PT on the Net Research

Muscle Fibers

Have you ever noticed your clients can exercise for long periods of time but get tired quickly when lifting heavy weights? Or they can lift heavy weights but can’t run for more than five minutes on the treadmill? The reason why some clients can run faster and get bigger muscles more easily, and others are able to run for longer periods of time without fatigue, lies in their muscles. Muscle fiber make-up contributes heavily to individual training strengths and weaknesses.

Types of Muscle Fibers

Humans have three different types of muscle fibers, the proportions of which are genetically determined (see Figure 1). Slow-twitch (ST) or Type I fibers are used for aerobic activities requiring low-level force production, such as walking and maintaining posture. Most daily activities use ST fibers, which have characteristics needed for good endurance, such as perfusion with a large network of capillaries to supply oxygen, ample myoglobin to transport oxygen, and lots of mitochondria — “aerobic factories” that contain enzymes responsible for aerobic metabolism. To excel at aerobic activities, an individual needs a large proportion of ST fibers.

Fast-twitch (FT) or Type II fibers are used for short, intense exercise, such as sprinting and weight lifting and are further divided into fast-twitch A (FT-A or Type IIa) and fast-twitch B (FT-B or Type IIb) fibers. FT-A fibers contain both endurance and power characteristics and represent a mid-point between ST and FT-B fibers. They are recruited for prolonged anaerobic activities with a relatively high-force output, such as running a long sprint and carrying heavy objects. FT-B fibers, which are sensitive to fatigue, are recruited for short anaerobic, high-force activities, like sprinting, jumping and lifting heavy weights. To be a good sprinter or jumper, an individual needs a large proportion of FT-B fibers since they contract about 10 times faster than ST fibers.

Figure 1

The differences in the contractile properties that give the fibers their names can be explained in part by a specific component of the myosin filament—the myosin heavy chain—that exists in three different varieties, or isoforms: Type I, IIa and IIb. In addition, the rate of release of calcium by the sarcoplasmic reticulum (the muscle’s storage site for calcium) and activity of the enzyme (myosin-ATPase) that breaks down ATP inside the myosin head influence the speed of contraction among the fiber types. Both of these characteristics are faster and greater in the Type II fibers.

Individuals are born with a specific percentage of ST, FT-A, and FT-B fibers and actual percentages vary from muscle to muscle and person to person. At any given velocity of movement, the amount of force produced depends on the fiber type. During a dynamic contraction, when the fiber is either shortening or lengthening, a FT fiber produces more force than a ST fiber. Under isometric conditions, during which the length of the muscle does not change while it is contracting, ST fibers produce exactly the same amount of force as FT fibers. The difference in force is only observed during a dynamic contraction. At a given velocity, the force produced by the muscle increases with the percentage of FT fibers and, at a given output, the velocity increases with the percentage of FT fibers. As the velocity of movement increases, the force produced by the muscle decreases, regardless of fiber-type distribution.

Recruitment of Muscle Fibers

Muscles produce force by recruiting motor units, a group of muscle fibers innervated by a single motor neuron. All muscle fibers of a motor unit are of the same type (ST, FT-A, or FT-B). During voluntary contractions, the pattern of recruitment is controlled by the size of the motor unit, a condition known as the size principle. Regardless of exercise intensity, ST motor units (with the lowest firing threshold) are always recruited first and if the exercise intensity is low, ST motor units may be the only fibers recruited. Demands for larger forces are met by the recruitment of increasingly larger motor units. If the exercise intensity is high, such as when lifting heavy weights or cycling fast, ST motor units are recruited first, followed by FT-A and, finally, FT-B, if needed.

Determining Fiber Type in Clients

The best way to directly determine your clients’ fiber type make up is with a muscle biopsy, during which a needle is stuck into the muscle and a few fibers are plucked out to be examined under a microscope. Since research has repeatedly shown that there is a significant, positive relationship between the proportion of FT fibers and muscular strength and power, it’s possible to estimate your clients’ fiber types (without biopsy) by measuring their muscular strength and/or speed (see Figure 2).

