Perception and control of upper limb movement: Insights gained by analysis of sensory and motor variability

Clinical research has presented evidence that chronic neck-shoulder pain is associated with impairments of proprioception (perception of limb position and movement without vision and touch) and motor control. Thus, assessment of proprioceptive and motor function of the upper limb may be powerful tools both for research and clinical practice. However, insufficient knowledge of certain features of human sensorimotor control hampers both development and interpretation of results of clinically relevant tests. For example, evidence is lacking which proprioception submodalities (position and movement sense) are reflected in common tests of shoulder proprioception. With respect to testing of upper limb motor function, a better understanding of the control of goal directed arm movements would be needed. The overall purpose of the thesis was to gain further insights into the sensorimotor control of the upper limb in healthy subjects, with implications for clinical testing and ergonomics. The main aims were: (1) to study relationships of outcomes of different psychophysical tests for assessment of proprioceptive acuity in the shoulder joint and (2) to study control strategies in repetitive bimanual pointing tasks by analysis of the structure of joint angle variability. Proprioceptive acuity was assessed in several variants of ipsilateral positionmatching and velocity-discrimination by testing subjects’ ability to repeat a memorized arm location and to discriminate between two different velocities of arm movement, respectively. Sensory discrimination thresholds were represented by Variable Errors (VEs) for position-matching and by Just Noticeable Differences (JNDs) for velocity-discrimination. The pattern of correlations of the VEs and JNDs was analyzed by Principal Component Analysis. The main finding was that two uncorrelated mechanisms based either on perception of position or movement might underlie perception of limb location in ipsilateral position-matching. This depended on the extent of arm movement and on association of the memorized arm location with an active location-focused searching task. The results provided important information for interpretation of common tests of shoulder proprioception with implications for design of novel tests allowing for specific proprioception submodalities to be addressed. Control strategies in bimanual pointing in 2D and 3D space were studied within the Uncontrolled Manifold (UCM) hypothesis. The structure of joint angle variance was computed with respect to the vectorial distance between the endpoints of the arms and with respect to the endpoint coordinates of each arm separately (selected task variables). Joint angle variability was decomposed in variance affecting (VUN) and not affecting (VCOMP) a task variable. The UCM hypothesis predicts that the central nervous system stabilizes a task variable by minimizing VUN, while allowing VCOMP to be high. Thus, the ratio of these variance components quantifies the degree of control of the task variable. The results showed that the variance in joint space was structured according to the predictions of the UCM hypothesis. It was also shown that the arms were united into one synergy to significantly larger degree than joints within each arm were united into single-arm synergies. It was concluded that the UCM method might quantify components of motor variability during repetitive motor tasks, which are not detectable by conventional performance measures.