Resistance bands are elastic loops and tubes that provide resistance which rises as the band stretches. They are light, cheap and portable, which is why they anchor most travel and home programmes.
A systematic review and meta-analysis in SAGE Open Medicine (2019) compared training with elastic resistance against conventional weights and machines and reported similar strength gains between the two across the pooled trials, in both trained and untrained participants. The authors conclude that elastic resistance can promote strength comparably to conventional resistance when effort and progression are matched. Because band tension varies through the movement, published strength-testing protocols still use fixed loads for measurement.