Co-contraction

A muscle can only pull, so every joint has muscles on both sides: the agonist that makes a movement and the antagonist that opposes it (the biceps and triceps at the elbow). When both pull together, that’s co-contraction. The joint’s torque is the difference of the two pulls,

$$\tau = r_{\text{ag}} F_{\text{ag}} - r_{\text{ant}} F_{\text{ant}}$$

so equal pulls cancel and the joint doesn’t move, but it is stiff: push it and one side is stretched and resists. That is how a person tenses a limb, and it is the only way a muscle body gets stiffness, where a servo has it built in.

Stiffness has a price in fast movement. A switched-on antagonist being stretched resists more than it can pull (force–length–velocity), so a fast punch or kick needs the antagonist to let go. The spinal cord does this automatically: reciprocal inhibition switches the antagonist off when the agonist fires.

In this project

In Muscles instead of motors every muscle pair rested at half excitation, so a zero action was a 50 % co-contraction. Through a jab the muscle fighter’s shoulder and elbow pairs both sat at 0.3–0.7, and its fists moved at 0.5–2.9 m/s, under the 3.5 m/s a hit needs. With the arm muscles resting at 10 % instead, the antagonists switched off during punches (the stretch reflex pushed them to zero, like reciprocal inhibition) and the first muscle hit came within 4 M steps.