Well at my practices (soccer) we practice right near a beach. so once and a while we run the sand. and wow do my calfs get so tired after i do that!! The next day my calfs always get sore. since its on sand your legs sink so it takes more effort so it works your calfs good. i have seen a difference in them already. but the best way to do it is to use cleats. they have the studs ar the bottem so your legs sink in deeper. and your killing 2 birds with one stone. Cardio and working your calfs!