The anaerobic energy contribution in short and middle-distance swimming is fundamental for higher level performances, but its analysis per sex remains unstudied. Thus, we have determined the AOD of male and female swimmers at the 50, 100, and 200 m front crawl distances, probably some of the most relevant events of the Olympic Games. The current study reported three unique findings: (i) absolute and relative AOD values increased from 50 to 200 m front crawl trials for both sexes, with female swimmers exhibiting higher values than males; (ii) AOD values at 10 and 30 s elapsed times in each distance-trial decreased for both sexes (concurrently with velocity reducing as distances increased), with higher values for female swimmers than males in the 200 m trial; and (iii) LBM and AOD values are inversely related for all distances, probably due to the differences in body composition between sexes.
Both groups showed time dependence for the AOD profile from 50 to 200 m front crawl trials, which is aligned to previous studies reporting AOD for swimming performances at 140, 127, and 108% V̇O2max (Troup et al., 1992) and for bouts lasting 30, 60, and 120 s (Ogita et al., 2003 and Peyrebrune et al., 2012). In these pioneer studies, conducted only with male swimmers, the AOD profile reaches its maximal hypothetical value of 3.2 LO2 when performing at 108% V̇O2max (Troup et al., 1992) or either during exhaustive trials performed at 100–110% vV̇O2max lasting 120–180 s (Ogita et al., 2003). In the current study, the mean absolute AOD values (in litres) for both sexes during the 200 m are close to the aforementioned maximal reference, and therefore, we evidenced that male and female swimmers only differed in AOD response when body mass differences are not taken in account.
Regarding the differences between sexes for AOD, only Ogita et al. (1996) reported the maximal AOD was higher for male (53–61 ml×kg− 1) than female swimmers (43–53 ml×kg− 1), and stated that AOD reaches its maximum value in swimming performances lasting 2–3 min, at a metabolic rate of 110% V̇O2peak. Although these results for maximum AOD values between sexes are unique in swimming, they are in line with the trend of values presented in other sports (Spencer and Gastin. 2001; Hill and Vingren, 2014; Weber et al., 2006). Despite the maximal AOD assessment is not in the scope of the current study, the AOD values for male and female swimmers during 200 m correspond to an overall energy demand approaching 107% V̇O2peak and 115% V̇O2peak (respectively), therefore the current data suggest that the 200 m might be an adjustable distance to demand the maximal AOD in female swimmers, while for males a longer distance at such metabolic rate would be advisable.
However, AOD increases over time, but its contribution to total V̇O2demand reduces as the period of swimming time is elongated, with values lower for male swimmers than females respectively in the 50 (65 vs 73%), 100 (47 vs 53%), and 200 m (32 vs 39%). These observed AOD rates of contribution are close to those for 30, 60, and 120–180 s reported by Ogita et al. (2003) and for 30 s tethered swimming bouts reported by Peyrebrune et al. (2012). Curiously, those rates for AOD contribution are also similar to those observed in cycling during 30, 60, and 120 s (60, 50, and 35%, respectively; Medbo and Tabata, 1989), and running the 400 (59% and 55%) and 800 m events (40 and 30%) for male and female participants (Duffield et al., 2005). Hence, these reports support the notion that the balance between AOD and V̇O2Ac in swimming is attained between 100 to 200 m, regardless of sex.
The results also showed that LBM related negatively to AOD (for each distance), V̇O2demand, and to the slope of V̇O2 vs velocity adjustment. While the LBM correlation to AOD reduced when controlled for sex-specific body composition, the V̇O2Ac, V̇O2demand, and the slope remained unchanged, enhanced, or became evidenced. These correlations suggest that swimmers with the greatest LBM can perform each distance with the highest oxidative energy release, which is supported by the lower V̇O2Ac and higher AOD for female swimmers when compared to males in each distance.
Studies relating muscle tissue mass to metabolic response also consider that the largest is the muscle mass engaged in exercise, the lowest is the intensity of fibre activation, reducing the glycolytic demand and increasing the oxidative energy supply (Beneke et al., 2001). Regarding sex-specific lean mass regionalisation, larger upper-limb muscle mass provides less peripheral restriction and higher O2 muscle extraction, increasing V̇O2Ac (Withers et al., 1991; Weber et al., 2006). Therefore, the current study corroborated the influence of muscle mass on AOD values, considering that sex-specific response is mandatorily lower among swimmers with greater LBM while performing trials at similar %vV̇O2peak during the same distance.
