Pop quiz: Joe weighs 150 lbs. and decides that he wants to start working out in the weight room on a regular basis to gain some muscle mass. Through consistent work at his new routine, Joe gained thirty pounds of muscle! By what percentage did the number of Joe’s muscle cells increase?
Answer: Zero percent! When a person gains muscle mass, the number of muscle cells does not change, but the size of each of those muscle cells increases! How does this work?
As we know from experience, people gain muscle mass and strength by working out the muscles that they are targeting. What is actually happening on a cellular level within each of the muscle cells of the body is the basis for gaining muscle mass, even though we don’t gain more muscle cells. It all starts with trauma to the muscle; that is what exercise does, even healthy amounts of careful exercise cause small amounts of damage to the muscle. This damage activates a different type of cells called “satellite cells” to come to the aid of the damaged muscle cells in efforts to repair them. These satellite cells fuse to the muscle cells and create new strands of protein within the muscle cell, called myofibrils. As this process is happening, it is still only happening in a single muscle cell, but that cell is accumulating more myofibrils and making existing myofibrils larger as they are repaired and this is what causes the muscle to grow in size. The extra muscle fiber can allow creation of more actin and myosin in muscle cells, which are the “contractile myofilaments” that contribute to muscle strength! (1).
Also, as you exercise muscles, the number of capillaries to that muscle increases, which allows more blood flow to that part of the body, and muscle cells develop more mitochondria due to regular exercise. The mitochondria “convert chemical energy into energy the cells can use” (2). The increased amount of both capillaries and mitochondria in the muscle cells also contributes to the increase the size of the overall muscle. A good thing to know if you lift weights regularly is that high repetition sets are good for building up more mitochondria, so while low reps with high weight are also beneficial for increasing muscle size, adding sets of high reps with lower weight will also be good for increasing muscle strength. As with any exercise routine, it’s all about balance and variety. Regularly exercising a muscle also increases the ability of the muscle cells to store glycogen, which is the storage form of energy in the body and can be broken down and used within the muscle to provide energy for working muscle cells when they are exercised (2).
The scientific term for increasing muscle mass is “muscle hypertrophy.” Muscle hypertrophy is affected directly by hormone levels specific to each person’s body, and one main hormone affecting muscle hypertrophy is testosterone. Testosterone “can stimulate growth hormone responses in the pituitary, which enhances cellular amino acid uptake and protein synthesis in skeletal muscle” (2). This is part of the reason that men typically gain muscle mass much more easily than women, because although both men and women have testosterone as a regulatory hormone in their bodies, the levels are much higher in men. Due to differences in other regulatory hormones, at the same BMI, healthy women will also have a higher level of body fat than men because this is required for normal bodily processes and for women to be able to have children (3).
This is a pretty basic understanding of how muscles grow, but I found it really interesting! I also found this video illustrating the things that were explained above in what I thought was a fun and simple way! But whether you’re trying to bulk up or not, don’t worry about the fact that working out damages your muscle fibers—the repair process is completely normal and necessary and virtually all cells in our bodies need repair on a regular basis. Just make sure to maintain proper form and safety precautions when lifting weights to prevent unnecessary damage to joints and tendons, which are much, much harder to repair!
Workout Wednesday blog posts are written by UNC Campus Recreation. Each Wednesday we swap blog posts with the Tar Heel Tone Up blog so that readers can view more diverse post topics that will benefit their health and wellness. Workout Wednesday blog posts can be found both here and on tarheeltoneup.com.