Durability of physiological and biomechanical variables during a marathon.

Durability is the ability to withstand the deterioration of physiological parameters and is associated with marathon performance. The aim of this study was to examine whether changes to biomechanical parameters are dependent on durability. Sixty-nine runners submitted data collected using a footworn accelerometer and heart rate (HR) recording device during a marathon (median finish time (IQR): 224.0 (60.4) mins). Biomechanical parameters (both speed-adjusted and absolute) including stiffness, duty factor, step frequency, step length, running speed, and HR were separated into eight 5 km segments. Decoupling was used to quantify durability, defined as the ratio between HR and running speed. The magnitude of the decoupling was determined from the last full 5 km segment of the race (35-40 km) and expressed relative to the 5-10 km segment, and used to group the participants into high, moderate and low decoupling groups. Greater biomechanical deterioration was observed in the high decoupling group, but this disappeared after adjusting for speed. More durable runners (i.e., low decoupling) exhibited distinct changes in speed-adjusted step frequency and step length across the marathon. These patterns may relate to fatigue resistance, though it remains unclear whether they reflect durability-enhancing adaptations or are traits of inherently resilient runners.