Automobile racing is one of the largest sports worldwide with a viewing audience equal to football (soccer), yet unlike football there are less than 30 peer-reviewed publications examining the physiological stress placed on the competitors (1–5). In fact, it was only in the 21st century that racing drivers were considered athletes in the scientific peer-reviewed literature (2,6). The limited number of studies related to motorsport physiology has shown that racing drivers compete at HR of 65% to 80% of their maximum (3,7–9) for 1 to 4 h depending on the race regulations. The increase in HR is due to an associated increase in oxygen consumption (7), emotional arousal (10), skin blood flow and sweat loss (8,11) due to a 2°C to 3°C increase in core body temperature (1).
While being exposed to physical stress, racing drivers must precisely maneuver a race car at speeds in excess of 273 km·h−1. Drivers are required to steer and brake the race car multiple times a lap necessitating high skeletal muscle strength to pilot the vehicle. Adding to the strength requirements drivers must hold themselves in the racing seat while being exposed to three to four times the force of gravity (1,3). If the skeletal musculature is not able to withstand the associated braking, steering, and gravitational loads fatigue can cause performance to decline, or the driver to crash the vehicle (1). Therefore, successful racing drivers must be aerobically fit, strong, fatigue resistant, and acclimated to hot environments (3).
A characteristic of automobile racing that is different from many sports is that female drivers compete alongside male drivers. In the last decade there has been an increase in female participants (12). The sport performance literature suggests that female athletes may experience fluctuations in skeletal muscle strength (13–15), cutaneous blood flow, and onset of sweating during thermal stress (16–19) due to hormone fluctuations associated with the menstrual cycle. As strength and thermoregulatory capability directly influence success in automobile racing (1,3) an important research question is, “does the menstrual cycle influence the physiological response to automobile racing?”
Further to this, social and popular media have questioned women’s (but not men’s) ability to compete safely under these physiologically challenging conditions (20). Specifically, it has been suggested that women may fatigue more quickly than men, causing their performance to deteriorate sooner and as such may present a risk to the safety of other racing drivers (20). These comments regarding women’s greater fatigability do not appear to be based on data from peer-reviewed scientific investigations (11) yet they have the potential to negatively influence the representation of women in motor racing. It is important that decisions regarding driver safety are made based on hypothesis-driven, empirical evidence. Although the physiological characteristics and capabilities of males and females have been compared in other athletic domains (21–24), the relative physiological responses of males and females to race car driving have not been documented.
Therefore, the purpose of this study was to examine the physiological response of male and female (during the follicular and luteal phase) racing drivers in two different classes of automobile racing. A major limitation of the literature on racing driver physiology is that all drivers are grouped into a single category of “race car driver,” despite there being a variety of racing series that stress drivers differently. Specifically, race cars where the driver is enclosed in the vehicle (closed cockpit cars) can produce greater thermal strain compared with open cockpit cars where the driver is exposed to the ambient environment (3,8,11). As such, car structure was considered as a covariate in this investigation.
This study was approved by the Institutional Review Board at Michigan State University. Before the start of testing, the protocols were explained and the participants provided an up-to-date institutionally approved health history and informed consent. All procedures performed were in accordance with the ethical standards established by the 1964 Declaration of Helsinki and its later amendments.
Study participants were licensed (IndyCar license for open cockpit cars and Sports Car Club of America Pro license for closed cockpit cars) male and female race car drivers. Participants reported their age, height, and weight from their annual physical to obtain a racing license. During the course of the investigation, female participants completed a menstruation journal which included date and length of menses to obtain a valid characterization of their menstrual cycle (25). Furthermore, female participants recorded their basal body temperature using a commercially available under-tongue thermometer before getting out of bed (13). This information was used to determine which phase of the menstrual cycle (follicular or luteal) the female participants competed in, during automobile races. None of the female participants were taking oral contraceptives and all had a normal monthly cycle.
To obtain a representative sample we recruited race teams that had one male and one female driver for both open and closed cockpit series. Thus, three race teams were recruited for the closed cockpit cars and three race teams recruited for the open cockpit cars. The open cockpit cars had a similar structure for all competing drivers in the series. whereas the closed cockpit cars raced a variety of GT4 race cars from different manufacturers (i.e., Porsche, Acura, and BMW). To limit variability induced by the structure of the closed cockpit race car, teams that raced the BMW M4 GT4 race car were recruited.
Participants competed in either open cockpit or closed cockpit cars. The open cockpit drivers competed in the US F2000 series. Female (n = 3) and male (n = 3) comparison drivers competed on the same team driving similar race cars. The closed cockpit drivers competed in the Pirelli World Challenge Series (PWC). Like the open cockpit drivers, in this series male (n = 3) and female (n = 3) teammates competed in similar cars.
