When young men lift weights, how much oxygen is in their muscles affects how much their muscles grow bigger. Less oxygen in the muscles during exercise seems linked to better muscle growth results.

When you exercise with less oxygen in your muscles—like training at high altitude or with special gear—you might build more muscle than when you train normally, because your muscles get more 'starved' for oxygen, which could trigger bigger growth.

Doing slow, controlled weight lifts with light weights can make your muscles use up more oxygen during the workout than doing the same lifts fast or lifting heavy weights—even if the heavy lifting feels harder.

A certain type of weight training keeps your muscles working non-stop, making them use more oxygen and possibly grow bigger because of changes inside your body.

When healthy young men do slow, low-effort weightlifting exercises, their muscles work longer and use more oxygen than during faster or higher-effort exercises in a single workout session.

When healthy young men lift weights until they're too tired to continue, their muscles grow about the same amount in both their arms and legs, no matter how heavy the weights are.