Sensory spikes act as mini acupressure points. Ribbed surface quickly relieves muscle knot for trigger point therapy. Stretchable cord ensures ball is positioned on the right spot. Can be used with full back against wall or laying down. Massage balls can be utilized before and after an intensive workout. Made of durable PVC with stretchable cord. This massage ball set can be used with full back against the wall or laying down. Best used before and after an intensive workout. Stretchable cord ensures the ball is positioned on the right spot.