Depending on the activity that you are doing throughout your training sessions, you can activate different types of muscle fibers.
Generally speaking, there are two of them:
1. Fast-twitch muscle fibers
2. Slow-twitch muscle fibers
At low levels of intensity (i.e jogging or using a light dumbbell) you only activate the slow-twitch muscle fibers.
Those fibers are fit for low-intensity work that is long in duration and are not really great at producing force and power, so think of these as your ‘endurance muscle fibers’.
The more your training intensity grows however, the more fast-twitch muscle fibers you recruit, in order to endure the load.
These fast-twitch muscle fibers were designed for high-intensity performance - Short, explosive bursts of power.
Think of the fast-twitch muscle fibers as the fibers you want to engage when building muscle, because these fibers have the greatest potential for growth as well as power output.
As we mentioned, training may seem pretty simple at first - You lift heavy weights consistently and, well… You grow bigger and stronger.
However, you can get big in different ways, because different styles of training lead to a different type of muscle growth.
It is considered that there are two main types of muscle hypertrophy:
1. Myofibrillar hypertrophy
2. Sarcoplasmic hypertrophy
Let’s have a look at each one, individually.
Myofibrillar hypertrophy is the increase in size of the muscle contractile units, called ‘myofibrils’ and commonly known as “muscle fibers”.
These muscle fibers are the contractile elements of the trained muscle groups, which allow contraction and relaxation.
Sarcoplasmic hypertrophy, on the other hand, is the growth of the ‘sarcoplasm’, which is basically a jelly-like fluid that surrounds the muscle fibers.
The sarcoplasm contains different non-contractile elements, which can grow in volume, depending on the type of training.
It appears that the myofibrils are more closely related to maximum output of strength and explosiveness, while their sarcoplasm is engaged during intense, yet longer loads.
In other words, myofibrillar hypertrophy is a result of a powerlifting approach to weight training, where you do 1-5 repetitions.
Oppositely, sarcoplasmic hypertrophy is a result of a bodybuilding approach to training, where you do 6-15+ repetitions with heavy weights.
Now of course, these are not mutually exclusive!
Though myofibrillar hypertrophy primarily results in increased maximum and relative strength, you have those adaptations with the other type of hypertrophy too, BUT… They are not as prominent.
Our musculature appears to be flexible, having the chance to adapt for both short, very intense training bouts, or, medium intensity, prolonged bouts.
This is possible because of the nervous system, our wonderful muscle fibers and their energy reserves!
Whatever your goal may be, try to stimulate both types of muscle growth, as well as other functions like balance, agility, coordination, etcetera.
Become a functional human being!