A key advantage that free weights have over stationery exercise equipment is the tremendous number of possible variations. The shifting masses of the XCO-TRAINERs produce other important benefits as well. The shifting mass causes a much different result than an inert mass is able to achieve. The most beneficial of these effects is realized at the end of each movement. The body absorbs the delayed impact of the shifting mass, which acts as a load charge on the muscle fibers. The connective tissues, including the ligaments and synovial joint capsules, all benefit alike from this delayed or REACTIVE IMPACT.