Thermoregulatory Responses of Wearing a Cycle Helmet
by Richard Stern email@example.com
One thorny issue that crops up in cycling circles is helmet use. The rules for bike racing state that all amateurs must wear helmets, while professionals only have to wear helmets in certain countries (e.g. Australia, Belgium).
However, cyclists often say that helmets increase body temperature and are uncomfortable. Therefore, they do not always wear helmets when training and often do not wish to wear them in races.
Previous research by Rasch et al., 1991 and Rasch and Cabanac, 1993 has shown that the head is able to lose heat during exercise induced hyperthermia. It has also been shown that cooling the head during exercise has a beneficial effect, whilst it is known that the head, neck and face are important areas for thermal comfort (Sheffield - Moore et al., 1997). Rasch and Cabanac (1993) demonstrated that covering the head with a wool cap during exercise prevents heat loss and evaporation and decreases performance.
Recent studies have examined the effects of wearing a helmet while cycling in hot dry environments (Gisolfi et al., 1988; and John and Dawson, 1989) and hot and humid environments (Sheffield and Moore et al., 1997). These studies have shown that wearing a cycling helmet does not affect thermoregulatory responses to submaximal exercise when cycling at intensities of between 60 and 70 percent of VO2max.
However, a paucity of data suggests that further research needs to be carried out as these recent studies used subjects who were well acclimatized to hot weather. Therefore, I hypothesize that subjects from cooler countries (e.g. England - where it rarely gets warm) might have different thermal responses while cycling in hot conditions with a cycle helmet.
This study, therefore, examined the effects of submaximal cycling in a hot temperatures while wearing a commercially available cycle helmet against not wearing the helmet.
Four competitive male cyclists volunteered to take part in the investigation. Subject selection was based on cyclists who had been training for more than two years and who competed regularly. Trials were conducted in the spring after subjects had become used to the ambient temperature. Subjects had previously completed maximal/peak oxygen consumption (VO2max) tests for an undergraduate dissertation (Sutton, 1998). Testing was performed in an environmental chamber at 35 degrees Celsius, 40% humidity. Subjects performed the test on a standard Monark Ergometer 616 in a randomized fashion.
In the helmet condition, subjects wore a Specialized Air Piranha Helmet (Specialized Bicycle Components, inc., Morgan Hill, California, USA). Standard helmet padding was used and padding did not obscure any ventilation ports on the helmet.
Subjects were asked to maintain their normal dietary habits and training regimens during the period of the study and to refrain from strenuous exercise the day prior to each trial. The day prior to testing subjects were asked to ingest one liter of water to ensure they were normally hydrated.
Subjects had their mass recorded nude and then with their cycling equipment (shoes, socks, cycling shorts, undervest). The same clothes were worn for each trial. Subjects were fitted with a Polar Sports Tester and a rectal thermometer. Skin temperature was recorded at five different sites (upper arm, chest, thigh, and lower leg). The fifth site was the superior region of the skull near the coronal suture. Subjects were then re-weighed with the added equipment.
The subjects rested in the environmental chamber for twenty minutes prior to the exercise to acclimatize to the temperature. Heart rate and VO2 were monitored and recorded at five-minute intervals, to ascertain baseline values.
The subjects were told to cycle for 60 minutes at approximately 65% of their peak aerobic power output, while maintaining about 90 rpm pedal cadence. Every ten minutes measurements of skin temperature, core temperature, rating of perceived exertion (Borg Scale), heart rate, and VO2 were collected. Heart rate was continuously monitored. A core temperature of 39.5 degrees Celsius or greater, or more than a two-degrees Celsius increase in core temperature terminated testing to prevent heat injury.
Upon completion of the 60-minute ride, subjects were immediately re-weighed with their equipment. The subjects then towel-dried their skin, and a post exercise nude mass was obtained. Temperature and heart rate were recorded at five-minute intervals for twenty minutes while subjects remained seated and rested.
Although the methodology stated that subjects exercised for one hour at approximately 65 percent of maximum minute power, all but one subject failed to do so. The last common minute for all subjects in both conditions was 40 minutes. Therefore, data are presented to 40 minutes. It was noted that in both conditions the environmental chamber was not able to maintain correct humidity and temperature in the exercise period. Humidity increased from about 40% towards 80%. The temperature fluctuated between 30 and 38 degrees Celsius. The chamber had to be opened occasionally to help regulate temperature and humidity.
There was no significant difference between rectal temperatures in the two conditions. However, in the helmet condition rectal temperature started from a lower temperature.
There was no significant difference for mean body temperatures in the two conditions. Body temperature peaked at a higher temperature in the no helmet condition.There was a tendency for heart rate to differ between conditions, with the helmet condition slightly lower. Heart rate differences approached significance. Peak heart rate for the no helmet condition was 177 bpm. while the helmet condition achieved a peak heart rate of 173 bpm.
