You don't have to lift super heavy to grow muscles. Studies show that you can gain muscle by using either light resistance OR heavy resistance, as long as the lift is taken close to failure.
However, there seems to be a minimum amount of resistance you need to use to grow muscles, and if you use any lighter, it doesn't seem to have much effect.
Dr. Payton and Dr. Shannon walk you through the process of calculating your one rep max, so you can work backward to figure out the minimum amount of resistance to use in each lift.
One rep max calculator
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