Race venue data collection
Data collection was completed over three separate occasions for both the open and closed cockpit drivers during the summer season. Both racing series competed on road courses consisting of left and right hand turns. Table 1 provides the characteristics of each course.
Physiological responses to race car driving
Physiological responses to driving were recorded during races using a lightweight ambulatory monitoring system around their chest (Equivital EQ02; Hidalgo Ltd., UK). Participants wore the Equivital Life Monitor under their racing attire. For the female drivers the Equivital Life Monitor did not interfere with the sports bra. The Equivital Life Monitor was put on an hour before the start of race when the drivers put on their racing suit. At this time drivers consumed 10 mL of water per kilogram body weight to ensure drivers were hydrated at the start of the race. The drivers did not consume fluids while inside the race car. The Equivital Life Monitor system recorded HR (bpm), breath rate (BR; ventilations per minute), and skin temperature (Tsk;°C). Core temperature (Tcore;°C) was obtained from an ingestible pill sensor (VitalSense, Mini Mitter, Philips Respironics, The Netherlands) that transmitted data to the Equivital Life Monitor at 15-s intervals for storage. Pill sensors were ingested 3 h before the start of the race, according to established guidelines to ensure reliability and validity of measurement (26).
All variables were recorded without interference from electrical or communication systems within the race car and there was no faults in data collection. A percentage of the driver’s maximum HR was calculated from the driver’s age predicted peak HR (peak HR = 220 − age). Physiological Strain Index (PSI) is a measures of heat stress and was calculated using Tcore (adjusted for core pill as opposed to rectal temperature) and HR in the following equation (27):
In equation 1, Tcoret and HRt are simultaneous measurements taken at any time during the data collection period and Tcore0 and HR0 are the initial measurements taken once the racing attire was put on but before entering the race car (27).
Physiological responses to motor racing were divided into green flag (racing) and yellow flag (caution) laps. Furthermore female drivers were divided into the follicular or luteal phase of the menstrual cycle. A repeated measures (race venue 1–3) ANOVA with the main effects of car (open vs closed cockpit), flag (green vs yellow), and phase of menstrual cycle (follicular vs luteal vs male) with an alpha level of 0.05 set a priori compared differences in HR, BR, Tsk, Tcore, PSI, and percent of maximum HR achieved. If significant differences were identified a Tukey Honest Significant Difference (HSD) post hoc test was run. All statistical procedures were conducted in JMP Pro v.13 (SASS, Carry, NC). All values presented are means ± standard error.
Female and male racing drivers were evaluated at three different races. Participant characteristics are shown in Table 2. The duration of the race and length of the race track were similar for all participants evaluated. There was a nonsignificant main effect of race venue in the ANOVA model (P > 0.05) and as such, all race data were pooled (all open cockpit races pooled together and all closed cockpit races pooled together). Our experimental design aimed to measure female drivers at three races to allow for evaluation of physiological responses during the follicular and luteal phase of the menstrual cycle. Despite our best efforts we were unable to collect data on open cockpit female drivers during the follicular phase. Open cockpit female drivers were evaluated on day 24.0 ± 2.0 (luteal phase) of the menstrual cycle. Closed cockpit female drivers were evaluated on days 8.5 ± 3.5 (follicular phase) and 22.0 ± 4.24 (luteal phase) of the menstrual cycle.
Physiological responses are presented during racing conditions (green flag laps) and caution periods (yellow flag laps, when race car speed was reduced). Yellow flag conditions elicited a lower HR compared with green flag conditions (Fig. 1A; P < 0.001) as well as a lower BR (Fig. 2; P < 0.001) except among closed cockpit female drivers in the luteal phase.
Differences by Menstrual Cycle
During closed cockpit green flag racing, females had a 7% lower (Fig. 1A; P < 0.001) HR in the follicular phase, compared with the luteal phase; HR during yellow flag laps was not different between follicular and luteal phases (P > 0.05). When expressing HR as a percent of age predicted maximum (Fig. 1B), the trends described above persisted (P < 0.001).
In closed cockpit cars the luteal phase elicited a 15% higher (P < 0.001) BR compared with the follicular phase during green flag laps (Fig. 2). During yellow flag laps, BR during the luteal phase was 37% higher (P < 0.001) than in the follicular phase.
Under green flag conditions, the Tsk in the luteal phase was 2% higher (Fig. 3A; P < 0.001) than in the follicular phase in the closed cockpit cars. During yellow flag conditions Tsk in the luteal and follicular phases were not statistically (P > 0.05) different from each other.
