This is an excerpt from Neuromechanics of Human Movement-5th Edition by Roger Enoka.
As suggested by the reduction in motor neuron number (figure 9.35a), the number of neurons decreases and the functional capabilities of the CNS decline as we age. The adaptations that influence movement most directly extend from spinal reflexes to the coordination of multiple muscles in the performance of voluntary actions. Several studies have demonstrated that modulation of reflex pathways can differ between young and old adults during some tasks. For example, the amplitude of the soleus H reflex (pathway shown in figure 7.10) is increased by a Jendrassik maneuver in all standing positions tested for young adults but in only some standing positions for old adults. Similarly, old adults modulate presynaptic inhibition of Ia afferent feedback (pathway shown in figure 7.17), the short-latency component of the stretch reflex (pathway shown in figure 7.12), and reciprocal Ia inhibition (pathway shown in figure 7.13) less than young adults do during low-force contractions compared with resting levels. However, there is no age effect in the modulation of the H reflex during walking, in the ability to reduce the amplitude of the H reflex with training, or in the capacity to depress the H reflex during a challenging standing posture.
In contrast to the lesser modulation of some spinal reflexes during voluntary contractions, old adults tend to exhibit greater levels of activity in the cerebral cortex during simple motor tasks compared with young adults. With the use of functional MRI, research has demonstrated that old adults have slower reaction times and recruit additional cortical and subcortical areas, especially in the ipsilateral sensorimotor and premotor cortex, when performing finger and hand movements. The greater ipsilateral distribution of activation in old adults is associated with less functional lateralization during the control of simple motor tasks, which suggests that the control of these actions is less automatic. Adaptations in the motor cortex have been probed with TMS; old adults have displayed smaller motor-evoked potentials in limb muscles, lesser intracortical inhibition, higher stimulus intensities for achieving maximal motor output, and greater differences between hands.
Due to the differential adaptations in spinal reflex pathways and cortical activity exhibited by old adults, they often accommodate changes in movement conditions differently than young adults. For example, Stéphane Baudry and colleagues (2010) compared the strategy used by young and old adults when they performed steady contractions while supporting two different types of loads with arm muscles. One task involved pushing against a rigid object with the back of the hand (wrist extensor muscles), whereas the other task required the person to maintain the position of the hand while supporting a mass with a comparable net muscle torque. We previously distinguished these two tasks as requiring force and position control, respectively (see figure 8.30). Although each person produced the same submaximal muscle torque during the two tasks, position control (supporting the mass) is more challenging than force control (table 8.3). Consequently, individuals use different activation strategies to maintain a steady contraction with each of the two loads. Force steadiness, as indicated by the force fluctuations, was similar for both young and old adults during the two loading conditions. However, each age group accommodated the change from force control to position control with a different activation strategy. To maintain force steadiness during the more difficult task (position control), young adults reduced the level of Ia presynaptic inhibition (figure 9.41a) and old adults increased the amount of antagonist coactivation (figure 9.41b). The preferred strategy for the young adults, therefore, was to increase the feedback provided by a spinal reflex pathway (Ia afferent feedback), whereas the old adults used a feedforward strategy to stiffen the wrist joint by increasing the activation of agonist and antagonist muscles. Old adults often use greater amounts of coactivation than do young adults during submaximal contractions (e.g., when performing steady contractions, tracking an object with limb muscles, and stepping down from a platform), which increases the energetic cost of performing these activities. Moreover, changes in the conditions in which a movement is performed, such as exposure to a stressor, have a greater effect on the quality of the performance in old adults than in young adults. (Figure 8.39 provides an example of the age-associated influence of a stressor on force steadiness.)
Activation strategies used by young and old adults when preforming steady submaximal contractions with the wrist extensor muscles. (a) When performing position control (dark gray), young adults reduced the level of Ia presynaptic inhibition (increase in the amplitude of the conditioned H reflex; pathway shown in figure 7.17) relative to the amount they used during force control (light gray). Old adults did not change the size of the conditioned H reflex (Ia presynaptic inhibition) across the two tasks. (b) Old adults used more coactivation (EMG amplitude for an antagonist muscle relative to an agonist muscle) than did young adults during the force task and increased it even more when changing from force to position control.
Adapted, by permission, from S. Baudry, A.H. Maerz, and R.M. Enoka, 2010, "Presynaptic modulation of Ia afferents in young and old adults when performing force and position control," Journal of Neurophysiology 103: 623-631.