Figure 1.

(A) Set-up used to stabilize the subjects. Note that subjects were only restricted in medio-lateral direction, as carts could freely move on rails in anterio-posterior direction, and vertical movements have little effect on the length of the springs. (B) Characteristics of the springs used (Red lines), and the force field generated by the spring used. Springs were pretensioned, so that in the initial position, the springs left and right exerted about 150 N. For estimation of the force field, a regression was fitted to the whole displacement curve of the spring, ignoring slight differences between lengthening and shortening that occurred as a consequence of damping of the spring.