Shock absorption during forefoot running and its relationship to medial longitudinal arch height.

Background: Overuse injuries to the lower extremity have often been connected with the repetitive loading of the foot and in particular its ability to absorb shock. The shock absorbing ability of the foot is thought to relate to its structure, particularly the height of the medial longitudinal arch. The purpose of this study was to investigate the shock absorption characteristics of the foot in forefoot running as measured by the dynamic load rate of the vertical ground reaction forces during the early stages of ground contact and to relate these characteristics to the height of the medial longitudinal arch.

Methods: Eighteen normal athletic adult volunteers were used as subjects and all had clinically normal feet. An Arch Index was computed from lateral radiographs taken with the foot in a full weightbearing position. Dynamic load rate was computed as the first differential of the vertical force as measured by a Kistler force platform. Each subject performed ten trials of running at a speed of 3 m.s-1 using forefoot running style.

Results: The dynamic load rate showed three definite peaks (mean 93, 18, and 16 kNs-1 respectively), and two intervening troughs (mean 18 and 3 kNs-1 respectively), showing that the process of shock absorption was one that was progressive over the foot loading phase. The time at which these features occurred indicated a consistency in process of shock absorption. However, none of the force peaks or load rate peaks correlated with the Arch Index.

Conclusion: It was concluded that the structure of the foot as characterized by the Arch Index, was not the major factor in determining the way in which force is transmitted to the musculoskeletal system in forefoot running. These findings support the concept that the height of the arch, although a commonly used clinical descriptor of foot type does not appear to be important in defining the functional capacity of the foot in action.