I use a Hyperice Hypersphere ball and recommend highly. I have bought them as gifts in fact. I use the larger one, they make minis. My last gifted one was a mini. My sis on a visit here tried mine and said if it was only smaller, so I got her a mini. Pick your poison.
Use for me is mainly on hip flexors, but gluts, quads, hamstrings as well. Occasionally on my back, and semi frequently on forearms for cubital tunnel woes. But Di2 has helped that loosing the lever swing which seemed to exacerbate that. For cabled I actually prefer ERGOs. Maybe I should shift less, I digress..
But for when you get some muscle imbalances on your larger groups with cycling it is great PT tool.
