Force and time gain interact to nonlinearly scale adaptive visual-motor isometric force control.

This study examined the influence of force-time gain on the visual-motor control of isometric force. The spatial lengths on the computer screen representing the unit of elapsed time (time gain) and force (force gain) of the force output were compressed or extended in a crossed fashion while subjects produced index finger abduction force to a sinewave and constant force target that was 20% of maximal voluntary contraction. The results revealed a U-shaped interactive influence of force-time gain on force performance, namely a particular combination of moderate force-time gains leads to optimal force performance. The nature of the interaction between the force and time gains also differed depending on the task demand. During constant force production, the best gain at one dimension (force or time) was invariant across the other dimension (time or force), whereas during sinewave force production, the best gain at one dimension varied with the gain at the other dimension. The results support the proposition that the control of force output is organized by the interactive influence of different categories of constraints where the influence of visual information gain depends on the dynamics of the force control and the task demand. The findings also provide implications for visual gain parameter settings for adaptive force control.