OPTP ActivMotion Bar 4.5lb | 4-foot Bar with Internal Rolling Steel Weights to Active Core Muscles for Fitness, Therapy, Pilates and Golf Exercise 4.5lb ActivMotion Bar features shifting, weighted ball bearings inside the bar Hollow bar partially filled with rolling steel weights Moving resistance creates instability that challenges and activates the core Improves core strength, flexibility, range of motion and balance Measures 4' L x 1.25" diameter