Energy exchange in downhill and uphill walking: a calorimetric study

Energy balance can be written as 1) M = sigma Q +/- W +/- S, expressed as power (W), where M is the rate of metabolic energy transformation, sigma Q the rate of heat loss, W the work rate, and S the rate of body heat storage. When submaximal treadmill exercise continues long enough, body temperature stops changing, S becomes zero, and the heat storage term is dropped from equation 1. For uphill walking the equation becomes 2) M = sigma Q + Wvert, and for downhill walking it becomes 3) M = sigma Q - Wvert. This study tested the energy balance equations with direct measurements of heat exchange using a suit calorimeter and M from standard measurements of respiratory gas exchange. Ten healthy men walked on a motor driven treadmill at 1.5 m.s-1 at grades of 0, 5, 10, -5, and -10% for 70-90 min to ensure a thermal steady state. As expected, +Wvert was identified as a power output, whereas -Wvert was accounted for as a power input, totally transformed to heat in the downhill walking subject. There also appeared to be a quantity of non-thermal energy, Wwalk, needed to satisfy the energy balance equation. This was significant at 0, 5, and 10% grades (P less than 0.01) but not significant at -5 and -10% grades (P greater than 0.05). The data confirm previous results for level walking and extend them to include uphill walking. While it had been suggested that Wwalk represents an externalization of energy at the foot, the present data suggest an alternative explanation.(ABSTRACT TRUNCATED AT 250 WORDS)