What Drives Muscle Growth & What Doesn't | Dr. Mike Roberts

Hypertrophy occurs mainly as radial hypertrophy, meaning an increase in the cross-sectional area of muscle tissue. Ultrasound imaging shows approximately 15% increases in cross-sectional area of muscles such as the rectus femoris and vastus lateralis. Muscle biopsies from the vastus lateralis reveal 20-25% increases in type 2 fiber cross-sectional area. Type 2 fibers show preferential growth. Type 1 fibers may show no hypertrophy or around 10% increase depending on training status.

Myofibrillar Addition05:30

Until recently, the internal changes within muscle fibers during growth were unclear. The conventional view held that as fibers increase in size, there is a proportional increase in myofibrillar protein content. Research with Professor Troy Hornberger at Wisconsin using fluorescent microscopy on mouse models and human training study samples showed that fiber cross-sectional area increases result from an increase in the number of myofibrils rather than existing myofibrils simply getting larger. Myofibrils maintain a relatively conserved size, and resistance training produces larger radial increases through addition of myofibrils.

Mitochondrial Response07:30

Resistance training produces proportional expansion of all cellular components. Some studies using citrate synthase activity as a marker showed decreased activity after 10 weeks of resistance training, suggesting a dilution effect where myofibrillar growth outpaced mitochondrial expansion. However, immunohistochemistry work by Brad Ruple in a 2021 paper demonstrated proportional expansion of mitochondria with hypertrophy, which aligns with the need for energy production in the growing cell.