Muscle function during locomotion on skis at varying speed and incline conditions

The human musculoskeletal system is well adapted to use energy efficient muscle-tendon mechanics during walking and running but muscle behaviour during on-snow locomotion is unknown. Therefore, we examined muscle and muscle-tendon unit behaviour during diagonal style roller skiing at three speed and incline conditions. We assessed lower leg muscle and muscle-tendon unit mechanics and muscle activity in thirteen high-level skiers during treadmill roller skiing using synchronised ultrasound, motion capture, electromyography and ski-binding force measurements. Participants skied using diagonal style at 2.5 and 3.5 m·s−1 at 5°, and at 2.5 m·s−1 at 10°. We found an uncoupling of muscle and joint behaviour during most parts of the propulsive kick phase in all conditions (P<0.01). Gastrocnemius muscle fascicles actively shortened ~9 mm during the kick phase, while the muscle-tendon unit went through a stretch-shortening cycle. Peak muscle-tendon unit shortening velocity was five times higher than fascicle velocity (375 vs 74 mm·s−1, P<0.01). Increased incline was met by greater muscle activity (24%, P=0.04) and slower fascicle shortening velocities (34 vs. 45 mm·s−1, P<0.01). Increased speed was met by greater peak muscle activity (23%, P<0.01) and no change in fascicle shortening velocity. Our data show that muscle behaviour was uncoupled from the joint movement, which enables beneficial contractile conditions and energy utilisation during diagonal style at different slopes and speeds. Active preloading in the end of the glide phase may benefit the mechanisms. Summary statement We examined muscle function during diagonal style cross country skiing in competitive cross-country skiers. Our data show an uncoupling of muscle and joint behaviour in the lower leg during skiing.