An Evidence-Based Narrative Review of Mechanisms of Resistance Exercise–Induced Human Skeletal Muscle Hypertrophy

Just because your muscles hurt or swell after lifting weights doesn’t mean that’s what’s making them grow—you can still get stronger and bigger even if you don’t feel sore or inflamed.

When you first start lifting weights, your body makes more of the tiny machines that build muscle proteins, which helps your muscles grow. But after a while, your body shifts focus and gets better at using those machines more efficiently instead of making more of them.

When you lift weights, your muscles sense the tension and turn on internal signals that help them grow bigger—and they might even become better at using leucine, a protein-building block, to help with that growth.

When you lift weights, your muscles turn on a specific molecular switch called mTORC1, which helps build more muscle protein. But just because this switch flips on right after a workout doesn’t mean you’ll definitely get bigger muscles over time—it doesn’t always line up.

Just because your testosterone, growth hormone, and IGF-1 levels spike after a workout doesn't mean those spikes are what make your muscles grow.

When your muscles contract or get stretched, the physical pull they feel is the main reason they grow bigger — it's like the muscle gets a signal from the tugging to grow more muscle fibers.