Step length and frequency effects on ground reaction forces during walking.

It is well established that the speed of walking or running significantly affects ground reaction force (GRF) characteristics. While it is sometimes assumed that the variations in step length (SL) and step frequency (SF) also affect GRF patterns, little documentation of this can be found in the literature. Ten young adults performed overground walking at 1.43 m s-1 across a force platform under five SL conditions: preferred SL and SLs that were longer and shorter than the preferred by 5 and 10% of greater trochanter height. The contact time, anteroposterior braking and propulsive force and impulse descriptors, and vertical impulse per step increased systematically as SL increased. In contrast, vertical peak forces and impulse per meter walked showed little change with SL manipulation. Despite the systematic effect of SL on several GRF descriptors, constraint of SL and SF in gait assessments is not recommended as this would prohibit the evaluation of representative gait kinematics and kinetics. Rather, these results suggest that researchers should report SL and SF data when comparing GRF characteristics between experimental groups or conditions, and should be alert to the association between SL/SF and GRF's when interpreting GRF trends.