In the closed cockpit cars during green flag conditions the follicular phase resulted in a 1% lower (Fig. 3B; P < 0.001) Tcore compared with the luteal phase. During yellow flag conditions, female driver core temperatures increased by 0.8% from green flag conditions, but there was no difference (P > 0.05) between the follicular and luteal phases.
Physiological strain index
In the closed cockpit cars the luteal phase elicited a 10% higher (P < 0.001) PSI compared with the follicular phase (Fig. 4) during green flag conditions. This difference in menstrual cycle phase was not observed during yellow flag conditions, under which there was an 11% decrease (P < 0.001) in PSI for the luteal phase. There was no difference (P > 0.05) in PSI between green and yellow flag conditions for the follicular phase.
Differences by Sex
There was no difference in HR between male and female open cockpit racing drivers (Fig. 1; P > 0.05) during green flag laps. During yellow flag laps male drivers’ HR was 11% lower (P < 0.001) than that of female drivers. During closed cockpit green flag racing, females in the follicular phase had a 3% lower (P < 0.001) HR compared with male racing drivers. The HR of female drivers in the luteal phase and male drivers were not different from each other (P > 0.05). However, during yellow flag laps in closed cockpit cars, HR during the luteal phase was 5% lower (P < 0.001) than that of male drivers. When expressing HR as a percent of age predicted maximum (Fig. 1B), the trends described above persisted.
During green flag conditions females had a higher BR compared with the males (Fig. 2; P < 0.001) for both open (18% higher compared with males) and closed (52% higher compared with males) cockpit cars. When under yellow flag conditions males had a 35% lower (P < 0.001) BR compared with the females in the closed cockpit cars. However, male open cockpit drivers had a 36% higher (P < 0.001) BR compared with female open cockpit racing drivers during yellow flag laps.
During green flag conditions the male open cockpit drivers had a 2% higher (Fig. 3A; P < 0.001) Tsk compared with the female open cockpit drivers. During yellow flag conditions, there was no difference in the skin temperature of male and female open cockpit drivers (P > 0.05). Green flag laps in the closed cockpit race cars resulted in the females having an 8% higher (P < 0.001) Tsk as compared with the male drivers. During yellow flag conditions, the luteal and follicular phase elicited a 2% higher (P < 0.001) Tsk than males.
During green flag racing there was no difference (P > 0.05) in Tcore between male and female drivers in the open cockpit cars. During yellow flag conditions, Tcore increased (P = 0.001) 1.5% from green flag conditions for female drivers, whereas it did not change in the male open cockpit drivers. In the closed cockpit cars Tcore was not different (P > 0.05) during green flag laps between male and female drivers in the follicular phase. Male drivers had a 0.6% lower (P = 0.001) Tcore compared with females in the luteal phase. During yellow flag laps the male drivers’ Tcore was 1.4% lower (P = 0.001) than the female drivers.
Physiological strain index
There was no difference (Fig. 4; P > 0.05) in PSI between male and female drivers in the open cockpit cars during green flag conditions. Male drivers had a 32% lower (P < 0.001) PSI during yellow flag conditions as compared with the female drivers. In the closed cockpit cars during green flag racing the males had a 12% higher (P < 0.001) PSI, as compared with females in the follicular phase, with no difference (P > 0.05) in PSI between male drivers and female drivers in the luteal phase. During yellow flag conditions in the closed cockpit car there was no difference (P > 0.05) in PSI between the male and female drivers.
Differences by Car
Green flag laps in open cockpit cars elicited a 6.2% higher (P < 0.001) HR among drivers when compared with closed cockpit drivers. However, during yellow flag laps the closed cockpit racing drivers had 19.9% higher (P < 0.001) HR than the open cockpit drivers. The race car structure (closed vs open cockpit) did not elicit a difference in BR (Fig. 2; P > 0.05). However, the closed cockpit cars yielded a 0.6% higher Tsk (P = 0.006; Fig. 3A) and 1.8% higher Tcore (P = 0.001; Fig. 3B) as compared with the open cockpit cars. The closed cockpit car elicited an 11% higher PSI compared with the open cockpit cars (P < 0.0001, Fig. 4).
