An investigation of the contribution of the multi-joint arm stiffness to the motor control defecit [sic] experienced in ataxia

Abstract

Prior research has shown that the control response of the limbs is affected by the mechanical properties of the limb and the feedback properties of the CNS. Cerebellar ataxia describes a situation in which damage to the cerebellum results in compromised motor control. It is characterized by such things as a clumsy or disturbed gait, a lack of balance and coordination, and unsteady speech patterns; for severe cases of ataxia, gross muscle coordination can degenerate to the point where successful, coordinated movements are not possible. In order to better understand the control deficit experienced by ataxic persons, estimates of the feedback properties of the CNS and the limb-muscle mechanical properties and will be necessary. Specifically, this investigation hopes to determine to what extent ataxia is cause by abnormal effective stiffness. Because ataxic patients do not exhibit deficits in strength or postural maintenance, we hypothesize a priori that the measured stiffness of ataxic subjects will be normal. We test this by conducting postural stiffness study on an ataxic subject, and measuring stiffness for two degrees of subject co-activation - minimal subject co-activation and maximal subject co-activation - and for different equilibrium postures.

(cont.) Because the observed kinematic trajectory following neuromuscular activation, as well as the ability of the limb to maintain a given posture in an external force field will be a result of the CNS reflex responses as well as the mechanical properties of the limb-muscle system, we expect all measurements of stiffness to be affected by CNS reflex responses. These reflex responses tend to be noticed between 20 msec (spinal reflexes) and 150 msec (long-loop reflexes) after an environmental disturbance, and because measurements of muscle stiffness require that we wait at least that long after external force application, we expect their contribution to the stiffness measurements to be represented. Our findings show the postural stiffness measured at six static positions in a 0.23 meter by 0.23 meter horizontal workspace and centered 0.45 m in front of the ataxic subject were within (something %) of those measured for a normal subject, and within the range reported by MussaIvaldi. As expected, however, the kinematics of cross-body hand movements were significantly different for the ataxic and normal subject. These results indicate an intact postural regulation for the ataxic subject but a deficit in dynamic control when compared to the normal subject.