Figure 2

Implications for Training

Your clients’ fiber type proportion will play a major role in the amount of weight they can lift, the number of reps per set they can complete, and the desired outcome. A client with a greater proportion of FT fibers won’t be able to complete as many reps at a given weight as a client with a greater proportion of ST fibers and, therefore, won’t attain as high a level of muscular endurance as the client with a greater proportion of ST fibers. Conversely, a client with a greater proportion of ST fibers won’t be able to lift as heavy a weight or run as fast as someone with a greater share of FT fibers.

There is no evidence that a ST fiber can be converted to a FT fiber as a result of training but training for endurance, strength, or power can cause changes in the expression of the enzyme myosin ATPase and myosin heavy chains. These changes in expression can result in an altered contractile function of myosin that favors the specific demand of training. So, through targeted training FT-B fibers can take on some of the endurance characteristics of FT-A fibers and FT-A fibers can take on some of the strength and power qualities of FT-B fibers. Heavy strength training (6 to 8 RM) has also been shown to decrease the percentage of Type IIB fibers while increasing the percentage of Type IIA fibers. It seems that any interconversion of fibers that exists is limited to the FT fiber subtypes.

Although the type of fiber cannot be changed from one to another, training can change the amount of area taken up by the fiber type in the muscle. In other words, there can be a selective hypertrophy of fibers based on the type of training. For example, your client may have a 50/50 mix of FT/ST fibers in a muscle, but since FT fibers have a larger cross-sectional area than ST fibers, 65 percent of that muscle’s area may be FT and 35 percent may be ST. Following an intense strength training program, the number of FT and ST fibers will remain the same (still 50/50); however, the cross-sectional area will change. This happens because the FT fibers will hypertrophy. Depending on the specific training stimulus, the muscle may change to a 75 percent FT area and a 25 percent ST area. The change in area will lead to greater strength but decreased endurance. In addition, since the mass of FT fibers is greater than that of ST fibers, your client will gain muscle mass. Hypertrophy only occurs in those muscle fibers that are overloaded, so the FT-B motor units must be recruited during training in order to be hypertrophied. Training with a low or moderate intensity will not necessitate the recruitment of FT-B motor units. Therefore, the training intensity must be high to achieve this result.

Alternately, if your client trains for muscular endurance, the FT fibers will atrophy while the ST fibers hypertrophy which can create a greater area of ST fibers. The area of the muscle, which began at 65 percent FT and 35 percent ST before training, may change to 50 percent FT and 50 percent ST following training. The endurance capabilities of the muscle will increase while its strength will decrease.

While most people workout to achieve a specific goal, your clients’ training should also reflect their physiology. If a client has more ST fibers, he or she is best suited for endurance activities and his or her training should focus on aerobic exercise or weight training using more reps and lighter weight. If a client has more FT fibers, he or she is best suited for anaerobic activities and his or her training should focus on anaerobic exercise and weight training, using fewer repetitions of a heavier weight. If a client wants to get stronger and faster, increase the intensity of weight training and speed in cardio workouts as his or her training progresses. Conversely, if your client wants to increase endurance, increase the duration of cardio workouts and the number of reps in his or her strength training workouts.

To maximize your clients’ training, tailor it to match their muscle fiber composition. If they train smart enough, not only will they have the best results, they’ll have something interesting to talk about with their friends.

