Expiratory flow limitation in highly trained endurance athletes: The role of FEF_25-75% and ventilatory capacity during treadmill running

Expiratory flow limitation (EFL) during exercise in highly trained individuals is thought to result from increased ventilatory demands that exceed the capacity of the respiratory system, which does not fully adapt to exercise training. Reduced forced expiratory flow between 25 % and 75 % of forced vital capacity (FEF_25-75 %), a marker of small airway function, may contribute to EFL by limiting the maximum expiratory flows available during the hyperpnea of exercise. This study investigated whether FEF_25-75 %, peak minute ventilation (V̇_E), and breathing patterns differ between highly trained endurance athletes with and without EFL.

Forty highly trained endurance athletes (20 males and 20 females; V̇O₂max: 59.6 ± 9.2 mL∙kg^-1∙min^-1) completed spirometry and a maximal incremental cardiopulmonary treadmill exercise test. EFL was assessed by superimposing tidal flow-volume loops within the maximum flow-volume loop according to end-expiratory lung volume.

During maximal exercise, 40 % of participants (n = 16: 7 males, 9 females) developed EFL, with no significant sex differences (P > 0.05). Athletes with EFL had significantly lower FEF_25-75 % (3.45 ± 0.78 vs. 4.16 ± 0.98 L·s^-1, P = 0.020, d = 0.802) and a higher ventilatory demand-to-capacity ratio (V̇_E/V̇_Ecap) (0.86 ± 0.14 vs. 0.66 ± 0.11, P = 0.002, d = 1.589) compared to those without EFL. There were no significant differences in absolute tidal volume, breathing frequency or V̇_E between groups (P > 0.05).

In a homogeneous cohort of highly trained endurance athletes, EFL during maximal treadmill exercise appears to be primarily driven by a reduced capacity to generate expiratory flow, as evidenced by lower FEF_25-75 %, rather than differences in ventilatory demand.