FORCE AND CONTRACTILE CHARACTERISTICS AFTER STRETCH OVERLOAD IN QUAILANTERIOR LATISSIMUS-DORSI MUSCLE

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 ch anges is not known. The purpose of this study was to determine if stre tch-induced hypertrophy alters function of the ALD. A tube weight corr esponding to 12% of the bird's body mass was added to one wing of 12 J apanese quail for 30 days. This increased muscle mass corrected for th e increase in nonmuscle tissue by 121 +/- 3%. In vitro contractile mea sures were made at 25 degrees C by direct stimulation of the ALD. Fibe r hypertrophy accounted for only 60% of the 95% increase in maximal fo rce in stretched muscles. Specific tension was similar in control (11. 1 +/- 0.2 mN/mm(2)) and experimental muscles (9.2 +/- 0.3 mN/mm(2)). S tretch-enlarged muscles had significantly greater twitch half-relaxati on time (161.9 +/- 7.4 vs. 187.7 +/- 9.7 ms) relative to control muscl es. 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 dec reased from 4.2 +/- 0.6 to 2.6 muscle lengths/s in stretched muscles. These data indicate that 1) twitch duration and shortening velocity ar e slowed in stretch hypertrophied muscle and 2) new fibers contribute to force production after 30 days of stretch overload.