THE EFFECT OF PROXIMAL JOINT TORQUE REVERSAL IN A TWO-SEGMENT MODEL OF FLAIL-LIKE THROWING

Abstract

The purpose was to investigate the possible advantages of torque reversal at the proximal joint in the late stages of the flail-like motion of a two-segment system such as a throwing arm. The maximum velocity of the distal endpoint (vEmax) produced with a shoulder joint reversal torque (with SJRT) was compared to the maximum value produced without a shoulder joint reversal torque (without SJRT). A planar two-segment model of an arm with a fixed proximal endpoint was created. For greater generality, interial parameters for both segments were equal, and based on the average values of the parameters for upper arm and forearm-plus-hand segments. Muscle torque generators at the shoulder and elbow were angular velocity-dependent. A series of simulations were run over a present range of motion with SJRT and without SJRT. Each simulation started with the activation of the shoulder joint torque generator (tSH = 0.000 s). The elbow joint torque generator was activated at various times after the onset of the shoulder joint torque generator (tEL = 0.000 s to 0.250 s), with increments of 0.001 s between successive simulations. For the simulations with SJRT, at various times after the elbow joint torque onset time the shoulder joint torque generator was deactivated and the shoulder joint reversal torque generator was activated (tREV = tEL to 0.250 s), with increments of 0.001 s between successive simulations. The optimal simulation for each condition (with and without SJRT) was considered to be the one that produced the largest vEmax value within the specified range of motion and prior to alignment of the two segments. The optimal simulation with SJRT produced only a slightly larger vEmax value than the optimal simulation without SJRT (12.17 m/s versus 11.44 m/s). In addition, it was found that small errors in the timing of the reversal torque led to smaller values of vEmax than the one produced without SJRT. Taking into account that instantaneous torque reversal is not physiologically possible, the results implied that the use of torque reversal in real life would provide either limited advantage or no advantage at all.