Muscle Activation in Reflex-Induced Contractions

In general, it can be stated that the same recruitment order is found in isometric contractions and in contractions that result from the reflex-induced activation of muscle. However, for a proper description of the reflex-induced activation of motor units, a distinction has to be made for the different reflex components.

It is well known that reflex activity is segmented into several reflex components, each with a different functional role and presumably mediated by different neural pathways. The precise pathways involved in all reflex components are not yet known. For the first reflex component (also called tendon reflex, M-l reflex, or short-latency reflex), it is generally accepted that it is predominantly caused by a short route involving a single neuron directly activated by stretch- sensitive muscle spindles (presumably I_a afferents); however, other pathways may also contribute to it.

The main function of the shortlatency reflex component is thought to be to compensate for the sudden decrease in muscle force caused by the break of actinmyosin bonds, which occurs when muscle fibers are stretched by more than 0.2% of their rest length. The gain of this reflex component is not constant, but is modulated in various phases of movements and for different motor tasks by presynaptic mechanisms.

The second reflex component is frequently called long-latency reflex component or M-2 reflex. Presumably more than a single type of sensor is involved. The experimental results so far have demonstrated a role for muscle spindle afferents (both /a and type II endings) and cutaneous afferents. The long-latency reflex component is thought to be mediated by pathways, involving many neurons in the spinal cord and presumably also a long-loop pathway involving the motor cortex.

The long-latency reflex component serves a role in the coordination of an adequate response. As explained before with regard to the activation of motor units in m. triceps, for torques in supination or pronation direction some muscles may be activated in isometric contractions even if the isometric torque is orthogonal to the torque that results from activation of that muscle. Similarly, m. triceps is activated in a long-latency reflex by torque perturbations in pronation direction, which do not stretch or shorten the length of m. triceps.

The time interval of activation of motor units in m. triceps after a torque perturbation in pronation direction corresponds to that of the longlatency reflex. This shows that muscles that are not stretched may reveal long-latency reflex activity if the adequate coordination of movements requires so. This reflex activity may be excitatory, as in the case of m. triceps in response to perturbations in pronation direction, or inhibitory, such as observed for motor units in m. brachialis in response to perturbations in pronation direction (Fig. 5).

FIGURE 5. Motor-unit activity in m. triceps (A, B) and brachialis (C) elicted by torque perturbations in flexion direction (A) and pronation direction (B, C). Motor-unit responses, plotted as the number of action potentials per unit of times in m. triceps, were tested for a preload in flexion direction. Motor units in brachialis were tested by torque perturbation superimposed on an extension preload. Note the excitation in m. triceps in the time interval between 50 and 100 msec and the decreased activity for the motor unit in m. brachialis in the same period

The amplitude of short- and long-latency reflex components may be different in different muscles; however, within each reflex component, the recruitment order appeared to be the same, equal to that in isometric contractions.