Leg stiffness decreases during a run to exhaustion at the speed at O_2max

Abstract

Vertical and leg stiffness are related to running speed. In endurance running, the ability to maintain stiffness might be more important than the absolute stiffness magnitude. The purpose of this study was to examine changes in vertical and leg stiffness during an exhaustive. Six sub-elite runners (24.2, s = 4.2 years; 1.81, s = 0.03 m; 73.4, s = 4.4 kg) participated in this study. They performed preliminary tests to determine lactate threshold, lactate turnpoint, O_2max, sO_2max and a series of isokinetic endurance tests. During the run to exhaustion runners were videoed (50 Hz) to determine contact and flight times, from which leg (K_leg) and vertical (K_vert) stiffness were calculated. During the run K_leg showed a significant decrease [P = 0.030, effect size statistics (ES) = 0.74], however, the decrease in K_vert was non-significant and of a small magnitude (P = 0.051, ES = 0.32). The distance covered during the run was correlated with ΔK_leg (r = −0.868) but not ΔK_vert (r = 0.684). ΔK_leg was very strongly related to Δ ground contact time (r = −0.937) and Δ step length (r = −0.957). The Δ ground contact time had a near perfect relationship with Δ step length (r = 0.995). Isokinetic measures were not significantly correlated with either ΔK_leg. The ability to maintain a short ground contact time appears to be a key determinant of maintaining performance during a run to exhaustion. Minimising this is important for maintaining K_leg. K_leg was not significantly related to isokinetic measures.

Keywords:

• Running
• spring–mass model
• fatigue
• strength
• biomechanics

Acknowledgements

We would like to acknowledge the assistance of Emma Davies and Sarah Bowen in the data collection for this study.