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Recovery and compensation in human movement

This is an excerpt from Neuromechanics of Human Movement-6th Edition by Roger M. Enoka.

Although motor recovery ­after damage to the CNS depends on changes in synaptic function, it can be accomplished in ­either of two ways: recovery or compensation. In terms of neuroplasticity, recovery refers to the restoration of function in neural tissue that was initially lost due to injury or disease, whereas compensation occurs when residual neural tissue takes over a function that has been lost due to injury or disease.

When recovery occurs, it is usually spontaneous and can be explained by the gradual removal of responses to the injury, such as edema, inflammation, and disturbances in blood flow (Ahuja et al., 2020). The amount of recovery that is pos­si­ble depends on the severity and location of the injury. The severity limits the availability of neuronal components that can be used to repair the damage, whereas the influence of location depends on the role (critical or less so) of the damaged tissue in the specific be­hav­ior (Fouad et al., 2015). For example, a small lesion in the ventral horn of the spinal cord usually has greater impact on locomotion than does a larger lesion in the dorsal horn. Nonetheless, the association between functional recovery and tissue loss is not linear, which means that the CNS can sustain small amounts of damage with a minimal decline in function, but once a threshold is reached the be­hav­ior (e.g., reaching to grasp a reward) ­will be abolished.

When the networks that typically control a specific task have been too damaged, other components in the CNS are able to substitute and generate a somewhat similar be­hav­ior. Compensation is pos­si­ble due to the redundancy that exists among neural pathways and networks, especially in the cortex. For example, sensory and motor maps exist in multiple locations across the primary somatosensory cortex so that when one area is damaged, such as due to a stroke, other areas can assume the lost function. Such redundancy provides a platform for compensation through functional reor­ga­ni­za­tion. However, compensation often results in a decline in ­performance during be­hav­iors in which the commandeered networks ­were involved.

Rehabilitation Strategies

Kleim (2011) suggests that strategies for improving motor function ­after brain damage should focus on restoration, recruitment, and retraining:

• Restoration involves recovery and refers to reactivating brain areas in which function has been compromised. ­Because restoration depends on the resolution of injury-­related responses, it occurs progressively but ­will not begin ­until sometime ­after the injury. If restoration is pos­si­ble, it is necessary for the rehabilitation program to engage the damaged neural tissue and facilitate recovery.
• Recruitment refers to enlisting brain areas that can produce the desired motor function but that ­were not involved in the action before the injury. Recruitment is achieved by compensation but does not involve learning a new function. Recruitment occurs when the damage to the neural tissue cannot be resolved and is likely not due to secondary effects related to the injury.
• Retraining is a form of compensation in which residual neural tissue is required to learn a new function—­for example, the emergence of a motor map for a body part in an area of the motor cortex that was previously related exclusively to another body part. The most successful intervention that can induce neuroplasticity in the damaged CNS is intense training in a meaningful pattern of activity.

The interactions among the three types of adaptations depend on the amount and location of the damage sustained by the CNS. Given its capacity for reor­ga­ni­za­tion, ­there is considerable practical interest in the ability of the damaged CNS to recover motor function and in the interventions that can facilitate this recovery. The capacity of the CNS to reor­ga­nize itself ­after damage is evident in the brains of elite athletes who compete with physical impairments (Nakazawa, 2022). As examples of this plasticity, we discuss current rehabilitation strategies used with individuals who have a spinal cord injury and ­those who have experienced an ischemic lesion in the ­cerebral cortex due to a stroke.

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