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The aim of this study was to evaluate the degree of contribution of supraspinal input to the generation of the compensatory leg muscle activation following stance perturbation. Therefore, evoked motor response (EMR) input-output relations of two different motor tasks were compared at 3 distinct periods: (1) the basic period of muscular activity during standing, i.e. when no additional cortical or spinal activity due to the different tasks is to be expected, (2) the pre-movement period with low background activity, when different spinal and cortical inputs to the motoneuronal pool can be assumed and (3) the period of plateau EMG activity of compensatory and voluntary motor task. Transcranial magnetic stimulation (TMS) just below the motor threshold was applied randomly at 19 different time-intervals before and during the onset of stance perturbation and for comparison during an equivalent voluntary foot-dorsiflexion task. Recordings of electromyographic (EMG) activity from the tibialis anterior (TA) and corresponding ankle-joint movements were made from both legs. Forward-directed displacements were induced by randomly-timed ramp impulses of constant acceleration upon a moveable platform. For comparison, leg muscle EMG was recorded during isometric foot dorsiflexion during stance while leaning back against a support. The stance perturbations were followed by a compensatory response (CR) in the TA with a mean onset time of 81 ms. During the basic period of muscular activity and the period of plateau EMG activity there was no significant difference of the input-output relation between stance perturbation and the voluntary motor task. However, in the voluntary task compared with the CR, there was significantly greater input-output relation (facilitation) of the EMR in the TA following TMS, which may be related to an increased cortical influence. In contrast to this result of the CR following stance perturbation, a facilitation of the EMR was described for hand muscles under corresponding conditions of automatic compensation for muscle stretch, suggesting a transcortical reflex loop. This difference in the results from upper and lower extremity muscles favors the assumption of a predominantly spinal generation of the TA-CR following stance perturbation. Copyright (C) 1998 Elsevier Science Ireland Ltd.

Some normal human subjects show definite synchronization of the motor units in hand muscles (i.e., the impulses from two or more motor units coincide in time more frequently than expected for independent random processes). Subjects who show synchronization tend to use their hands to exert large, brief forces, either in their work (e.g., manual laborers) or recreational activities (e.g., weightlifters). In this study all seven weightlifters examined showed a significant degree of synchronization. Furthermore, after 6 weeks of using the first dorsal interosseus muscle of the hand to exert maximal, voluntary contractions, the level of synchronization increased substantially in four control subjects, and the average level became significantly different from zero. Weightlifters also showed greater late reflex responses than control subjects, but no significant difference in earlier spinal reflexes. Two late reflex waves are described which probably involve fast pathways to and from motor cortes. We suggest that supraspinal connections from motor cortex directly to spinal motoneurons may be enhanced as a result of training to the point where they produce a significant synchronization of motor units during steady, voluntary contractions.

The unloading reflex of abductor hallucis brevis was elicited by removal of the external load during voluntary contraction. The duration of the unloading pulse was varied experimentally from 1 to 32 msec. With all stimulus durations, the response was a measurable decrease in the level of EMG activity. Expressed as a percentage change from baseline, the mean size of the unloading reflex was a logarithmic function of the unloading pulse duration. The results demonstrate that the hand muscles are sensitive to very brief mechanical stimuli. Although the motoneurons of the intrinsic hand muscles receive strong projections from the cerebral cortex, they exhibit reflex behavior similar to that of the large, postural muscles.

Simultaneous recording was performed from pairs of motor units either in a small intrinsic hand muscle (first dorsal interosseus, 6 subjects, 37 records) or in two synergistic calf muscles (gastrocnemius and soleus, 6 subjects, 50 records). The two motor units were recorded by means of two electrodes inserted into the muscle during stationary isometric contractions of different strengths. Cross-correlograms were calculated in two ways: (1) between spike trains of two motor units; (2) between either spike train and the peaks of the tremor record. The cross-correlation between pairs of motor units showed a consistent tendency to synchronization in the hand as well as in the calf muscles. This synchronization was higher than could be expected from the random activity of asynchronously firing motor units. The amount of synchronization was correlated with the amplitude of physiological tremor: the stronger the tremor, the higher the synchronization coefficient. The cross-correlation between a spike train and the peaks of the tremor records showed a higher probability of unit firing 30-60 msec prior to the tremor beats. It is concluded that the tremor force produced by one muscle or a pair of synergistic muscles is the result of the synchronized activity of motor units. This hypothesis is supported by some characteristic changes in the amplitudes and rates of tremor in Parkinson's disease and myopathies. In these diseases the pathological changes in tremor rates simply reflect underlying changes of the motoneuronal firing rates.