Feature:: Easily adjustable, soft surface Plyo Boxes reduces stress on joints during landing Non-skid vinyl ensures the stack won\'t move under pressure. Easily adjustable heights offers exercisers of all fitness levels progression and regression options. Made from high density foam and covered with durable, non-skid vinyl, these soft Plyometric boxes absorb landing impact to decrease stress and strain on joints. Adjustable height when stacked from 3 - 39\" make them a clear favorite among all grades of gym users and athletes. Stacks secured with Velcro strips.