This claim says that lifting lighter weights until you're tired, done regularly for six weeks, can build muscle just as well as lifting heavier weights. It specifically found growth in arm and chest...

If you do push-ups or bench presses with light weights until you can't do another rep, twice a week for two months, your arms and chest will grow just as much — so push-ups can be just as good as gym machines for building muscle, if you push yourself just as hard.

Even if you lift the same weight every time without making it heavier, your arms can still get noticeably bigger in just two months — at least when you're just starting out and you're a young woman.

Even lifting light weights really slowly until you can't do another rep can make your muscles bigger, just like lifting heavy weights.

When untrained young men lift lighter weights many times until they can't lift anymore, it builds just as much muscle in their calf muscles as lifting heavier weights fewer times. Both ways made their muscles about 10% bigger after 8 weeks.

In men who train casually, lifting heavier weights leads to faster and greater thickening of the triceps muscle compared to lifting lighter weights over nine weeks, even though both methods may result in similar overall muscle growth.