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When you engage in strength training, your body builds muscle. However, not everyone builds the same type of muscle fibers. Whether you develop fast-twitch or slow-twitch muscle fibers depends on the types of exercises you do when lifting weights. Understanding your weightlifting goals and the exercises you should do to achieve them will give you insight into which type of muscle fibers you are developing, or if you are building both.
Types of Muscle Fibers
There is one type of slow-twitch muscle fibers, and there are two types of fast-twitch muscle fibers. Slow-twitch fibers have a high resistance to fatigue. This is because they work by breaking down oxygen, which can consistently generate energy using oxidative metabolic processes. Type I fast-twitch fibers use a combination of oxygen and glycogen for energy. They contract a little faster than slow-twitch fibers and are somewhat fatigue resistant. Type II fast-twitch muscle fibers primarily use glycogen for energy, so they fatigue quickly due to the lactic acid buildup that results when the body breaks down glycolytic enzymes for energy.
Some weightlifters train for endurance rather than strength and will have more slow-twitch muscle fibers. These athletes train aerobically and usually include distance runners, swimmers and cyclists. Since slow-twitch muscle fibers contract slowly, they provide consistent, steady energy, allowing you to continue using them for extended periods of time. This is ideal for exercises that require endurance. Slow-twitch fibers do not account for explosive movements that require quick bursts of energy, but they can continuously feed oxygen to the muscles to prevent the lactic acid buildup that makes your muscles feel fatigued.
Those weightlifting for strength and power, like sprinters and powerlifters, usually have more fast-twitch muscles fibers. These athletes train anaerobically, so they do not activate slow-twitch fibers which require oxygen for energy. Those who lift extremely heavy loads tend to develop more type II fast-twitch fibers. This is because these types of fibers contract quickly and can efficiently provide bursts of energy for powerful movements. However, these fibers fatigue quickly due to lactate accumulation, so have to rest often.
Developing Different Muscle Fibers
To develop slow-twitch fibers you have to train aerobically. When strength training, lift weights that are light enough for you to complete 12 to 20 reps per set, and rest for about 60 to 90 seconds between reps. To develop fast-twitch muscle fibers you need to train anaerobically. Lift weights that are heavy enough so that you can only complete about four to six reps per set, and rest for about three to five minutes between sets. To get a balance of both strength and endurance, which will develop slow-twitch and both types of fast-twitch fibers, you need to do a combination of both forms of training. In this case, lift weights that will allow you to complete eight to 12 reps per set, and rest two to three minutes between sets.