How Long Does it Take for Muscles to Grow After Working Out?

Your muscles start to change almost immediately after a strength-training session, but you won't notice those changes for several weeks or even months. Len Kravitz, program coordinator of exercise science at the University of New Mexico in Albuquerque, notes that protein synthesis -- the process by which muscles grow bigger and stronger -- occurs within two to four hours after a workout. After six to eight weeks of regular strength-training, this protein synthesis process should reveal changes in your body, provided you create the right muscle-building environment.

A group of adults lifting weights together
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Create the Conditions

Changes in your muscles occur when you challenge them with resistance exceeding 60 to 70 percent of their maximal force potential, sometimes referred to as a one-repetition maximum. You must train regularly -- at least three or four times per week -- and allow at least 48 hours of recovery between sessions. During this "rest" time, muscles repair themselves from damage caused by weight lifting. The fibers need time to grow back stronger and denser.

Dr. Melina Jampolis, a diet and fitness expert, recommends taking in 250 to 500 calories more than you burn daily, depending on how easily you gain fat. Your muscles use this fuel to grow. A paper included in the 2013 Nestle Nutrition Institute Workshop Series notes that consuming 20 to 25 grams of a high-quality protein -- such as whey or eggs -- near the time of your workout maximizes muscle growth. Adequate sleep and hydration are other factors that support fast muscle-building results.

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