As physical therapists and strength coaches know quite well, resistance bands are perhaps the least expensive and most versatile exercise tools for improving muscle strength and flexibility. These strips and loops of natural or synthetic rubber, which resemble large rubber bands, come in a range of resistance levels, each of which is indicated with a different color. The amount of resistance, which ranges from about 3 pounds to 22 pounds, varies throughout exercise. That is, there is less resistance at the beginning of the stretch, and more resistance at full stretch. By looping these bands around feet and hands, exercisers and physical therapy patients can produce muscle- strengthening resistance that is similar to that produced by lifting weights.