The forces generated by the cat diaphragm (DIA) during different ventilatory and nonventilatory behaviors were determined by measuring transdiaphragmatic pressures (Pdi). The Pdi generated during eupnea was only approximately 12% of the maximum Pdi (Pdimax) generated by bilateral phrenic nerve stimulation. When the animals breathed a gas mixture of 10% O2 and 5% CO2, the Pdi increased to approximately 28% of Pdimax. During total airway occlusion, the Pdi generated by the diaphragm increased to approximately 49% of Pdimax. Only during the gag reflex and sneezing did Pdi reach maximal levels. A model for diaphragm motor unit recruitment during these different behaviors was presented based on the proportion of different motor unit types within the diaphragm, the relative tetanic tensions produced by each unit type, and the assumption of an orderly pattern of motor unit recruitment.

Dysfunction of the muscles of ambulation contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). Men with COPD have high prevalence of low testosterone levels, which may contribute to muscle weakness. We determined effects of testosterone supplementation (100 mg of testosterone enanthate injected weekly) with or without resistance training (45 minutes three times weekly) on body composition and muscle function in 47 men with COPD (mean FEV(1) = 40% predicted) and low testosterone levels (mean = 320 ng/dl). Subjects were randomized to 10 weeks of placebo injections + no training, testosterone injections + no training, placebo injections + resistance training, or testosterone injections + resistance training. Testosterone injections yielded a mean increase of 271 ng/dl in the nadir serum testosterone concentration (to the middle of the normal range for young men). The lean body mass (by dual-energy X-ray absorptiometry) increase averaged 2.3 kg with testosterone alone and 3.3 kg with combined testosterone and resistance training (p < 0.001). Increase in one-repetition maximum leg press strength averaged 17.2% with testosterone alone, 17.4% with resistance training alone, and 26.8% with testosterone + resistance training (p < 0.001). Interventions were well tolerated with no abnormalities in safety measures. Further studies are required to determine long-term benefits of adding testosterone supplementation and resistance training to rehabilitative programs for carefully screened men with COPD and low testosterone levels.