Since 1888 automobile racing has included female participants who have competed alongside males, and in the past decade there has been a surge in female driver competitors particularly in the developmental series (12). Following this, there have been main stream and social media conversations questioning whether females should participate with males. These conversations are sensationalized and geared toward public conversation and not the basis for hypothesis-driven research. However, in the last 2 yr these conversations have questioned “if females are more physiologically fatigable than their male counterparts.” Speculation has centered on the female menstrual cycle leading to a more pronounced thermal stress and skeletal muscle weakness which could produce a decrease in performance and increase the risk of a crash. To date, there have been no scientific investigations into this topic. It is crucial to evaluate the physiological stress of male and female drivers to ensure that any racing regulations regarding safety and racing participants are based on scientific evidence. As such, we compared the physiological responses of female and male racing drivers.
The nature of automobile racing makes controlled studies difficult because the race venue, mechanical problems with the car, and incidence of a crash can hinder data collection (11). We were able to minimize these variables by selecting male and female racing drivers from the same race team. We ensured that the race cars were as identical as possible. Therefore, one car was not more difficult to drive than the other due to different mechanical components/engineering set ups for competition. Additionally the thermal stressed induced by the engine and structure of the vehicle was the same by utilizing cars from the same team. Specifically, in PWC (closed cockpit) racing teams may use different race cars (Porsche, BMW, Acura) with varying engines which could produce more heat inside the cockpit.
Previous research on the physiological responses to automobile racing has typically evaluated noncompetitive practice sessions or only one racing event (7,8). This has been shown to hinder reproducibility of data, making evidence-based decisions regarding driver safety and performance difficult (1). Therefore, racing drivers in this investigation were evaluated at three different racing venues which had similar race duration and track layout.
These controls were applied to both open and closed cockpit racing drivers. It is necessary to evaluate open and closed cockpit cars separately as open cockpit cars typically have more down force and lack power-assisted steering and braking, requiring greater skeletal muscle strength to hold the body in the driving position while piloting the vehicle (1,28). Open cockpit drivers are exposed to thermal strain from the insulating fire protective race suit however, there is greater thermal strain placed on closed cockpit drivers due to limited air flow and heat dissipation inside the race car (3,8).
Closed cockpit cars elicited a higher Tcore and Tsk as compared with open cockpit cars (Fig. 3). The female closed cockpit drivers in the luteal phase had a higher Tsk compared with the males. Yet, interestingly the luteal phase for the open cockpit drivers elicited a lower Tsk compared with the male counterparts. This discrepancy potentially indicates that the structure of the race car has a stronger influence on the thermoregulatory response than menstrual cycle.
The menstrual cycle has been shown to influence thermoregulatory responses in previous sport performance literature (29). Specifically, the increase in progesterone in the luteal phase is associated with an increased resting ventilation, Tcore and threshold for cutaneous vasodilation and sweating (16–19,30). As such, during the luteal phase, women potentially have more pronounced heat storage compared with the follicular phase (29) which could increase the challenge of performance under heat stress (driving a race car). Kuwahara et al. (16) evaluated the thermoregulatory response of physically trained and untrained women during a 42°C heat challenge (16). In the untrained group, the luteal phase demonstrated an increased Tcore and threshold for cutaneous vasodilation and sweat rate (16). However, in the trained group there was no difference in Tcore, onset of cutaneous vasodilation, and sweat rate between the follicular and luteal phase. In fact throughout the heat challenge the trained women had a lower Tcore, onset of cutaneous vasodilation and sweat rate, regardless of phase of the menstrual cycle. In the Kuwahara study there was no difference in total sweat loss between trained or untrained women and phase of the menstrual cycle (16). Therefore, although the menstrual cycle has been shown to influence thermoregulatory responses, any potential impairment in sport performance could be mitigated by physical conditioning.
The menstrual cycle’s influence on ventilation is uncertain; data indicate that the luteal phase is associated with increased minute ventilation, potentially due to elevated progesterone leading to increased central and peripheral chemosensitivity to hypercapnia and hypoxia (30). Yet, during exercise the observed hyperventilation appears to be due to an increased Tcore resulting in increased minute ventilation and tidal volume to aid in cerebral cooling (30). Closed cockpit female drivers demonstrated an increase in BR (Fig. 2) during the luteal phase which persisted during the yellow flag laps compared with males. During yellow flag laps work to drive the car is reduced but thermal load inside the cockpit remains elevated due to the radiant heat from the drive train as well as reduced air flow through the cockpit to cool the driver. As such, this observation provides support to the suggested mechanism of increased ventilation aiding in body cooling.
