Muscle coordination of movement: a perspective.

Multijoint movement requires the coordination of many muscles. Because multijoint movement is complex, kinesiological data must be analyzed and interpreted in the context of forward dynamical models rich enough to study coordination; otherwise, principles will remain elusive. The complexity arises because a muscle acts to accelerate all joints and segments, even joints it does not span and segments to which it does not attach. A biarticular muscle can even act to accelerate one of the joints it spans opposite to its anatomical classification. For example, gastrocnemius may act to accelerate the knee into extension during upright standing. One powerful forward dynamical modeling method to study muscle coordination is optimal control theory because simulations of movement can be produced. These simulations can either attempt to replicate experimental data, without hypothesizing the purpose of the motor task, or otherwise generate muscle and movement trajectories that best accomplish the hypothesized task. Application of the theory to the study of maximum-height jumping has provided insight into the biomechanical principles of jumping, such as: (i) jump height is more sensitive to muscle strength than to muscle speed, and insensitive to musculotendon compliance; (ii) uniarticular muscles generate the propulsive energy and biarticular muscles fine-tune the coordination; and (iii) countermovement is often desirable, even in squat jumps, because it seems both to prolong the duration of upwards propulsion, and to give muscles time to develop force so the body can move upwards initially with high acceleration. The effort necessary to develop forward dynamical models has been so high, however, that model-generated data of jumping or any other task are meager. An interactive computer workstation environment is proposed whereby users can develop neuromusculoskeletal control models, generate simulations of motor tasks, and display both kinesiological and modeling data more easily (e.g., animations). By studying a variety of motor tasks well, each within a theoretical framework, hopefully muscle coordination principles will soon emerge.