Sprint triathlons: why your swim pace is critical

While long- and medium course triathlon events like Ironman and Olympic distances remain extremely popular among triathletes, the interest in shorter ‘sprint’ events – typically comprising of a 750m swim, 20km bike and 5 km run – has grown rapidly in recent years, no doubt fueled in part by the inclusion of mixed relays in the Olympic Games⁽¹⁾. Another reason is that training for and competing in a sprint-distance triathlon is a far less onerous and time-consuming affair, which makes it easier for those who have demanding or busy lives but still want to keep their hand in the sport.

The sprint dilemma

If you look at the proportion of time required on average to complete a sprint triathlon, broken down into a leg-by-leg basis, the actual time spent in the water is actually very short – typically 18 minutes or less for recreational triathletes and 15 minutes or less for more elite competitors⁽²⁾. Research also indicates that finishing the swim leg of a triathlon in among the leaders significantly improves the likelihood of a good final placing, because it enables better positioning and potential drafting during that all-important cycling leg⁽³⁾. Because of the short duration in the water, and the importance of exiting the swim leg among the leaders, the common belief is that triathletes need to swim that leg at maximal possible pace in to exit the water in the lead pack.

The problem is that such an aggressive strategy comes with a heavy physiological penalty; sprinting the swim leg creates large amounts of metabolic stress – for example, high levels of blood lactate. This cost is then carried through to the bike and run legs, and which can impair an athlete’s ability to cycle and run efficiently (meaning more oxygen is required to maintain a sub-maximal pace) and reduce power output⁽⁴⁾ Moreover, other studies have identified that the transition from the swim leg to the bike leg is a critical period where performance often suffers if athletes are excessively fatigued after the swim leg⁽⁵⁾.

All in all, while there’s a common belief that triathletes should swim the leg of a sprint triathlon with all guns blazing’, there’s evidence that this might be counterproductive. And even if a sprint-swim strategy might be best for some triathletes, there’s also little data on whether it applies equally to men and women. That’s because men and women exhibit distinct physiological and metabolic profiles. For example, recent research has suggested that female skeletal muscle tends to naturally have increased oxidative capacity as evidence by higher mitochondrial density and capillarization⁽⁶⁾. In plain English, female muscle fibers may have a natural bias towards endurance capacity whereas as male fibers seem to lend themselves more readily to high-intensity work. This could mean that a sprint-swim strategy for female triathletes is less of a problem than for males since female competitors are more able to draw on fat burning for fuel in the cycling and running legs.

New research

To try and get to the truth, a team of Brazilian researchers has investigated the impact of fast swim leg in a sprint triathlon, and the impact on subsequent cycling and running performance, as well as the overall race outcome. They hypothesized that intense swimming would impair performance overall, and also that the cycling and running legs would be less impacted by swimming intensity in women than in men⁽⁷⁾.

Twenty 20 well-trained amateur triathletes (12 men and 8 women) were recruited for the study, which involved five laboratory visits overall. On the first visit, each participant underwent body composition analysis using a highly accurate measuring technique known as Dual-energy X-ray Absorptiometry (DXA). Testing was also carried out to measure maximal oxygen consumption (VO2max) and maximal aerobic power (MAP - the highest power output an athlete can sustain using primarily aerobic metabolism, typically measured for about 4-7 minutes.

On the second visit, the researchers calculated each athlete’s ‘critical velocity’ (CV) – see figure 1. CV is the highest speed an athlete (in this case, while swimming) can theoretically maintain without exhaustion based on the linear relationship between distance and time. Importantly CV is known to be a reliable marker for setting training and racing intensities⁽⁸⁾. To help determine critical velocities for each participant, the athletes performed maximal 200m and 400m swim trials. In the final three visits, athletes performed three separate simulated sprint triathlons.

Figure 1: Example of critical velocity plot using runners