Introduction Swimming represents a small proportion of total triathlon duration (18% to 10% from the shortest to the longest distances) and was therefore the subject of very few experimental studies (e.g., Delextrat et al., 2003). However, recent reviews on triathlon determinants highlighted that the metabolic demand induced by swimming could have detrimental effects on subsequent cycling or running adaptations (e.g., Bentley et al., 2002). Within this framework, one of the strategies currently used by triathletes in order to decrease swimming metabolic load is drafting (i.e., swimming directly behind another competitor). Bassett et al. (1991) have shown that swimming in a drafting position was associated with significant reductions in oxygen uptake (VO2,10%), heart rate (HR, 6.2%) and blood lactate concentration (LA, 31%). The objective of the present study was therefore to investigate the effects of drafting during swimming on energy expenditure in the context of a swim-bike trial. Methods Eight well-trained male triathletes underwent two sub maximal sessions conducted in a counterbalanced order. These sessions comprised a 15-min ride on a bicycle ergometer at 75% of maximal aerobic power and at a freely-chosen cadence, preceded either by a 750-m swim performed alone at competition pace (SAC trial), or by a 750-m swim performed in a drafting position at the same pace as during SAC (SDC trial). Blood lactate concentration was measured after swimming and cycling, and cardio-respiratory parameters (VO2, HR, expiratory flow: VE and respiratory frequency: RF) and pedal rate were continuously recorded during cycling. Physiological solicitation of cycling was assessed using oxygen kinetics analysis and energy expenditure was analyzed by gross efficiency calculation (GE: ratio of work accomplished to metabolic energy expended). Differences between SAC and SDC trials were analyzed using a Student t-test for paired samples. The level of confidence was set at P<0.05. Results The results show a significant decrease in swimming HR (-7%) and post-swim LA (-29%) when swimming in a drafting position. This lower swimming metabolic load in the SDC trial involved two main modifications during cycling: (1) a significant improvement in GE, and (2) a significant slowing down of the oxygen uptake kinetics at the onset of exercise (i.e., higher time constant) in comparison with the SAC trial (Table 1, P<0.05). Discussion/Conclusion The higher cycling GE in the SDC trial as compared to the SAC trial could be partly explained by the lower relative intensity observed during swimming in a drafting position, involving a lower solicitation of anaerobic metabolism, and probably a lower state of fatigue in the muscles of the lower limbs. Furthermore, the acceleration of oxygen uptake kinetics in the SAC trial as compared to the SDC trial could be related to an improved perfusion of the active muscle mass (Hughson et al., 1991), resulting on the one hand, from the high metabolic acidosis induced by the swim performed alone and on the other hand, by the significantly higher pedal rate adopted during cycling in this trial. This study suggests the relative importance of swimming condition and highlights the advantage of drafting during the swimming portion of a sprint triathlon.
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