Figure 2: (I) Set-up for generating sudden perturbations to the upper part of the subject’s trunk. The wire is fastened to a rigid bar fastened by a vest at the upper part of the trunk. The movement of the trunk is measured by a potentiometer mounted on a reel. (II) Details of the perturbation apparatus (right top: 90 degrees rotation) and standing position (left): (a) cylinder, (b) 5 kg load, (c) gripping device, (d) solenoid for activating gripping device, (e) holding magnets, (f) load-bearing construction, (g) 5 kg load, (h) anchor plate for magnet, (i) holding magnet, (j) bearing construction, (k) vertical adjustable reel with potentiometer, and (l) horizontal adjustable reel to adjust wire length to individual subject height. Generation of sudden unloading ((III) left): first the computer activates the magnet (i) and releases the load (b, h, and g) causing the weight of the load applied to the wire to suddenly decrease from 5.4 kg (a) to 0.1 kg (i). Generation of the sudden loading ((III) right): first the computer activates the solenoid (d) causing the gripping device (c) to fix the load (b) to the cylinder (a) and secondly deactivates the holding magnets (e). This releases the load (b, h, and g) causing the weight of the load applied to the wire to suddenly increase from 5.4 kg (a) to 10.9 kg (a–d).