You may have heard some bodybuilders talk about how weight training has a permanent effect on your muscle fibers. Two different ways that the muscles can change through vigrous weight training routines over a course of a few weeks, is through muscle fiber composition changes and hyperplasia.

Before we look at studies and theories, let’s understand how the muscle is made up. There is two major types of muscle fibers, type I and Type II. Type I are functionally best for endurance. Type II are good for strength and power, but not good for endurance. Type II can actually be dividedfurther into fiber subtypes. Type IIB and Type IIA. Type IIB are white colored, unlike the other fibers IIA and I. They are also the most responsive to hypertrophy (Muscle growth). There are many other fiber sub-types in the body, but the most common are Type I, IIA, and IIB.

There is evidence from studies I have found, that muscle fibers type changes from weight training. In other words, One Type can become more like another fiber type, depending on your type of training. This tells us that our muscles adapts to weight training, so it can try to gain muscle easier in the future. When there is more Type II and less Type I acting fibers, it means it’s easier to stimulate future muscle gains. If their is more type I and less Type II, than it becomes harder to gain muscle. It also tell us, that we have to make sure we do the right kind of training to stimulate the right changes in fibers for maximum muscle growth.

Muscle Fiber change research studies

A study published in Eur J Appl Physiol. 2002 Nov;88(1-2):50-60. Epub 2002 Aug 15. 32 men who were untrained were put into low rep, intermediate rep, high rep, and control groups. High rep group did between 20 to 28 reps with 1 minute rest. The intermediate group did between 9 to 11 reps with 2 minutes of rest for 3 sets. The low rep group did 3 to 5 reps with 3 minutes of rest for 4 sets. The excercises chosen for all groups were squat, leg press, and knee extension done 2 times a week for first 4 weeks, then 3 times a week for the final 4 weeks. The low and intermediate rep group had muscle growth (hypertrophy), but the control and high rep group did not have nearly any. Most interesting however, is that all resistance trained groups had a decrease in IIB fibers and increase in IIAB fibers. Type IIAB sub-type fibers are slightly less effective at building muscle and strength than pure IIB fibers. This study tells us that although high reps around 20 or so do not do much for hypertrophy, they induced the same muscle fiber type changes as even the very low rep ranges(3 to 5 reps).

Another study J Gerontol A Biol Sci Med Sci. 2000 Jul;55(7):B336-46 was done on 18 older men. Half did resistance training, the other half served as control. The routine was leg press, half squat, and leg extension for 16 weeks with 6 to 8 reps to muscular failure and 1 to 2 minutes of rest. Results showed that everyone’s muscles hypertrophied, but IIB fibers decreased and IIA fibers increased. Once again, the fibers are changing away from the IIB fibers.

One other study I found published in Eur J Appl Physiol Occup Physiol. 1990;61(1-2):37-41 They had 12 college age men do strength resistance training for 7.5 weeks, than 5 week break to then go onto 5.5 weeks of endurance training (high reps light weight). The other group started with endurance then went to strength. Results showed that both groups in phase 1 had hypertrophy and increase in sizes of all the major fiber types. The endurance group that did strength in the 2nd phase, also had more hypertrophy gains in all fibers during the 2nd phase. However, the group that went from strength to endurance in 2nd phase, reversed all hypertrophy gains in all fibers. This tells us that endurance training after strength training reverses all hypertrophy in all the muscle fiber types. The reason the endurance group probably had some hypertrophy even in phase one, is probably cause they were untrained (not previous lifters).

The results are stunning for me from these studies. I thought it would show a change to more IIB at the lower rep ranges. Instead, all the common rep ranges of lifting for bodybuilders, whether they be in the 6 rep range or the 20 rep range, all had similiar changes in muscle fiber types. The studies also show us that the common rep range bodybuilders use between 6 to 12 reps, all stimulate the major 3 types I, IIA, and IIB for hypertrophy simultaneously. This is well known by bodybuilders and this is why it is the best rep range for muscle gains. The last study cited is useful in that it tells us that switching to endurance training after strength training, will lead to muscle shrinking. This is something bodybuilders have known all along from experience. You can’t maintain muscles lifting weights at the 15 + rep range, as you had gained from lifting in the 6-12 rep range. This is one reason why I don’t advocate high reps during dieting.