Sensorimotor deficits in patients with central nervous system lesions: Explanations based on the λ model of motor control

Abstract

Reflex and motor deficits such as spasticity, limitations in force regulation, inappropriate agonist/antagonist coactivation and movement segmentation are common sequelae of central nervous system (CNS) lesions. However, the disturbed control mechanisms underlying such deficits are still unclear. Data are presented according to which sensorimotor deficits in the elbow of adult stroke patients and children with cerebral palsy (CP) may be related to limitations in the regulation of stretch reflex (SR) thresholds ( λs) for elbow flexors and extensors. The presence of sensorimotor deficits is discussed within the framework of the λ model of motor control by the analysis of: (1) velocity-dependent stretch reflexes in elbow flexors and extensors in stroke and CP patients and (2) angular ranges in which reciprocal or coactivation patterns of elbow flexor and extensor activity occur during reflex and voluntary activation in stroke patients. At least one boundary of the SR threshold range for both flexors and extensors was located within the physiological range of the elbow joint in all children with CP. In hemiparetic adults, SR thresholds were found within the physiological range for flexors in all but one subject and for extensors in 1/3 of the subjects. In addition, in hemiparetic adults, the angular ranges in which either agonist or antagonist muscle activity could be generated were decreased in all but one subject suggesting a limitation in the range of regulation of central commands responsible for reciprocal coordination between muscles. Hemiparetic adults with the most severe motor deficits were unable to specify anticipatory coactivation in the arm muscles to stabilize final arm position after sudden unloading. Limitations in SR threshold regulation may be a basic mechanism underlying sensorimotor deficits in patients with spastic movement disorders such as stroke and CP. This may account for abnormal movement patterns and decreases in voluntary ranges of motion, maximal torques and speeds of movement seen in these patients. It also may account for muscle weakness and spasticity in parts of the physiological range.