Metabolically Optimal Gait Transitions in Cross-country Skate Skiing

Cross-country skiing is a rare example of how humans can efficiently use all four limbs for propulsion. Effectively becoming a quadruped, a skate cross-country skier uses skis in a skating motion while the arms are fixed to poles which are pushed into the snow for propulsion. Cross-country skiers use the 2-skate technique (similar to a galloping horse) when going slow, switch to the 1-skate technique (which has no animal equivalent) at intermediate speeds, but then, in contrast to everything known about locomotion in humans and animals, they revert back to the previously rejected 2-skate technique at very high speeds of locomotion. This pattern of gait transitions suggests that the metabolic efficiency curves for 1-skate and 2skate skiing intersect twice, rather than just once as they do for bipeds and quadrupeds for any known gait transition.