After receiving verbal and written explanation of the purpose, procedures, and potential risks of the present study, 18 international level professional road cyclists gave their written informed consent to participate in this investigation. All subjects were members of the same cycling team and had finished at least one of the main professional 3-wk stage races (i.e., Tour de France, Giro d’Italia, or Vuelta a España). The main physical characteristics of the participating subjects are presented in Table 1. Values shown in the table were computed from each cyclist’s best laboratory test (i.e., that in which the highest maximal power output was attained). All experimental procedures were approved by the ethics committee of the Universidad del País Vasco.
Three to four weeks before the beginning of the race in which each TT was studied, cyclists performed an incremental maximal laboratory test on a mechanically-braked cycle ergometer (Monark 818 E, Varberg, Sweden) adapted with a racing saddle, drop handlebars, and clip-in pedals. Initial resistance was set at 110 W, and it was increased by 35 W every 4 min, with 1-min recovery intervals between workloads. Pedal rate was maintained constant at 75 rpm throughout the test. Subjects kept cadence with a metronome. Testing continued until the subjects were no longer able to maintain the required pedal rate. Heart rate was recorded every 5 s during the whole test (Sport Tester PE 3000, Polar Electro Oy, Kempele, Finland). Forty-eight hours prior to each laboratory test, subjects were instructed to refrain from strenuous physical training and to ingest a standardized food and fluid plan based on individual body mass. Training and nutritional logs were kept to assess subjects’ compliance with these requirements. No food intake was allowed during the 3 hr prior to testing.
Maximal power output (Wmax) was determined as the highest workload a cyclist could maintain for a complete 4-min period. When the last workload was not maintained 4 full min, maximal power output was calculated as follows (17):MATH 1 in which Wf is the value of the last complete workload (W), t is the time the last workload was maintained (s), and 35 is the power output difference between the last two workloads (W).
Based on individual laboratory data obtained in the last test prior to each race under investigation, a HR-power output relationship was computed for each cyclist from a linear regression equation. Each cyclist’s individual equation was used to estimate mean power output during actual TT racing from competition mean HR values. As in previously reported investigations, the HR-power output relationship was considered to be linear (3,23,27).
Immediately after completion of each workload, capillary blood samples (25 μl) were withdrawn from a previously hyperemized ear lobe (Finalgon, Laboratorios FHER, Barcelona, Spain) for the determination of blood lactate concentration ([La]), using an electroenzymatic technique with an automatic analyzer (YSI® 1500 Sport, Yellow Springs Instruments, Yellow Springs, OH). Following the recommendations of the manufacturer, the analyzer was calibrated before each test with standard solutions of known lactate concentrations (0, 5, and 15 mmol·L−1).
The exercise intensity corresponding to the onset of blood lactate accumulation (OBLA) was identified on the [La]-power output curve by straight line interpolation between the two closest points as the power output eliciting a blood lactate concentration of 4 mmol·L−1 (24). The lactate threshold (LT) was identified on each subject’s [La]-power output curve as the exercise intensity that elicited a 1 mmol·L−1 increase in [La] above average baseline lactate values measured when exercising at 40–60% of the maximal aerobic power output (11). Heart rate and power output values at OBLA (HROBLA and WOBLA, respectively) and LT (HRLT and WLT, respectively) were determined by straight line interpolation. Based on these values, two exercise intensity zones were considered: OBLAZONE and LTZONE, described respectively as HROBLA ± 3 beats·min−1 and HRLT ± 3 beats·min−1.
Competition time trials.
For the purpose of the study, competition TT were classified into five categories according to their distance, type of terrain, and racing format. Prologue TT (PTT) were the race opening TT, which are most often raced over flat terrain and cover distances no longer than 10 km. Individual TT raced over predominantly level terrain were classified as short (STT) and long (LTT) when the distance covered was shorter or longer than 40 km, respectively. Individual TT in which the altitude change between the start and the finish line was higher than 500 m were considered as uphill TT (UTT). Finally, team TT (TTT) were the TT races in which, instead of racing individually as in all previously described competition formats, all members of the team rode together. Table 2 indicates the professional races during which this investigation was carried out, their date of initiation, the number of cyclists studied in each of the races, as well as the distance of each TT.
In professional road cycling, not all TT are raced at the fastest possible pace. Indeed, due to team racing strategies and according to their specific role in competition (21), some cyclists ride all out in the competition TT (TTLIMIT), whereas some other cyclists choose, following the indications of the team coach, a more conservative approach (TTSTRATEGY). In fact, the only cyclists racing TT all out are those who have a real chance of winning the stage, are competing for a top position in the final overall standings, or are involved in the fight for the “best team” standings. This represents no more than 15–20% of the racers. Therefore, possible differences in the physical and physiological responses to these two approaches were studied.
