ORGANIZING PRINCIPLES FOR VOLUNTARY MOVEMENT - EXTENDING SINGLE-JOINTRULES

1. Four subjects performed fast flexions of the elbow or shoulder over three different distances. Elbow flexions were performed both in a ho rizontal, single-degree-of-freedom manipulandum and in a sagittal plan e with the limb unconstrained. Shoulder flexions were only performed i n the sagittal plane by the unconstrained limb. We simultaneously reco rded kinematic and electromyographic (EMG) patterns at the ''focal'' j oint, that which the subject intentionally flexed, and at the other, ' 'nonfocal'' subject had been instructed to not flex. 2. Comparisons of the elbow EMG patterns across tasks show that agonist and antagonist muscles were similar in pattern but not size, reflecting the net muscl e torque patterns. Comparisons at the shoulder also revealed similar E MG patterns across tasks that reflected net muscle torques. 3. Compari sons of EMG patterns across joints show that elbow and shoulder flexor s behaved similarly. This was not true of the extensors. The triceps E MG burst was delayed for longer distances but the posterior deltoid ha d an early, distance-invariant onset. 4. Similarities in EMG reflect t orque demands required at the focal joint to produce flexion and at th e nonfocal joint to reduce extension induced by dynamic interactions w ith the focal, flexing joint. These similarities appear despite very d ifferent kinematic intentions and outcomes. This argues against a stro ng role for length-sensitive reflexes in their generation. 5. These re sults support the hypothesis that movements are controlled by muscle a ctivation patterns that are planned for the expected torque requiremen ts of the task. This general rule is true whether we are performing si ngle-joint or multiple-joint movements, with or without external const raints. The similarities between single-joint and multijoint movement control may be a consequence of ontogenetic development of multijoint movement strategies that prove useful and are therefore also expressed under the constrained conditions of specialized tasks such as those p erformed in single-joint manipulanda.