Splits not gains- Muscle hypertrophy

As a trainer, weight loss was often the main focus for many of my clients. I would often have non-obese women come to me with this goal and I had to explain that they didn't necessarily need lose weight they and they instead should shift their goal to lose fat. Education on this became especially important when these clients started to increase weight due to muscle mass.  My biggest selling point was the fact that increasing muscle mass, increases metabolic rate. In fact, skeletal muscle uses on average 13.0 kcal/kg body weight per day, where as adipose tissue uses only 4.5 kcal/kg of body weight per day (Heymsfield et al., 2002). 

If humans are good at hypertrophy (myofibril splitting), but not so good at hyperplasia (generating new myofibrils), how does muscle mass actually increase under hypertrophic conditions? Is it the contractile proteins that are attributing completely to this muscle mass or is it something else?

To start, hypertrophy is a complex process that contributes to muscle growth through myofibril splitting rather than generation of new muscle fibers (hyperplasia). Activities such as resistance exercise overload can induce hypertrophy though mild trauma or damage to the muscle itself. Satellite cells reside on the outer surface of muscle fibers. They are usually inactive but upon muscle trauma or damage, become activated. Once activated, satellite cells multiple and fuse to existing muscle fibers. Satellite cells donate their nuclei and allow the damaged muscle fiber to regenerate (Hawke & Garry, 2001). This process increases the size of the muscle fiber and also increases the amount of contractile proteins within them. Various growth factors also contribute to hypertrophy. For example, insulin-like growth factor (IGF) stimulates protein synthesis as well as satellite cell proliferation and differentiation (Hawke & Garry, 2001). 

Many factors contribute to an increase in muscle mass after hypertrophic conditions. Increased muscle fiber size allows for space available for glycogen storage, and a larger area for water volume. Additionally, contractile proteins such as myosin and actin proliferate and contribute to some of the mass gain. While exercise may result in weight gain in certain situations this weight gain is due to non-lipid sources and even contribute to increase metabolic rate and subsequent fat loss.

Hawke, T. J., & Garry, D. J. (2001). Myogenic satellite cells: Physiology to molecular biology. Journal of Applied Physiology, 91(2), 534–551. https://doi.org/10.1152/jappl.2001.91.2.534

Heymsfield, S. B., Gallagher, D., Kotler, D. P., Wang, Z., Allison, D. B., & Heshka, S. (2002). Body-size dependence of resting energy expenditure can be attributed to nonenergetic homogeneity of fat-free mass. American Journal of Physiology. Endocrinology and Metabolism, 282(1), NaN-NaN. https://doi.org/10.1152/ajpendo.2002.282.1.E132