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The New High Intensity # 2.

Remember, when the force of a repetition becomes too great for the type I fibers, the type II fibers must be recruited to continue moving the weight. Find out how what you need to know about type IIb fibers!

NCSF Certified Personal Trainer
NCSF Certified Nutritionist/Weight Management Specialist

Note: This is part two, click here for part one!

In my last article I said I think our definition of intensity needs to be changed, however intensity in training is definied by the percentage of maximum effort exerted in a single moment. Therefore, the closer a weigh is to a maximal lift, the higher the intensity will be. In addition, true high intensity in a non-maximal set will only be achieved when a set is taken to failure, and maximum effort will only be achieved on the last repetition.

What's interesting to note in both of these cases is that when maximum intensity is achieved, the type IIb muscle fibers are activated at the greatest level. Therefore, the type IIb muscle fibers must be involved in high intensity training. This may seem to contradict my previous suggestions of training in a high repetition range some weeks, and of course extremely low repetition training is not the most conducive method for building muscle.

What Rep Range Is Best For Gains In Muscular Size?

Bodybuilders seem to realize the best gains in muscular size when keeping their repetition range between 6-10, while strength gains are most obvious with lower repetition ranges. Due to progression and the correspondance of strength to muscular size, one would assume there is no reason to lift in a higher repetition range, but I have explained that lifting in higher repetition ranges will improve recovery by bringing more blood into the muscle and giving the tendons more time to recover.

The higher repetition ranges stress the type I (slow-twitch) muscle fibers more so than lower repetition ranges, and increase mitochrondria and red blood cell density in the muscle. Both of these factors can limit overall muscular growth, but the individual needs to find a balance between working in repetition ranges that are more effective for muscle hypertrophy and working in repetition ranges that that are more effective for recovery and priming the muscle cells for greater hypertrophy.

Since the type II fibers experience a greater increase in hypertrophy than the type I fibers, it makes sense to focus training on the type II fibers. However, there are both type IIa and type IIb muscle fibers, and research has detected even more varieties of type II muscle fibers. Type IIb fibers produce maximal force, but are fatigued quickly and are slower to recover. Type IIa fiber are fast-twitch fibers that have endurance properties similar to type I fibers. Also, type IIb fibers can change to type IIa fibers and type IIa fibers can change to type IIb fibers.

It is impossible to use type II fibers exclusively. During any muscular contraction, type I fibers are activated first. If the type I fibers are not strong enough to get the job done, type IIa fibers and then type IIb fibers are called upon. Therefore type IIa fibers and especially type IIb fibers are only recruited if the intensity is sufficient enough. During the most intense contractions, all fiber types are used in conjunction.

Type IIb Fibers

So now that I've explained intensity as it relates to fiber recruitment, one may think that to maximize muscular gains they should increase IIb fibers as the total percentage of muscle fiber, however this is not the case. Strength athletes have the greatest percentage of type IIb fibers, but they are certainly not the most muscular. They do exert more intensity on their maximal sets, but the duration is not sufficient enough to cause maximum muscular hypertrophy. If these same lifters were to lift in higher repetition ranges, they would achieve greater hypertrophy, however some of their type IIb fibers would become type IIa fibers.

This may not be a bad thing since type IIa fibers are recruited before type IIb fibers and would receive more work in sub-maximal training, therefore reulting in a greater increase in size for more muscle which would be able to become type IIb fibers once the repetition range was lowered. This conversion between the type IIa and IIb fibers could benefit the bodybuilder as well. By training in extremely low rep ranges, type IIb fibers will be maximized and strength will be increased.

When the lifter returns to higher repetition ranges, they will be able to use more weight and thus increase their intensity for greater gains. The repetition range progression I detailed before took advantage of this, but improvements for the individual could be made. Individuals respond differently to different repetition ranges and their recovery varies. An ideal HIT program for maximum muscular development would have an individual varying between a repetition range of 1-5 reps for building strength and a repetition range of 6-10 for building size.

Now, you may ask, what happened to my suggestions of a week at 18-30 reps and a week at 13-17 reps? I already explained that such repetition ranges are for recovery and priming the muscle cells for greater hypertrophy. It is now up to the individual to decide when and how often these repetition ranges should be used. For recovery, the 18-30 repetition range should be used. To prime the muscle cells for new growth if strength or hypertrophy begins to stagnate, the 13-17 repetition range should be used. In my HIT program, the 18-30 rep range week follows the 3-7 rep range week to make sure my tendons recover from the heavy lifting.

The next week I continue with a 13-17 rep range to prime my muscles for maximum hypertrophy during the 8-12 rep range week. I then follow it up with a 3-7 rep range week to continue building muscle while focusing on building strength. The high rep range weeks are not wasted time either as muscle hypertrophy still occurs.

Remember, when the force of a repetition becomes too great for the type I fibers, the type II fibers must be recruited to continue moving the weight. Even if the repetition range is high, if the force of the last few reps is sufficient (which it should be if a reasonably heavy weight is taken to failure), the type II fibers will be recruited.

About The Author

Ian is available for personal training at Lifestyle Family Fitness locations throughout the Tampa Bay area. He is also available for online nutritional consultation regardless of where you live. You can contact Ian at

Note: This is part two, click here for part one!