Muscle Activation in Voluntary Movements

To study in detail the activation of motor units in voluntary contractions, single motor-unit activity has been studied in movements at constant velocity against a constant force acting in flexion or extension direction at the wrist. Because the relative activation of muscle changes as a function of joint angle, the recruitment behavior and firing rate were studied in a small range of positions centered around the same fixed elbow angle. This procedure also eliminates difficulties in interpretation due to the variable mechanical advantage of muscle at various joint angles.

The activation of motor units in voluntary movements is quite different from that in isometric contractions. Already at very slow shortening velocities (as low as 2 deg/sec flexion in the elbow joint), the recruitment threshold of motor units in m. biceps is considerably decreased (about 30%) with respect to the isometric recruitment threshold. This decrease was found at the lowest contraction velocities that could be tested. For larger shortening velocities the recruitment threshold decreases more gradually, in quantitative agreement with the well-known force-velocity relation. The difference in recruitment threshold in isometric and isotonic conditions increases proportionally to the isometric recruitment threshold, and the ratio of recruitment thresholds in isometric and isotonic contractions at 2 deg/sec is approximately constant for all motor units in m. biceps.

In other muscles the recruitment behavior in isometric and isotonic contractions may differ from that observed in m. biceps. For example, in m. brachialis, the recruitment threshold in isometric and isotonic conditions was the same. If the angular velocity in the elbow joint is used to calculate the shortening velocity of muscle fibers in m. biceps and m. brachialis, the shortening velocity of fibers is approximately the same in the two muscles. This fact and the observation of a difference in recruitment behavior in isometric contractions and in movements in biceps and brachialis indicate that the relative activation of human arm muscles is different in isometric contractions and in movements.

At a higher level, the coordination of all muscles involved in a multijoint movement reflects a rather stereotyped behavior. There is good evidence that the organization of limb movements involves a series of sensorimotor transformations between intrinsic and extrinsic coordinates. The central nervous system has at its disposal extra degrees of freedom in the performance of any motor task. Thus a given position of the hand in space, relative to the trunk, can be achieved with an infinite variety of postural orientations of the arm because the arm has seven degrees of freedom (three at the shoulder, one at the elbow, and three at the wrist), whereas only three parameters are required to describe hand location. Nevertheless, in the case of arm movements at the shoulder and elbow, it has been found that any given task is performed in a stereotyped manner with a low variability within and between subjects.

This observation is reflected in invariances in movements that hold true for entire classes of limb movements. For example, in drawing or handwriting, the magnitude of shoulder and elbow angular motions scales roughly with the size of the figure drawn. Furthermore, although shoulder and elbow movements are tightly coupled (constant phase relation), motions at distal joints are loosely coupled to those at the proximal joints (variable phase relation). Motion at distal joints increases the accuracy of the movement as indicated by the smaller variability of finger relative to wrist trajectories.

In general the relative activation of muscles in single- and multijoint movements is very much constant. However, an exception has to be made for very fast movements, when muscles that have predominantly “fast” muscle fibers may be activated preferably at the expense of muscles with predominantly “slow” muscle fibers. An example has been found in rapid movements in the cat ankle, where gastrocnemius seems to be activated almost exclusively, with m. soleus almost silent.