There was no significant difference between VO2 for the two conditions. Mean values were almost identical between the two conditions. Oxygen uptake increased from approximately 65% VO2max at 10 minutes to approximately 77% VO2max at 40 minutes. There was a tendency for rating of perceived exertion (RPE) to differ between conditions. The no helmet condition produced a lower RPE which approached significance. Peak RPE for the no helmet condition was 18 compared 18.5 for the helmet condition.
There was a highly statistical significant difference between scalp temperatures in the two conditions. At rest, the two conditions were similar, however, as soon as exercise began the scalp temperature increased, with the magnitude of increase greater in the helmet condition, resulting in a higher peak scalp temperature. Peak scalp temperature was 36.13 degrees Celsius for the no helmet condition as opposed to 37.58 degrees Celsius for the helmet condition. Sweat rates were not statistically different between the two conditions.
In the present study, it was attempted to discern if a difference in physiological and psychological variables would occur when comparing submaximal cycling in hot conditions with and without a helmet. It was hypothesized that wearing a helmet might cause an increased heat strain in the helmet condition.
Previous research has produced equivocal results. Gisolfi et al. (1988) found that cycling in an environmental chamber at 33 degrees Celsius at 20% humidity at a workload that elicited 70% VO2max for 120 minutes did not affect any physiological or psychological parameters. The present study partially supports that of Gisolfi et al. (1988) in that apart from scalp temperature there was no difference. However, it is interesting to note that subjects in the Gisolfi et al. (1988) study were able to complete 120 minutes of cycling. In the present study, subjects generally fatigued to volitional exhaustion at approximately 40 minutes, by which time VO2 values had risen from about 65% to about 78% of maximal values.
John and Dawson (1989) found that cycling at 65% VO2max for 75 minutes in a hot temperature affected only scalp temperature in the helmet conditions which supports the present study. The authors also had a high subject drop out rate in both conditions conferring with the present study.
Sheffield and Moore et al. (1997) found no difference in cycling at 60% VO2max for 90 minutes in 35 degrees Celsius at 20% and 75% humidity with and without a helmet. The present results partially contradict this study as an increase in scalp temperature was noted along with tendency for rating of perceived exertion to differ between conditions. This study also found no difference in head temperature between conditions which contradicts the present study and that of John and Dawson (1989).
In the present study subjects fatigued at approximately the same time in both conditions, i.e. there was no difference between the two conditions. However, scalp temperature was increased, but this did not appear to affect other variables. Interestingly, there was a tendency for heart rate to be lower in the helmet trial. This is contrary to what would be expected.
The increased scalp temperature could have had a detrimental effect on sweat rate, by increasing plasma volume loss and therefore, producing a higher stroke volume and heart rate to counter what would have been a decreasing cardiac output (Astrand and Rodahl, 1986). Sweat rates were similar between conditions and averaged 1.2 liters an hour and 1.26 liters per hour for the non-helmet condition and the helmet condition, respectively. These figures are similar to those of Sheffield and Moore et al. (1997) and could explain the cardiovascular drift, resulting in increased heart rates and VO2 seen during both conditions.
In conclusion, cycling in a hot environment was not significantly affected by wearing a commercially available helmet, with the exception of scalp temperature. It is postulated that the helmet acted as a thermal barrier, causing an increase in scalp temperature, which is supported by John and Dawson (1989). There was a tendency for rating of perceived exertion to be raised in the helmet trial and this is supported by subjects reporting that they felt greater heat discomfort in the helmet trial.
As 3 of the 4 subjects stopped early (about 40 minutes) this suggests that the exercise intensity combined with the heat stress were of such a magnitude that the effects of wearing a helmet were overridden by these factors. The present study was limited in that only four subjects took part and the environmental chamber was not able to keep pace with the changes in humidity and temperature. Solar radiation was not taken into account in this study and might have increased the magnitude of differences between the two conditions (John and Dawson, 1989).
Further research is needed, with both heat and non-heat acclimatized male and female subjects who are both trained and untrained. Females are likely to have lower sweat rates and this might affect results (McArdle et al., 1991). Further, it is noted that all previous studies have examined the effects of helmets at relatively low intensities (about 60 and 70% VO2max). Cycle racing is likely to include much greater intensities for longer periods of time (Astrand and Rodahl, 1986) which might accentuate the differences between helmet and non-helmet wearing. In elite cycling, races often traverse mountain ranges (e.g. Alps, Dolomites) at low speeds lowering evaporation, causing increased thermal stress and further accentuating differences between helmet and non-helmet conditions.
Present and previous research, however, suggests that cycle helmets do not prove detrimental on most physiological parameters, with the exception of scalp temperatures. There is no scientific reason to not where a helmet.
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Richard Stern, South East England, coaches all forms of cycling (road, track, time trial, MTB, touring) and offers a consultancy/education service. Training programmes are based on scientific principles