Assessment division, INFS
• Most people are born with an equal split of fast twitched and slowly twitched fibers and in fact, almost all the muscle groups in your body have an even split of both.
• Muscle fiber recruitment is dictated by the Henneman’s size principle. Type I, which have lowest firing threshold, are activated first during any activity and as the force requirement increases, Type II are also recruited.
• No clear evidence that different types of training can lead to the preferential development of Type I or II muscle fibers. Both heavy and lighter weights, low and high lifting velocity can result in equal muscle gain when training to volitional failure.
Keywords: Muscle, fiber, training, failure, tempo
Every muscle in our body is made up of bundles of muscle fibers. As discussed in the first article, muscle fibers can be fractioned as type I and type II. Type I muscle fibers are recruited during the long duration and low-intensity activities whereas type II muscle fibers are employed during short duration, high-intensity activities. Now, most people are born with 50:50 split of fast twitched and slowly twitched fibers and in fact, almost all the muscle groups in our body have even split of both. However, few muscle groups are dominant in either of them. Generally, upper body muscles are higher in Type II and lower body has more type I. So, does that mean that different muscle groups need to be trained differently based on their fiber composition? Let’s understand.
The size principle:
During resistance training, muscle fibers are activated based on the Henneman’s size principle, in which the low firing threshold motor units are recruited first, followed by high firing threshold motor units . Therefore, Type 1, which have lowest firing threshold, are activated first during any activity and as the force requirement increases, Type II are also recruited. Now if the workload intensity is low, Type I may be the only one to be recruited. Since fiber recruitment is the key factor to generate hypertrophy, training to failure or close to failure should, therefore, produce the maximum results.
Can different training types give different results?
It has been postulated that hypertrophy of muscle fibers may be load-specific. Since type II fibers have the maximum potential for growth, greater force production ability and have fast contraction time, high load-low repetition training with fast lifting velocity should result in maximum hypertrophy of type II fibers while training with lower loads and greater time under load (TUL) should favor the growth of type I.
The current evidence, however, is mixed on this. For example, this study  supported the above hypothesis by showing some very interesting results:
➢ The greater increase was observed in type I muscle fibers with light weight training (50% of 1RM) and type II with heavy weight training (80-85% of 1RM).
➢ Greater overall hypertrophy in the low load training group.
On the other hand, this study  showed exactly contrasting results:
➢ A similar increase in both the muscle fiber types in high load (70-90% of 1RM) and low load (30-50% of 1RM) training.
➢ Similar muscle growth in both the groups.
Thus, it’s still unclear whether different fibers respond better to different types of training but both heavy and light load training have been found to elicit a similar muscle growth response when training volume is equated. This implies that there is no “hypertrophic range” practically. You can absolutely grow effectively when training with low reps and high reps [2,3].
The effect of lifting velocity can be understood by the force-velocity relationship. The velocity of the lifting during an exercise is inversely related to the load you are lifting. Velocity is faster while lifting lighter loads and decreases gradually with the increment in the training load. Now different studies have shown almost equal muscle gain at high load low velocity and low load high-velocity training when trained to volitional failure . Thus, it seems that lifting velocity has no influence on the hypertrophy. In fact, moving the weight too fast can create too much momentum which can decrease the muscle contraction by displacing the stress to joints, leading to increased risk of injuries.
Like lifting velocity, TUL is also load-dependent. Conceivably, lower TUL is the high load condition whereas higher TUL can be achieved with a lighter load. Greater TUL has shown to induce greater hypertrophy in type I fibers  but the significance of TUL is again debatable. For example, when the load is very low, although the TUL will increase significantly but the overall hypertrophic response seems to suffer . More than time, the magnitude of the load and overall training volume, thus, are the important factors for gains in strength and muscle mass.
While training, our goal is to work maximum muscle fibers since recruiting more muscle fibers implies greater gains in strength and muscle size.
Most people have a pretty even split of fast-twitched and slow-twitched muscles and the jury is still out on whether different fibers respond better to different types of training. Both heavy load and high load training, with fast and slow lifting velocity, when trained till volitional failure, can result in the similar amount of muscle growth. In fact, many studies have shown maximum benefits of working out with a variety of weight and repetition ranges than just working in one range . Thus, periodizing your workout also makes sense here.
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2. Campos, G. E., Luecke, T. J., Wendeln, H. K., Toma, K., Hagerman, F. C., Murray, T. F., … & Staron, R. S. (2002). Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones. European journal of applied physiology, 88(1-2), 50-60.
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8. Grgic, J., Homolak, J., Mikulic, P., Botella, J., & Schoenfeld, B. J. (2017). Inducing hypertrophic effects of type I skeletal muscle fibers: a new hypothetical paradigm in resistance training aimed at muscular hypertrophy. Medical Hypotheses.
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