Determining the right number of sets and repetitions in a workout is an important job and a very difficult one at the same time. Yes, you can choose a number randomly, but that won’t guarantee maximal strength and muscle increase over time. To successfully find your magic number, you first need to look at yourself, your body type and the physical make up of your muscles. All of this can be determined by knowing all about fast twitch and slow twitch muscle fibers.
It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch muscle fibers and fast twitch muscle fibers. Fast twitch fibers can be further categorized into Type II-a and Type II-b fibers.
This division of muscle fibers affects how muscles react to physical training because every type has a unique ability to contract and every person has a genetically fixed mixture of these two muscle types.
Slow Twitch Muscle Fibers
Slow-twitch muscle fibers have slow contraction time and are very resistant to muscle fatigue (that is why they are also known as endurance muscle fibers). They have a very strong ability for aerobic oxidation. Functionally, slow-twitch muscle fibers are used for aerobic activities requiring low-level force production, such as walking. Most activities of daily living use slow-twitch fibers.
When weight training, you should know that slow twitch muscle fibers react to high rep training in ranges of 12 – 20 repetitions per set.
Fast Twitch Muscle Fibers
Fast twitch fibers have fast contraction time and low resistance to muscle fatigue.They have high contents of glycogen and creatine phosphate, high cross section area and low oxidation capacity. These muscle fibers are used in activities such as sprinting, powerlifting, and bodybuilding i.e. activities where high-level force production for short periods of time is needed.
Fast twitch fibers react to low rep – high weight training in ranges of 3 – 5 repetitions per set.
How to know your muscle fibers type?
There is a method that you can use to determine your muscle composition. First find out your 1RM (max weight that you can do 1 full repetition with) for a certain muscle group. Calculate the weight that equals 80% of your 1RM and try to do as many repetitions as you can. If you can do 1-7 reps then that muscle group is composed of more than 50% fast twitch muscle fibers. If you can do more than 12 reps then the muscle group has more than 50% of slow twitch fibers. If the repetitions you can do are between 8 – 12 then the fiber contents is probably 50 – 50.
In conclusion, training for mass and strength requires working in the 6 – 12 reps range with 80% of your 1RM while training purely for strength requires training in the lower rep range (1 – 5 reps).