The effects of diaphragmatic breathing learning on chest wall motion, mechanical efficiency of the respiratory muscles, breathing pattern, and dyspnea sensation were studied in seven patients with severe chronic obstructive pulmonary disease (COPD) (FEV1 34 +/- 7% of the predicted value) during loaded and unloaded breathing. Chest wall motion was studied focusing on amplitude and phase relation of rib cage and abdominal motion. Mechanical efficiency was defined as the ratio of added external power output and added oxygen consumption during inspiratory threshold loading (40% maximal inspiratory pressure [Plmax]). After 2 wk run-in, all subjects participated in a diaphragmatic breathing program for 3 wk. Variables obtained during diaphragmatic breathing were compared with those obtained during natural breathing. During diaphragmatic breathing the ratio of rib cage to abdominal motion decreased significantly during unloaded (0.86 versus 0.37; p < 0.01) as well as during loaded breathing (0.97 versus 0.50; p < 0.01). Chest wall motion became more asynchronous during diaphragmatic breathing in the unloaded conditions (mean phase difference for natural breathing 3.5 versus 10.4% for diaphragmatic breathing; p < 0.02) and loaded conditions (mean phase difference for natural breathing 6 versus 11.4% for diaphragmatic breathing; p < 0.02). Surprisingly, mechanical efficiency decreased significantly during diaphragmatic breathing (2.57 +/- 0.76%) in comparison with natural breathing (3.35 +/- 1.48%; p < 0.01). Tidal volume, respiratory frequency, and duty cycle did not change significantly during diaphragmatic breathing. Dyspnea sensation tended to increase during diaphragmatic breathing.(ABSTRACT TRUNCATED AT 250 WORDS)