Fig. 6. Internal work rate (_int, W kg^-1), measured in two subjects as a function of the speed (v, m s^-1). W_int increases with the speed in both conditions: with fins (FL, open circles) or without (L, closed circles). At paired speeds, _int is larger when swimming without fins than when swimming with them. The differences in _int are mainly attributable to differences in kick frequency (about 40 % lower when fins are used; see text for details).

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