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58 Elite Physique
Muscle Growth: What Scientists Know
These days we have handheld phones that are more powerful than computers in the
1960s, which took up entire rooms. That computing power, combined with advances
in imaging technology, allows researchers to examine muscle tissue in ways never
imagined by the pioneers of exercise science in the 20th century. But for all those
advancements, we still don’t know much about the physiological and metabolic
adaptations that make your muscles grow. Two things scientists are pretty sure of:
the roles played by satellite cells and the mTOR pathway.
After a bout of resistance training, your muscle fibers have trauma in the form of
microscopic tears through the fibers and surrounding structures. This microtrauma
signals the muscle’s satellite cells (i.e., muscle stem cells) to activate and move to
the site of damage. The satellite cells donate their nuclei, which starts a cascade of
events that allow the fibers to increase their size (Egner, Bruusgaard, and Gundersen
2016). The mTOR pathway is thought to be the master regulator of muscle growth
(Thomas and Hall 1997). Short for mammalian target of rapamycin, mTOR performs
numerous roles involving insulin, growth factors, and amino acids, as well as the
muscle’s nutrient, oxygen, and energy levels (Hay and Sonenberg 2004; Tokunaga,
Yoshino, and Yonezawa 2004).
One question that still baffles scientists is, Can adult muscles split to form new
fibers?
To get the idea, think of a stock you bought in a Fortune 500 company. If the
stock price increases rapidly, the company may decide to split the stock, so instead
of owning one share worth $100, you own two shares, each worth $50. Muscle
fibers are like the stock shares in the first half of the analogy. We know they can
get larger. But we don’t know if a single fiber can split into two smaller fibers (a
process called hyperplasia) when it reaches a critical level of growth (Jorgensen,
Phillips, and Hornberger 2020).
How Does Muscle Grow or Shrink?
A muscle’s physiological cross-sectional area can expand (hypertrophy), shrink
(atrophy), or stay the same size. Throughout the day, the body alternates between
periods of muscle protein synthesis and muscle protein breakdown. Hypertrophy
occurs when synthesis is higher than breakdown over the course of a day or
more. The muscle pulls amino acids from the blood, which it then uses to build
muscle proteins. Atrophy occurs when protein breakdown is higher than protein
synthesis. In this case, muscle proteins break down into amino acids, which are
then released into the blood, where they’re used for other metabolic processes.
These processes are shown in figure 4.1.
After a bout of high-intensity resistance exercise, muscle protein breakdown
and synthesis increase (Phillips et al. 1997). But during the subsequent 24 to 36
hours, synthesis is greater than breakdown. The result is a net increase in muscle
protein (i.e., hypertrophy), assuming the person has eaten enough protein for
that to occur (Cermak et al. 2012; West et al. 2016).
As most of us know, increasing a muscle’s size through hypertrophy not only
makes you look better on the beach but increases your performance as well,
assuming you don’t also gain a lot of fat. Larger muscles can produce more