Transient decrease in number of motor units after immobilisation in man.

Abstract

SUMMARY On the day after the removal of a long leg cast, when the patient could just bend his knee, the electrical activity in the disused quadriceps muscle showed changes that indicated a reduction in the number of motor units. At 10 to 75%/, of maximum force the number of turns and the mean amplitude of the needle-recorded EMG were reduced in the disused muscle. Eight days later, when half the initial loss of force had been regained, the electrical activity was normal. The electrical activity produced during a constant force of 5 kg did not differ in the disused and in the contralateral muscle. From the findings in normal subjects it was deduced that the compensatory increase in turns in the EMG pattern to be expected from the decrease in cross-sectional area of the muscle fibres was within the error of the method. The transient decrease in the number of turns and in mean amplitude of the EMG of the disused muscle are an indication of the plasticity of the motor system.Inactivity of a muscle results in a diminished force usually attributed to atrophy of the muscle fibres (Adams, 1975). Our study was initiated by the observation that the force of the atrophic quadriceps muscle increased from 40% to 80% of the force of the contralateral muscle within one week after immobilisation. This rate of increase did not seem compatible with the time required for recovery of the cross-sectional area of the muscle fibres. To distinguish between a diminished cross-sectional area of muscle fibres and a reduced number of activated motor units, we analysed the pattern of electrical activity in the muscles of eight patients during recovery after immobilisation. The difference in the EMG pattern obtained during a force adjusted relative to the maximum force and during a standard force allows this differentiation to be made (Fuglsang-Frederiksen and MWnsson, 1975;Fuglsang-Frederiksen et al., 1977). Patients and methodEight patients, six men aged 20-32 years and two women aged 20 and 42 years, were investigated. The right or the left leg was immobilised in a long

Quadriceps Atrophy after Knee Traumatisms and Immobilization: Electrophysiological Assessment

“…The main results of our study after immobilization of the knee joint were a transient decrease in the number of turns in T/A analysis, such as it has been reported in other series [18], and a more persistent decrease in MFCV. The number of turns returned to normal values within the first weeks, whereas the recovery of MFCV was progressive throughout several weeks, bound up with the gradual increase in muscle strength.…”

Neuromuscular function in limb girdle dystrophy.

“…4 The normal twitch speeds found in the present study were therefore unexpected, although similar results had been obtained in the adductor pollicis by Desmedt.' It is possible that the dystrophic process affects twitch speed to different extents in various muscles of the body; however, an equally plausible explanation would be that conclu- of whom examined the adductor pollicis.…”

“…4 Since the weakness in this disorder is most evident in larger, more proximal, muscles than in smaller distal ones, there is added interest in studying the former. Accordingly, in the present study measurements of voluntary and evoked contractions have been made in the dorsiflexor and plantarflexor muscles of the ankle, together with the associated muscle impulse activity.…”

Effects of immobilization on contractile properties, recruitment and firing rates of human motor units.

“…5 which also shows that the decrease in A firing rate after immobilization is larger in motor units of low recruitment threshold and large frequency modulation as compared to motor units of higher threshold and lower frequency modulation. Decrease in maximal firing rate could be explained by changes in proprioceptive afferents on the motoneurons (Mayer, Burke, Toop, Hodgson, Kanda & Walmsley, 1981) and/or MOTOR UNIT BEHA VIOUR AFTER IMMOBILIZATION reduced ability to activate motor units (Fuglsang-Frederiksen & Scheel, 1978;Sale et al 1982). This last point was suggested by the finding after immobilization of a smaller reflex potentiation which is closely controlled by the central drive (Upton, McComas & Sica, 1971).…”

“…Possible neural adaptations to disuse are difficult to approach experimentally in animal preparations, but they have been proposed on the basis of experiments performed in humans (Fuglsang-Frederiksen & Scheel, 1978; Sale, McComas, MacDougall & Upton, 1982). In a previous paper (Duchateau & Hainaut, 1987), we compared maximal voluntary contractions MS 7717 (MVCs) and electrically evoked contractions of the human adductor pollicis and concluded that the functional alterations observed during immobilization resulted from changes in the peripheral processes associated with the contraction and also from changes in central and/or peripheral afferents on the motoneurons.…”