Spreading of fatigue-related effects from active to inactive parts in the medial gastrocnemius muscle of the cat.

In the medial gastrocnemius muscle of the decerebrate cat, the spatial spread of fatigue between active and inactive muscle parts was studied. Conditioning fatiguing stimulation (CFS) was applied to a part of the muscle to test whether it had an effect on the contraction efficiency in an unstimulated part. To exclude somato-sympathetic reflexes during CFS, a full rhizotomy of the lumbo-sacral spinal cord was performed. The same ipsilateral ventral root, either L7 or S1, was divided into seven filaments, one of which was used for the test stimulation, and four or five for CFS. The CFS consisted of 12 s sessions of distributed stimulation of five (or four) filaments at a rate of 40 s(-1), the sessions were repeated, every 40 s, 15 or more times. The test consisted of 12 s of regular stimulation at a rate of 10 s(-1), preceded and followed by a single stimulus. The tests applied just after CFS showed a strong decline of both tension and electromyogram (EMG), amounting to only [mean (SD)] 0.45 (0.18) and 0.51 (0.19) (n = 15), respectively, of the corresponding values in the tests before CFS. It thus turned out that depressive fatigue-related effects could spread within the muscle. At the same time, control reactions recorded in the lateral gastrocnemius during stimulation of its cut nerve did not change. Subsequent repetitions of the tests usually revealed a tendency towards restoration. The EMG reactions recovered more quickly than tension. The depression of EMG after CFS was accompanied by a slowing of the constituent M-waves; their latencies decreased during restoration. Distinct changes in the systemic blood pressure were observed during CFS. These changes were usually correlated well with muscle tension changes. The factors possibly underlying the observed effects may include diffusion of metabolites from active to inactive muscle fibres, lowering of the efficiency of neuro-muscular transmission due to squeezing of efferent motor terminals and changes in outer metabolite content, as well as local hypoxia due to increases in intramuscular pressure.

Analysis of the electromyographic activity of human elbow joint muscles during slow linear flexion movements in isotorque conditions.

Electromyograms were recorded by surface electrodes from the mm. biceps brachii (caput longum et breve), brachioradialis and triceps brachii (caput longum) in 10 healthy human subjects during slow linear movements in the elbow joint against a weak extending torque. The test movements were carried out under visual control through combining on a monitor screen a signal from a joint angle sensor with an appropriate command generated by a computer. The movements were fulfilled against a weak constant extending torque (0.5-2.5 Nm) and the extensor muscles were inactive. Surface electromyograms were full-wave rectified, filtered and averaged within sets of 10 identical tests. For the test movements in the range from 20 degrees to 100 degrees (0 degrees corresponds to a completely extended joint in these designations) the dynamic components of the efferent commands to actively contracted muscles frequently had a well-expressed monotonous increase. The electromyography intensity during movement quite often increased exponentially in all three investigated flexors. At the same time, the averaged electromyograms in one or two muscles could contain non-monotonous oscillations, thus showing a well-expressed decrease in the intensity of the efferent inflow within a middle range of a movement phase. The non-monotonous oscillations could occur in some subjects under minimal loads (0.5-0.75 Nm); they usually appeared initially only in one muscle, whereas the frequency of their occurrence was not high. The probability of finding the non-monotonous oscillations in the electromyograms from the muscles under study increased with heightening the extending load. Under the loads of 2.0-2.5 Nm this type of reaction could be found in almost half of all records (for the three muscles in each of the 10 subjects under testing). The presence of noticeable non-monotonous components in the electromyograms of the elbow flexors during their contraction is probably connected to their biomechanical arrangement within the joint. It is known that the arms of the forces generated by elbow flexors are maximal in the middle range of joint angles and decrease with a change in angle in both directions. Thus, we can suppose that non-monotonous decrement components in the electromyograms of the elbow flexors are presumably connected with an obvious necessity for a subject to decrease the excitatory efferent inflow to the muscles in the middle range of the joint angles. The pattern of electromyograms in the flexors acting around the joint was also dependent on a redistribution of activity between agonists. In the subjects showing stable non-monotonous components on the averaged electromyogram records in two agonists, a redistribution of the activity between these muscles has been demonstrated when phases of the electromyogram diminishing in one of them coincided with the appropriate increments in the other.