When scientists check muscle growth from exercise, taking tiny muscle samples doesn't match up well with other methods like ultrasound scans or strength tests, so using just the sample data isn't a great way to measure progress.

Scientists aren't sure if lifting more weights always makes muscles grow bigger, because their research shows the results are often unclear and sometimes show no effect at all.

Scientists looked at only 8 small studies about how different weight training affects muscle growth. They found there isn't enough research yet to say for sure which exercises work best for building specific types of muscle fibers.

After doing strength training, men might see a tiny bit more muscle growth in their slow-twitch muscles than women, but we’re not super sure because the studies were small and measured muscle samples in different ways.

When untrained young men lift weights until they can't anymore, both heavy and light weights make their muscles grow about the same amount (6% bigger) if they do the same total work. This goes against the common belief that heavier weights are always better for muscle growth.