Graded Walking Energetics under Cold Strain

Introduction Although research supports selecting steeper routes to minimize metabolic rate (M) in mountain racers, the "steeper is cheaper" strategy has yet to be confirmed for slower, more typical graded walking speeds under cold stress. Purpose This study aimed to confirm whether "steeper is cheaper" is true for typical graded walking speeds in individuals exposed to incremental cold stress. Methods Fourteen healthy, military-aged adults (age, 24 ± 6 yr; height, 1.72 ± 0.08 m; body mass, 72 ± 16 kg) completed four 20-min treadmill walks in three ambient temperatures (20°C, 10°C, and 0°C) in light clothing (i.e., shorts, t-shirt, light gloves). Each walk involved five stages at incremental vertical speeds (0.00, 1.93, 3.86, 5.79, 7.79 m min^-1) but variable treadmill speeds (0.54, 0.72, 1.07 m s^-1). To verify the "steeper is cheaper" strategy, we compared M between treadmill speeds at matched vertical speeds in each temperature. We also tested if the 90% confidence interval around the mean percent paired difference between Load Carriage Decision Aid (LCDA) metabolic model predictions and measured M was within ±10%. Results M was significantly higher for the faster treadmill speed at matched vertical speeds in all but two comparisons at 20°C, all but two comparisons at 10°C, and all comparisons at 0°C (P < 0.05). LCDA metabolic model predictions were statistically equivalent to measured M during graded walking at 20°C (90% confidence interval, -3.1% to 0.6%) and 10°C (-7.8% to -2.6%) but not 0°C (-16.2% to -9.5%). Conclusions Route planners should recommend steeper but shorter routes to minimize M in individuals that walk in temperate-to-cold environments. The LCDA metabolic model provides accurate M predictions in lightly dressed individuals in temperatures down to 10°C, but users should expect underestimated M in 0°C or colder.