The best way to strengthen any muscle group is to increase the weight being pulled, one way is simply placing this Sporti Pull Buoy between your knees allows your arms to pull your whole weight through the water. Features Great for doing muscle conditioning, endurance training, and many other fitness activities. For more information on pull buoys, read Swim Goggles Buying Guide Details Material: EVA foam Dimensions: 9.5" x 3.5" x 6" Country of Origin: Imported