1994 Project Reports | Contents | Previous | Next | Home |
Muscle Coordination of Multijoint Motor Tasks: an interactive computer workstation environment
Felix E. Zajac, PhD; J. Peter Loan, PhD; Michael Gordon, PhD
Objective - Multijoint movement involves a complex coordination of many muscles. 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 in the direction opposite to its anatomical classification. For example, gastrocnemius, which anatomically is classified as a knee flexor, can act to accelerate the knee into extension during upright standing (Figure 1).
Figure
1. The biarticular gastrocnemius muscle generates a flexor torque at the
knee and an extensor torque at the ankle.
The underlying neuromuscular principles of these complex multijoint movements will remain a mystery unless kinesiological data can be analyzed and interpreted in the context of dynamic musculoskeletal models sophisticated enough to study coordination. The effort required to develop models has been so large, however, that few simulations of motor tasks have actually been performed. To facilitate the creation of such models, we are developing an interactive computer workstation environment that will allow researchers to develop neuromusculoskeletal control models, generate simulations of motor tasks, and display both kinesiological and modeling data in an animated format.
Approach - Our interactive workstation environment includes several modules (Figure 2). The SIMM module (Software for Interactive Musculoskeletal Modeling, by Musculographics, Inc.) is used to model the skeletal geometry, the joint kinematics, the muscle attachments and lines of action, and the force-generating parameters of the muscles and tendons. The SDFast module (by Symbolic Dynamics, Inc.) is used to generate dynamic models that simulate the movement of the limb segments in response to particular muscle activation patterns.
Figure 2. Interactive workstation environment for generating simulations of motor tasks.
The heart of the dynamical model are the equations of motion (State Equations) which define how the joint torques produced by the muscles, and by gravity and segmental motion, contribute to the angular acceleration of the joints. Because of the coupling of the segments, soleus, a uniarticular muscle which only produces a torque at the ankle, nevertheless contributes to the angular acceleration of the hip and the knee as well. Moreover, gastrocnemius, a biarticular muscle which produces torques at both the knee and the ankle, can contribute either positive or negative accelerations at the these joints and thus can act either to flex the knee and extend the ankle, (consistent with its anatomical classification), to flex both joints, or to extend both joints.
The dynamical models can be used in conjunction with empirical or theoretical muscle activation patterns to generate simulations of movement, or they can be used with optimization procedures to predict what muscle activation patterns are needed to accomplish certain movements. Both experimental and simulated data can be displayed either as time plots or by means of animated stick figures. Feasible acceleration sets can also be computed and displayed.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact Felix E Zajac.
