Felix E. Zajac, III - US grants

The overall aim of this project is to develop a technique for specifying the motor coordination strategies that the central nervous system (CNS) ought to employ in order to execute a specific, but complex motor task. The technique under development is based on optimal control theory which is a highly developed framework for analysis of complex dynamical systems. Once developed, the technique may well lead to improved methods for rehabilitative training and physical therapy, for reconstructive orthopaedic surgery, and for the training of athletes. Optimal control theory requires a mathematical formulation of the task to be optimized. The major problem associated with the few previous attempts employing such theory is that the performance criterion cannot be specified with certainty. This project circumvents this problem by studying maximum jumps and pedaling at high effort. With this problem resolved, this project will focus on the development of a mathematical, computer-implemented representation of the musculoskeletal system of the human leg. The emphasis will be on the development of computer descriptions of muscle and tendon based on their architecture and the properties of sarcomeres. This development depends on the comparison, to be made in this project, of body trajectory, ground reaction forces and joint torques that humans ought to produce with the trajectory, forces and torques actually generated by human subjects who pedal or jump. The optimal control model will then be used to determine, among other things, how physique, coordination and elastic storage of energy in muscle and tendon affect overall performance and the details of body movement. The coordination strategy to be found using the above technique will be the one that is optimal, given that there is no restriction on the motor output pattern that the CNS can generate. This assumption may be invalid for unfamiliar tasks. Comparison of the coordination used by subjects who perform novel pedaling tasks with the coordination that ought to be used in the absence of CNS constraints will be analyzed to find the operational characteristics of the constraints.

The ongoing aim of this project is to develop a novel, computer- based technique that can be used to delineate, in the human, the neural control and biomechanical mechanisms associated with complex lower extremity motor tasks. Eventually, it is our goal to be able to use this technique to specify the motor coordination strategies that the human central nervous system ought to employ to stand and walk, first in able-bodied persons and later in neurologically- or musculoskeletally-disabled persons. Another goal is to use this technique to assist in the design of patient- specific rehabilitation strategies. Once developed, therefore, this technique may well lead to improved methods of neurological and physical rehabilitation. The computer technique under development is based on optimal control theory, which is highly-developed framework for analysis of complex dynamical systems. However, this theory requires a mathematical formulation of the task to be optimized. One major problem associated with the few previous attempts employing this theory is that the performance criterion cannot be specified with certainty. Our study of maximum jumps and pedaling at high effort circumvents this problem and has let us focus on the development of a mathematical, computer-implemented representation of musculotendon dynamics and musculoskeletal geometry. This representation is needed for computer studies of jumping and pedaling, as well as any motor task involving the lower extremities. The goals are, by using jumping and pedaling as examples of complex motor tasks, to conduct computer studies and experiments: 1. to comprehend how intermuscular coordination, inertial coupling among body segments, musculotendinoskeletal dynamics, and energetics interact to produce synergistic movement; and 2. to understand how muscle strength and speed, elasticity in tendon and muscle, and kinematic constraints affect human coordination and energetics. In contrast to experimental data alone, it is hypothesized that our computer studies will augment one's ability to understand complex movement, such as how and to what degree muscles need to be coordinated, and how limb structure and biomechanical constraints affect coordination.

Our overall goal is to understand muscle coordination of the human lower limbs. Muscle coordination is constrained by both the mechanics of the motor task and the ability of the nervous system to generate muscle activity patterns. Our past work has focused on how the biomechanical constraints of the musculoskeletal system and its interaction with the environment impact muscle coordination. The role of computer models has been important to our studies on multijoint lower limb motor tasks (jumping, posture, walking, pedaling). These computer models, based on mechanics and physiological concepts, have been pivotal to the formalization of biomechanical principles, which must be accounted for in the neural strategy controlling the motor task. The focus in this next project period is on how the nervous system generates muscle activity patterns to control interleg coordination. Emphasis is placed on the identification of the nervous system constraints and how sensory information from the ipsilateral and contralateral legs is used. The constraints to be considered are muscle synergies (i.e., similar excitation of different muscles in the same limb) and interlimb synergy coupling (i.e., exultation of synergies in the two legs to be alternating or in-phase). Pedaling a stationary ergometer is a motor task well suited to the study of interleg multijoint muscle coordination. Our past work on pedaling biomechanics and muscle coordination suggests that the ability to pedal can be realized by grouping all the muscles in each leg into three agonist-antagonist synergy pairs, where the two synergies of each pair alternate with each other in the crank cycle and with their counterparts m the other leg. A simple conceptual model of how sensory information of position and motion is used to phase the synergies is being proposed. Backward and forward pedaling, initiation of pedaling, and servomotor- assisted one-legged pedaling (where the other, mechanically-decoupled leg is either stationary or moving, and either passive or active) will be studied to develop these concepts of muscle synergies, interlimb synergy coupling, and afferent control. The computer sensorimotor control model to be developed will provide a framework for the diagnosis and rehabilitation of persons with neurological impairments of the lower limbs.