Spiky ball used for massaging:
Feet - rolling under foot after a long day at work, hiking, skiing, etc.
Back - placing on the ground or on wall and laying or leaning against to massage out those hard to reach problem areas and pressure points.
Also used on other parts of the body by pressing or rolling between palms, on arms, neck, etc.
Ball has a hole for adding or releasing stiffness by changing or pressure with basic ball pump.