recurrent perturbations were to a large extent mimicked by a rubber band model with a longitudinal resonance frequency of about 40 Hz.7. It is concluded that segmentation of reflex e.m.g. responses to sudden joint displacements and other types of brisk muscle perturbations to a large extent depends on the inherent resonance characteristics of musculo-tendinous structures. Primary spindle endings with their high vibration sensitivity and their segmental projections to alpha-motoneurones are believed to be the receptors primarily responsible for reflex entrainment of the motor impulses.