Physiological Basis to Use Neuroplasticity for Rehabilitation after CNS Damage

In patients suffering from a movement disorder after a damage of the central nervous system, improvement in walking or hand function can be achieved by providing functional training. After a stroke or a spinal cord injury (SCI), neuronal centers below the level of lesion exhibit plasticity that can be exploited by functional training paradigms that include assisting stepping or hand/arm movements of the affected side. Training of locomotor function, the focus of this chapter requires body-weight support (BWS), while the subjects stand on a moving treadmill. In these individuals, human spinal locomotor centers become activated if an appropriate afferent input is provided.

Load- and hip-joint-related afferent input seems to be of crucial importance for the generation of a locomotor pattern and, consequently, the effectiveness of the locomotor training. Rehabilitation robots enable longer, more intensive training. In addition, they also offer the ability to standardize training approaches and to obtain objective feedback within training sessions allowing to monitor functional improvements over time.

Pre-requisites for a successful functional training:

In a successful training program for stroke and SCI subjects, first, spastic muscle tone must be present as a partial compensation for paresis, and second, the spinal central pattern generator must be activated by the provision of an appropriate afferent input and proprioceptive feedback to induce plastic neuronal changes.

Role of appropriate afferent input:

Body unloading and reloading are considered crucial to inducing training effects on the spinal locomotor centers because the afferent input from receptors signaling contact forces during the stance phase (corresponding to the initiation of newborn stepping by foot-sole contact, see above) is essential to activate spinal neuronal circuits underlying locomotion. Therefore, a cyclic loading is considered essential for achieving training effects in cats and humans. Overall, observations of healthy subjects, small children, and patients with paraplegia indicate that afferent input from load receptors essentially contribute to the activation pattern of leg muscles during locomotion. This suggests that proprioceptive input from extensor muscles, and probably also from mechanoreceptors, in the foot sole provides load information. In addition, afferent input hip joints, coming from muscles that act around the hip, obviously is important for the leg muscle activation during locomotion. The significance of hip joint afferents was also emphasized for cat locomotion. This afferent activity from load and hip joint receptors is to shape the locomotor pattern, to control phase transitions, and to reinforce ongoing activity (Fig. 1). Short-latency stretch and cutaneous reflexes may be involved in the compensation of small irregularities and in the adaptation to the actual ground conditions.

Fig. 1: Essential afferent input for the generation of a locomotor pattern. To evoke a locomotor pattern in complete SCI subjects, load and hip-joint-related afferent input were shown to be crucial (Color figure available online only).

In severely affected subjects, the muscle force produced by the leg muscle activation (small EMG amplitude) is insufficient to support the body during walking at the initial stage after stroke or SCI. Therefore, partial body-weight unloading is necessary to allow for the performance of stepping movements. During daily locomotor training, the amplitude of leg extensor EMG activity increases during the stance phase, while an inappropriate tibialis anterior activation decreases. This is associated with a greater weight-bearing function of the leg extensors, i.e., body unloading during treadmill locomotion can be reduced. Several studies indicate that following an acute, incomplete SCI in humans, an improvement of locomotor function can be attributed to the locomotor training in addition to the spontaneous recovery of spinal cord function that occurs over several months following an SCI.

Patients with a low motor score undergoing a locomotor training can improve locomotor function without or with little change in motor scores. In these cases, a relatively low voluntary force level in the leg muscles (reflected in the ASIA motor score) is required to achieve the ability to walk. Interestingly, elderly SCI subjects have greater difficulties to translate a gain in motor score into function compared to younger subjects suffering an SCI. This requires specific training programs for elderly subjects focusing on a few rehabilitation goals.

This manuscript is based on a chapter published recently, where appropriate references can be found:

References:

[1] Dietz, V (2012) Clinical aspects for the application of robotics in Neurorehabilitation. In: Neurorehabilitation Technology, edited by V Dietz, T Nef and WZ Rymer. Springer, London, pp.291 – 302.