How do muscles grow?

When you work a muscle hard, especially against resistance, it creates microscopic tears in the muscle fibres. In response, the body repairs these fibres and adds extra protein to them, making them thicker and stronger, a process called muscle hypertrophy. This repair happens during rest, which is why recovery time and sleep are important for building muscle. Adequate protein in the diet supplies the building blocks, and hormones help drive the process. Over weeks and months of regular training, the accumulated growth of many fibres makes the whole muscle larger and more powerful.