THE SPATIOTEMPORAL STRUCTURE OF CONTROL VARIABLES DURING CATCHING

The discrepancy between traditional (force scaling models) and the mor e recently conceived dynamic explanations of load compensation (lambda model) was the departure point for the present study. By using the co mplex ''open'' motor skill of catching a ball - rather than the tradit ional ''closed'' skills - under ''normal'' (baseline) conditions and u nder conditions where a spring load was applied to the catching hand ( thereby changing the dynamics of the skeleto-muscular system) it was h oped to provide further clarification of this issue. Traditional force scaling models, in this respect, would predict that maximal closing v elocity of the grasp action, and movement time would not be significan tly different between a control and a spring-load condition. In contra st, a dynamic system perspective would maintain that spring loading wo uld be compensated for by a change in the rate of shift of the recipro cal command (R-command). The obtained results showed a significant dif ference for conditions with regard to the maximal closing velocity of the grasp action, the baseline condition being higher than the two spr ing-load conditions. Furthermore, a significant difference was found f or the aperture at moment of catch, the aperture at moment of catch be ing smaller in the baseline condition than that under the two spring-l oad conditions. With regard to the temporal variables, no significant differences were obtained. A comprehensive overall explanation of the obtained data in terms of the force scaling models was not realisable. It may be that findings supporting such theories are task specific an d that for constrained tasks - such as catching a ball different under lying organisational principles apply. The lambda model, however, coul d explain adequately the obtained results. It was concluded that, exce pt for the preparatory phase associated with load compensation before the onset of the movement of the ball, the spatiotemporal structure of the control pattern underlying catching remains the same (invariant) in both baseline and load conditions. Thereby, the spatiotemporal stru cture of the resulting movement changes under the influence of the loa d and thus is not the same for load and baseline condition.