Compared with level walking or running, progressive downhill walking or running requires a decreasing energy cost to some minimum where the cost again increases with further decrements in grade. Margaria estimated this minimum occurs at a −9% grade. In this study an attempt was made to more precisely track the energy cost curve in progressive downhill treadmill walking. Ten men, mean age 22.0 ± 2.5 yr, volunteered as subjects. After VO2max determinations the subjects attended two downhill walking sessions. Each subject performed 14 randomly ordered walking bouts of 6 min in duration, at speeds of 90 and 105 m.min−1, The grades used were 0, −3, −6, −9, −12, −15, and −18%. Gas exchange measurements were obtained by open circuit spirometry during each work bout. Heart rate was monitored continuously and the stride frequency was counted by direct observation during each walking bout. Net Vo2 values decreased with decrements in grade to −9, −12% for the respective speeds of 90 and 105 m.min−1, The group mean net Vo2 minimums at −9 and −12%, however, were not significantly different (P > 0.05) from the group mean values at −6 and −15% at 90 m.min−1, or between −9 and −15% grades at 105 m.min−1. Group mean net Vo2 values at O, −3, and −18% were significantly different (P < 0.05) from net Vo2 values for the other grades at 90 m.min−1 walking. At 105 m.min−1, mean net Vo2 values at 0, −3, −6, and −18% were significantly different (P < 0.05) from net Vo2 values at the other grades. We conclude that the energy cost curve for downhill walking does not precisely conform with the view that the minimum cost occurs at −9% grade. While a reasonable approximation, our group data suggest that this point is variable, occurring between −6 and −15% grade depending on an individual's walking characteristics and walking speed. We also present a rationale to explain the energy cost curve observed during progressive downhill walking.
©1993The American College of Sports Medicine