Expiratory loading modulates the quadriceps muscle pump-induced venous return during rhythmic isometric exercise.

During exercise, venous return is enhanced by the skeletal muscle pump. Expiratory loading increases intra-abdominal pressure, which may impede venous return from the lower limbs. However, its effect on venous return mediated by the quadriceps muscle pump remains unclear. The purpose of this study was to elucidate the effect of expiratory loading on venous return facilitated by the quadriceps muscle pump. Healthy young participants performed rhythmic unilateral isometric knee-extension exercise of the right quadriceps (10% maximal isometric voluntary contraction; 2 s contraction/2 s relaxation), and data was successfully obtained from twelve participants. Controlled breathing (breathing frequency: 15 breaths/min; inspiratory-to-expiratory ratio: 1:1; tidal volume: twice the resting tidal volume) was performed with and without expiratory load. The expiratory phase was synchronized with the contraction phases. Expiratory load was adjusted to produce a mouth pressure of 20 cmH2O during expiration. Quantity of blood flow in the right femoral vein (Qfv) was continuously measured using Doppler ultrasound. No significant differences were observed in the increases in force and electromyography during expiration between the non-loading and loading conditions. Quadriceps muscle contraction increased Qfv; however, the contraction-induced increase was smaller (P = 0.005) with expiratory loading (497.6 ± 350.5 mL/min, mean ± standard deviation) than without expiratory loading (694.8 ± 358.9 mL/min). These findings suggest that increased expiratory loading may partly reduce the effectiveness of the quadriceps muscle pump in facilitating venous return during light-intensity exercise.