The eccentric action of a muscle refers to a resisted lengthening of that muscle; a muscle exerting force while it's being lengthened. This type of action is also called the yielding or negative action (as opposed to the overcoming action which refers to the actual lifting of the resistance).
Eccentric action is present in most free-weight and machine exercises. However, since concentric strength potential is lower than the eccentric strength potential the yielding portion of a movement is rarely fully stimulated. In other words, the relative weakness of the overcoming portion prevents a complete overload during the yielding portion of the exercise.
As I will explain, it is the yielding portion of an exercise which gives us the greatest bang for our buck. So an individual seeking maximum result should plan training methods emphasizing eccentric overload.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.