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Wed, Nov 2

Speaker: Devjani Saha

Title: The Role of Effort in Motor Control. Location: Ward 5-230, Physiology Conference Room

Abstract: The effort applied by muscles determines how our musculoskeletal system interacts with the environment. Differential activation of antagonistic muscles leads to movement while coactivation of the muscles increases our body’s resistance to perturbation. However, muscular effort is a costly commodity and prolonged muscle activation can cause fatigue, pain, and joint degeneration. The overarching goal of this thesis was to determine how our brain controls for muscular effort during stabilization, force application, and for movement. The first aim of this thesis was to determine whether the central nervous system (CNS) applies a minimum muscle force criterion to plan for forces applied by the hand. If this is the case, we can assume that the brain represents contact forces in terms of muscle forces, instead of on joint torques, or on the position of the hand. We developed a redundant force task in which subjects were asked to generate forces of a given magnitude but the direction of the force remained unconstrained. Our results showed that subjects generated the desired force magnitude with the least amount of muscle force. This suggests that the CNS considers the musculoskeletal geometry of the arm when planning for forces applied by the hand.

The second aim of this project was to investigate the role of muscular effort on the choice of a stabilization strategy. There are two main alternatives for stabilization, one based on position feedback and another based on high stiffness. The former control policy is computationally more challenging but also required less muscular effort than the later strategy. In order to determine whether the CNS has a bias towards a particular control policy, we designed a stabilization task that could cater to both polices. We found that novice subjects are capable of adopting both strategies and their choice is dependent on a trade-off between effort and stability rather than on physical factors such as arm strength and gender.  In the final aim of this thesis we extended the idea of stability and muscular effort to the control of a prosthetic device. Several mechanisms have been proposed for the control of arm movement in able-bodied subjects, including the equilibrium point hypothesis (EPH) and inverse dynamics. With EPH the brain exploits the viscoelastic properties of muscles to generate movement, while with inverse dynamics the brain explicitly controls for forces and torques that lead to movement. In the final aim we compare the effectiveness of these two theories for the development of a myoelectric interface that controls for the movement of an external object. The inverse dynamics mechanism is intrinsically less stable but also requires less muscle activity than the equilibrium point controller. We found the stabilizing mechanism associated with the equilibrium point controller contributes to simultaneous control of multiple degrees of freedom, faster transition times to the target, and more accurate movements in the absence of vision. These three studies demonstrate the importance of muscular effort and stability on the control of human motor behavior. Both factors shape how we plan for forces applied by the hand, our choice of a stabilization mechanism during learning, and our ability to coordinate movements with myoelectric control.