Force asymmetry deteriorates complementary force production during joint action.

The present study aimed to determine the effects of force asymmetry on interpersonal force production. This study consisted of an individual task executed by one participant at a time in a pair, and three joint tasks executed by two participants simultaneously under conditions of 1:1, 1:0.75, and 1:0.5. Two individuals produced discrete forces at the same time so that the sum of forces they produced was the target force in the joint task. Under the 1:1 condition, the target force was the sum of the maximum voluntary contraction (MVC) produced by the index finger of each participant × 0.1 (10% MVC). Under the 1:0.75 condition, the investigators manipulated the force produced by only one of the pair, for example, B, but not A. The feedback was also scaled as a result. The target force was the MVC of participant A + the MVC of participant B × 0.75 × 0.1. Similarly, the target force under the 1:0.5 condition was the MVC of participant A + the MVC of participant B × 0.5 × 0.1. The present study found that forces produced by pairs were negatively correlated and the correlation value was higher under the 1:1 condition than the 1:0.75 and 1:0.5 conditions. The absolute error was smaller under the 1:1 condition than the 1:0.5 condition. Complementary force production was attenuated and the error increased as differences between forces produced by two participants increased. Thus, asymmetry of forces produced by pairs deteriorated complementary force production and interpersonal performance.