Cross-country is the most popular mountain-biking event, and it was introduced as an official Olympic sport during the 1996 Atlanta Summer Games. Cross-country is a mass-start endurance competition characterized by off-road circuits with continuous climbs and descents on gravel roads and field trails. Off-road cyclists usually use bikes with front suspension to decrease muscular stress on arms and legs as several isometric muscle contractions are necessary to absorb shock caused by terrain conditions, and bike handling and stabilization (20). The International Cycling Union (UCI) suggests an optimal competition winning time of 105–135 min. The UCI calendar includes up to 260 international cross-country competitions every year. Off-road cyclists usually compete at least once a week for 9 months a year, for a total of 30–40 competitions, sometimes in the form of short stage races.
Despite its increasing popularity, to the best of our knowledge, there are no studies that have analyzed the exercise intensity profile of cross-country races. The only physiological investigations of mountain biking are on the anthropometric and functional characteristics of off-road cyclists (1,24), the influence of different suspensions system on the cost of cycling (11,15,20), and the energy cost of noncompetitive off-road cycling in low-level athletes (12). However, there are in the literature several studies describing the exercise-intensity profile of road cycling (4,10,17,18,19).
The exercise-intensity profile can be useful to understand the physiological demands of these particular cycling competitions. From a practical point of view, this information could also help to design proper training programs. Furthermore, as many coaches include some races as a part of training, the exercise-intensity profile could be useful to understand the training load imposed on athletes. The aim of this study was to quantify and describe the exercise intensity of cross-country races monitoring the heart rate (HR) responses of a group of high-level mountain bikers during four different competitions.
The subjects of this study were nine mountain bikers, six under 23 and three elite (UCI categories). The athletes of these two categories compete together in the same race, or at different times but on the same tract. The subjects involved in this study were informed of its aims and procedures, and they gave their written consent to participate. This study was approved by the Ethical Committee of Institute of Biomedical Technologies and was carried out in agreement with the Policy of Italian National Research Council. All the subjects have familiarity both with the laboratory setting and the physiological tests utilized in this study.
The athletes were tested two times during the season: winter and summer. Each testing session was within 2 wk of the corresponding two winter or two summer competitions. After the prediction of % body fat using a skin-fold technique (7), they performed an incremental maximal exercise test on an electromagnetically braked ergometer (SRM-Science, Welldorf, Germany). The position adopted by the cyclists on their own bicycle was reproduced on the ergometer. The test started at 100 W, and the power output was increased 40 W every 4 min. Subjects were asked to keep the cadence at 96 rpm (range 94–98). The test was terminated upon voluntary exhaustion or incapacity to maintain a cadence above 90 rpm for more than 10 s. The choice of this arbitrary cadence was based on our previous experience with this group of mountain bikers who prefer a high rather than a low cadence. This is in agreement with the results of Takaishi et al. (23), who showed that well-trained cyclists prefer higher pedalling rate to minimize neuromuscular fatigue. Furthermore, in a previous investigation on the physiological profile of off-road cyclists, a similar cadence (90–100 rpm) was used (24). During the test, athletes were verbally encouraged by the lab technicians and the coach of their team.
If the work rate was not completed, the peak power output (PPO) was calculated using the formula of Kuipers et al. (8):MATHwhere Wf is the last completed workload, and t is the time in seconds of the uncompleted workload.
Expired respiratory gases were measured using a breath-by-breath automated gas-analysis system (Vmax29, SensorMedics, Yorba Linda, CA). Flow, volume, and gases were calibrated before each test. HR was recorded every 5 s with an HR monitor (Vantage NV; Polar Electro, Kempele, Finland).
In the last 15 s of every workload, capillary blood samples (25 μL) were collected from ear lobe and immediately analyzed using an electroenzymatic technique (YSI® 1500 Sport, Yellow Springs Instruments, Yellow Springs, OH). Before each test, the analyzer was calibrated following the instructions of the manufacturer.
