DOES STRETCH INDUCE FIBER HYPERPLASIA?
by Jose Antonio PhD
...This animal model was first used by Sola et al. (38) in 1973. In essence, you put a weight on one wing of a bird (usually a chicken or quail) and leave the other wing alone. By putting a weight on one wing (usually equal to 10% of the bird's weight), a weight-induced stretch is imposed on the back muscles. The muscle which is usually examined is the anterior latissimus dorsi or ALD (unlike humans, birds have an anterior and posterior latissimus dorsi). Besides the expected observation that the individual fibers grew under this stress, Sola et al. found that this method of overload resulted in a 16% increase in ALD muscle fiber number.
Since the work of Sola, numerous investigators have used this model (1,2,4-8,10,19,26,28,32,43,44). For example, Alway et al. (1) showed that 30 days of chronic stretch (i.e., 30 days with the weight on with NO REST) resulted in a 172% increase in ALD muscle mass and a 52-75% increase in muscle fiber number! Imagine if humans could grow that fast!
More recently, I performed a study using the same stretch model. In addition, I used a progressive overload scheme whereby the bird was initally loaded with a weight equal to 10% of the its weight followed by increments of 15%, 20%, 25%, and 35% of its weight (5). Each weight increment was interspersed with a 2 day rest. The total number of stretch days was 28. Using this approach produced the greatest gains in muscle mass EVER recorded in an animal or human model of tension-induced overload, up to a 334% increase in muscle mass with up to a 90% increase in fiber number (5,8)! That is pretty impressive training responsiveness for our feathered descendants of dinosaurs.
But you might ask yourself, what does hanging a weight on a bird have to do with humans who lift weights? So who cares if birds can increase muscle mass by over 300% and fiber number by 90%. Well, you've got a good point. Certainly, nobody out there (that I know of), hangs weights on their arms for 30 days straight or even 30 minutes for that matter. Maybe you should try it and see what happens. This could be a different albeit painful way to "train."
But actually the physiologically interesting point is that if presented with an appropriate stimulus, a muscle can produce more fibers!
What is an appropriate stimulus? I think it is one that involves subjecting muscle fibers to high tension overload (enough to induce injury) followed by a regenerative period.
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