Effects of incremental ambulatory-range loading on arch height index parameters

Deformation of the medial longitudinal arch under body weight loading is often assessed using the Arch Height Index Measurement System. This system assesses change in arch height between sitting and standing, estimated to be 10% and 50% of body weight, respectively. However, body weight forces during ambulation exceed these loads, therefore limiting our understanding of arch deformation under ambulatory load ranges. Thus, the study aims were (1) to assess if sitting and standing arch height differed from that seen under 10% and 50% body weight using a force target-matching procedure, and (2) to quantify the assumed linearity of arch stiffness, measured statically, throughout an ambulatory load range. Established sitting and standing arch height measurements were taken from 25 healthy subjects, who also underwent testing from 10% to 120% body weight in sequential 10% increments. Arch deformation in sitting was less than for 10% body weight, whereas the standing and 50% condition did not differ. The incremental loading data revealed linear and curvilinear trends between arch deformation and loading through the ambulatory range, such that further deformation beyond that seen at 120% would be minimal using these procedures. These data suggest that sitting arch loads and deformation are less than those seen at 10% body weight, which affects known parameters such as arch stiffness. Further, the curvilinear trend in the arch height data suggests that most arch deformation occurs in the ambulatory load range for a healthy foot.