Introduction The amount of metabolic energy spent for transporting the body mass over a unit of distance is defined as the energy cost of locomotion (C). C is the most appropriate way for quantifying the economy of progression and it is calculated from the ratio of metabolic power expenditure (E`) to the speed (v). In swimming, it has been usually investigated in athletes specialised in pool competitions. In long distance competitions (up to 25 km), the economy of swimming is particularly important, since the more economical athletes would swim at a smaller fraction of their maximal aerobic power sparing energy substrates whose availability may become the limiting factor of performance. The aim of our study was: i) to assess C in a group of female and male, open-water elite swimmers; ii) to evaluate the possible effect of fatigue on C. Methods Five female (24 ± 5.9 yy; 167 ± 1.6 cm; 59 ± 5.4 kg) and five male (28 ± 4.0 yy; 180 ± 4.8 cm; 77 ± 9.6 kg) elite long distance swimmers were studied. The subjects performed three experimental runs swimming 400-m in a 50-m-long pool at increasing speeds. Then, they swam a 2-km trial, simulating a long distance competition, at a velocity corresponding to 10-km race pace. At the end of this trial, C was assessed again (fatigue sessions, F) during three runs swum at the same speeds as those of the pre-fatigue (PF) sessions. C was calculated dividing the oxygen uptake at steady state (V`O2ss) by the average speed maintained during the 400-m run. V`O2ss was estimated by means of the "back extrapolation" method recording breath-to-breath oxygen uptake (V`O2bb) during the first 30 s of recovery after each run (Montpetit RR et al, 1981). Results C increased as a function of the speed both in female and male subjects and its values were comparable to those obtained previously, at similar speeds, in elite swimmers (Capelli et al, 1998). C after the 2-km training test increased, on the average, by 7% in male (0.05<P<0.1) and by 11% in female subjects (0.05<P<0.1) (Fig. 1). Stroke frequency (cycles per minute, CPM) increased by 2.1% and by 2.5% in male and in female subjects, respectively (Fig. 2) Discussion In swimming, the energy requirement is mainly dictated by the work spent to overcome hydrodynamic resistance and by the efficiency wherewith total mechanical work is transformed into useful work for propulsion (i. e. drag and propelling efficiency, respectively). Since drag is unlikely affected by fatigue, the reported increase of C after fatiguing exercise is probably due to a decrease of propelling efficiency. This is also suggested by the increase in stroke frequency found after 2-km swim. Indeed, the ratio of speed to stroke frequency yields the distance per stroke, a parameter which was suggested to be directly related with propelling efficiency in swimming humans (Craig and Pendergast, 1979).
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