How do the antagonistic muscles contract during modulating the output force?

Muscle forces or joint loads during daily living are important data for designing assistive devices or interfaces. Since it is difficult to measure them, they are usually calculated by static or dynamic simulations. However, such simulation requires some assumptions to solve the statically indeterminate problem, which is, the number of muscles is larger than the number of the static or dynamic conditions. One reason for the indetermination is the simultaneous contraction of the antagonist muscle. Antagonist muscles are considered to be exerted to increase the stiffness of the joint, however, it is not used as the condition to solve the indeterminate problem yet, because there is no quantitative evidence. We have assumed that the antagonist would contract during modulating the output force sensitively. Although we already confirmed the assumption by simulation, the experimental support to show the validation had been lacking. Therefore, we performed the experiment to confirm the assumption using the upper limb. Test subjects were asked to grasp a handle with a force sensor and push/pull it; by maximum force, about half of the maximum force, and the half force by monitoring the output of the force sensor with inputting some unpredictable external force. EMGs of the biceps and triceps brachii were measured. As a result, the antagonist was exerted only in the condition of monitoring the force to modulate the output. Our future plan is to confirm the relation between the magnitude of the antagonist exertion and the sensitivity to modulate the output force.