The subject descriptive data are presented in Table 1. During the laboratory testing, the heart rate at the ventilatory threshold occurred at 83.3 ± 2.5% of VO₂max with no differences between the men and women. The women completed their six kilometre competition trial in 23.28 minutes (± 0.56) and the men completed their 10 kilometre competition in 34.4 minutes (± 0.61).

There were no gender-related differences in measured maximum heart rates) from TR HR, COMP HR or maximum heart rates obtained during the GXT (Table 2).

However, TR HR (207 ± 5 b·min^-1; presented as pooled data for all subjects) and COMP HR (206 ± 4 b·min^-1) were significantly higher than maximum heart rates obtained during the GXT (194 ± 2 b·min^-1) (p = 0.035 and p = 0.021, respectively).

The primary finding of this experiment is that the maximal heart rate measured during a treadmill graded exercise test is considerably lower than the maximal heart rate obtained during either competition or during endurance interval training in distance runners. These data suggest that the exercise setting in which heart rates are measured should be considered when maximal heart rates are used for prescribing training intensity.

Similar to the present findings, Boudet et al., 2002 observed that 13 of 16 male endurance athletes attained higher maximal heart rates in a field or competition setting than in a laboratory setting, and that the intra-individual variation in maximal heart rate was ± 6 beats·min^-1. Furthermore, Vergès, Flore and Favre-Juvin (2003) observed that the blood lactate concentration at a given heart rate during a field test was higher than during a laboratory setting. Therefore, a laboratory based value of maximal heart rate may not be the optimal choice for designing a training program for enhancing endurance performance.

Rather than prescribing an exercise intensity based on a percentage of maximal heart rate for optimal endurance performance adaptations, other submaximal measures of heart rate could be used. For instance, there is the Conconi method (Conconi, 1982) for identifying the HRDP which may accurately reflect the lactate threshold in some athletes. Hofmann et al., 2001 further clarified the issue of the application of heart rate for prescribing an exercise intensity for endurance athletes when they observed that in many athletes, the heart rate - workload curve can be irregular and must be evaluated carefully in order to correctly identify which measure of heart rate (e.g. percentage of maximal, or HRDP) should be used. Finally, Boudet et al., 2004 observed that during high intensity running, such as during competition or intense training, it may be better to prescribe the running intensity based on running speed rather than heart rate. Thus, the utility of heart rate for optimizing training adaptations in athletes requires very careful consideration and evaluation of many factors and measurements, and not simply a single measurement of maximal heart rate.

The present observation that the maximal heart rate measured in a laboratory based treadmill exercise test is considerably lower than the maximal heart rate measured during training or competition is of considerable importance for athletes and coaches when designing a training program. For instance, if the laboratory based maximal heart rates from the present investigation were used for prescribing an exercise intensity of 85% of heart rate reserve (American College of Sports Medicine, 1990) the intensity would be only ~79% of the heart rate reserve based on the training or competition maximal heart rates. Similarly, if the laboratory based maximal heart rates were used to identify the ventilatory or lactate thresholds for training purposes, the athletes would be exercising at a heart rate that does not correspond to the lactate threshold during training or competition situations (Zhou et al., 1997). The considerably lower training intensity based on treadmill maximal heart rate compared to training or competition based maximal heart rate could reduce the adaptations to training and impair performance (Potteiger and Weber, 1994).

A possible reason for the athletes obtaining higher heart rate maximum levels during the high-intensity interval training day and during competition when compared to the treadmill GXT in a laboratory setting could be due to differences in temperature during test conditions (Potteiger and Weber, 1994). In this regard, temperature is always a controlling factor for chemical reactions and since heart rate is a function of chemical processes, when temperatures increase upward from 21 degrees Celsius, heart rate increases in a correlative manner by about one beat per minute. In the case of laboratory testing, the temperatures during the testing were controlled at between 23 and 25 degrees Celsius whereas during training and competition, the temperatures were not as controlled or measured and may be much higher than during laboratory testing resulting in increased heart rate levels. However, the temperature in the training facility (23.9 degrees Celsius) was very similar to the temperature in the laboratory, so it seems unlikely that differences in temperature would account for the differences in maximal heart rate. For the same reasons as increased temperature, a higher relative humidity will increase maximal heart rate. Further, during training and competition, runners typically lose over one kilogram of water per hour. This results in blood volume decreases and less blood is pumped by the heart per beat. Specifically, for every 1% loss in body weight due to dehydration, heart rate increases by approximately seven beats per minute (Lambert et al., 1998).

The duration of the interval training session (~30 minutes, including warm up) was considerably longer than the duration of the typical treadmill test (~12 minutes). Thus, the cardiovascular drift (Frangolias et al., 2000) associated with the longer exercise times in the training and competition situations could be partially responsible for higher maximal heart rates observed in these conditions. However, the duration of the interval training session was between the time of competition for the women (~23 minutes) and the men (~34 minutes). Furthermore, the interval training session consisted of ~5 minutes of warming up at a low intensity followed by ~25 minutes of discontinuous exercise, both of which are unlikely to contribute to cardiovascular drift in a similar way as does continuous intense running, such as during the competition (Zavorsky et al., 1998). In the present investigation, the maximal heart rates attained during the first intense interval run were not different from subsequent intervals, and the maximal heart rates during the interval training were not different from the maximal heart rates during competition in spite of the more prolonged, intense effort required during competition. When evaluating the data from competition for each athlete the heart rate would occasionally increase by ~4-5 beats·min^-1, presumably due to a burst of speed to pass a competitor or due to changes in terrain, and then decrease by the same amount, thus making it difficult to identify a slow component of cardiovascular drift. Therefore, the lack of difference between the maximal heart rates in the training and competition setting suggest that the differences in maximal heart rate between the laboratory and non-laboratory settings is not due to cardiovascular drift.

It is likely that the increased maximal heart rates observed during the high-intensity interval training day and during competition are due to an increased state of arousal during the uncertain and exciting training session and competitive meet. This uncertainty and excitement during training and competition could have then result in a heightened cardiac responsiveness (Johnston et al., 1990). The sympathetic nervous system plays a strong regulatory role in the increases in heart rate that occur during exercise (Kawada et al., 2006) and engaging in competition or high intensity training are strong activators of sympathetic nervous activity (Ruttkay-Nedecky, 1980). Simulated competition has been demonstrated to elicit oxygen consumption and heart rates higher than those observed during laboratory fitness testing (Foster et al., 1993). Also, those who participate in competitive orienteering at international levels achieve higher heart rates than those at the club or local level (Bird et al., 2003). As such, the present data suggests that the level of psychological stimulation presented in a laboratory environment does not effectively mimic the stimulation level during intense training or competition.

• A percentage of maximum heart rate is commonly used to prescribe and measure exercise intensity. However, maximum heart rate may be greater during competition or training than during laboratory exercise testing.
• Heart rates during training and competition were significantly higher than maximum heart rates obtained during laboratory exercise testing.
• To optimize training-induced adaptation, training intensity for NCAA division 2 distance event runners should not be based on laboratory assessment of maximum heart rate, but instead on maximum heart rate measure obtained either during training or during competition.