However, the findings that the maximal AOD values differed between males and females in running and cycling have been attributed to the specificity of sport demand and conditioning level, with reduced interference (4%) accounting for the sex difference regarding active muscle mass (Hill and Vingren, 2014). Nevertheless, the sex-specific amount of active muscle influences peak oxygen deficit in leg-cycling (Weyang et al., 1993). Furthermore, large muscle mass engagement also supports the findings that anaerobic capacity measured by the Wingate (30 s “all-out”) test with leg and arm ergometers differed between sexes only for upper-limbs, supporting the statement that body weight does not account for sex differences, exceptionally for body regions with distinct LBM distribution (Weber et al., 2006).
For the current study, both statements suggest that differences in body weight and LBM between sexes were associated with AOD increase in 50, 100, and 200 m. However, body composition differences between sexes neither constraints the absolute AOD values nor the rate of oxygen deficit (e.g., AOD measured at fixed elapsed times) during each bout, suggesting that female swimmers have higher AOD demand than males while performing supramaximal trials regardless of the distance and duration of the effort.
Regarding the slope, which is a rate relating V̇O2 to submaximal velocities, it is an index of the oxygen cost for the increment in exercise intensity (Green and Dawson, 1995; Pessôa Filho et al., 2017), which accounts for more than half of the variance in the AOD estimation (Medbo et al., 1988). In the current study, the difference between sexes for the slope was about 8.8%, with female swimmers showing higher values than males. The observed difference in slope between sexes might be considered not too large, when comparing with other studies in swimming that reported slopes differing from 13 to 25% between sexes (Fernandes et al., 2005; Pessôa Filho et al., 2017).
In general, male swimmers possess higher slope values than females when reported in absolute terms (i.e., ml×min− 1), which is a feature of high hydrodynamics drag during higher velocity for men (Fernandes et al., 2005). However, the current slope values were presented relative to body mass, and hence best aligned to the notion that higher cost for female swimmers might suggest a premature demand upon less economical fibre types while exercise intensity increases (Pessôa Filho et al., 2017), and therefore also aligned to the increased oxygen deficit and reduced exercise tolerance at such a high intensity condition (Weber et al., 2006). Notwithstanding, the effect of low LBM on O2 availability to muscles at high-intensity exercise (Falz et al., 2019) is also aligned to the notion of the earliest recruitment of fast-type fibres, and hence explains the inverse relationship of slope and V̇O2Ac with LBM observed in the current study. In addition, the difference between sexes regarding body fat content was also reported to have an effect on energy cost of running during middle-distance maximal performance (Hill, 1999).
A limitation could be related to the number of trials applied to the AOD assessment, which is not similar to the former AOD assessment. However, studies with similar procedures have been applied satisfactorily for submaximal steps during incremental tests for the assessment of AOD in swimming, running and kayaking (Reis et al., 2010; Duffield et al., 2005; Li et al., 2018). Also, the number of trials and the exercise intensities (e.g., %V̇O2peak) applied in the current study is in accordance with the recommendation to avoid the non-linearity of the slope and affect the robustness of the extrapolation of the V̇O2demand (Noordhof et al., 2010). The use of the New Aquatrainer® to measure the V̇O2Ac might be considered another limitation to reproduce physiological demand ecologically if considering the delays of the actual swimming velocity as an effect of turning and gliding constraints (Chaverri et al., 2016). However, the swimmer is enabled to stroke at a maximum rate when required, and therefore, task impairments with the New Aquatrainer® would not affect the muscle mass engagement, as well as the level of exertion while swimming (Ribeiro et al., 2016).
In conclusion male and female swimmers performed 50, 100, and 200 m with similar relative pacing above maximal aerobic velocity, but required different AOD values. The higher relative AOD values per unit of body weight in female rather than male swimmers might be explained by the sex-specific LBM content, since LBM has shown an inverse effect on all estimates for the assessment of AOD (i.e., V̇O2demand, V̇O2Ac, and slope). Although LBM plays a role in distinguishing the AOD response between sexes, future studies should better understand the influence of body composition on anaerobic conditioning after training planned to increase regional and total lean mass.