The Contribution of Muscle-Tendon Interactions to Superior Running Economy: An Uphill Versus Level Running Paradigm.

Trained runners exhibit better running economy (RE) during level running than untrained individuals, though the underlying contributors remain unclear. Using an indirect approach, we explored whether more effective muscle-tendon interactions might explain this superior RE. We measured RE, biomechanics, and lower limb muscle activation in 16 runners and 16 aerobically fit nonrunners during level (1.7 and 3.5 m/s) and uphill (+8.1°, 1.7 m/s) running. Unlike level running, uphill running requires net positive mechanical work, reducing the contribution of fine-tuned muscle-tendon interactions to RE. Therefore, we hypothesized that RE differences between runners and nonrunners would be smaller during uphill running. Our results confirmed that runners had better RE on level ground (P ≤ .046) but showed no RE advantage during uphill running (P = 1.00). Across all conditions, runners ran with higher average positive and negative ankle joint power (P ≤ .010) and a greater relative contribution of the ankle to the total positive joint power (P = .037). Despite this increased ankle reliance, relative triceps surae muscle activation was similar between groups. These findings suggest that superior RE observed in runners on level ground may stem from more optimized interaction between the triceps surae muscles and the Achilles tendon.