Vince made an awesome machine that replicates what a dumbbell does. The machine he made is superb. No question. But there was no problem to solve.
Let's be honest. Just about all exercise machines re-invent the wheel.
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http://www.rippeder.com/content/how-i-built-my-20-inch-arms-larry-scott"Incline Dumbbell Curl: the dumbbells are to be raised only up to the point where gravity takes the strain off the biceps and the dumbbells want to fall back to the shoulders. Slowly lower the weights until the arms are fully extended again, and
be extra sure to keep the palms up all the time and keep the wrist so the inside of the palm is level with the outside. Don’t let the dumbbell pressure turn the wrists in.
This tip will enable you to keep the strain on the belly of the muscle, and will make the difference between good gains and none at all. There is generally one key point in any exercise that will make the difference between it being a good one and a bad one. In this case it is the palm position, so keep this in mind every time you do your curls."