Does Stance Time Predict Running Economy at Relative Speeds in Highly Trained Distance Runners?

Abstract

Running economy (RE) is an important component of distance running performance. Recent evidence suggests shorter stance times (ST) are related to superior RE and practitioners often recommend shorter ST for this benefit. However, overall, reports are equivocal, perhaps because comparisons are usually made at absolute running speeds, which fails to account for differences in relative fitness. To date, this relationship has not been explored at relative intensities. PURPOSE: To determine the relationship of ST and RE at absolute and relative running speeds. METHODS: 20 highly trained distance runners (13 M, 7 F; VO2max = 67.6 vs 53.5 ml/kg/min) consented to procedures approved by EMU-HSRC. Session 1 included a critical speed (CS) field test to determine relative fitness and establish relative testing speeds. Session 2 included two treadmill tests for kinematic and metabolic measurements. For both tests, subjects ran at 10, 12, 14, and 16 km/h (absolute speeds) and at 75, 85, and 95% CS (relative speeds). Kinematic data were collected for 60 sec using lower-body optical motion capture (Plug-In-Gait; Vicon, Centennial, CO). Metabolic data were collected using open circuit spirometry (TrueOne 2400, ParvoMedics, Sandy, UT) during three-minute trials, where the last 60 sec of each stage was used to calculate RE. Linear regressions were used to determine the relationship between ST and RE expressed as mL/kg/km, %VO2max, and kcal/kg/km, done separately for absolute and relative speeds. α was set at .05. RESULTS: For absolute speeds, significant negative relationships were found between ST and RE expressed as mL/kg/km (β = -.374), %VO2max (β = -.461), and kcal/kg/km (β = -.459) (all p < .01). For relative speeds, a significant negative relationship was found for RE expressed as %VO2max (β = -.318) (p < .05) but not mL/kg/km or kcal/kg/km (p > .05). CONCLUSION: Contrary to some reports and recommendations, for comparisons using absolute speeds, shorter ST was related to inferior RE. Comparisons using relative speeds showed weaker or non-significant relationships. Therefore, changes in RE mediated by ST may depend more on individual exercise strain than absolute speed. Future investigations should consider using relative intensities for a more appropriate comparison between individuals.