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How to gain more muscle mass and strength
Monitor your progress.
As you start gaining strength and building muscle, keep track of how much weight you gain, how much weight you can lift, and what exercises you do from week to week. This will help you figure out what works for your body and what doesn't, and keep you from getting into a workout rut.
If you find that a certain muscle group doesn't seem to be making significant gains, switch up your exercises to see if something else works better. Alter your diet as necessary to help you lose fat and gain muscle.