From blood lactate curves, the lactate threshold (LT) and the onset of blood lactate accumulation (OBLA4) were calculated for each rider. LT was defined as the intensity that elicited a 1-mmol·L−1 increase in blood lactate concentration ([La]) above values measured during exercise at 40–60% of V̇O2max (6). OBLA4 was identified as the intensity corresponding to an [La] of 4 mmol·L−1 (21). HR, power output (PO), and V̇O2 at LT (HRLT, POLT, V̇O2LT) and at OBLA4 (HROBLA4, POOBLA4, V̇O2OBLA4) were identified by straight-line interpolation between the two closest points to LT and OBLA4. From these data, three intensity zones were established (5) to describe the exercise intensity profile of cross-country competitions:
1) EASYZONE for intensity below HR corresponding to LT;
2) MODERATEZONE for intensity between HR corresponding to LT and OBLA4; and
3) HARDZONE for intensity above HR corresponding to OBLA4.
The individual relationship between %V̇O2max and %HRmax was also determined.
During the competitions, HR was recorded every 5 s using HR monitor with an individually coded HR transmitter (VantageNV). After every race, HR data were downloaded on a portable PC using the specific software and subsequently analyzed.
Field data were collected during the winter and the summer competitive period. During the winter period, two international races (both classified as E2 by UCI) were studied. During the summer period, an international race (E1) and the Italian Championship were studied. Some of the best riders of the world participated in the three international races.
Descriptive data are presented as mean ± standard deviation (SD). Due to the small sample size, nonparametric tests were used for data analysis. To compare the intensity profile of the four races and the individual lap data (HR and time) collected during the third and fourth competition, a Friedman test was used. Post hoc analysis was conducted using Wilcoxon paired tests. To compare the winter and summer testing sessions, a Wilcoxon paired test was employed. The level of statistical significance was set at P < 0.05. For the statistical analysis the software package STATISTICA® (StatSoft, Inc., Tulsa, OK) was used.
Five of the nine mountain bikers completed all four competitions. The choice to include in the study only these five subjects was to reduce for intersubject variability and to increase statistical power. Their anthropometric and functional characteristics are shown in Table 1. The results of the summer and winter testing session were not statistically different. This was expected because athletes were well trained for both competitive periods.
In the first and second race, all athletes had to cover a part of the track walking instead of pedalling for the extremely muddy terrain. This is a typical condition in winter mountain-bike races. In these two races, ambient temperature was respectively 12°C and 15°C, weather was cloudy, and the walking distance was respectively 0.9 km (3% of the 33-km race length) and 0.8 km (2% of the 40-km race length), respectively. The two summer races (third and fourth competitions) were conducted in hot, sunny weather conditions (29°C and 32°C, respectively).
Table 2 shows the characteristics of the four different races and the average exercise intensity expressed as %HRmax, %V̇O2max, and percentage of race time (relative time) spent in the three intensity zones. Absolute time spent in the EASYZONE was 27 ± 16 min. Absolute time spent in the MODERATEZONE was 75 ± 19 min, and in the HARDZONE was 44 ± 21 min. Relative and absolute times spent in the EASYZONE during the first summer competition (race 3) was significantly lower (P < 0.05) than the two winter competitions (race 1 and 2), whereas relative and absolute time spent in the HARDZONE during race 2 was significantly lower (P < 0.05) than both the other winter race (race 1) and the two summer competitions (race 3 and 4). No other parameters were statistically different between races. Overall, no clear effect of season was found. Therefore, the differences found are likely to be related to the specific characteristics of the different circuits. The average HRmax (191 ± 6 beats·min−1) recorded during the competitions was not significantly different from the average HRmax recorded in the lab (192 ± 5 beats·min−1). Therefore, we decided to calculate exercise intensity as percentage of the HRmax measured in the field. Figure 1 shows the total race time spent at various exercise intensities for both %HRmax and %V̇O2max.
