As one of those people who has always needed to wear two bras to be able to run, this is a really useful product. Forget marginal gains, for me the Boobuddy significantly reduces vertical movement for a more comfortable running experience, with a reduction in breast pain and discomfort during and after running. I have found that it is sufficient to wear it in combination with a robust compression bra, which is better than having to wear two layers in hot weather.
Small