The use of esophageal and gastric balloons limits measurement of the tension-time index of inspiratory muscles (TTI) during exercise. The aim of this study was to assess whether a noninvasive tension-time index, TT(0.1), given by P(0.1)/PI(max) x TI/Ttot (where P(0.1) is mouth occlusion pressure, PI(max) is maximal inspiratory pressure, and TI/Ttot is duty cycle) could reliably assess TTI during exercise. In seven healthy young men and nine patients with COPD we measured TT(0.1) and TTI (i.e., Pes/Pes(max) x TI/Ttot where Pes is mean esophageal pressure and Pes(max) is maximal static Pes) at rest and during an incremental exercise test. A significant linear correlation (p < 0.02) was found between TT(0.1) and TTI in all normal subjects and patients with COPD. An equation for estimating TTI from TT(0.1) was established for each group. In the normal subjects there was good agreement between estimated and observed data. In five additional normal males studied prospectively, the agreement was also satisfactory and reproducible. In the COPD patients the agreement was poor. In conclusion, in young healthy subjects the changes in TT(0.1) during exercise reflect the changes in TTI, allowing satisfactory estimation of TTI from noninvasive measurements of TT(0.1).