Ankle muscle stiffness in the control of balance during quiet standing.

This research presents new data and reanalyzed information to refute the criticisms of our model of stiffness control during quiet standing. A re-review of their references to biomechanical research on muscle ankle stiffness confirmed muscle stiffness estimates of the ankle series elastic elements that agreed closely with our estimates. A new technique is presented that directly estimates the muscle stiffness from the ankle moment (N. m) and sway angle (deg). The linear regression of 10 subjects standing quietly for 10 s estimated the stiffness (N x m/deg) to be safely above the gravitational spring. The R(2) scores for this linear regression averaged 0.92, confirming how closely the model approached a perfect spring that would have an R(2) = 1. These results confirm our model of a simple muscle stiffness control and refutes the criticisms.