by Andrew R. Coggan, Ph.D.
As many readers of this blog are undoubtly aware, the skeletal muscles of humans and other animals can be classified into various "types". A number of such classification schemes exist, but the most common approach is to characterize muscle fibers based on their speed of contraction, which is primarily determined by the isoform of myosin protein they express. Thus, in simplest terms muscle fibers can described as slow-twitch, or type I, or fast-twitch, or type II. In addition to being slower to contract (and relax), type I muscle fibers tend to be smaller, but have more mitochondria and are surrounded by more capillaries, than type II muscle fibers located within the same muscle. As a result of these (and other) differences, "tonically-active" type I fibers tend to be less powerful but more resistant to fatigue, whereas "phasically-active" type II fibers are generally more powerful but also fatigue more rapidly. (Note that many, if not all, muscle fiber properties mentioned in this blog entry change in response to exercise training. However, the inherent differences between type I and type II fibers, even if markedly diminished, will generally tend to remain.)
Given the above, it is perhaps not surprising that, at least at the elite level, endurance athletes tend to have more type I fibers than average, whereas athletes in sprint sports tend to have more type II fibers. For example, in 1976 Costill and coworkers obtained biopsy samples from the gastrocnemius (calf) muscle of 40 male and female international-caliber track-and-field athletes (1). Although the fiber type distribution of those competing in field events was notably quite unexceptional, the gastrocnemius of the distance (5000 m to marathon) runners was composed of ~70% type I and ~30% type II fibers, whereas that of the sprint (100 m) runners was ~25% type I and ~75% type II. (The gastrocnemius of the average untrained individual usually contains 55-60% type I and 40-45% type II fibers(2).) As a result of the study by Costill et al., as well as numerous others, it is now well-established that muscle fiber type distribution can be an important determinant of athletic performance.*
Presented with the above information, it is natural for any athlete to wonder about their own personal fiber type distribution – in fact, it was partially because of such curiousity that I first volunteered for a research study involving muscle biopsies approximately 30 y ago. The muscle biopsy procedure, however, is somewhat invasive, and although it is generally quite safe, it is not entirely without risks. As well, the variability in determining the percentage of type I and type II fibers based on a single biopsy can be quite large (3), meaning that multiple samples may need to be obtained (ideally from multiple muscles) to really “nail down” someone’s true fiber type distribution. Thus, few, if any, exercise physiologists would argue that it is worth having a biopsy performed simply to satisfy an athlete’s curiousity, or even in hopes of improving their performance by altering their approach to training, the tactics they use in races, the events they choose to enter, etc. On the other hand, if information regarding an individual’s muscle fiber type were more easily obtained, at least in theory it could prove valuable in this regard, and if nothing else, might help satisfy their curiousity.
The purpose of this series of blog entries, then, is to describe two equations for predicting an individual's muscle fiber type distribution based on data easily collected using a powermeter. Specifically, in part 2 I will discuss how to do so based on force-velocity (really, power-velocity) measurements. This method is the more precise of the two, but requires use of an SRM powermeter, as none of the other devices currently on the market appear to provide data with sufficient fidelity and temporal resolution to utilize this approach. Thus, in part 3 I will describe how to estimate fiber type based on measurement of fatigue resistance. Being based on a secondary characteristic (i.e., fatigability vs. contractile properties) of the different muscle fiber types, this method is less precise, but has the advantage of being available to all powermeter users, not just those who own SRM cranks.
*Interestingly, however, this influence seems to be less evident in cycling than in running. For example, in a study of road cyclists Burke et al. (4) found no difference in fiber type distribution of the v. lateralis (thigh) muscle between those who had achieved national or international success and those who had not. Along the same lines, Mackova et al. (5) found that although international caliber match sprint cyclists had a greater percentage of type II fibers in the v. lateralis than non-athletes, the difference observed was less than previously reported for track-and-field sprinters by Costill et al. (1). The reason for this is not known. It may, however, be because in road racing the dynamics of pack cycling would tend to disfavor those who have an extremely high percentage of type I fibers, whereas in track racing access to different gears on a bicycle would tend negate some of the advantage provided by having an extremely high percentage of type II fibers.
1. Costill DL, Daniels J, Evans W, Fink W, Krahenbuhl G, Saltin B. Skeletal muscle enzymes and fiber composition in male and female track athletes. J Appl Physiol 1976; 40:149-154.
2. Coggan AR, Spina RJ, Rogers MA, King DS, Brown M, Nemeth PM, Holloszy JO. Histochemical and enzymatic comparison of the gastrocnemius muscle of young and elderly men and women. J Geront 1992; 47:B71-B76.
3. Nygaard E, Sanchez. Intramuscular variation of fiber types in the brachial biceps and the lateral vastus muscles of elderly men: how representative is a small biopsy sample? J Anat Rec 1982; 203:451-459.
4. Burke ER, Cerny F, Costill D, Fink W. Characteristics of skeletal muscle in competitive cyclists. Med Sci Sports 1977; 9:109-112.
5. Mackova E, Melichna J, Havlickova L, Placheta Z, Blahova D, Semiginovsky B. Skeletal muscle characteristics of sprint cyclists and nonathletes. Int J Sports Med 1986; 7:295-297.