Biomechanical comparison of carpet-stretching devices

Peak impact forces, measured from seven male carpetlayers using a knee-kicker over 39 trials were 2933 N (SD 397), taking 9.5 ms (SD +/- 1.1) to reach peak. The knee-kicking cycle involved high knee decelerations of 880 m.s-2 (SD +/- 271). Angles at peak force for the knee (63 degrees SD +/- 10 degrees) and hip (80 degrees SD +/- 35 degrees) showed considerable variation between individuals. Kneeling forces on the non-kicking leg during the kick cycle were 894 N: comparative forces were 368 N and 476 N measured during use of the power stretcher and crab re-stretcher, respectively. Under dynamic conditions the effective mechanical advantage of the knee-kicker was less than 1.0. In comparison, the mechanical advantage of the power stretcher was approximately 14 times greater over the stretch action. When tested with a crab re-stretcher, the breaking strength of the smooth-edge to which the carpet attaches averaged 2384 N (SD +/- 245). Design guidelines for an improved carpet stretching device are provided.