Torsional stiffness (TS), the tendency of an object to resist twisting, has been studied in footwear. Most studies have explored the links between torsional stiffness and injury susceptibility, as well as gait [1,2,3]. However, there is a lack of standardization between the definition and measurement methods of TS. Some studies have quantified the torque at specific angles of longitudinal angular displacement (twist) between the forefoot and heel [2,4], while others have characterized the torque throughout a range of twist . While the latter method may be more comprehensive, it requires an instrumented torsional load cell, which may not be feasible for many field or clinical studies. However, it is unknown whether TS is sufficiently linear to support torque measurement at one angular displacement, or whether TS differs between inversion and eversion. Therefore, the purposes of this study are (1) to identify whether standard-issue Army boots demonstrate a linear relationship between TS and angular displacement, and (2) to see if there are any differences between the boots’ eversion and inversion stiffness properties. Army boots are of interest due to their potential to act as personal protective equipment during combat, and the desire to characterize their material properties.
The right side only of six pairs of boots (varying sizes and brands) were each tested using an Instron multi-axis measurement system fit with a custom jig. The system captured torque (T) and angular displacement ( throughout +/- 15 degrees of forefoot eversion and inversion while the heel was held stable. The total length of each boot (L) was also recorded to calculate torsional stiffness: . Each trials data sets for were graphed with respect to . A linear trendline was used to identify the R2 values and slopes for each trial.
Results and Discussion
All boots had similar R2 values ranging from 0.982 – 0.999. Despite the differing sizes and brands, TS values were very similar between boots, and there was very little variation between eversion and inversion values (Figure 1).
Boot TS appears to be linear over many angular displacement values. Therefore, one displacement angle and its corresponding torque may be sufficient to characterize the boot’s TS. This suggests that portable devices can characterize footwear using measurements at one angular displacement. The similarity of TS between the eversion and inversion directions should be explored further with a larger sample size and with additional footwear types.