COORDINATING 2 DEGREES OF FREEDOM DURING HUMAN ARM MOVEMENT - LOAD SPEED INVARIANCE OF RELATIVE JOINT TORQUES

1. Eight subjects performed three series of pointing tasks with the un constrained arm. Series one and two required subjects to move between two fixed targets as quickly as possible with different weights attach ed to the wrist. By specifying initial and final positions of the fing er tip, the first series was performed bg flexion of both shoulder and elbow and the second by shoulder flexion and elbow extension. The thi rd series required flexion at both joints, and subjects were instructe d to vary movement speed. We examined how variations in load or intend ed speed were associated with changes in the amount and timing of the electromyographic (EMG) activity and. the net muscle torque production . 2. EMG and torque patterns at the individual joints varied with load and speed according to most of the same rules we have described for s ingle-joint movements. 1) Movements were produced by biphasic torque p ulses and biphasic or triphasic EMC bursts at both joints. 2) The acce lerating impulse was proportional to the load when the subject moved ' 'as fast and accurately as possible'' or to speed if that was intentio nally varied. 3) The area of the EMG bursts of agonist muscles varied with the impulse. 4) The rates of rise of the net muscle torques and o f the EMG bursts were proportional Co intended speed and insensitive t o inertial load. 5) The areas of the antagonist muscle EMG bursts were proportional to intended movement speed but showed less dependence on load, which is unlike what is observed during single-joint movements. 3. Comparisons across joints showed that the impulse produced al the shoulder was proportional to that produced al the elbow as both varied together with load and speed. The torques at the two 2 joints varied In close synchrony, achieving maxima and going through zero almost sim ultaneously. 4. We hypothesize that ''coordination'' of the elbow and shoulder is by the planning and generation of synchronized, biphasic m uscle torque pulses that remain in near Linear proportionality to each other throughout most of the movement. This linear synergy produces m ovements with the commonly observed kinematic properties and that are preserved over changes in speed and load.