Femoral venous blood flow is depressed by expiratory loading during light-intensity isometric knee extension exercise

Background: During exercise, the intermittent compression of leg veins by muscle contraction (i.e., the muscle pump) significantly enhances venous return, thereby contributing to an increase in cardiac output. Previous studies have shown that respiratory muscle pressures influence blood flow velocity in the femoral vein both at rest and during leg exercise. Additionally, increased expiratory resistance, such as observed in patients with chronic obstructive pulmonary disease, has been associated with reduced venous return under resting conditions. However, the effect of expiratory resistance on venous return during lower limb exercise remains unclear. Purpose: The aim of this study is to clarify the effect of expiratory resistive breathing on changes in blood flow in the femoral vein (Qfv) during intermittent isometric quadriceps muscle contractions. Methods: Nine healthy young adults (5 males and 4 females; 22±1 yrs, mean±SE) participated in rest and exercise trials. In both trials, participants controlled their breathing for 1-2 minutes with a tidal volume approximately 1.5 times their spontaneous breathing and a breathing frequency of 15 breaths/min (duty cycle 1:1, i.e., 2s inspiration, 2s expiration), with or without an expiratory load of +20 cmH 2 O. The exercise trials consisted of intermittent isometric knee extension of the right leg at 10% maximum voluntary contraction (2 seconds of contraction and 2 seconds of relaxation). During the exercise trial, the expiratory phase was synchronized with the contraction phase. Femoral venous blood velocity and diameter were continuously measured using Doppler ultrasound. Results: During the resting trial, Qfv during the expiratory phase was lower (P=0.008) with voluntary hyperventilation and expiratory resistance (-22±24 ml/min) than without resistance (139±44 ml/min). In the exercise trials, quadriceps muscle contractions increased Qfv. However, the contraction-induced Qfv was smaller (P=0.007) during voluntary hyperventilation with expiratory resistive breathing (289±115 ml/min) compared to without expiratory resistance (629±93 ml/min). Conclusion: These results suggest that expiratory resistance may modulate venous return during light-intensity intermittent isometric exercise of the quadriceps. This study was supported by JSPS KAKENHI #23K10633, and #23K24736. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.