Fig. 4. The effects of passive tools in augmenting human locomotion in water. World records in terms of average speed and performance duration are here represented for front crawl and monofin swimming. Iso-duration and iso-cost curves are as in Fig. 2 and Appendix. The cost of front crawl well matches the data published elsewhere (di Prampero, 1986), showing an increase with speed, while the cost of monofin swimming seems to be speed independent in a wide range of speeds/durations.