Godinez A, Liston DB, Ayzenberg R, Toscano WB, Cowings PA, Stone LS. G-loading and vibration effects on heart and respiration rates. Aviat Space Environ Med 2014; 85:949–53. Background: Operational environments expose pilots and astronauts to sustained acceleration (G loading) and whole-body vibration, alone and in combination. Separately, the physiological effects of G loading and vibration have been well studied; both have effects similar to mild exercise. The few studies of combined G loading and vibration have not reported an interaction between these factors on physiological responses. Methods: We tested the effects of G loading (+1 and +3.8 G_x) and vibration (0.5 g_x at 8, 12, and 16 Hz), alone and in combination, on heart and respiration rate. Results: We observed an effect of G loading on heart rate (average increase of 23 bpm, SD 12) and respiration rate (average increase of 5 breaths per minute, SD 5), an effect of vibration on heart rate, and an interaction on heart rate. With vibration, we observed heart rate increases of 4 bpm (SD: 3) with no increase in respiration rate. In the +1 G_x condition, the largest heart rate increase occurred during low-frequency (8 Hz) vibration, while at +3.8 G_x, the largest heart rate increase occurred during high-frequency (16 Hz) vibration, demonstrating interaction. Discussion: Consistent with previous reports, our G-loading and vibration effects are similar to mild exercise. In addition, we observed an interaction between G loading and vibration on heart rate, with maximum heart rates occurring at a higher vibration frequency at +3.8 G_x compared to +1 G_x. The observed interaction demonstrates that G-loading and vibration effects are not independent and can only be properly assessed during combined exposure.