Goal-directed arm movements. II: A kinematic model and its relation to EMG records.

In a previous study the EMG records of shoulder and elbow muscles during goal-directed arm movements were discussed. Here an analysis of the kinematic signals is presented and the relation between EMG and kinematic signals is assessed in a correlation analysis. The displacement as a function of time (movement trace) is analyzed. The movement trace is described by a model with five parameters that are estimated using a least squares criterion. Four parameters describe the timing of a triphasic muscular input and a fifth parameter describes neuromuscular dynamics. The parameters provide a means to compare the shape of movement traces recorded under different experimental conditions. After scaling, the shapes of movement traces with different step sizes and velocities/durations can be compared. The scaled parameters reveal a significant dependence on the movement direction of maximally fast movements. Furthermore, the scaled parameters depend significantly on the maximum velocity obtained by the subjects. This dependence can be interpreted in terms of neural inputs and muscular dynamics, as the parameters have been defined in these terms. The parameters estimated from the kinematics have a high correlation with EMG timing. A further analysis of these correlations indicates that the movement obtained is closely coupled to activity of the four prime movers of the upper arm and of three out of four scapular muscles.