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The classic Muscle Confusion Principle states that muscle fibers need constantly varied stimulation to best tap into their growth potential. That is true to a point, but varied doesn’t always mean better sports scientists at the Federal University of Rio de Janeiro in Brazil say. Researchers measured the muscle activation in the biceps of 22 subjects who performed a standing dumbbell biceps curl, incline dumbbell curl, and a dumbbell preacher curl. The dumbbell biceps curl and incline dumbbell curl yielded similar results, but the preacher curl lost stimulation during the last phase of the concentric portion and the first phase of the eccentric, when the elbow is fully flexed and the hands are close to the shoulders. During the dumbbell preacher curl, activation in the biceps was maximized only when the arm was close to being extended.