If you perform weight lifting exercises using constant resistance devices, you can notice changes in leverage during the joint movement. For example, when doing dumbbell curls the amount of muscular force becomes much greater at the bottom of the movement (when the dumbbells are at waist level) than it is at the top of the movement (when the dumbbells are near the chin). When the dumbbels are at the top of the movement the leverage increases and you can't train with proper intensity. This means that you won't be able to achieve visible results during the 'easy' portion of the movement. You need to stress your muscles in order that they could gain strength, so with the relatively diminished stress of constant resistance devices some of the benefit is gone. Some experts state that constant resistance exercise is more natural than any other weight lifting program because leverage imbalances looks like the actual day-to-day movement of the body. Variable Resistance Devices
"When you hoist a weight by pulling on a cable that goes over the top of a pulley and is attached to a weight, you're engaged in constant resistance training" (source: International Sports Sciences Association; 2001). Some exercise equipment manufacturers have used pulleys. They aren't round or don't have the hole in the exact middle of the pulley. As a result different levels of resistance were felt during different points in the exercise movement. Variable resistance fitness equipment doesn't match the natural human body workout. That's why some professionals consider this as disadvantage because it may lead to disturbance in brain centers that interpret force and movement patterns. The advantage of variable resistance fitness equipment is that it amplifies the stress level which has been put on your muscles. Your muscles are forced to work equally intense throughout the full range of motion. Due to the fact that each person is different is size, stature and strength it is virtually impossible to match everybody's leverage with machine leverage. Static Resistance Devices
"Static contraction is the muscle contracting without movement. The term isometric exercise is used to describe this form of stress" (source: International Sports Sciences Association; 2001). In the 1950's and early 1960's many people have been looking for a new effective way to lose excess weight. As a result the isometric exercise has been adopted as their new weight lifting workout. It has been proved by the scientists that isometrically contracting a muscle results in that muscle only gaining strength in that position. So if you want to gain strength throughout the entire range of movement, you need to isometrically contract the muscles throughout every conceivable angle in the entire range!
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