A treadmill is great for cardio, but it doesn't do anything for the upper body. Dumbbells are a good choice for upper body strength exercises. How to put them together for a challenging workout? One way is to turn running time on a treadmill into a circular workout, using dumbbells for upper-body exercises. To prepare, place a pair of dumbbells weighing between 2.5 and 5 kg (depending on your strength and experience) next to the treadmill.