Arthur Jones, founder of Nautilus Sports Medical Industries and later the MedX Corporation succinctly stated, "Man is a rotary animal." When we contract our muscles, we are causing rotational movement around a joint or a series of joints. Herein lies the primary limitation of a free-weight; while our joints are causing rotary movement, gravity acts on a free-weight in only one direction. The end result is that in most free-weight exercises, the targeted muscle is exposed to direct resistance for only a small portion of the range of motion. An intelligently designed machine includes mechanical elements that make the resistance omnidirectional; thus our muscles must create limb movement that directly opposes the resistance throughout the entire range of motion. This is the foundational advantage of machines when compared to free-weights; an advantage that the vast majority of exercisers and fitness professionals are completely unaware of. Arthur Jones says it best: "Since the "direction of movement" of the involved body-parts is constantly changing, the "direction of resistance" must change in exact accord, automatically, simultaneously, instantly; again, this requirement can only be provided by a rotary form of resistance."
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