Muscle specific force testing.

Methods for assessing DIAm isometric force have been described previously (15, 22, 26, 29, 38, 39, 42). Briefly, an ~2-mm-wide strip was suspended within a tissue bath, with the costal margin clamped and the central tendon tied with silk and attached to a force transducer (6350; Cambridge Technology, MA). Optimal DIAm length (L_o) and supramaximal stimulus settings were established in a manner identical to past rat studies (15, 38, 39). Electrical field stimulation was achieved via platinum plate electrodes placed on either side of the muscle, with stimulation current provided with a stimulator (701C; Aurora Scientific, ON, Canada). Supramaximal (~150 mA) stimulus pulses (0.05-ms duration) were delivered at 5, 10, 20, 30, 40, 50, 75, and 100 Hz in 1-s trains (15, 38, 39). Output of the force transducer data was digitized (1 kHz sampling rate) and recorded in LabChart software (ADInstuments, Dunedin, New Zealand). Specific force of the DIAm was calculated by normalizing force to the estimated cross-sectional area of the DIAm strip {muscle cross-sectional area = muscle strip weight (g)/[L_o (cm) × 1.056 g/cm³]} and expressed as newtons per square centimeter. In past studies using Fischer 344 rats, no differences were observed between males and females in isometric force generation of the DIAm, so these groups were combined for analysis (20, 22, 39).