Stimulating motor wisdom.

COMPARED WITH MOST NEUROPHYSIOLOGICAL phenomena, muscle contraction is a slow process. After an action potential has arrived at the muscle fiber membrane, it takes some 30 ms to fully start up the contractile machinery, and it takes even longer to stop it, i.e., to relax force. The contractile muscle properties differ between fast and slow muscle fibers, and they also change during contraction: the durations of contraction and relaxation increase as the muscle fatigues. Because of the asymmetric character of the mechanical muscle response, slowing of relaxation may functionally be far more relevant than slowing of contraction. To control muscle force, the central nervous system chooses moments of motor unit activation. Obviously, this timing is most relevant in dynamic contractions that require interplay between agonist and antagonist contraction and relaxation. Still, a major part of literature concerning motor control deals with isometric conditions. The reason is that during movement, most experiments become complicated and/or uncomfortable. But, even from isometric contractions, it appears that spinal motorneurons have evolved to match central drive with the contractile properties of their muscle fibers, as shown by Kernell (3). For the special case of a steady contraction with maximal force, the firing rate of a spinal motorneuron should be such that a steady state of contraction and relaxation processes is just reached. Increasing its firing rate beyond that level then yields no extra force, and it wastes energy for neuronal and muscular membrane activity. The observation of decreasing firing rate during fatigue to match slowing of relaxation has been termed " muscle wisdom " (1, 4). Although it may be wise to decrease firing rate, it apparently is not the best way to elicit maximum force from a fatigued muscle (2). Stimulation upstream of the muscle can be used to show this as the deficit of muscle activation, expressed as central fatigue. Such stimulus can be given electrically to the peripheral nerve, but also it can be given with transcranial magnetic stimulation (TMS) of the motor cortex (6, 8). In more general terms: we do know the changing output of the motor system, but it is a challenge to establish if a certain task is limited by mechanical, energetic, or neuronal constraints. To know how far motor control adjusts optimally to properties of the activated muscles, the muscle's properties need to be measured during the task. In this issue of the Journal of Applied Physiology, …