Moderate-to-loud in-ear music worsens ventilatory efficiency and running biomechanics during fixed-speed treadmill running: a sex-stratified randomized crossover study

: Despite widespread use of in-ear music during running, its effects on ventilatory efficiency and biomechanics remain unclear. This study examined whether moderate-to-loud (85 dB SPL) in-ear tempo music affects ventilatory efficiency, running economy (RE), and spatiotemporal biomechanics during fixed-speed treadmill running, and whether responses differ by sex. Thirty-two recreationally active participants (15 women, 17 men; age 22.7±2.2 years) completed two randomized, counterbalanced sessions of 20-min treadmill running at 8 km·h⁻¹ with and without 128-bpm in-ear music. Breath-by-breath gas exchange, spatiotemporal gait parameters, and RPE were collected and analyzed via mixed-model ANOVA. In the whole sample, music induced a fast-shallow breathing pattern: breathing frequency rose by 10.0% and tidal volume fell by 4.6%, with a 5.5% increase in minute ventilation (V̇E; all p≤0.001). Ventilatory efficiency was reduced, with a 4.3% higher V̇E/V̇CO₂ and a 4.5% higher V̇E/V̇O₂ (both p<0.001). Biomechanics shifted toward a more aerial gait: stride time (+1.3%) and stride length (+1.0%; both p=0.002) increased, as did vertical oscillation (+30%, p=0.015) and flight time (+16%, p=0.002), while duty factor fell (-1.3%, p=0.006), with the aerial-gait shift more pronounced in women. Music also slightly raised RE (+1.1%; p=0.009). RPE was lower with music (12.7±2.6 vs. 13.4±2.4; p<0.001). Thus, moderate-to-loud in-ear music during treadmill running elicits an altered, fast-shallow ventilatory pattern that reduces ventilatory efficiency, also shifting running biomechanics toward a less economical pattern. The concurrent lowering of perceived exertion despite a slightly higher metabolic cost underscores a paradoxical dissociation between subjective effort and objective strain. Trial registration: NCT03506282.