Experimental Evidence for the Independent Modulation of Step Frequency and Duty Factor during Endurance Running.

Abstract

Running patterns have been conceptualized as a continuum defined by step frequency (SF) and duty factor (DF), two variables proposed to act as independent axes within a dual-axis framework. However, their experimental independence at a fixed speed has not yet been verified. This study aimed to directly test the independence of SF and DF during treadmill running at 11 km·h -1 using a real-time visual biofeedback system. Fifteen trained runners performed a preferred baseline trial followed by 16 randomized conditions in which either SF (±6%, ±12%) or DF (±8%, ±16%) was imposed while the other variable was free to vary. SF manipulation did not significantly affect DF ( P = 0.14), with adjustments in contact and flight time occurring in parallel and preserving the ratio underlying DF. In contrast, extreme DF manipulations (±16%) significantly altered SF ( P ≤ 0.03), as opposing adjustments in contact and flight time were insufficient to stabilize step time. These findings provide the first direct experimental evidence that SF and DF can be modulated independently within a functional range, partly confirming the dual-axis model. Practically, SF and DF represent distinct levers for modifying running mechanics, offering clinicians and coaches flexible strategies for performance optimization.