Sarcoplasmic Hypertrophy: What High-Volume and Heavy Training Build

Overview

A muscle fiber grows mainly in two ways. It can add contractile protein (myofibrils), or it can expand the fluid-and-enzyme compartment around them (the sarcoplasm). This lecture covers a review of the evidence and human biopsy research showing that the type of training shifts which one dominates.

Sarcoplasmic hypertrophy is a real adaptation

A review of the methods and evidence describes three ways a fiber can grow.^[1]

• Conventional hypertrophy: myofibril protein is added in proportion to fiber growth.
• Myofibril packing: myofibril protein is added before the fiber grows.
• Sarcoplasmic hypertrophy: the sarcoplasm expands out of proportion to myofibril protein.

The authors present evidence that sarcoplasmic hypertrophy happens during resistance training, and they set out how future studies can separate the three.

What high-volume training did inside the fiber

Thirty previously trained men did 6 weeks of very high-volume training.^[2] In the 15 clear responders:

• Fiber size: mean fiber cross-sectional area rose 23%.
• Contractile proteins: actin and myosin concentrations fell by about 30%.
• Mitochondria: citrate synthase activity fell 24%, suggesting less mitochondrial volume.
• Sarcoplasm: sarcoplasmic proteins increased, with the glycolytic (energy-producing) pathway upregulated.