Browsing by Author "Killick, Anthony"

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    Energetic Considerations in Cross-Country Skiing
    (Springer, 2015-01) Herzog, Walter; Killick, Anthony; Boldt, Kevin Rudi
    Cross-country skiing is a four-legged gait, and some gait patterns, such as 2-skate skiing, are similar to those adopted by animals (galloping horse). Four-legged animals change gait patterns with increasing speeds of locomotion, at least in part, to minimize metabolic energy expenditure. For example, a horse will switch from a walk, to a trot, and finally to a gallop as speed of locomotion increases. Similarly, skate cross-country skiers will switch from a 2-skate gait to a 1-skate gait with increasing speeds of locomotion, but then unlike any other animal, will revert back to the previously-rejected 2-skate gait pattern at very high speeds. We used oxygen uptake measurements, force measurements in poles and skis, 3-dimensional movement analysis and functional muscle properties to explain this result. We found that propulsion in 1-skate skiing comes primarily from the arms/poles, while propulsion comes primarily from the legs/skis in the 2-skate technique. We also found that ground contact times for the skis are virtually independent of the skiing speed while pole contact times decrease dramatically with increasing speeds. Furthermore, propulsive forces from the arms dropped from skiing at 15 km/h to skiing at 30 km/h while simultaneously requiring much more metabolic energy. Finally, the cost of transport curves for 1-skate and 2-skate skiing intersected twice, indicating better efficiency for the 2-skate technique at slow and very fast speeds, and better efficiency for the 1-skate technique at intermediate to fast speeds. Combined, these results suggest that arm/pole action is optimized at intermediate speeds, thereby providing an advantage to the 1-skate technique which relies primarily on arm propulsion, while arm/pole action is highly inefficient at very high speeds, thus switching to the 2-skate technique which relies primarily on leg propulsion, is good strategy.
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    Is There an Optimal Pole Length for Double Poling in Cross Country Skiing?
    (Human Kinetics Journal, 2017-07) Onasch, Franziska; Killick, Anthony; Herzog, Walter
    The aim of this study was to determine the effects of pole length on energy cost and kinematics in cross country double poling. Seven sub-elite male athletes were tested using pole sets of different lengths (ranging between 77% and 98% of participants' body height). Tests were conducted on a treadmill, set to a 2% incline and an approximate racing speed. Poling forces, contact times, and oxygen uptake were measured throughout the testing. Pole length was positively correlated with ground contact time (r = .57, p < .001) and negatively correlated with poling frequency (r = -.48, p = .003). Pole length was also positively correlated with pole recovery time and propulsive impulse produced per poling cycle (r = .36, p = .031; r = .35, p = .042, respectively). Oxygen uptake and pole length were negatively correlated (r = -.51, p = .004). This acute study shows that increasing pole length for double poling in sub-elite cross country skiers under the given conditions seems to change the poling mechanics in distinct ways, resulting in a more efficient poling action by decreasing an athlete's metabolic cost.
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    Metabolically Optimal Gait Transitions in Cross-Country Skate Skiing
    (2014-09-04) Killick, Anthony; Herzog, Walter
    Animals change gait patterns to minimize metabolic cost. Horses walk at slow speeds, trot at intermediate speeds, and gallop at high speeds. Likewise, cross-country skiers use the 2-skate technique at slow speeds, the 1-skate technique at intermediate speeds, but then, in contrast to everything known about animal locomotion, they revert to the previously rejected 2-skate technique at high speeds. I determined the oxygen cost for the 1- and 2-skate technique while measuring forces in the skis and poles for eight athletes skiing at speeds ranging from 6-35km/h. I found that the oxygen cost curves for the two techniques intersected twice, and that propulsion comes primarily from the poles for the 1-skate and the skis for the 2-skate technique. Furthermore, the arm action becomes metabolically much more costly with increasing speeds of skiing than the leg action, thereby partly explaining the non-intuitive gait transitions in skiers.
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    Quadrupedal Locomotion-Respiration Entrainment and Metabolic Economy in Cross-Country Skiers
    (2016-02) Boldt, Kevin Rudi; Killick, Anthony; Herzog, Walter
    A 1:1 locomotion-respiration entrainment is observed in galloping quadrupeds, and is thought to improve running economy. However, this has not been tested directly in animals, as animals cannot voluntarily disrupt this entrainment. The purpose of this study was to evaluate metabolic economy in a human gait involving all four limbs, cross-country skiing, in natural entrainment and forced non-entrainment. Nine elite cross-country skiers roller skied at constant speed using the 2-skate technique. In the first and last conditions, athletes used the natural entrained breathing pattern: inhaling with arm recovery and exhaling with arm propulsion, and in the second condition, the athletes disentrained their breathing pattern. The rate of oxygen uptake (VO2) and metabolic rate (MR) were measured via expired gas analysis. Propulsive forces were measured with instrumented skis and poles. VO2 and MR increased by 4% and 5% respectively when skiers used the disentrained compared with the entrained breathing pattern. There were no differences in ski or pole forces or in timing of the gait cycle between conditions. We conclude that breathing entrainment reduces metabolic cost of cross-country skiing by approximately 4%. Further, this reduction is likely a result of the entrainment rather than alterations in gait mechanics.