In closed cockpit racing, under green flag conditions, the luteal phase elicited a significant yet minimally higher Tcore than the follicular phase and male drivers. There was no difference in Tcore during green flag laps for open cockpit drivers. To address driver fatigue as a function of thermal stress we calculated PSI, which accounts for baseline HR and Tcore to yield an arbitrary value indicating thermal stress (9,27,31). The literature has shown that a value of 6 or higher is considered “highly stressful” (31). Green flag racing yielded a PSI (Fig. 4) above 6 for both male and female drivers. Differences in male and female drivers were seen in the closed cockpit cars only where PSI during the follicular phase was less than during the luteal phase and that of male drivers. Thus, the menstrual cycle does elicit a higher thermal strain during the luteal phase, but the strain is not greater than that experienced by male drivers. Interestingly, PSI in yellow flag conditions remained elevated in the closed cockpit cars (whereas it decreased in drivers in open cockpit cars), potentially reflecting the minimal heat dissipation of the closed cockpit cars.
Thermal stress is only one component of the physical stress placed on racing drivers. Metabolic demand of contracting skeletal muscle to pilot the car also contributes to the elevated HR observed during green flag laps (3,7,10). The open cockpit cars elicited higher HR (Fig. 1) compared with closed cockpit cars which can be partially explained by the open cockpit cars not having power-assisted steering or brakes, requiring drivers to generate more skeletal muscle force (increased metabolic demand) to pilot the vehicles. When the open cockpit cars reduced speed (less metabolic demand) during yellow flag laps, HR decreased.
Media speculation has suggested that the luteal phase of the menstrual cycle reduces skeletal muscle strength compared with the follicular phase (13), and has led to the question of whether female drivers fatigue more in open cockpit cars that lack power-assisted steering and braking. The literature on strength during the menstrual cycle is far from conclusive with studies indicating (i) no difference in strength between phases, (ii) the luteal phase eliciting higher strength than the follicular phase, and (iii) the follicular phase eliciting higher strength than the luteal phase (13–15). Typically, females have less skeletal muscle mass than males, but force per fiber cross-sectional area is not different (32), thus at this time, conclusions cannot be made regarding how the menstrual cycle influences skeletal muscle strength and performance in automobile racing.
In all sports, success is often defined by winning. In automotive racing, female drivers have won races and championships, yet less often than their male counter parts. There are far fewer female competitors which may offer an explanation for this discrepancy, but another suggestion for this difference is that women experience a greater level of physiological fatigue. Our data do not indicate greater physiological fatigue in female drivers. However, consistent with the observation that male racing drivers have won more championships and races than females; in all races evaluated in this study, male drivers had faster lap times than females. The characteristics of the drivers in this study show that male drivers had nearly twice as many years of racing experience than female drivers. This difference in driving experience provides a logical explanation for the differences in lap times. An additional 10 yr of developing racing skills would increase performance (1).
This study shows that race car structure produces more thermal stress than the menstrual cycle. A limitation of this investigation is that, despite our best efforts, we were unable to collect data from open cockpit drivers during the follicular phase. However, we obtained data on the luteal phase in open cockpit drivers and both the follicular and luteal phase in closed cockpit drivers. The luteal phase is when popular media have suggested fatigue will be greater among female drivers, and our data allow us to address the validity of this suggestion. We chose to proceed with reporting these data instead of endeavoring to collect data on the missing phase from another race series or during a different year, as such factors could confound the data. Despite this limitation, this is the first investigation of its kind in automobile racing and it provides a foundation for future studies on this topic.
An additional limitation is that we did not measure hormone levels to confirm phase of the menstrual cycle. The methods we used to determine phase of cycle are valid (13) but we acknowledge measuring hormone concentration is ideal. In our initial study design, we aimed to collect blood samples from participants during the race event to confirm hormone concentration; however, the medical staff at the track did not have the man power to aid us in sample collection and expressed concerns with nonrace track medical personnel collecting blood. This resulted in the sanctioning body officials requesting that the study did not involve the collection of blood samples.
The results of this study suggest that race car structure and years of experience influence thermal stress and performance to a greater extent than the menstrual cycle. Previous suggestions that women experience greater physiological fatigue than males due to the menstrual cycle are not supported by the data in this study. Research on motorsport physiology is limited and more work is needed to enable racing sanctioning bodies and organizations to make evidence-based decisions for the benefit of the sport and the competitors. We hope this study can encourage collaborations between racing organizations, racing teams and academics to support the generation of valid information needed to inform the progression of the sport of automobile racing.
The authors wish to thank the members of the Spartan Motorsport Performance Lab and Human Energy Research Lab at Michigan State University. Funds for this study were provided by the Michigan State University Start-Up Funds. The authors also thank the drivers, engineers, medical staff, public relationship personal and officials in U.S. F2000 and PWC for their generosity in helping with this study. A very special thank you is reserved for Anne Roy in aiding in completion of the study.
The authors do not have any conflicts of interests related to this study. The results of the present study do not constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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