Figure 1.

Experimental setup and force production under different initial conditions. a, For the motion initial condition, subjects tapped the surface five times before pushing against it to maximize vertical force. The blue traces show vertical force data from all subjects. Data from the first 10 ms (gray box) were excluded to remove high-frequency impact sensor noise transients that subdued typically by 5 ms, and always well within 10 ms. b, For the relaxed (green) and preactivated (red) initial conditions, subjects produced force-ramps in a static posture (no previous finger motion), and from nearly zero, or low force (∼25% of maximal voluntary force), respectively.