In all studied competition TT, the HR of the cyclist was recorded every 5 s throughout the race using a HR monitor (Sport Tester PE 3000, Polar Electro Oy, Kempele, Finland). Recorded data were then transferred to a computer and analyzed with a computer program (Polar Precision Performance Software, Polar Electro Oy, Kempele, Finland). HR files including more than 5% of the values falling outside a physiological range of 50–220 beats·min−1 were excluded from the study. Moreover, files included in the computations were filtered to eliminate values falling outside the above-mentioned range using the correction function of the software (23). Competition HR values were expressed as a percentage of HR values determined in the laboratory test (%HRmax, %HRLT, and %HROBLA) using the following equation:MATH 2 in which 1, 2, and 3 are HRmax, HROBLA, and HRLT, respectively, HRT is the average HR during competition, and HRB is the basal HR (16). The latter was determined as the average of the HR values recorded immediately after awakening for the seven consecutive days prior to the laboratory tests with the cyclists still lying in bed.
The distribution of the exercise time during competition TT was analyzed in relation to the above-mentioned OBLAZONE and LTZONE exercise intensity indices. In addition, the training impulse (TRIMP) values, an integrative marker of the exercise load undertaken during competition, were estimated from TT competition HR measurements and race duration, using the following formula, as previously described by Banister (1):MATH 3 in which A is competition time (in min), B is [(HRT − HRB)/(HRmax − HRB)], and C equals 0.64·e1.92B.
Descriptive statistics are expressed as mean ± SD. ANOVA, followed by Fisher’s post hoc test, was used for all comparisons among different TT categories, as well as between TTLIMIT and TTSTRATEGY. Correlations between variables were computed from linear regression. The level of statistical significance was set at P < 0.05.
As shown in Table 1, the present group of cyclists attained Wmax values of 439 ± 45 W and 6.4 ± 0.3 W·kg−1 in the incremental laboratory test, and HRmax was 194 ± 5 beats·min−1. Submaximal laboratory power output and HR values (WOBLA, WLT, HROBLA, and HRLT) can also be seen in Table 1.
A comparison among the different TT categories (Table 3) showed that competition speed was significantly faster in TTT than in all other categories. PTT were raced significantly faster than STT and UTT, but not than LTT. UTT were also slower than STT and LTT.
Competition heart rate.
PTT elicited significantly higher mean HR, %HRmax, and estimated %Wmax than all other TT categories. These three variables were also different between STT on the one hand and LTT and UTT on the other hand. Mean HR was also statistically different between STT and TTT and between UTT and TTT. The latter group difference was detected in the %Wmax values as well (Table 3). Individual examples of raw HR profiles recorded during different competition TT categories can be seen in Figure 1.
When mean competition HR was expressed relative to HROBLA and HRLT, PTT values were significantly higher than STT, LTT, UTT, and TTT. %HROBLA values were also significantly higher in STT than in LTT and UTT, whereas %HRLT was higher in STT than in UTT (Table 3).
Figures 2 and 3 show the distribution of racing time during the different TT categories relative to OBLAZONE and LTZONE, respectively. During PTT, percentage time spent above (42 ± 28%) and at (40 ± 22%) OBLAZONE (Fig. 2) was significantly longer than during all other TT categories (20 ± 29% and 25 ± 18% during STT, 1 ± 2% and 14 ± 19% during LTT, 0% and 2 ± 3% during UTT, and 7 ± 6% and 21 ± 11% during TTT). Percentage cycling time above OBLAZONE was longer during STT compared with LTT and UTT. Moreover, relative time above and at OBLAZONE was longer in TTT than in LTT and UTT. When expressed in absolute values, the longest time above OBLAZONE was spent in STT (412 ± 612 s); this time was significantly longer than in LTT (32 ± 83 s) and UTT (0 s) but was not significantly longer than in PTT (234 ± 159 s) and TTT (310 ± 255 s). However, time at OBLAZONE in TTT (937 ± 469 s) was the longest compared with PTT (229 ± 129 s), STT (568 ± 451 s), LTT (589 ± 858 s), and UTT (97 ± 138 s), although only the difference with the latter reached statistical significance. As depicted in Fig. 3, 92 ± 7% of the racing time was spent above LTZONE in PTT, which was higher than in STT (71 ± 32%) and significantly higher than in LTT (51 ± 38%), UTT (47 ± 25%), and TTT (57 ± 35%). In absolute values, TTT required the longest cycling times above LTZONE (2539 ± 1550 s), followed by UTT (2140 ± 1221 s), LTT (2126 ± 1674 s), STT (1527 ± 742 s), and PTT (528 ± 92 s); the difference between TTT and PTT reached the level of statistical significance.
