Academic journal article Human Factors

Foot Flare and Foot Axis

Academic journal article Human Factors

Foot Flare and Foot Axis

Article excerpt

Most commercial footwear is designed and manufactured on a curved last, although the amount of curvature of the last and the turning point of the last centerline have not been formally determined. In this study, we used principal component analysis to determine the foot axis so that lasts that match feet can be produced, resulting in a good fit. In evaluating 50 Hong Kong Chinese participants, we found that the center of the foot is located at approximately 52% of the foot length measuring from the back of the foot (SD = 0.65%) and that Hong Kong participants have a mean inflare (inward curvature) of 3.2[degrees]. The foot center and inflare measures will help determine the fit between footwear and feet. Applications of this research include the ability to incorporate foot flare into the design and manufacture of footwear.

INTRODUCTION

Footwear are built using a three-dimensional mold called a shoe last (Figure 1). During manufacture, the upper part of the shoe is forcefully pulled around the last, thereby giving the footwear its characteristic shape. Consequently, the last size and shape determine the fit and comfort of a shoe (Cheskin, 1987; Hawes et al., 1994; Kouchi, 1995; Rossi, 1988).

One striking feature of a last is the piecewise linear centerline, as shown in the bottom view of Figure 1. One can identify this feature by looking at the shoe outsole or the bottom of a last. Two important characteristics of this centerline are evident: the point at which the turn begins and the amount of the turn. However, researchers have characterized only the amount of turn in a last, using an indirect measure that quantifies the longitudinal curvature of the last (Cavanagh, 1980; Cheskin, 1987). This measure, called flare, is shown as Angle [beta] in Figure 2. Without knowing the point at which the centerline shifts, it is clear that for any one value of the flare angle, [beta] (Figure 2), an infinite number of possibilities exist for Angle [alpha] in Figure 1. Thus it is not surprising that footwear manufacturers have downplayed the importance of flare in the design and manufacture of a last.

Foot flare has been defined in several different ways, and each definition is based on a somewhat different measurement approach. One of these, foot flare angle, involves angular measurement, which is itself specified according to different reference points (Yavatkar, 1993) and is similar to the measurement of the longitudinal curvature on a last; another, foot flare ratio, utlilizes the ratio between key linear distances on the foot (Freedman et al., 1946; Randall, Munro, & White, 1951). The foot flare ratio has no equivalent on the last because the points on the last corresponding to the anatomical landmarks used in figuring the ratio cannot be determined.

Figure 3 illustrates these two approaches to foot flare measurement (Freedman et al., 1946; Yavatkar, 1993). Both assume that flare can be represented with respect to the heel centerline (e.g., obtained by joining the two center points of the lines located 10 mm and 50 mm from Point H in Figure 3 or using a similar approach). However, they differ on how the reference line is defined and how the amount of flare is quantified.

The lack of a proper metric to quantify foot flare in relation to a last has resulted in mismatches between feet and shoes, causing fitting problems and discomfort.

Impact of Flare Mismatch

Figure 4 shows examples of flare and its effect on shoe fit. In figure 4a, we depict a matching foot and shoe and how the foot "sits." In Figure 4b, a clear mismatch between shoe and foot flare/turning point can be seen. If this shoe is worn, the forefoot will deviate from its neutral position in order to slide into the shoe, resulting in pressure on the lateral side of the metatarsal phalangeal joint (Figure 4c). Prolonged wearing will result in stretching of the upper material and possible overhang and/or a permanent deformation of the foot. …

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