Fiber type proportion

Muscle fibers can be classified into types according to various criteria. One key factor that differs between muscle fibers is the maximum shortening speed that they are capable of achieving. Indeed, some (slow twitch) muscle fibers can only reach very slow maximum shortening speeds, while other (fast twitch) muscle fibers can reach very fast maximum shortening speeds.

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#1. Fiber type proportion affects the velocity-specificity of force production

Some (slow twitch) muscle fibers have a slow maximum shortening speed (approximately 0.5 – 1.0 fiber lengths per second). Other (fast twitch) fibers have faster maximum shortening speeds. However, some fast twitch (type IIA) fibers have only moderately fast maximum shortening speeds (approximately 3 – 4 fiber lengths per second), while other fast twitch (type IIX) fibers have extremely fast maximum shortening speeds (approximately 5 – 6 fiber lengths per second). This makes for big differences between the maximum shortening speeds of most muscle fiber types. Such differences are much larger than the differences in muscle fiber size or maximum strength.

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When we test strength in high-velocity contractions, force can only be produced by those fibers that are capable of shortening at the required speed. Thus, in many high-velocity strength tests (and in many athletic movements such as vertical jumping), force is only produced by fast twitch muscle fibers. The slow twitch muscle fibers are activated, but they cannot produce force at those fast speeds, since their maximum shortening speeds are too slow.

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Consequently, when we test strength at a range of speeds, we find that individuals who have a higher proportion of fast twitch muscle fibers typically achieve superior performances (relative to other individuals) at fast speeds, but not necessarily at slow speeds.

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Conversely, fiber type proportion does not affect powerlifting performance. Individuals with a greater proportion of fast twitch muscle fibers do not necessarily display superior maximum strength. This is because there are only minor differences between muscle fibers of each type in terms of the force that they produce per unit cross-sectional area. In contrast, there are very large differences in maximum shortening velocity.

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#2. What causes fiber type shifts? 

Muscle fiber type shifts commonly occur after most types of exercise, including aerobic exercise and strength training. The fastest and most common shift after all types of exercise is from the fastest fiber type (type IIX) to a moderately-fast type (type IIA). When targeting improvements in the ability to produce high-velocity force, training programs should avoid fiber type shifts. Recent research has shown that fiber type shifts are greater when higher volumes of strength training are performed.

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Moreover, it has been demonstrated that while higher volumes of strength training are beneficial for improving maximum strength (which is measured at low velocities), lower volumes are actually superior for improving the ability to produce force at fast speeds, which is essential for athletic performance.

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Also, training closer to muscular failure appears to produce greater shifts in muscle fiber type (from type IIX to type IIA) as well as smaller improvements in high-velocity strength measures.

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The exact mechanisms that are involved in fiber type shifts are not known. Given that higher volumes and a closer proximity to failure (which involves a higher number of reps in each set) seem to produce larger fiber type shifts, researchers have suggested that the key stimulus could be either [A] the total amount of time that the muscle fiber spends being activated in the workout, or [B] the fatigue that the muscle fiber experiences. Recent research suggests that it is more likely to be the fatigue rather than the time spent being activated, because when total daily exercise volume is controlled, more sustained contractions (involving less fatigue) cause greater fiber type shifts.

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Indeed, if the key mechanism that leads to fiber type shifts is fatigue rather than exercise volume, this would explain why cluster set configurations are superior to normal strength training configurations for improving the velocity end of the force-velocity profile, even when the exact same number of reps are performed in each case.

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Overall, it seems very likely that the mechanism by which muscle fiber type shifts occur towards more oxidative types is by means of calcium ion accumulation (read more).

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Conclusions

Athletes who have larger proportions of fast twitch muscle fibers are able to produce larger forces at fast speeds compared to athletes with smaller proportions of fast twitch muscle fibers, even when they have similar levels of maximum strength. This happens because slow twitch muscle fibers only contribute to whole muscle force if their maximum shortening speeds are faster than the shortening speed of the muscle in the movement. For this reason, slow twitch muscle fibers cannot contribute to whole muscle force production in fast movements like jumping and throwing, even though they are still activated. Since the maximum shortening speed of the working muscle fibers is the key to achieving high movement speeds and a high degree of high-velocity force production, it makes sense to preserve the fastest (type IIX) fibers as much as possible during strength training programs. Muscle fibers shift from type IIX to type IIA during strength training programs that involve higher volumes and closer proximities to failure (most likely because the mechanism by which muscle fibers shift towards a more oxidative type is by means of an accumulation of calcium ions during exercise). Consequently, lower workout volumes, velocity-based training (with small losses in bar speed over a set), and cluster set configurations are able to cause smaller muscle fiber type shifts and enhance gains in high-velocity strength.​