T Nation

Estimating Percentage Muscle Fiber Type

Other than a muscle biopsy, I know there is no accurate way of assessing fiber type percentage. However, I came across the following estimation process and was wondering if anyone thinks it’s valid.

Do as many reps as possible with 80% of your 1 RM.

If you did less than 7 repetitions the muscle group is likely to be 50% or more fast-twitch fibers.

If you did greater than 12 repetitions the muscle group is likely to be 50% or more slow-twitch fibers.

I can’t imagine how anyone could possibly be able to get more than 12 reps using 80% of 1 RM!

How does everyone rate?


P-dog: knowing your muscle fibre type per bodypart will allow you to create a program that better suits your goals. There are at least 4 ranges to be concerned with, type I, type IIa, type IIb and type IIc. If your soleus is determined to be mostly type I fibre, then training in a range that best stimulates the type IIc fibres is not going to get you the max. results. This is why some people swear by using 4-6r to grow best and yet others swear by 12-15r. Neither is right or wrong for their individual type. This is why women should generally use a higher rep range than men. They don’t have quite as much fast twitch fibre as their male counterparts, same with inexperienced lifters. Neural capacity can improve with training years, however even with years of experience, some people will always be better off using higher reps. This is due to genetic muscle fibre make-up. You can’t make a dominant change in this area. You can improve the efficiency of your lesser fibre type make-up, but the dominant fibre type will always prevail. This is why it is widely recognized among track coaches that a world class sprinter is born, not made. There is always room for improvement among those with lesser genetics, but the best were born to be the best. Maurice Green would never have been a top distance runner, no matter what type of training he did. In general, it is recognized that a resistance of at least 70%max is necessary to illicit a growth response in muscle. If one performs 6RM for an exercise, yet that weight is only 60%of max., there is no growth response. This is the case with a highly fast twitch dominant trainee. He will not get the best results with this weight. He will have to lower his reps, therefore increasing his %of max.weight up to say 75% at 4reps. He will now see a growth response in that muscle, while at 6reps and above he did not. Now, a predominantly slow twitch individual will have to use a weight that is more like 95% max to fail at 4RM, therefore he could, and should, use much higher rep ranges to target those slow twitch fibres and make due with the remaining 25%of max. weight that he has in reserve. He could use reps in the 15range and still be at a work capacity above 70%, therefore taking advantage of this growth potential.