Association between foot thermal responses and shear forces during turning gait in young adults

Abstract

Background: The human foot typically changes temperature between pre and post-locomotion activities. However, the mechanisms responsible for temperature changes within the foot are currently unclear. Prior studies indicate that shear forces may increase foot temperature during locomotion. Here, we examined the shear-temperature relationship using turning gait with varying radii to manipulate magnitudes of shear onto the foot.

Methods: Healthy adult participants (N = 18) walked barefoot on their toes for 5 minutes at a speed of 1.0 m s^-1 at three different radii (1.0, 1.5, and 2.0 m). Toe-walking was utilized so that a standard force plate could measure shear localized to the forefoot. A thermal imaging camera was used to quantify the temperature changes from pre to post toe-walking (ΔT), including the entire foot and forefoot regions on the external limb (limb farther from the center of the curved path) and internal limb.

Results: We found that shear impulse was positively associated with ΔT within the entire foot (P < 0.001) and forefoot (P < 0.001): specifically, for every unit increase in shear, the temperature of the entire foot and forefoot increased by 0.11 and 0.17 °C, respectively. While ΔT, on average, decreased following the toe-walking trials (i.e., became colder), a significant change in ΔT was observed between radii conditions and between external versus internal limbs. In particular, ΔT was greater (i.e., less negative) when walking at smaller radii (P < 0.01) and was greater on the external limb (P < 0.01) in both the entire foot and forefoot regions, which were likely explained by greater shear forces with smaller radii (P < 0.0001) and on the external limb (P < 0.0001). Altogether, our results support the relationship between shear and foot temperature responses. These findings may motivate studying turning gait in the future to quantify the relationship between shear and foot temperature in individuals who are susceptible to abnormal thermoregulation.

Keywords: Biomechanics; Curved path walking; Feet; Locomotion; Thermoregulation.

Figure 1. The experimental setup involved pre- and post-temperature measurements (A) as participants (n = 18) walked on their toes along a curved path with varying radii (B).

The order of the radii conditions (1.0, 1.5, and 2.0 m) was randomized for each participant. Participants were instructed to walk on their toes around each curved path while maintaining a fixed speed (1.0 m s⁻¹). A force platform embedded on the floor was used to quantify shear forces acting on the foot.

Figure 2. A representative image of one participant’s pre- (A) and post- (B) temperature measurements.

The outlines (manually traced for visualization) represent the areas to gather the entire foot (white) and forefoot (gray) temperature. To create a contrast difference between the foot and background temperature, a cold towel was placed above the subject’s leg (no contact between towel and limb) before taking the thermal image.

Figure 3. The anteroposterior (AP) (A) and mediolateral (ML) (B) components of the ground reaction forces (time-normalized to stance phase) were utilized to gather resultant shear impulse extrapolated to the 5 min walking trial for each limb within each radii condition

AP and ML forces are relative to the coordinate system of each foot, where medial forces on the external limb and lateral forces on the internal limb indicate forces directed towards the center of the radii, respectively. The resultant shear impulse was greater within the external limb compared to the internal limb (p < 0.0001, denoted by an asterisk) and greater as the radii condition decreased (p < 0.0001, denoted by a + symbol) (Fig. 2C). Pairwise comparisons are denoted by horizontal lines above each condition. (Values are means ± S.D.).

Figure 4. Difference between post- and pre-walking temperatures (ΔT) were analyzed for subjects’ (n = 18) between limbs and radii conditions for the entire foot (A) and forefoot (B) region within all subjects.

ΔT was greater (i.e., less negative) as the radii decreased within the entire foot (p < 0.01) and within the forefoot (p < 0.01) (significant radii effect denoted by an asterisk). Greater ΔT was observed in the external limb compared to the internal limb within the entire foot (p < 0.01) and in the forefoot (p < 0.01) (significant limb effect denoted by a + symbol). Pairwise comparisons are denoted by horizontal lines above each condition. (Values are means ± S.D.).

Figure 5. A linear mixed models analysis was used to determine the relationship between temperature difference between post- and pre- walking (ΔT) and shear within the entire foot (A) and forefoot (B) of participants (n = 18).

Resultant shear impulse (extrapolated to the 5 min walking trial) was a significant predictor of entire foot ΔT (p < 0.0001) and forefoot ΔT (p < 0.0001). A greater slope for forefoot ΔT was predicted compared to entire foot ΔT. (Open circle = external limb, X = internal limb; blue = 1.0 m, red = 1.5 m, and green = 2.0 m). Marginal variance (i.e., R² for our fixed effects) was calculated for both regions of the foot.