Changes In Step Time And Length Between Kilometer Eleven And Thirty-nine Of A Marathon

Abstract

PURPOSE: Marathon running invokes neuromuscular fatigue which has been shown to result in kinematic changes in a laboratory setting. However, there are limited studies on changes that take place during a race. The purpose of this study was three-fold: 1) to evaluate the step time and step length at an early and late time point in a full marathon and quantify the change between the two points; 2) to identify differences in step time and length associated with sex; and 3) to determine if step time and length are predictors of race finish time. METHODS: This is an observational study in which runners were filmed at two stations, at kilometer 11 (S1) and kilometer 39 (S2) of a full 42.2 kilometer marathon. Each station incorporated two cameras, one in the sagittal plane to assess kinematics and the second to identify the runners’ bib numbers. A 5-meter section of roadway was marked with chalk, delineating each meter, to allow for assessment of step length using Dartfish 5.5 Video Analysis software (Dartfish, Fribourg, Switzerland). RESULTS: Step time was slower at S2 compared to S1 with a mean difference ± SD of 0.290 ± 0.403 m/s (95%CI 0.246 – 0.334; p < 0.001). Step length was shorter at S2 compared to S1 with a difference of 0.098 ± 0.111 m (95%CI 0.086 – 0.110; p < 0.001). There was no interaction in step time for males or females between S1 and S2 (S1, Male 0.35 ± 0.02 s, Female 0.33 ± 0.02 s; S2, Male 0.35 ± 0.02 s, Female 0.34 ± 0.02 s; p = 0.099) however, an interaction was detected for step length (S1, Male 1.16 ± 0.13 m, Female 1.05 ± 0.11 m; S2, Male 1.05 ± 0.13 m, Female 0.98 ± 0.11 m; p = 0.01). A regression model to predict finish time found that step length at S1 accounted for 47% of the variability (F (1,323) = 283.7; p < 0.001), this increased to 68.3% when S1 step time was included (F (2,322) = 350.4; p < 0.001), S2 step length increased this to 75.2% (F(3,321)=328.4; p<0.001) while including S2 step time increased it to 76.7% (F(4,320)=268.4, p<0.001). CONCLUSION: Step time was slower and step length was shorter at the 39-kilometer point of the full-marathon compared to the 11-kilometer point. Step time did not differ at either time point in the race for males or females; however, they both demonstrated a reduction in step length. Step time and step length at both points in the race are able to account for a significant amount of finish-time variability.