Force measurements: A height adjustable strain gauge, placed beneath the bed, were secured to the metal framing of the bed and connected horizontally (180 degree) to the subjects with a non-extensible strap placed around the angle of the subject. To minimize leg movement a second non-extensible strap were attached between the ankle and a fixed metal bar, pulling directly opposite the strap holding the strain gauge. A bridge circuit was used to detect changes in relative resistance in the strain gauge and transformed voltage changes, proportional to the torque generated by the activated muscle, were then A/D converted, sampled at 1 kHz and subsequently stored on a personal computer for further offline analysis.