Role of exercise in inducing increases in skeletal muscle fiber number

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Abstract
Twenty cats were conditioned to lift weights with their right forelimb against increasing resistance to receive a food reward. Sixteen cats were used as controls. The exercise intensity at which muscle fiber number increased in the flexor carpi radialis muscle (FCR) during the 34-wk training period, and what functional alterations occurred in exercise performance that could be correlated with the onset of the increase in muscle fiber number was determined. There was a 20.5% (P < 0.02) increase in the number of muscle fibers of the exercised FCR. The increase in fiber number, possibly due to fiber splitting, was restricted to those cats lifting weights greater than 1 kg (high-resistance exercise group, which lifted 1.25 .+-. 0.03 kg). The low-resistance exercise group lifted 0.71 .+-. 0.13 kg with no significant increase in muscle fiber number. The average daily work increased from 1.1-7.0 J (P < 0.001) and the average power per event increased from 0.19 to 3.4 J/s (P < 0.001) during the prolonged exercise period. The event time (time required to lift the weight) decreased from 651-205 ms (P < 0.025) especially after the 1-kg level was passed. There was a significant increase in oxidative fiber diameters for the low-resistance exercise group, whereas the high-resistance group exhibited increased diameters in all fiber types. The maximum increase in diameter was about 11% for the high-resistance group, hence the gain in muscle cross-sectional area was due to increases in fiber number and cross-sectional area. There was no significant shift in fiber types or fiber proportions when compared with controls.