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When you use lighter weights, the muscles you are targeting are able to manage the load by themselves, without reliance on compensatory muscles to assist in the movement, hence more hypertrophic gains occur in the muscles you want to grow. A good example is the triceps pressdown. When the weight is too heavy, the shoulder muscles, being larger than the triceps, overpower the smaller muscles in order to press the weight down. The result? Your shoulders get a great workout, but your triceps are left in the dust.