During the third and the fourth race (summer races), the time spent in each lap was recorded. HR and time of each lap during these competitions are presented in Figure 2. Statistical analysis showed a significant increase in lap time during the third (P < 0.05) and the fourth (P < 0.01) competition. On the contrary, average lap HR showed a significant decrease as the races progressed (P < 0.05 in the third race;P < 0.01 in the fourth).
To our knowledge, this is the first study describing the exercise-intensity profile of mountain-biking cross-country competitions. The HR method utilized in this study had been used by several authors to describe the physiological demands of road cycling (4,10,17,18,19).
The group of mountain bikers studied have similar V̇O2max compared with the athletes of the United States National Off Road Bicycle Association investigated by Wilber et al. (4.99 L·min−1) (24). Relative to body weight, the maximum aerobic power of our group is even higher than the one recently reported by Baron (1) in a group of elite Austrian mountain bikers (68.4 mL·kg−1·min−1). The high V̇O2max of our subjects not only confirms their high level but further suggests that cross-country competitions, like road cycling races (9,17), require athletes to possess high aerobic power. Also the maximal power found in this group of athletes is in agreement with the values shown in the studies of Wilber et al. (5.9 W·kg−1) (24) and of Baron (5.5 W·kg−1) (1).
Coaches and mountain bikers empirically refer to cross-country as an intense activity, for which a near maximal effort is necessary at the start of the race. This study confirms this widespread opinion. In fact, the average HR of the four competitions was 90 ± 3% of HRmax that corresponded to 84 ± 3% of V̇O2max (Table 2). This exercise intensity is similar to short road cycling time trials (TT) (17) but higher than on-road cycling stages of longer duration (18). For example, the mean HR found by Padilla et al. (18) in professional cyclists during semi- and high-mountainous stages (mean duration of 302 ± 57 min and 355 ± 67 min, respectively) was 58 ± 6% and 61 ± 5% of HRmax. On the other hand, the average exercise intensity of cross-country competitions lasting 147 ± 15 min is similar to the exercise intensity reported by Padilla et al. (17) for TTs lasting 10 ± 2 min and 39 ± 11 min (89 ± 3% of HRmax and 85 ± 5% of HRmax, respectively). However, the data reported by these last authors were average data and also included mean HR of cyclists not specialized in TTs. Well-motivated TT specialists can maintain exercise intensity similar to the off-road cyclists of this study for a longer time than the TTs reported by Padilla et al. (17). In fact, Lucia et al. (10) showed that a professional cyclists’ winner of a TT during the Tour of France could sustain 70 min at an intensity above 90% of V̇O2max.
Another way to describe the exercise intensity profile of cycling competitions is the use of lactate thresholds determined in laboratory tests, such as LT or OBLA4 (17,18). In our study, 44 ± 21 min was spent in the HARDZONE (>OBLA4). This value is higher than the one reported by Padilla et al. (18), who investigated road cycling events of longer duration (>302 min). In these cycling events, absolute time spent at and above OBLA4ZONE (similar to the HARDZONE of this study) was 13 ± 14 min, during semi-mountainous stages, whereas in high-mountainous stages it was 16 ± 21 min. The time spent in the HARDZONE during cross-country events studied is even higher than the time spent at and above OBLA4 (9 ± 8 min and 7 ± 10 min, respectively) during short TTs (17).
Taken together, these results suggest that cross-country competitions are conducted at higher intensity than road stage races. This can be easily explained by the much shorter duration of cross-country competitions compared with on-road stage races. The importance of duration is also underlined by the fact that exercise intensity expressed as percentage of V̇O2max found in this study (84 ± 3%) is similar to the exercise intensity found in other endurance activities of similar duration. For example, Sjodin and Svedenhag (22) reported an exercise intensity between 80 and 86% of V̇O2max in good and elite runners during marathon races of about 128–157 min. However, the fact that exercise intensity of cross-country lasting more than 2 h is similar to TTs lasting no more than 70 min suggests that factors other than duration may influence exercise intensity during cross-country competitions.
