Motoneuronal drive during human walking

Recent technical advances have made it possible to reveal some of the inputs that drive spinal motoneurones during normal human walking. These techniques are based either on a temporary removal of the drive to the motoneurones or on an analysis of the coupling of motor unit activity. During walking a sudden unloading of the plantarflexor muscles leads to a pronounced drop in the soleus EMG activity. This unloading effect is caused by cessation of activity in the sensory afferents, which mediate positive feedback from the active muscles in the stance phase. Somewhat surprisingly the drop in EMG activity following unloading is still observed when Ia afferents are blocked, suggesting that these afferents do not make an important contribution to the motoneuronal drive. It would seem that gr. Ib and/or gr. II afferents are the main contributors to the positive feedback. It has been known for a long time that transcranial magnetic stimulation (TMS) at low intensities may selectively activate local inhibitory circuits in the cortex. At such low intensities TMS applied over the motor cortex may thus inhibit the output from the cortex. The removal of the corticospinal drive in this way may be revealed as a drop in EMG activity from the active muscle. During walking TMS may evoke such a drop in EMG activity from the active muscles, which demonstrates that the corticospinal tract makes a contribution to the muscle activity. Time- and frequency domain analysis of motor unit activity have been shown to be effective tools in the analysis of synaptic drive to spinal motoneurones during tonic voluntary contraction. Applying these techniques to human walking reveals that motor units recorded from the same muscle or from close synergists show short-term synchrony and coherence in the 15-20 Hz frequency band. However, motor units from muscles acting at different joints show no coupling. This suggests that leg muscles are generally activated relatively independently of each other during human walking. These techniques show great promises for revealing changes in the sensory and corticospinal drive to motoneurones in relation to different tasks as well as in patients after injury to the central motor system.

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