Figure 3


  1. Adams, G.R., Hather, B.M., Baldwin, K.M., and Dudley, G.A. (1993). Skeletal muscle heavy chain composition and resistance training. Journal of Applied Physiology. 74(2):911-915.
  2. Andersen, J.L. and Aagaard, P. (2000). Myosin heavy chain IIX overshoot in human skeletal muscle. Muscle Nerve. 23:1095-1104.
  3. Andersen, J.L., Schjerling, P., and Saltin, B. (2000). Muscle, genes, and athletic performance. Scientific American. 283(3):48-55.
  4. Bacou, F., Rouanet, P., Barjot, C., Janmot, C., Vigneron, P., and d’Albis A. (1996). Expression of myosin isoforms in denervated, cross-reinnervated, and electrically stimulated rabbit muscles. European Journal of Biochemistry. 236(2):539-547.
  5. Barjot, C., Laplace-Marieze, V., Gannoun-Zaki, L., Mckoy, G., Briand, M., Vigneron, P., and Bacou, F. (1998). Expression of lactate dehydrogenase, myosin heavy chain and myogenic regulatory factor genes in rabbit embryonic muscle cell cultures. Journal of Muscle Research and Cell Motility. 19(4):343-351.
  6. Bosco, C. and P.V. Komi, P.V. (1979). Potentiation of the mechanical behavior of the human skeletal muscle through pre-stretching. Acta Physiologica Scandinavica. 106(4):467-472.
  7. Coyle, E.F., Costill, D.L., and Lesmes, G.R. (1979). Leg extension power and muscle fiber composition. Medicine and Science in Sports and Exercise. 11(1):12-15.
  8. Cress, N.M., Peters, K.S., and Chandler, J.M. (1992). Eccentric and concentric force-velocity relationships of the quadriceps femoris muscle. Journal of Orthopaedic and Sports Physical Therapy. 16(2):82-86.
  9. Fitts, R.H., and Widrick, J.J. (1996).Muscle mechanics: adaptations with exercise training. Exercise and Sport Science Reviews. 24:427-473.
  10. Froese, E.A., and Houston, M.E. (1985). Torque-velocity characteristics and muscle fiber type in human vastus lateralis. Journal of Applied Physiology. 59:309-314.
  11. Gerdle, B., Wretling, M.L., and Henriksson-Larsen, K. (1988). Do the fiber-type proportion and the angular velocity influence the mean power frequency of the electromyogram? Acta Physiologica Scandinavica. 134(3):341-346.
  12. Gregor, R.J., Edgerton, V.R., Perrine, J.J., Campion, D.S., and DeBus, C. (1979). Torque-velocity relationships and muscle fiber composition in elite female athletes. Journal of Applied Physiology. 47(2):388-392.
  13. Griffin, J.W., Tooms, R.E., VanderZwaag, R., Bertorini, T.E., and O’Toole, M.L. (1993). Eccentric muscle performance of elbow and knee muscle groups in untrained men and women. Medicine and Science in Sports and Exercise. 25(8):936-944.
  14. Harigaya, S., and Schwartz, A. (1969). Rate of calcium binding and uptake in normal animal and failing human cardiac muscle. Membrane vesicles (relaxing system) and mitochondria. Circulation Research. 25:781-794.
  15. Henneman, E., Clamann, H.P., Gillies, J.D., and Skinner, R.D. (1974). Rank order of motoneurons within a pool: law of combination. Journal of Neurophysiology. 37(6):1338-1349.
  16. Staron, R.S., Malicky, E.S., Leonardi, M.J., Falkel, J.E., Hagerman, F.C., and Dudley, G.A. (1990). Muscle hypertrophy and fast fiber type conversions in heavy resistance-trained women. European Journal of Applied Physiology and Occupational Physiology. 60(1):71-79.
  17. Suter, E., Herzog, W., Sokolosky, J., Wiley, J.P., and Macintosh, B.R. (1993). Muscle fiber type distribution as estimated by Cybex testing and by muscle biopsy. Medicine and Science in Sports and Exercise. 25(3):363-370.
  18. Thorstensson, A., Grimby, G., and Karlsson. J. (1976). Force-velocity relations and fiber composition in human knee extensor muscles. Journal of Applied Physiology. 40:12-16.
  19. Wilke, K.E., Johnson, R.D., and Levine. B. (1987). A comparison of peak torque values of the knee extensor and flexor muscle group using Biodex, Cybex, and Kin-Com isokinetic dynamometers. Physical Therapy. 67:789-790.