Differences between TTLIMIT and TTSTRATEGY are presented in Table 4. Average speed was consistently faster in the former in all TT categories, although significance was only reached in STT and LTT. Average HR in TTLIMIT was significantly higher in all TT categories, %HRmax was higher in STT, LTT, and TTT; and %Wmax was also higher in STT and UTT. Moreover, time spent above OBLAZONE in PTT, STT, and LTT was longer in TTLIMIT than in TTSTRATEGY, both in percentage and absolute values; the differences were statistically significant in the latter two.
Table 3 shows the amount of TRIMP corresponding to each TT category. TTT (146 ± 6) showed significantly higher values than PTT (21 ± 3), STT (77 ± 23), and LTT (122 ± 27). TTT values were also somewhat higher than UTT (129 ± 14), although this difference was not statistically significant. UTT values were significantly higher than PTT and STT values, and LTT had significantly higher values than PTT and STT.
Significant inverse relationships were observed between the duration of the TT and the average competition HR (r = −0.67), and between the duration of the TT and %HRmax values (r = −0.64). Competition speed, on the other hand, did not significantly correlate with exercise intensity-related variables such as mean HR (r = 0.34), %HRmax (r = 0.39), percent time above OBLAZONE (r = 0.31), and TRIMP (r = −0.03).
This investigation was undertaken to study the exercise intensity of road cycling TT performed during professional stage races. To achieve this goal, and in an attempt to attain a fair reflection of the actual physiological demands of professional road cycling, the study was carried out on a group of world-class cyclists (see the subjects’ physiological characteristics in Table 1, and the outstanding average race speeds in Tables 3 and 4) during some of the most important stage races in the world (Table 2). Considering %HRmax as a valuable index of exercise intensity (2,9,10,14,15,25), the present results indicated that professional road cycling TT were performed at quite high exercise intensity levels, as all of the studied events were raced above 76%HRmax. This average value is much higher than the 50% and the 60%HRmax at which level ground stages and mountain stages, respectively, are usually raced (Padilla et al., unpublished observations).
Several factors could have an influence on exercise intensity during competition TT, such as duration, whether they are raced individually or in a group, team strategies, and race profile. In the present study, there was an inverse relationship between exercise intensity estimated using average HR and %HRmax values and the duration of the TT. Thus, PTT were the events raced at the highest relative intensity (88–90%HRmax). This value was close to those reported by Foster et al. (7) for skating events of a similar duration (between 84 and 92%HRmax). It is worth mentioning that one of the PTT described as TTLIMIT, lasting 492 s, raced at an average speed of 49.7 km·h−1 and resulting in an event victory, elicited in the average 94%HRmax. STT were raced at an average value of 85%HRmax, which falls within the range of maximal steady-state values observed by Snyder et al. (25) for 30-min exercise tasks (83%, 86%, and 88%HRmax at 10, 20, and 30 min of exercise, respectively). Palmer et al. (22), on the other hand, reported an average HR value of 90.3 ± 2.4%HRmax during a 20-km TT simulation in the laboratory. Moreover, the same authors reported average values of 94%HRmax for a 16.6-km field TT (23). The discrepancy between these and the present STT results could be explained by the longer duration of the latter (16–20 km vs 28 km) and by the influence of team strategies on TT intensity. Indeed, TTLIMIT values measured during STT reached 88%HRmax, much closer to those values previously reported (22). More to the point, 27-km and 36.5-km STT were respectively raced at 93%HRmax (Fig. 1A) and 90%HRmax by individual cyclists in this study, resulting in a stage victory and a top-three position.
LTT events, which had a duration of about 60 min, were raced at exercise intensity values that were in keeping with those previously published for 40 km. Hopkins and McKenzie (13) reported an estimated power output of 359 W, whereas Hoogeveen and Schep (12) reported a power output of 347 W, estimated using the laboratory HR-power output relationship. Using the same method, an average power output of 347 W was estimated in the present investigation for LTT (average distance 54 km). Coyle et al. (5) reported, for 1-h laboratory simulations, power outputs of 346 and 311 W in elite and national level cyclists, respectively. Lindsay et al. (19) observed an average intensity of 326 W (90%HRmax) during a 40-km TT simulated in the laboratory. Because of the variability in exercise conditions (laboratory setting, field test, and amateur or professional competition) and type of resistance that the athlete must overcome (fixed resistance in most laboratory settings vs variable resistance in the field), it is difficult to compare the exercise intensity values and to justify the differences between the present and previously published data.
