Stretch overload has been shown to increase muscle mass by increasing fiber size and fiber number in the anterior latissimus dorsi (ALD) of adult Japanese quail. However, the functional significance of these changes is not known. The purpose of this study was to determine if stretch-induced hypertrophy alters function of the ALD. A tube weight corresponding to 12% of the bird's body mass was added to one wing of 12 Japanese quail for 30 days. This increased muscle mass corrected for the increase in non-muscle tissue by 121 +/- 3%. In vitro contractile measures were made at 25 degrees C by direct stimulation of the ALD. Fiber hypertrophy accounted for only 60% of the 95% increase in maximal force in stretched muscles. Specific tension was similar in control (11.1 +/- 0.2 mN/mm2) and experimental muscles (9.2 +/- 0.3 mN/mm2). Stretch-enlarged muscles had significantly greater twitch half-relaxation time (161.9 +/- 7.4 vs. 187.7 +/- 9.7 ms) relative to control muscles. The maximal velocity of loaded shortening decreased from 2.6 +/- 0.4 muscle lengths/s in control muscles to 1.1 +/- 0.1 muscle lengths/s in stretched muscles. The maximal velocity of unloaded shortening decreased from 4.2 +/- 0.6 to 2.6 muscle lengths/s in stretched muscles. These data indicate that 1) twitch duration and shortening velocity are slowed in stretch hypertrophied muscle and 2) new fibers contribute to force production after 30 days of stretch overload.