Sprint triathlons: why your swim pace is critical

The red plot shows a critical velocity curve (in runners) for different race durations. Shorter races permit higher critical velocities. The green line shows the critical velocity at which (in theory), these runners are not forced to reduce their pace over a very long distance. A critical velocity curve must be calculated separately for each athlete (in the study above, it was calculated for all the triathletes).

Each trial consisted of a 750m swim followed immediately by a 20km cycle and then a 5km run. In all three trials, the athletes were told to complete the cycling and running legs as fast as possible, and the conditions were identical apart from the intensity of the 750m swim. These were as follows:

·        3% slower than CV

·        Exactly at CV

·        3% faster than CV.

The order of the three trials was randomized in order to reduce the risk of bias and in all trials, constant measurements were taken of the athletes’ heart rates, levels of blood lactate, ratings of perceived exertion (RPE), subjective muscle pain, and shortness of breath. Figure 2 sums up the trial protocol used.

Figure 2: Trial protocol

What they found

The key finding was that the male and female triathletes responded quite differently to different swimming intensities. When the females swam at a 3% slower pace than their CV, their triathlon race times overall were significantly slower compared to swimming either bang on CV or 3% faster than CV. In essence, pacing themselves a little more slowly in the swim leg didn’t result in a big enough payback during the bike and run legs to claw back the lost time during that slower swim leg. In fact the reverse was true; pacing the swim leg at high intensity bought the women more extra time than they lost in the bike and run legs as a result of extra fatigue, resulting in a faster time overall.

Contrast these findings to the male triathletes. For them, there was no significant difference in the total race time across the three swimming-leg intensities. In other words, when pacing the swim leg more slowly, the men seemed able to make good the lost time by maintaining a sufficiently high pace in the bike and run to ensure their total time remained stable. When the men swam the swimming leg faster, they experienced higher blood lactate and greater perceived exertion - but any drop in pace across the bike and run legs was not big affect to change the total race time. In short, any lower or extra effort in the water neither helped nor hindered the final clock time. Figure 3 sums up these findings in graphical form.

Figure 3: Swim leg speed vs. total triathlon time for men and women

Red plot = female triathletes; blue = make triathletes. Intensities 1, 2 and 3 = swim leg at -3% CV, exactly CV and +3% of CV respectively. When the females paced the swim leg slowly (at -3% of CV), their overall race times (shown in seconds) were significant higher (ie slower) than when they paced it at CV or +3% of CV. This trend was NOT seen in the men who performed the same overall regardless of swim leg pace.

Practical implications

The authors proposed a number of explanations as to why these findings differed between men and women. One is that women can often able to sustain a higher percentage of their maximal aerobic capacity for longer periods than men⁽⁹⁾. This might explain why women in this study didn’t suffer a significant slowdown in the bike and run legs after a fast swim; their bodies were better equipped to handle the subsequent high-intensity effort without a significant drop in efficiency. When the women swam slowly however, the time loss created in the water might have been harder for them to claw back compared to men. This is likely the case since men (generally) have the ability to generate higher absolute power outputs during the cycling stage, which therefore creates more opportunity to mitigate the time loss of a slower swim.

Either way, the take-home message for female triathletes is clear – don’t hold back in the swim! The time you lose by swimming at a slower, more comfortable pace is unlikely to be clawed back by and improved cycling or running performance. Instead, the women should aim to swim at or slightly above CV. A faster swim time could allow you to be positioned well to draft in the cycling leg without disproportionately fatiguing the legs for the bike and run stages.

Male triathletes however can afford to be more flexible with the swim leg pace since swimming 3% faster or 3% slower than CV did not seem to significantly change the total finish time. However, you might need to make more of a tactical choice; if you’re a strong swimmer, will likely be confident swimming at or above CV to secure a spot in the lead draft pack, which will help later on. Conversely, if you’re a weaker swimmer, and find yourself in between two packs of swimmers, you may be better off settling into a pace at or slightly below CV, and then using your extra capacity to generate power to catch up during the bike and run legs.

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