Association between muscle force production during steady-speed running and running economy in competitive athletes

Introduction Running economy (RE) is a key determinant of performance in distance running, and it is of interest to understand how RE is associated to biomechanical characteristics of running gait. Previous studies showed that RE is proportional to the rate of muscle force production during running (Kram & Tailor, 1990; Pontzer 2007). In steady-speed running, the highest portion of muscle force is applied to support one’s own body weight (Kram & Tailor, 1990; Chang & Kram, 1999), but a significant amount of force is used to accelerate the center of mass (Chang, 1999) and to swing the oscillating limb (Modica & Kram, 2005). Therefore, the purpose of this study was to analyze the association between RE and the overall rate of force production, as estimated using a mathematical model. Methods RE was measured in ten high-level athletes while running on a track at the steady speed of 14 km/h. Furthermore, ground reaction force (GRF) data were collected with a force plate while running at the same speed. A mathematical model, adapted from Pontzer (2007), was used to estimate the rate of force production during running as a combination of i) the rate of the vertical component of GRF, ii) the rate of the horizontal component of GRF, and iii) the average force required to swing the limb in a stride, determined using a specific equation. A linear regression was conducted to assess the proportion of variance in RE explained by the modeled force values. Results The mean rate of the vertical component of GRF, when considered alone, explained a large portion of variability in RE (R2 = 0.71). The explained variance increased when the horizontal component of GRF and the force required to swing the limb were included in the model (R2 = 0.79 and 0.88, respectively). Discussion The present results confirm the previous conclusions that the force needed for supporting the runner’s body weight is a major determinant of RE (Kram & Taylor, 1990). Furthermore, including the horizontal component of GRF and the force needed for swinging the limb allows to obtain an index of overall force production that is highly associated with RE in trained distance runners. This finding provides further support to the observation that RE is proportional to the rate of force produced during running. From an applied perspective, the model used here can provide a reasonably simple evaluation tool when analyzing an athlete’s running mechanics with the goal to improve the economy of his/her running style. References Chang YM, Kram R (1999). J Appl Physiol, 86, 1657-1662. Kram R, Taylor CR (1990). Nature, 346, 265-267. Modica JR, Kram R (2005). J Appl Physiol, 98, 2126-2131. Pontzer H (2007). J Exp Biol, 210, 484-494.