My question is regarding the friction use trace (ratio between forward and downward forces) for normal stepping. I have noticed in two studies investigating slip-and-falls (Perkins and Wilson, 1983 & Strandberg 1983) that for subjects taking a normal step, the following occurs: first, heel-strike presents the expected generation of a force in a forward direction and then toe-off generates a backward force. However in the friction use traces of the papers I mentioned above, I see a quick, sharp peak in the backward direction during heel-strike. The force rapidly changes direction and goes forward again. Is this a consistent feature of friction use traces for normal walking?

Can anyone explain what causes this peak? An advisor and I discussed it, and we thought it might be attributable to one of three things: a subject who digs his heels backward at heel strike, a backward force generated by hip extension as the body's center of mass is transferred over the leading foot, or perhaps artifact. Any information would be much appreciated.

Rachel Beyer