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Physiological Strain of Stock Car Drivers During Competitive Racing

Date: 
October 21, 2016
By 
Lara A. Carlson, David P. Ferguson, Robert W. Kenefick

HIGHLIGHTS

  • Drivers endure increased thermal and cardiovascular stress during stock car racing.
  • Competitive stock car racing in hot conditions may lead to large fluid losses.
  • Drivers should consider strategies to meet the thermoregulatory challenges.

ABSTRACT
Heat strain experienced by motorsport athletes competing in National Association for Stock Car Automobile Racing (NASCAR) may be significant enough to impair performance or even result in a life-threatening accident. There is a need to carefully quantify heat strain during actual NASCAR race competitions in order to faithfully represent the magnitude of the problem and conceptualize future mitigation practices. The purpose of this investigation was to quantify the thermoregulatory and physiological strain associated with competitive stock car driving. Eight male stock car drivers (29.0710.0 yr; 176.273.3 cm, 80.6715.7 kg) participated in sanctioned stock car races. Physiological measurements included intestinal core (Tc) and skin (Tsk) temperatures, heart rate (HR), blood pressure, and body mass before and after completion of the race. Pre-race Tc was 38.170.1 °C which increased to 38.670.2 °C post-race (p1⁄40.001). Tsk increased from 36.170.2 °C pre-race to 37.370.3 °C post-race (p1⁄40.001) whereas the core-to-skin temperature gradient decreased from a pre-race value of 2.070.3 °C to 1.370.3 °C post-race (p1⁄40.005). HRs post-race were 8070.1% of the drivers' age-predicted maximum HR. Physiological Strain Index (PSI) post-race was 4.9, which indicates moderate strain. Drivers' thermal sensation based on the ASHRAE Scale increased from 1.370.5 to 2.870.4, and their perception of exertion (RPE) responses also increased from 8.471.6 to 13.971.8 after competition. Heat strain associated with competitive stock car racing is significant. These findings suggest the need for heat mitigation practices and provide evidence that motorsport should consider strategies to become heat acclimatized to better meet the thermoregulatory and cardiovascular challenges of motorsport competition.

INTRODUCTION
During National Association for Stock Car Automobile Racing (NASCAR) competition, motorsports athletes may be exposed to severe heat strain due to the summer months and southern climate of the competitive season. Motorsports athletes can drive at speeds of 322 km/h for 3–4 h of competition in a cockpit that can reach temperatures of $50 °C (Falkner, 1972; Walker et al., 2001a). The high temperature is due in part to the stock car design. The thin construction materials used to keep weight down and the aerodynamic design, force heat from the drive train (engine and transmission) into the car cockpit in order to reduce drag. Furthermore, motorsport athletes are required to wear fire protective clothing that adds insulation and impedes heat loss. It has been suggested that motorsport competition may increase heat stress, challenge the cardiovascular system, adding to driver fatigue, and possibly leading to catastrophic injury (Brearley and Finn, 2007; Carlson, 2013). In a recent review, Potkanowicz and Mendel (2013) stated that the goals of sports science research are to improve the fitness levels in all competitive athletes in order to minimize stress on the body during competition, but unfortunately, less is known regarding the stress motorsports drivers are faced with during competitive racing. To date, the thermoregulatory and cardiovascular stress and strain of actual stock car race competitions has not been carefully quantified.

