Runners with lower dynamic stability exhibit better running economy.

Maintaining dynamic stability during running incurs an energetic cost that does not contribute to forward propulsion. Despite this, dynamic stability has received little attention as a potential factor influencing running economy. To understand the relationship between dynamic stability and running economy, nineteen trained runners were tested on a treadmill across three individualized speeds. Whole-body dynamic stability was quantified via a single maximum Lyapunov exponent (MLE) computed from a 21D state-space embedding, which incorporated 3D angular velocities from seven body segments (bilateral: shank, upper torso, forearm; unilateral: lower torso) and running economy was measured as cost of transport (COT) using metabolic gas-exchange data. Linear mixed-effects models were used to assess the relationship between MLE and COT as well as the effects of running speed on MLE. MLE was negatively associated with COT (p = 0.049), while running speed had no significant effect on MLE (p > 0.579). This study is the first to demonstrate that the MLE calculated from a multivariate state-space is negatively associated with COT, indicating that runners with lower dynamic stability exhibit better running economy. Further, MLE was not affected by running speed, indicating that this measure of whole-body dynamic stability can be robustly assessed at a range of running speeds. These results may hint at a previously unexplored avenue to improve running economy through alteration of dynamic stability characteristics.