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What Makes Some People Faster or Stronger?

Ever notice that some of your clients can do cardiovascular exercise for long periods of time but tire quickly when lifting weights? Or that others can lift heavy weights but run only 5 minutes on the treadmill? The reason why some clients can run faster or longer or get bigger muscles more easily than others lies in their muscles. The specific types of fibers that make up individual muscles greatly influence the way your clients adapt to their training programs. Humans have different types of muscle fibers (as well as gradations between them). The proportions from person to person are genetically determined, although the exact ratios may be amenable to change with specific training.

  • Slow-twitch (type I) fibers are recruited for aerobic activities and therefore have many characteristics needed for endurance, such as perfusion with a large network of capillaries to supply oxygen; lots of myoglobin to transport oxygen; and lots of mitochondria–the aerobic factories that contain enzymes responsible for aerobic metabolism. True to their name, slow-twitch fibers contract slowly but are very resistant to fatigue.
  • Fast-twitch (type II) fibers are recruited for anaerobic activities and therefore have many characteristics needed for strength, speed and power, such as large stores of creatine phosphate and glycogen and an abundance of enzymes involved in the anaerobic metabolic pathway of glycolysis. They contract quickly but fatigue easily. Fast-twitch fibers come in two forms: fast-twitch A (type IIa) and fast-twitch B (type IIb). Fast-twitch A fibers, which represent a transition between the two extremes of slow-twitch and fast-twitch B fibers, have both endurance and power characteristics. They are recruited for prolonged anaerobic activities that require relatively high forces, such as running a long, controlled sprint and carrying heavy objects. They are more fatigue-resistant than the fast-twitch B fibers, which are recruited only for short, intense activities, such as jumping, sprinting at full speed and lifting very heavy weights.

It is well known that aerobic athletes have a greater proportion of slow-twitch fibers, while anaerobic athletes have more fast-twitch fibers (Ricoy et al. 1998). The greater proportion of fast-twitch fibers in anaerobic athletes enables them to produce greater muscle force and power than their slow-twitch-fibered counterparts (Fitts & Widrick 1996). Fast-twitch fibers are the main contributors to force production during maximal ballistic movements, such as sprinting and jumping.

How This Affects Your Clients

Fiber type proportions will play a major role in the amount of weight your clients can lift, the number of repetitions they can complete per set, and the desired outcome (e.g., increased muscular strength or endurance). For example, a client with a greater proportion of fast-twitch fibers won’t be able to complete as many repetitions at a given percentage of his or her one-repetition maximum (1-RM) as will a client with a greater proportion of slow-twitch fibers–and therefore will not attain as high a level of muscular endurance as will the slow-twitch-fibered client.

Similarly, a client with a greater proportion of slow-twitch fibers won’t be able to lift as heavy a weight or run as fast as will a client with a greater proportion of fast-twitch fibers–and therefore won’t be as strong or powerful as will the fast-twitch-fibered client.

Tailoring Training to Dominant Fiber Type in Clients’ Programs. To focus on a specific goal, your clients’ training should reflect their physiology. For example, if a client has more slow-twitch fibers, he or she is best suited for endurance activities, and the program should focus on aerobic exercise or muscular endurance training, using more reps of a lighter weight. If a client has more fast-twitch fibers, he or she is best suited for anaerobic exercise and weight training for muscular strength, using fewer repetitions of a heavier weight.

However, if a client has more slow-twitch fibers but wants to get stronger and faster, you should try to increase the intensity of the weight training workouts and the speed of the cardio workouts as training progresses. Conversely, if a client has more fast-twitch fibers but wants to increase endurance, you should try to increase the duration of the cardio workouts and the number of repetitions in the resistance training program as training progresses.

To determine whether a specific client has more slow-twitch or fast-twitch fibers, and to view the full list of article references, see “A Primer on Muscles” in the online IDEA Library or in the May 2010 issue of IDEA Fitness Journal.

Jason Karp, PhD

A professional running coach, freelance writer, fitness consultant and PhD candidate in exercise physiology at Indiana University. He coaches runners of all levels through RunCoachJason.com.

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