Does the Heel’s Dissipative Energetic Behavior Affect Its Thermodynamic Responses During Walking?

Most of the terrestrial legged locomotion gaits, like human walking, necessitate energy dissipation upon ground collision. In humans, the heel mostly performs net-negative work during collisions, and it is currently unclear how it dissipates that energy. Based on the laws of thermodynamics, one possibility is that the net-negative collision work may be dissipated as heat. If supported, such a finding would inform the thermoregulation capacity of human feet, which may have implications for understanding foot complications and tissue damage. Here, we examined the correlation between energy dissipation and thermal responses by experimentally increasing the heel’s collisional forces. Twenty healthy young adults walked overground on force plates and for 10 min on a treadmill (both at 1.25 ms−1) while wearing a vest with three different levels of added mass (+0%, +15%, & +30% of their body mass). We estimated the heel’s work using a unified deformable segment analysis during overground walking. We measured the heel’s temperature immediately before and after each treadmill trial. We hypothesized that the heel’s temperature and net-negative work would increase when walking with added mass, and the temperature change is correlated with the increased net-negative work. We found that walking with +30% added mass significantly increased the heel’s temperature change by 0.72 ± 1.91 ℃ (p = 0.009) and the magnitude of net-negative work (extrapolated to 10 min of walking) by 326.94 ± 379.92 J (p = 0.005). However, we found no correlation between the heel’s net-negative work and temperature changes (p = 0.277). While this result refuted our second hypothesis, our findings likely demonstrate the heel’s dynamic thermoregulatory capacity. If all the negative work were dissipated as heat, we would expect excessive skin temperature elevation during prolonged walking, which may cause skin complications. Therefore, our results likely indicate that various heat dissipation mechanisms control the heel’s thermodynamic responses, which may protect the health and integrity of the surrounding tissue. Also, our results indicate that additional mechanical factors, besides energy dissipation, explain the heel’s temperature rise. Therefore, future experiments may explore alternative factors affecting thermodynamic responses, including mechanical (e.g., sound & shear-stress) and physiological mechanisms (e.g., sweating, local metabolic rate, & blood flow).

Walking on a treadmill can cause an irregular rise in heel temperature. That is because the friction between the running belt, the rollers, and the platform inside the treadmill's deck can cause the belt to heat up and transfer additional heat to the heel. This supplementary experiment aimed to examine the validity of the heel temperature measurements acquired from the treadmill trials. Therefore, we aimed to answer two questions; first, does the heel temperature increase after 10 minutes of barefoot overground walking with +30% of added body mass, and secondly, are the heel's temperature changes between overground and treadmill walking similar?

Methods
To achieve that, we recruited a subset of twelve participants (N=12, 7 females, 5 males; age = 23.7 ± 3.2 yrs; height = 1.68 ± 0.21 m, mass = 74.5 ± 17.4 kg; means ± standard deviation). One female participant's age, height, and mass are missing; therefore, means and standard deviations are calculated from N=11. Participants walked barefoot for 10 minutes in a randomized order on a treadmill and on an overground track while carrying (via weight vest) two different levels of added mass: +0% (no added body mass) and +30% relative to their body mass. The order of the added mass levels was also randomized. We controlled the walking speed for the treadmill and overground track trials at 1.25 m s -1 (the same speed as the treadmill and overground trials mentioned in the Experimental Protocol section). We controlled the walking speed for the overground track trials by dividing the track into four equal distance sections and providing verbal feedback about the time taken to walk each section. We obtained all measurements precisely the same procedures described in the Analysis: Foot and Heel Temperature (Treadmill Walking) section. In summary, the participants rested on a treatment table for 30minutes before each walking trial to allow the foot temperature to stabilize. We measured the before and after walking temperature for all walking trials at the same sites mentioned in the Analysis: Foot and Heel Temperature (Treadmill Walking) section. We computed temperature change (∆ ; ℃) as the difference between the temperature after and before each walking trial. Additionally, we computed the difference in temperature change between added mass conditions (∆ ∆ ; ℃) as: +30% added body mas − +0% added body mas and +30% added body mas − +0% added body mas , where ∆ 30 is the difference between the temperature after and before walking with +30% added mass and ∆ 0 is the difference between the temperature after and before walking with +0% added mass.
We used paired-samples t-tests to compare the heel's temperature changes between +0% and +30% added body mass overground walking and between +0% and +30% added body mass treadmill walking. Also, we compared the difference in temperature change (i.e., +30% − +0% ) between treadmill and overground walking. We analyzed the data using MATLAB R2021a (MathWorks, Natick, MA, USA). The significance level was considered as p < 0.05
In conclusion, this experiment showed that 10 minutes of barefoot walking with +30% of added body mass increases the heel's temperature during overground and treadmill walking (Sup. Figure 1; panel A. & panel B.; respectively). Furthermore, the mechanical characteristics of the treadmill's moving parts (e.g., belt and rollers) affect to some extent but do not compromise the ability to detect temperature changes due to walking with added mass (Sup. Figure 1; panel C.). Overall, this supplementary experiment supports the validity of our main findings that the added mass affected heel's temperature, which was not explained by the artificial stimuli of the treadmill surface.
Sup. Figure 1: The differences in heel's temperature changes (i.e., + % − + % ) between overground and treadmill walking were not statistically different (p=0.272; panel C.), supporting our main experiment's validity regarding heel temperature measurements. Walking with +30% added body mass significantly increased the heel's temperature change after 10 minutes of overground barefoot walking (p = 0.014; panel A.) and after 10 minutes of barefoot treadmill walking (p=0.021; panel B.). The horizontal square brackets indicate the adjusted significant pair-wise comparisons (N=12). Figure 2: Average temperature measurements for additional anatomical sites obtained before and after the treadmill walking trial of the main experiment (N=20). The vertical error bars represent one standard deviation.

Figure
3: Heel temperature measurements immediately before and after each treadmill trial for all the participants of the main experiment (N=20).
Sup. Table 1: We performed an explanatory analysis using a linear mixed model to examine the effect of additional variables on the heel temperature change for the data set of our main experiment (N=20).