Control and Co-ordination in Multi-Joint Movements

Abstract

Our interactions with the world often entail a wide range of multi-jointed movements. We are bestowed with an abundant degree of freedom, that a movement can be achieved by recruiting multiple joints, muscles, and neural pathways, while it is unclear as to how our central nervous system resolves this redundancy to arrive at a single solution, is formerly known as the problem of motor redundancy. In this thesis I investigated redundancy in the skeletal system, where a simple reaching task could be performed by traversing along multiple trajectories between the start and the end locations and have various joint configurations to achieve the same endpoint (location of the hand). At first, I looked at the trajectories of the endpoint, where, despite the evidence for invariant kinematic signatures such as approximate straight-line trajectories with bell-shaped velocity profiles, a fundamental unresolved question is whether such trajectories are planned or derived from a trajectory-free online control. I developed statistical measures using Spearman’s rank correlation, zero-crossing rate (ZCR), and z-scores to capture the extent of correction during the movement. I found that the endpoint trajectories during whole arm reaching movements had similar signatures of rapid control to a task that necessitated trajectory control in the context of finger movements. The control measures were sensitive to the presence or absence of a goal and pathological conditions such as Cerebellar Ataxia and Parkinson’s Disease. Still, they were similar across varied skills and speeds of movement. Such control signatures, accompanied by the evidence of rapid corrections during saccadic eye movements, suggest that the central nervous system (CNS) potentially utilizes fast feedback and internal feedback processes to implement corrections towards a planned trajectory. Further, I extended the above analysis onto the trajectories of serially linked joints of the upper limb. While some theories, such as the uncontrolled manifold hypothesis (UCM), suggest that the control is enforced through joint co-ordination in the service of the endpoint, recent theories, such as the leading joint hypothesis (LJH), advocate for control over fewer leading joints in generating the movement. I found distinct signatures of kinematic trajectory control among the shoulder and elbow joints versus the control of the wrist joint. Nevertheless, the extent of joint control was significantly less and was delayed compared to the endpoint. In the context of the UCM hypothesis, although I found robust signatures of joint synergy/co-ordination, comparing the exploration of joint subspaces with my measures of kinematic control, I showed that, while the shoulder and elbow joints regulated the task space, the distal wrist joint, surprisingly, regulated the exploitation of redundancy. Further, in the context of LJH, I found that, depending on the direction, either the shoulder or the elbow contributed to generating most of the movement, and that my measures of kinematic control on joints interestingly corroborated with it. In conclusion, I found that different joints took over different objectives during the movement; while proximal joints, such as the shoulder or the elbow, took over the role of generating whole arm movement, the distal joints, such as the wrist, were responsible for exploring redundancy.