Your muscles are made up of two different types of muscle fibers: fast-twitch and slow-twitch. Fast-twitch muscles are associated with activities that require short, powerful bursts of activity. Prolonged endurance type of activities is most closely associated with slow-twitch muscle fibers. While you are born with a finite number of each type of muscle fiber, training specificity can create some crossover.
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Slow-twitch muscle fibers rely on the aerobic energy system. The aerobic energy system can be further divided based on the predominant fuel source. In aerobic glycolysis, carbohydrate sources are the primary source of fuel. This track of the aerobic system is used in events such as running a 5k or 8k. In events of longer duration such as a marathon, aerobic lipolysis – body fat, is the primary fuel source.
There are different physical characteristics that determine whether a muscle fiber is primarily slow-twitch or fast-twitch. Slow-twitch muscle fibers tend to be dark red in color which can be attributed to a greater density of capillaries – increased blood flow is needed to transport oxygenated blood to the active muscle as well as a larger amount of myoglobin and higher number of mitochondria. Myoglobin is a protein found in the muscles that is utilized for transporting oxygen. Mitochondria are considered the power-houses of the muscle fibers as this is where energy is generated.
Slow-twitch muscle fibers excel with prolonged endurance activities. Therefore, that is going to dictate the way that you train. Choose long, slow runs or other activities like extended cycling or swimming sessions. If you are unable to exercise for long periods of time, gradually build up your endurance with shorter, more frequent sessions.
Strength training can be directed toward two different avenues: strength or endurance. Muscle strengthening exercises typically utilize heavier weights and fewer repetitions. That type of training is more conducive to fast-twitch muscle fibers. Muscle endurance training uses lighter weights and a greater number of repetitions. When building muscular endurance your strengthening program should be a minimum of three days per week and should include eight to 10 exercises that target the major muscle groups of your body with two to three sets of 15 to 20 repetitions. The weight that you choose to lift should be challenging, but you should be able to lift it in excess of 10 repetitions without experiencing fatigue. Your body weight can be used in lieu of lifting weights when performing squats, lunges and pushups, for example.
- Nutrition for Health, Fitness & Sport – 7th Edition: Chapter Three – Human Energy
- College of the Holy Cross: Muscle Fiber Types
- Journal of Applied Physiology: Single Muscle Fiber Adaptations with Marathon Training
- American College of Sports Medicine: Progression Models in Resistance Training for Healthy Adults