The calves are loaded with both slow-twitch and fast-twitch muscle fibers. Slow-twitch fibers are stimulated through the use of high-repetition, low-weight training. Until you get past the 12th repetition, these fibers remain dormant, untrained, and therefore incapable of growth. Fast-twitch fibers (those most trained by bodybuilders and powerlifters) are used during very heavy sets (80% or above 1-rep maximum) are trained primarily during low-rep sets. If you train using under 10 repetitions, you are hitting the fast-twitch fibers. If you train using over 10 reps, you are hitting the slow-twitch fibers. Both types of training are required to simulate both types of muscle fibers, and create the most potential for growth in the muscle group.

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