Producing Stability through Antagonistic Architecture

To hold a coffee cup steady requires a different approach: The motor system sends commands to activate both biceps and triceps. This coactivation results in both flexion and extension torques on the elbow joint. If the net torque, which is the sum of these two torques, equals zero, then the forearm will remain still. Why not simply shut down both muscles rather than waste the torques (and energy) to no effect? The answer has to do with limb stability. Consider what happens if an object suddenly hits a person's hand and causes the arm to flex at the elbow. During coactiva-tion, the muscle that gets stretched because of the impact (triceps) will vigorously resist because intrinsic muscle stiffness increases with activation level. Without coactivation (i.e., with an inactive triceps), the impact would result in a much larger flexion of the elbow. Thus, coactivation promotes limb stability.

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