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  1. Downhill skiing uses many different muscles, and requires a combination of slow aerobic fibers and fast and slow anaerobic fibers. Skiing is overall an endurance sport, because people remain active for hours at a time and are breathing hard using full body muscles. Backcountry skiing, or ski touring, in particular requires a high aerobic capacity because the skier is transporting themselves and all their gear on the approach, uphill, and downhill. The approach is often mostly flat or gently sloping uhill, and the uphill portion is made of switchbacks up a steep slope. The descent is when the fast anaerobic fibers are fired. When skiing down, the skier is in a squat-like position and is mostly using aerobic fibers. At the initiation of the turn, however, very fast movements have to be made explosively requiring the fast anaerobic fibers to be fired.
    Additionally, there are several types of strength used in backcountry skiing. Muscles contractions are divided into isometric and isotonic contractions. Isometric contractions involve the muscle length not changing during contraction, and isotonic contractions the length of the muscle changes while overall tension doesn’t change. Isotonic contractions are then divided into two types of contractions: eccentric and concentric contractions. Concentric contractions involve the muscle tension meeting the resistance and then staying stable, while eccentric contractions involve a lengthening of the muscle when the resistance is greater than the muscle’s force.
    Eccentric contractions absorb energy that is exerted on muscles from external sources, which forcibly lengthens the tendon-muscle complex. Eccentric contractions are usually characterized by actions like running downhill, or walking down the stairs. The muscles contract eccentrically when they support the body against gravity and absorb shock, as well as to store elastic recoil for future concentric contractions. These contractions require less oxygen and energy, “approximately fourfold lower” and with “fewer motor units” than with concentric contractions (Hody).
    The major muscles used in ski touring are: biceps, triceps, deltoids, latissimus dorsi, lateral hamstring, rectus femoris, gastrocnemius, and tibialis anterior. Other muscles used include the trapezius, pectorals, lower and upper abdominal muscles, and the external oblique muscles. These muscles are used to move the body as well as stabilize movements.
    Aerobic respiration is the primary means of energy production. This type of respiration involves the use of oxygen, pyruvic acid, free fatty acids, amino acids from protein catabolism and glucose to create large amounts (28-32) of ATP within the mitochondria. The by-products are water and CO2. Since most of the respiration is aerobic, the majority of the muscle fibers in a ski tourer’s muscles would be slow oxidative fibers.
    Explosive movements during the downhill portion of the ski tour at the initiation of downhill turns. This means that there will be a portion of muscle fibers that are fast and slow oxidative and fast glycolytic fibers. The energy conversions used by fast oxidative fibers are through the use of glucose. There are mostly aerobic fibers, but some anaerobic fibers that go through the process of glycolysis in the cytosol which produces 2 ATP and two pyruvic acid molecules. Pyruvic acid gets converted into lactic acid which is released into the blood. Fast glycolytic fibers also use this method, but lack the anaerobic fibers. The final method of energy production is the use of creatine phosphate, which donates a phosphate to ADP directly transforming it into ATP. Creatine kinase is the enzyme that performs the transfer of phosphate. Fast contractions don’t produce very much ATP so they are not sustainable for long-term use, however for the short and powerful movements they are very effective.
    Sources

    Hody Stéphanie, Croisier Jean-Louis, Bury Thierry, Rogister Bernard, Leprince Pierre. Eccentric
    Muscle Contractions: Risks and Benefits. Frontiers in Physiology, Volume 10. 2019. https://www.frontiersin.org/article/10.3389/fphys.2019.00536.DOI: 10.3389/fphys.2019.00536

    Padulo, J., Laffaye, G., Chamari, K., & Concu, A. (2013). Concentric and eccentric: muscle
    contraction or exercise?. Sports health, 5(4), 306.
    https://doi.org/10.1177/1941738113491386

    American Nordic Walking Association, March 4 2020. “Beginner’s Guide to Nordic Pole Walking”
    https://www.americannordicwalking.com/blog/2017/3/4/eo4yagte05ql8orxo5vnti12ze0d3
    n

    Gwendalynn Macander

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