Effect of Fatigue on Preferred and Optimal Stride Rates in Running

Abstract

Introduction: Experienced runners naturally optimize stride rate in a manner that minimizes oxygen uptake at a given running speed (Cavanagh and Williams, 1982). However, as runners become fatigued, preferred stride rate often decreases (Bates and Ostenig, 1977). Whether such changes with fatigue occur in parallel with changes in optimal stride rate is unknown. This study's focus was on determining whether experienced runners were able to self-optimize stride rate initially and at the end of a near-maximal one-hour run. Methods: Seventeen subjects completed a one-hour near-maximal effort run on a treadmill at a fixed speed slightly slower than each individual's 10 km race pace. After the first five minutes, preferred and optimal stride rates were measured. Ground reaction force data from an instrumented treadmill were used to determine preferred stride rate averaged over 10 strides. After this measurement, optimal stride rate was determined in a manner similar to that of Cavanagh and Williams (1982). Runners completed five two-minute segments of running at preferred stride rate, 4% above, 4% below, 8% above and 8% below their preferred rate. Oxygen uptake was measured during the second minute of each two-minute segment. Fitting a second-degree polynomial through the five oxygen uptake versus stride rate data points provided a minimum value for oxygen uptake from which optimal stride rate was determined. The runners then continued without having stride rate manipulated for 35 minutes, (50 minutes elapsed from the beginning of the run). The final 10 minutes involved a manipulated stride rate stage like that occurring near the beginning of the run but with a new randomization of order for the altered stride rates. Results: Paired t-tests showed no difference between preferred and optimal stride rates at the beginning (p = 0.213) and at the end of the run (p = 0.635), but a decrease in optimal stride rate was observed from beginning to end (p = 0.003). Half of the subjects decreased preferred and optimal stride rate over the course of the hour run while half showed little or no change. The average run was completed at 77% max VO2. Discussion: Experienced runners have demonstrated the capability to self-optimize stride rate at the beginning of a run and when very fatigued near the end of an hour run. This consistent ability was observed for runners with substantial shifts of stride rate with fatigue as well as for runners with no change of stride rate with fatigue. The ability to self-optimize despite changing optimal rates helps runners maintain given speeds for longer times than would be possible if constant stride rate were maintained for the full time.Figure 1: Mean preferred and optimal stride rate. Error bars represent standard deviation across subjects.Some subjects of this study demonstrated decreases in preferred stride rate across the hour run while others did not change. At this point, it is unclear what leads to rates decreases in some cases but not others. We speculate that leg stiffness may be influenced by fatigue and this may lead to decreased stride rate. Why this would be affecting some runners but not others is unknown.