In some competitive sports like rowing and swimming, an interesting phenomenon occurs in the generation of the propulsive force. That is, the push-off cannot be made against a fixed object but against water, which will give way. The propelling force is thus generated by giving masses (M) of water a velocity change (L::.v). These water masses acquire a kinetic energy change (?i M • .t.if) and, as a result, consume part of the external power the swimmer delivers. The energetic consequences of this phenomenon have been studied more extensively in the field of animal locomotion, for example, in fish (Webb, 1971). In human swimming, however, this power loss in the generation of the propulsive force has been overlooked. When calculating efficiency of swimming humans, only the ratio of power needed to overcome drag and the power equivalence of the measured oxygen uptake was taken into consideration (Adrian, Singh, & Karpovich, 1966; Holmer, 1972; Pendergast, Di Prampero, Craig, Wilson, & Rennie, 1973; Di Prampero, Pendergast, Wilson, & Rennie, 1974). In this study, the power that is lost to the water when generating propulsive forces during swimming is estimated. This power loss is then taken into account when the propelling efficiency (ep) is defined and calculated as the power needed to overcome drag (Pa) divided by the total external power (P.), that is, power needed to overcome drag and power lost in the generation of the propulsive force (P.) (Groot & Ingen Schenau, 1987; Toussaint et al., 1983).
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