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CAVANAGH, PETER R. and KEITH R. WILLIAMS. The effect of stride length variation on oxygen uptake during distance running. Med. Sci. Sports Exercise, Vol. 14, No. 1, pp. 30–35, 1982. Ten recreational runners (mean VO2max) 64.7 ml.kg-1.min-1) underwent a 5-d acclimation period to treadmill running at a 7 min.mile-1 pace (3.83 m.s-1) for 30 min each day. During these runs their freely chosen stride lengths were determined and expressed as a percentage of leg length (%LL). On two subsequent testing days stride length was systematically varied over a range of ± 20% LL about the freely chosen value. O2 uptake was determined by the Douglas Bag method. All subjects exhibited a stride length at which O2 uptake was minimized, although the individual profiles varied considerably. The mean increases in VO2 were 2.6 and 3.4 ml.kg-1.min-1 at the short-and long-stride length extremes, respectively. During unrestricted running deviations from optimal stride length caused a mean increase in VO2 of 0.2 ml.kg-1.min-1. The relatively efficient running patterns used by the subjects during unrestricted running indicate either an adaption to the chosen stride length through training or a successful process of energy optimization.