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.(ABSTRACT TRUNCATED AT 250 WORDS)