This study tested two theories about the relationship between voluntary changes in muscle tension and pulmonary function in asthma. Kotses has theorized that decreased facial muscle tension decreases respiratory impedance via a hypothesized vagaltrigeminal reflex, but that muscle tension in other muscle groups has no such effect. Others have suggested that decreased thoracic muscle tension improves pulmonary function. Subjects were 19 volunteer asthmatic adults. They performed 3-minute cycles of deliberate muscle contraction, alternating two each for the shoulder and forehead muscles, followed by dominant forearm contraction. Surface EMG was measured from the frontalis and right trapezius areas. Airway impedance was measured by forced oscillation pneumography. Cardiac interbeat interval and respiratory sinus arrhythmia were measured to assess vagal tone. Frequency dependence of respiratory impedance increased during shoulder tension, giving some support to the theory relating thoracic tension to impairment in pulmonary function. Correlational analyses suggested a negative relationship between changes in cardiac interbeat interval and both frontalis muscle tension and decreased compliance of tissues in the airways. These findings are the opposite of those predicted by the vagal-trigeminal reflex theory.