All motor activity, even the most elementary reflex type, requires the synchronous activity of many muscles. Analysis of a relatively simple movement, such as clenching the fist, conveys some idea of the complexity of the underlying neuromuscular arrangements. In this act the primary movement is a contraction of the flexor muscles of the fingers, the flexor digitorum sublimis and profundus, the flexor pollicis longus and brevis, and the abductor pollicis brevis. In the terminology of Beevor, these muscles act as agonists, or prime movers. For flexion to be smooth and forceful, the extensor muscles (antagonists) must relax at the same rate as the flexors contract (reciprocal innervation, or Sherrington law). The muscles that flex the fingers also flex the wrist. If it is desired that only the fingers flex, the extensors of the wrist must be brought into play to prevent its flexion; the extensors are synergists. During this action of the hand, appropriate flexor and extensor muscles stabilize the wrist, elbow, and shoulder; muscles that accomplish this serve as fixators. The coordination of agonists, antagonists, synergists, and fixators is effected mainly by segmental spinal reflexes under the guidance of proprioceptive sensory stimuli. In general, the more delicate the movement, the more precise must be the coordination between agonist and antagonist muscles.
Motor activities include not only those that alter the position of a limb or other part of the body (isotonic contraction) but also those that stabilize posture (isometric contraction). Movements that are performed slowly are called ramp movements. Very rapid movements, which are too fast for sensory control, are called ballistic (also termed phasic).
All voluntary ballistic movements toward a target are accomplished by the activation of ensembles of motor neurons. Large motor units participate mainly in triphasic movement, which are characterized by an initial burst of activity in the agonist muscles, then a burst in the antagonists, followed by a third smaller burst in the agonists. The strength of the initial agonist burst determines the speed and distance of the movement, but there is always the same triphasic pattern of agonist, antagonist, and agonist activity (Hallett et al). Smaller motor units are more efficiently activated by sensory afferents from muscle spindles, more tonically active, and more readily recruited in reflex activities, postural maintenance, walking, and running. The basal ganglia and cerebellum set the pattern and timing of the muscle action in any projected motor performance. These points are discussed further in Chaps. 4 and 5.
Unlike the phasic movements just described, certain basic motor activities do not involve reciprocal innervation. In support of the body in an upright posture, when the legs must act as rigid pillars, and in shivering, agonists and antagonists contract simultaneously. Locomotion requires that the extensor pattern of reflex standing be inhibited and that the coordinated pattern of alternating stepping movements be substituted; the latter is accomplished by multisegmental spinal and brainstem reflexes, the so-called locomotor centers. Suprasegmental control of the axial and proximal limb musculature (antigravity postural mechanisms) is mediated primarily by the reticulospinal and vestibulospinal tracts. These aspects of motor function are elaborated further on.