Inspiratory muscle function has been shown to be related to general muscle weakness, weight loss, blood gas tensions, airway obstruction and hyperinflation. The aim of this study was to define (1) the factor that is the main determinant of the tension-time index of the inspiratory muscles ( T tmus), and which thus increases the risk of inspiratory muscle fatigue; and (2) whether a breathing strategy is adopted to avoid inspiratory muscle fatigue. Twenty-seven normal volunteers and 35 stable COPD outpatients (FEV 1% predicted, range: 21–89%; and FRC/TLC, range: 49–77%) were studied. The T tmus was determined as follows: T tmus = P i/P imax · T i/ Ttot, where P i is the mean inspiratory pressure calculated from the mouth occlusion pressure ( P 0·1), P imax is the maximal inspiratory pressure, T i is the inspiratory time, and Ttot is the total time of the breathing cycle. COPD patients showed significantly lower P imax and higher P 0·1, P i, P i/ P imax, and T tmus than normal subjects. No patient had a T tmus value higher than the inspiratory muscle fatigue threshold of 0·33. The FEV 1 was significantly correlated with T tmus and all its components in the patients. The FRC/TLC was also correlated with all components except P i. Body weight was only correlated with P imax. In a forward and backward stepwise regression analysis, FEV 1 appeared to be the only significant factor explaining the variance of log ( P i/P imax) and log ( T tmus), whereas FRC/TLC was the principal determinant of P imax. In COPD patients, a non-linear relationship was found between T i and P 0·1. A negative linear relationship was found between T i/ Ttot and P i/P imax. In conclusion, although hyperinflation predominantly affected inspiratory muscle strength in a group of stable COPD patients with a wide range of severity, airway obstruction was the principal factor determining the magnitude of T tmus. In addition, in order to remain below the inspiratory muscle fatigue threshold, as the severity of airway obstruction increased, patients adopted a breathing strategy characterized by decreased T i/ Ttot as inspiratory pressure demand increased.