In vivo effects of theophylline on diaphragm, bicep, and quadricep strength and fatigability

Aminophylline has been demonstrated to increase in vitro contractility in skeletal muscle, including diaphragm. In vivo studies report significant increases in diaphragm contractility in patients with chronic obstructive pulmonary disease but only small increases in control subjects. The present study determined the effects of aminophylline on strength and fatigability in the diaphragm, the biceps brachii, and the quadriceps of normal individuals. Seven healthy subjects were tested with placebo and drug conditions on separate days in a randomized, double-blind fashion. Mean theophylline levels of 15 ± 2 mg/L SD were maintained by constant intravenous infusion. Strength of the diaphragm was measured as maximum inspiratory pressure. Strength of the biceps and quadriceps were measured isometrically during arm flexion (90 degrees) and leg extension (115 degrees) against an electronic load cell. Fatigue was measured as the decrease in tension during a 30-second contraction and during a 6-minute period- of alternating 5-second maximal contraction and 5-second rest. Therapeutic levels of theophylline had no effect on strength or fatigability during a maximal contraction in any muscle group studied.