Little racetrack looking gadget that holds lacrosse balls for self massage. The balls stay in one of three tracks allowing you to self massage or to find a tight spot and hold. The tracks are slightly different widths, which allow you to switch to what makes sense for you. They keep the balls in a more stable position (because it limits the directions the ball can roll) but roll up and down well. Stays stable on hard floor during use. You might be able to roll on it but all I have done is find tight spots and hold.
As far as the usefulness by parts of the body:
Shins: best tool I have ever used. Allows me to switch between the narrow track and the wider track based on leg position. Having two balls and leaning on one or the other (or both) as necessary is giving me the best massage on my shins I have had.
Back: ok, somewhat useful. balls are further apart than a two ball peanut, which is useful and having the groove is nice. Still prefer a single ball for this but after a while I might be convinced otherwise.
Neck: moderately useful. A bit more stable than a single ball
Calves: somewhat easier than the ball alone
Essential? No but it is small and only $30 at amazon. Worth it if you have shin issues, might be useful if you don't. I like it and will continue to use it.
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