PROGRESSIVE STRETCH OVERLOAD OF SKELETAL-MUSCLE RESULTS IN HYPERTROPHY BEFORE HYPERPLASIA

Intermittent stretch of the anterior latissimus dorsi (ALD) muscle produces fiber hypertrophy without fiber hyperplasia (J. Appl. Physiol. 74: 1893-1898, 1993). This study was undertaken to determine if a progressive increase in load and duration of stretch would induce extremely large muscle fiber areas or if the fibers would reach a critical size before the onset of fiber hyperplasia. Weights ranging from 10 to 35% of the bird's mass were attached to the right wing of 26 adult quail while the left wing served as the intra-animal control. The stretch protocol was as follows: day 1 (10% wt), days 2 and 3 (rest), day 4 (15% wt), days 5- 7 (rest), day 8 (20% wt), days 9 and 10 (rest), days 11-14 (25% wt), days 15 and 16 (rest), and days 17-38 (35% Wt). Birds were killed after 12, 16, 20, 24, and 28 days of stretch not including rest days. Muscle mass increased 174% (12 days), 196% (16 days), 225% (20 days), 264% (24 days), and 318% (28 days). Muscle length increased 60% (12 days), 34% (16 days), 59% (20 days), 50% (24 days), and 51% (28 days). Mean fiber area increased 111% (12 days), 142% (16 days), 75% (20 days), 90% (24 days), and 39% (28 days). Fiber number, which was measured histologically, increased significantly by 82% only in the 28 days of stretch group. The percentage of slow tonic fibers did not change for any of the time points examined. These results suggest that the initial muscle mass increase in progressively overloaded ALD muscle is due to an increase in muscle length and muscle fiber cross-sectional area; moreover, fibers reach a critical size and then fiber hyperplasia occurs.