Timing accuracy in human throwing

This study examines the precision required in the timing of muscle activati ons and projectile release to hit a target of 20 cm in diameter oriented ho rizontally either 6 or 8 m away. Over-arm throws, constrained to the sagitt al plane, were simulated using a muscle-actuated, two-segment model represe nting the forearm and hand plus projectile. The parameters defining the mod eled muscles and the anthropometry were specific to two male subjects. An o bjective function specified that throws must be both fast and accurate. Onc e an optimal solution had been found, the sensitivity of these timings was investigated. The times of activation or release were changed and the simul ation model re-run with the new timings, and it was determined whether the projectile would still have struck the target. For one set of simulations, to hit the target at 8 m, the optimal throw was achieved with a time delay between the onset of wrist activation and elbow extensor activation [Proxim al-distal (PD) delay] of 49 ms and a release time of 83.4 ms. At this optim al point in the solution space, the launch window was 1.2 ms (assuming the original PD delay). The launch window was the time available within which t he projectile must be released and still strike the target. The window duri ng which the wrist flexors could be activated was 10.41 ms (assuming the pr ojectile was released at the pre-planned optimal time). The control scheme which required the least timing precision had a PD delay of 56 ms and a rel ease time of 89.4 ms. Errors in timing could occur in activation and releas e simultaneously under this scheme, the timing windows were 4 ms in PD dela y and 2.4 ms in release. Similar results were found for a second set of sim ulations. These simulations revealed the precise timings required in muscle activations and release required for fast accurate throws. (C) 1999 Academ ic Press.