Although, according to race distance and duration, TTT were similar to LTT, the fact of racing in a group instead of individually makes this TT format a unique competition event. Average intensity during TTT was higher than during LTT and UTT, despite a similar duration. As well, the average speed attained in TTT was significantly higher than in all individual TT categories (Table 3). This was undoubtedly due to the recovery periods allowed by the drafting strategies used by all cycling teams during TTT, as shown by the fluctuations in individual HR values during this type of event (Fig. 1C). The graph shows repeated HR peaks and falls, which are indicative of increased and reduced exercise intensity periods. These periods, however, did not imply changes in competition speed because they corresponded to periods during which a cyclist was, respectively, leading the group or drafting behind his teammates (23). HR values during TTT were therefore much more irregular than during individual STT (Fig. 1A) or LTT (Fig. 1B).
UTT was the competition category eliciting the lowest HR response, even in TTLIMIT events. Even though these HR values as well as estimated power output values were not significantly different from LTT, the average speed was remarkably slower in UTT. This discrepancy between speed and markers of physiological demands was mainly due to two factors: i) UTT were in the average 13% longer in duration than LTT; ii) during LTT, which are raced on a predominantly level terrain, a high percentage of the power exerted by the cyclist is converted into linear displacement, quantified as speed. Meanwhile, in UTT, part of this power is used to perform gravitational work, which depends on the cyclist’s body mass and is not quantified as linear speed (6,21,26).
Thus, the average speed attained during competition TT does not seem to be a fair marker to estimate exercise intensity (15) because speed of locomotion in cycling depends on multiple factors, such as the type of terrain, environmental conditions, or physiological and anthropometrical characteristics of the cyclists (6,21,26). Indeed, the fastest average competition speed in the present study was attained in TTT, but the average exercise intensity, as indicated by %HRmax and estimated power output, was lower than in the next fastest events (i.e., PTT and STT). In addition, quite low correlations were found between speed on the one hand, and such markers of the physiological demands of exercise as %HRmax, time spent above OBLAZONE, or the amount of TRIMP.
As can be inferred from the discussion above, it appears to be very difficult to estimate TT competition intensity on average race speed values, or even on a single physiological marker of exercise intensity such as %HRmax. Exercise physiologists often attempt to describe how “hard” an exercise task is using such physiological markers, but “hardness,” in the case of professional road cycling TT, should integrate not only physiological markers of competition intensity, but other variables such as the duration of competition as well. Other important but less easily quantifiable variables include environmental conditions and race profile. The TRIMP (1) could be considered as an integrative marker of exercise load. According to this unit, TTT were the “hardest” TT category of all (Table 3), despite the drafting strategies that allow intermittent periods of recovery. The absolute amount of time spent at and above OBLAZONE, which has been considered as a hard exercising zone (9,10), was indeed longest in TTT (20 min), followed by STT (17 min), LTT (10 min), PTT (7.5 min), and UTT (1.5 min). As a general rule, the organizers of professional road cycling races choose undulating race profiles for STT and level roads for LTT in an attempt to balance hardness by means of the type of terrain in the former and by duration in the latter.
The percentage racing time spent at the different metabolic zones described in relation to OBLAZONE (Fig. 2) and LTZONE (Fig. 3), and the average HR values in relation to HROBLA and HRLT (Table 3), indicate that for TT shorter than 30 km (duration below 30–40 min), such as PTT and STT, HROBLA could be a fair metabolic index to establish appropriate training and competition intensities. HRLT, on the other hand, would be a useful tool to indicate suitable intensity levels for TT lasting ≥60 min, such as LTT and UTT of the present investigation. However, the fact that PTT and STT were, respectively, raced at average values of 100% and 95%HROBLA, and UTT, LTT, and TTT at 101%, 103%, and 105% of HRLT, does not imply that the different TT categories were raced at steady states at the corresponding reference intensities, as shown by the detailed analyses of the time distribution in relation to each intensity zone, both in relative and absolute values.
In summary, exercise intensity was not fairly reflected by the average race speed in competitive road cycling TT. Markers such as %HRmax, TRIMP, or the time distribution in relation to the different metabolic zones described using HROBLA and HRLT seemed to be more accurate indicators of the physiological demands of this cycling competition format. Other variables, including duration, whether racing is individual or in a group, team strategies, and race profile can also influence the physiological demands of the race. In TT events lasting ≤30 min, HROBLA could be a valuable metabolic index to determine appropriate training and competition pace. For TT lasting ≥60 min, suitable intensity levels would be fairly indicated by HRLT.
This investigation was supported by a research grant from IBERDROLA.
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