Extreme heat has the potential to cause pronounced challenges to the cardiovascular system due to the increased demand for greater skin blood flow in order to dissipate heat. The increased heart rate (HR) and cardiac output associated with exercising in the heat is necessary to maintain blood pressure (Crandall, 2008) while perfusing active muscle, including the myocardium (Gonzalez-Alonso et al., 2008) and peripheral vessels. Jacobs et al. (2002) found that open-wheel (wheels are outside car's body in contrast to stock cars which have wheels under the fenders) drivers during two non-competition, paced driving sessions on a road course, were reported to elicit oxygen uptake (VO2) and HR responses of approximately 79% and 82% respectively, of their maximal responses from treadmill testing. Additionally, drivers' rating of perceived exertion (RPE) responses ranged from 15 to 17 (hard to very hard) after the road course testing session (Jacobs et al., 2002). Jacobs et al. (2002) concluded that the physiological responses assessed during the driving sessions required similar energy expenditures and heart rates as observed in other competitive sports activities such as basketball and soccer. Brearley and Finn (2007) likewise reported that V8 Supercar drivers averaged HRs of 167 and 169 beats/min during both short and long road course races. Furthermore, amateur kart drivers exhibited heart rates of 150 beats/min during kart racing and significantly decreased blood pressure post-racing (Yamakoshi et al., 2010). Despite some cardiovascular monitoring during different types of motor sports, there is clear indication that this has not been documented in stock car racing, especially during actual competition. The constant pedal work required during a stock car race, in addition to the isometric muscular activation (Falkner, 1972) of the neck, trunk, abdomen, and legs to counter against the acute exposure to gravitational (G) forces may not only contribute to an increase in metabolic heat (Brearley and Finn, 2007), but cardiovascular strain (Convertino, 2001). Allan and Crossley (1972) reported that elevated cockpit temperatures in military aircraft, which induced modest pilot core and skin temperatures, reduced grayout threshold (Grayout threshold is defined as the level of þGz acceleration when maintained for 15 s, caused the subject to lose peripheral vision for at least 5 s without losing central vision) by $1G. Stock cars, depending on the track, can reach between 3 and 4Gs for several hours. The cardiovascular strain associated with driving in hot conditions (Walker et al., 2001a) combined with exposure to G accelerations for several hours, may adversely affect driver ability. A consequence of this combination may possibly result in a lower blackout tolerance (Allan and Crossley, 1972) and greater likelihood of making a mistake on the track resulting in a poor performance, or even a life-threatening accident.

Thermoregulatory stress for motorsport athletes is likely increased due to the fact that: (1) drivers are subjected to a hot environment within the vehicle; (2) the moderate to high intensity work ($45–80% of VO2max during open-wheel racing on road and oval courses (Jacobs et al., 2002)) generates metabolic heat and; and (3) evaporative cooling is inhibited by heavy, protective clothing composed of a Nomexs fire-retardant suit, gloves, underwear, socks, boots, and a full helmet that drivers are required to wear. The increased metabolic expenditures of drivers coupled with prolonged heat exposure will ultimately result in elevated body temperatures and substantial sweat losses. In fact, Jareno et al. (1987) reported that during the Grand Premio de España at the Jerez Speedway in Spain where temperatures in the pitboxes averaged 31 °C and the relative humidity was 53%, two drivers were clinically diagnosed with heat stroke, despite electric cooling devices in their helmets (Jareno et al., 1987). In addition, V8 Supercar drivers competing in hot conditions experienced elevations in core temperatures to approximately 39 °C, despite the utilization of torso cooling shirts (Brearley and Finn, 2007), and kart racing drivers (Yamakoshi et al., 2010) have also experienced increases in core temperatures of 0.5 °C at the end of a driving session. Thermoregulatory responses to these different modes of motorsports driving in different scenarios have been documented, yet none have quantified the core and skin temperatures of drivers in stock cars during a sanctioned competition on an oval track, and without the use of any cooling devices, which are not typically used during this type of competition. Given all of these factors, it is reasonable to assume that motorsport drivers undergo significant thermoregulatory stress; however, this has not been quantified in competitive stock car racing. Moreover, the heat strain during summer months adversely affects performance. Anecdotally it has been noted that during June to September with the accompanying elevated ambient temperatures, there are increased wrecks at the end of the race (Fielden, 2007). Furthermore personal communications with infield car center staff state that during the summer months, there is increased number of drivers requesting intravenous fluids. Thus it stands to reason that during stock car racing there is increased thermal load that leads to performance impairment and possible increased risk of accidents. Accordingly, the purpose of this investigation was to quantify the thermoregulatory and physiological strain associated with stock car automobile driving during competition. There is a need to carefully quantify heat strain during actual NASCAR race competitions in order to accurately quantify the problem and thereby aid in the development of countermeasures.

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