Changes in Running Gait Biomechanics Under Heat Stress

Abstract

Millions of Americans regularly participate in outdoor physical activity for exercise (Statista, 2022). Running is one of the most common and popular exercises because of its accessibility and health benefits (Nystoriak & Bhatnagar, 2018). Part of running’s appeal is that it can be done nearly anywhere outdoors. Furthermore, running is a significant component to many popular sports performed outdoors including football, soccer, and many other organized and unorganized physical activities. Many organized and unorganized activities involving running and international sporting competitions are held during the summer (Fields et al., 2010). However, the hot and humid weather during summer months in many regions of the United States and worldwide makes outdoor physical activity challenging and potentially dangerous because of an increased risk for heat stress injury (Nichols, 2014). The mean global temperature in 2020 was 1.2ºC greater than the temperature during 1850-1990 (World Meteorological Organization, 2021). With sufficient exposure, exercise in hot and humid conditions poses serious health risks of heat-related illnesses, such as heatstroke which can be fatal (Bergeron, 2014). According to the US Natural Hazard Statistic report, heat is the leading cause of weather-related fatalities in the United States (The U.S. Natural Hazard Statistics, 2022). The growing effects of global climate change exacerbate the risk for heat stress injury, which makes heat stress an inevitable problem that must be overcome by elite runners and recreational athletes alike.

Running and exercising in hot and humid conditions will result in physiological changes detrimental to physical performance and accelerate the occurrence of fatigue (Dill et al., 1931; Febbraio et al., 1996; Galloway & Maughan, 1995). Prolonged exercise in the heat is associated with elevated body core temperature, decreased maximal aerobic metabolic rates, and increased rates of body water loss due to sweating (Périard et al., 2021). Combined, these physiological changes and responses to exercising in the heat can reduce performance and increase perceived exertion levels compared to exercising in cold or moderate temperatures (James et al., 2017; Tatterson et al., 2000). These physiological changes are similar to the changes experienced during prolonged exercise under controlled environmental conditions, but heat exposure accelerates the rate and severity of these changes (Tatterson et al., 2000). The physiological effects of exercising under heat stress conditions likely cause functional movement changes such as altered gait biomechanics. Previous studies have primarily focused on characterizing biomechanical changes under heat stress conditions during movements such as walking lifting, striking, and pulling (Hostler et al., 2021) or investigated how footwear mechanical properties change in hot environments (Dib et al., 2005). A study investigating the relationship between running biomechanics and core temperature revealed significant increases in kinetics, such as shock, impact, and braking segmental acceleration, as well as in runners' core temperature throughout the outdoor race completed when the heat index ranged from 21–42ºC (DeJong Lempke et al., 2024). Furthermore, another study has indicated that heat stress could impair proprioception, although it also suggests that heat stress may not affect foot strike angle at initial contact. However, the lack of observed gait changes may be attributed to the fact that participants in this study were able to freely adjust their paces in both hot and control conditions, ultimately resulting in different pace and perceived exertion levels between conditions. Overall, these studies found that human movements can be affected by heat stress (Demura et al., 2010; Hostler et al., 2021). Based on this research, it can be assumed that heat stress potentially impacts running gait biomechanics. However, no studies to date have documented these changes in athletes or recreationally active individuals while running at the same pace and perceived exertion. It is particularly important to study alterations in gait biomechanics while exercising in the heat because alterations in gait due to fatigue are hypothesized to increase the risk of developing musculoskeletal injury (Winter et al., 2016). For example, prolonged running under controlled environmental conditions alters gait mechanics, such as increased foot eversion and dorsiflexion at initial contact (Urbaczka et al., 2022), which may be related to overuse injury development (Powers et al., 1995). Additionally, changes to footwear mechanical properties may affect the ability of the shock absorption of a shoe (Dib et al., 2005; Kinoshita & Bates, 1996), and thus may further exacerbate heat- and fatigue-related changes in gait. Given that the physiological response to heat stress is similar to but more severe than the physiological response to prolonged exercise (Tatterson et al., 2000), running under heat stress may result in larger alterations of runners’ movement that occur earlier during a run and thereby contribute to the risk of developing a running-related musculoskeletal injury.