Using arms when running counterbalances the legs. The natural motion when running or walking is to have the arm swing coincide with the opposite step. This is to keep things balanced and help enable you to move in a straight line. If a runner has poor upper body form, the lack of an arm swing can slow the legs because they are working harder and against the upper body. Having them balanced with the same intensity is the key to getting faster. The arms should start bent at the elbows at a 90-degree angle. The range of motion will largely depend on how fast the runner is going. Faster running calls for a higher knee raise while a distance runner who is going slower can afford to keep the hands low and have less arm swing. Though the arm motion will be fast and vigorous, it should be done with relaxed muscles and joints. A weak upper body will make it difficult to swing your arms aggressively enough to keep up with your legs, which causes you to work harder and tire sooner.