Each muscle fiber is made up of chains of these myofibrils surrounded by a fluid (called sarcoplasm) that contains things like glycogen (stored carbohydrates), water, ATP (energy source), minerals (like calcium), fats, and oxygen-binding proteins (called myoglobin).
Although the sarcoplasm is needed for muscle function, it does not directly generate the force/tension that can move a resistance.
The majority of space inside a muscle fiber is filled with the myofibrils (usually 75% or more) with other things like the sarcoplasm filling the remaining area. In a single muscle fiber there may be 1,000 myofibrils or more.
The myofibrils are mostly made up of two proteins (called actin and myosin) that can contract in a muscle fiber. This just means these proteins work together to generate force/tension to move a weight.
During Myofibrillar Hypertropy the body first increases the amount of these contractile proteins (actin and myosin) in each individual myofibril causing it to grow.
But a single myofibril can only become so large until it is forced to split and create new myofibrils.
When the myofibrils grow or increase in number, so does the size of each individual muscle fiber and the overall muscle itself.
Because there are larger and more myofibrils available to generate more force/tension, the muscle also becomes stronger.
Since the muscle is stronger, Myofibrillar Hypertrophy is typically called functional hypertrophy because the muscle is better able to generate force.
Even small increases in myofibril size and number can lead to noticeable strength increases.
Causes of Myofibrillar Hypertrophy
There are several ways to cause Myofibrillar Hypertrophy (muscle growth) and trigger the amount of contractile proteins to increase (also called protein synthesis).
One way is through an increase in the body’s hormones (such as testosterone) as happens during puberty or when using anabolic steroids.
The other way is through training that causes muscle damage or microtrauma followed by rest with proper nutrition (protein) so the body can overcompensate. In this case the damaged fibers are replaced with larger and stronger fibers (made up of the myofibrils).
Myofibrillar Hypertrophy Workout and Training
Microtrauma (muscle damage) occurs when the myofibrils are sufficiently stressed. This stress typically comes from the muscle having to support and move heavy loads as in normal strength training programs like 3X3 (3 sets of 3 reps using weights that are typically 90% or more of a one rep maximum weight).
But the stress can also come from the muscle being fatigued with sub-maximal loads. As the muscle wears down and reaches failure, some muscle fibers are damaged since they can no longer support the loads.
The weights are still heavy but slightly lighter to allow more reps. Programs like 5X5 (5 sets of 5 reps using the straight sets method) work well when starting with weights at least 75% or more of a one rep maximum weight.
Myofibrillar vs. Sarcoplasmic Hypertrophy
Sarcoplasmic hypertrophy is the growth or increase in the amount of sarcoplasm (the fluid that surrounds the myofibrils).
Just like Myofibrillar Hypertrophy, sarcoplasmic hypertrophy would also cause a muscle to become bigger since the muscle fibers are made up of both the myofibrils and sarcoplasm.
There is some debate if sarcoplasmic hypertrophy can even occur. But the fact is that the sarcoplasm is needed for muscle function.
As the myofibrils increase, the sarcoplasm will increase as necessary to support the amount of Myofibrillar Hypertrophy that occurred. Again, the myofibrils need the sarcoplasm to function.
How much the sarcoplasm will increase is influenced by genetics/nature and possibly the type of training used.
The sarcoplasm is also made up of things determined mostly from diet like glycogen, water and minerals so there will be natural fluctuations in the amount present. Increases in these things would increase the size of the sarcoplasm without changing the myofibrils.
But the real debate and myth is that the type of training you use can cause Myofibrillar Hypertrophy and sarcoplasmic hypertrophy to occur totally separate from each other.
The myth goes that a bodybuilder’s training causes just sarcoplasmic hypertrophy (using lighter weights with high reps and volume). That is supposedly why they are weaker but bigger than the powerlifters and Olympic lifters that train for Myofibrillar Hypertrophy (heavy weights for low reps and volume).
Since the sarcoplasm is not actual muscle fibers, bodybuilders are also said to have fake or puffy muscles unlike the supposedly dense muscles of strength athletes.
But if the muscles on bodybuilders were 100% sarcoplasmic hypertrophy, bodybuilders would not be any stronger than if they never lifted weights. Following the same logic, endurance athletes would have the largest muscles from all the reps and volume they do.
Of course that’s not the case.
There are many things to consider but primarily that bodybuilders are concerned with muscle isolation not just the amount of weight used. Strength athletes just want to lift more weight, regardless of the muscles used.
Bodybuilders also train and develop more muscle fiber types, like Type I fibers, by using a wider variety of weights and reps than strength athletes do.
When a muscle is sufficiently stressed to cause hypertrophy (muscle growth), both sarcoplasmic AND Myofibrillar Hypertrophy will occur. They are dependent on each other, it’s just how the body works.
The amount each increases can be influenced by the type of training used since the body adapts as needed to its stress environment.
A strength athlete will definitely provide enough stress from heavy weights to trigger Myofibrillar Hypertrophy. But there will also be some sarcoplasmic hypertrophy to support the new/larger myofibrils.
And a bodybuilder still needs to use sufficiently heavy weights to trigger muscle growth (both myofibrillar and sarcoplasmic). Although the weights used might be somewhat lighter, muscle fatigue with heavy loads can still cause the necessary muscle damage.
If that same bodybuilder uses higher volume in their training, the sarcoplasm may grow even larger than in a strength athlete because they need more energy as glycogen and ATP, both of which are stored in the sarcoplasm.
In the end if a muscle is properly stressed, BOTH sarcoplasmic and Myofibrillar Hypertrophy will occur. Bodybuilders don’t just cause sarcoplasmic hypertrophy to occur completely by itself.
The amount of growth of the myofibrils vs. sarcoplasm can be influenced by the type of training used but there are many other factors to consider as well such as time under tension.
In general, the heavier weights are needed for Myofibrillar Hypertrophy while higher volume favors more sarcoplasmic hypertrophy.