The standard protocol for developing more muscle mass and size is to perform compound exercises with a heavy amount of weight, keeping your repetition range between 6 and 8. This method of training will develop muscle fiber thickness while increasing the volume of the sarcoplasm, the area of the muscle between the fibers and cell wall.
However, some muscles have a greater percentage of red muscle fibers (slow-twitch). These red fibers have more endurance capacity compared to the white fast-twitch fibers. Although most muscle size is developed with the fast-twitch fibers, if you're training a muscle that consists of more slow-twitch fibers, the normal 6-8 rep scheme might not be effective.
To develop muscles that are comprised of more than 50% red slow-twitch fibers, you might have to perform more than 6 to 8 reps per set. The obvious solution is to use less weight, but lighter resistance doesn't build mass as well as heavier weight.
Here's an alternate solution: Incorporate constant tension movements into exercises for muscle groups with a greater percentage of slow-twitch fibers. This best of both world's approach utilizes red slow-twitch fibers that aren't sufficiently activated with heavier resistance by keeping the muscle under tension for a longer time.
The easiest way to accomplish this is to substitute free weights for machines that use a cable and pulley system to deliver resistance. It's a simple concept. If you don't allow the weight plate on the bottom of the stack touch the base during the set, your muscles have to supporting the load keeping them under constant tension. When training muscle groups that require higher volume, make the switch to machines for 6 to 8 weeks and see what develops!
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