First, the relative lower speed, larger tires, bad terrain conditions, and continuous climbs and descents that characterize cross-country competitions require off-road cyclists to spend the most part of their effort against the force of gravity and presumably greater rolling resistance compared with on-road cycling (16). In addition, reducing energy expenditure using the draft of others cyclists (13) does not greatly influence mean exercise intensity because situations in which drafting could be useful (high speed and group cycling) are less frequent than in on-road cycling.
Second, during mountain biking, intense and repeated isometric contractions of arm and leg muscles are necessary to absorb shock and vibrations caused by the terrain, and for bike handling and stabilization. Because isometric muscle contraction significantly increases HR response to submaximal cycling (3), this may partially explain the higher mean HR during off-road cycling compared to on-road cycling. However, Seifert et al. (20) showed that the use of front suspension bikes similar to the ones used by our athletes significantly reduces the mean HR during a flat looped course with fabricated bumps compared with rigid bikes, whereas V̇O2 was not different between the two conditions. In addition, the data presented by Cable (3) suggest that during submaximal cycling the absolute and relative HR increase induced by isometric contraction is smaller at higher workload. Although a significant influence of isometric muscle contractions on the high HR response during cross-country competitions cannot be excluded, the results of Seifert et al. (20) and Cable (3) suggest that their influence in our study was minimized by the use of front suspension bikes and because of the high power output during off-road cycling (11).
The strategy used in cross-country competitions can also partially explain the high exercise intensity found in this study. Figure 3 shows the HR raw data of one subject during the fourth race. From this trace, it is possible to note a decrease of HR during the race and that maximum HR is reached soon after the start. In fact, in cross-country competitions, the start has a fundamental importance to the strategy of the whole race. The athletes try to start in the first position to avoid slowing down when the road narrows and to enter in the single-track trails in a good position because on these tracks overtaking can be difficult. In fact, a “starting loop” is added at the start of many cross-country competitions to spread out the riders in this initial part of the course. This allows the best cyclists to start in the front positions. For these reasons, the mountain bikers are used to beginning the competition at very high exercise intensity. This was confirmed by our field laps data, collected during the summer races (Figure 2), which showed a significant decreased of average lap HR and a significant increased of lap time. In a similar analysis, Mognoni et al. (14) found a different HR profile during cross-country ski competition. In fact, they found a decreasing velocity in the last lap of the race, corresponding to an increase of the HR. According to these authors, this cardiovascular drift could reflect an increased energy cost or a higher HR due to dehydration. In our study, the third and fourth race was performed in hot conditions, but despite this, we found a lowering of HR during the race. The reasons for this discrepancy are not clear at present, and further research is needed to clarify the causes of fatigue during cross-country races.
The knowledge of cross-country competitions’ exercise-intensity profile can be used by coaches as starting point to design effective training programs. For practical purposes, Figure 1 allows a better description of the time distribution of HR and estimated V̇O2 expressed relative to HRmax and V̇O2max. Our results can also assist coaches who are used to introducing competitions of minor importance as a part of the training program to quantify the training load imposed to their athletes. Despite the increasingly popular use of power output to prescribe and describe exercise intensity in endurance sports, we believe that HR monitoring remains the most practical tool for coaches and athletes. Furthermore, during prolonged exercise, HR is linearly related to V̇O2 whereas power output is not (2). This suggests that HR is a better indicator of exercise intensity during endurance training and events compared with PO.
In conclusion, this study shows that off-road competitions are conducted at very high intensity. The physiological profile of this group of athletes and the duration of these events (>120 min) suggest that cross-country races require high aerobic power. However, the start strategy and the long time spent in the HARDZONE suggest that cross-country competitions also require efforts in which anaerobic pathways play a significant role. The exercise-intensity profile described in this study can be used by coaches to prescribe training programs specific for mountain bikers involved in cross-country competitions.
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Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
HEART RATE; LACTATE THRESHOLD; MOUNTAIN BIKE; RACES