DOES THE APPLICATION OF GROUND FORCE SET THE ENERGETIC COST OF CROSS-COUNTRY SKIING

We tested whether the rate at which force is applied to the ground set s metabolic rates during classical-style roller skiing in four ways: 1 ) by increasing speed (from. 2.5 to 4.5 m/s) during skiing with arms o nly, 2) by increasing speed (from 2.5 to 4.5 m/s) during skiing with l egs only, 3) by changing stride rate (from 25 to 75 strides/min) at ea ch of three speeds (3.0, 3.5, and 4.0 m/s) during skiing with legs onl y, and 4) by skiing with arms and legs together at three speeds (2.0-3 .2 m/s, 1.5 degrees incline). We determined net metabolic rates from r ates of O-2 consumption (gross O-2 consumption - standing O-2 consumpt ion) and rates of force application from the inverse period of pole-gr ound contact [1/t(p(arms))] for the arms and the inverse period of pro pulsion [1/t(p(legs)))] for the legs. During arm-and-leg skiing at dif ferent speeds, metabolic rates changed fn direct proportion to rates o f force application, while the net ground force to counteract friction and gravity ((F) over bar) was constant. Consequently, metabolic rate s were described by a simple equation ((E) over dot (metab) = (F) over bar.1/t(p).C, where (E) over dot (metab) is metabolic rates) with cos t coefficients (C) of 8.2 and 0.16 J/N for arms and legs, respectively . Metabolic rates predicted from net ground forces and rates of force application during combined arm-and-leg skiing agreed with measured me tabolic rates within +/-3.5%. We conclude that rates of ground force a pplication to support the weight of the body and overcome friction set the energetic cost of skiing and that the rate at which muscles expen d metabolic energy during weight-bearing locomotion depends on the tim e course of their activation.