Fluctuations in the stride interval time series of unconstrained walking are not random but exhibit long range correlations that decay as a power law [Hausdorff JM, Peng CK, Ladin Z, Wei JY, Goldberger AL. Is walking a random walk? Evidence for long range correlations in stride interval of human gait. J Appl Physiol 1995;78:349-58]. Here, we examine whether the long range correlations are present in the stride interval time series of running. Recreational female runners ran 8min trials at their preferred running speed as well as 10% and 20% slower and faster than their preferred speed. Both the average time and the amount of variability of the stride interval decreased with increasing speed. Detrended fluctuation analysis (DFA) showed that there were long range correlations present in the stride interval time series and these correlations followed a quasi U-shaped function, with the minimum at the preferred running speed. These results are consistent with the hypothesis that the preferred running speed, falling as it does between the upper and lower limits of possible running speeds, is the speed at which the most dynamical degrees of freedom are available for adaptive control of locomotion.