Muscle hypertrophy, or an increase in size, occurs in response to a challenging workload. This occurs when you lift weights, perform body-weight exercises, or use other strength-training equipment like resistance bands at an appropriate frequency and intensity. The increase in size is primarily due to an increase in the cross-sectional area of the muscle fibers you already have; as a result, you get stronger.
If you reduce your training or stop working out, you’ll experience a loss of muscle, or atrophy. Without an adequate training stimulus, the cross-sectional area of your muscles will shrink, resulting in a smaller muscle appearance and a loss of strength and function.
Following general guidelines for strength-training — working the major muscle groups with 1-3 sets of 8-12 repetitions, 2-3 times a week — is not enough to cause excessive hypertrophy in men or women. If you’re currently doing more than this, or lifting very heavy loads, scale back on your workload for a few weeks and re-evaluate. You could decrease the amount of weight you’re lifting by 5%, perform 1-2 sets instead of 3, and work out twice a week instead of three times — and reduce size while still maintaining a healthy level of strength. You could also add variety and challenge your muscles in different ways by using different types of equipment.
Work with a certified personal trainer, at least for a few sessions, to make sure you’re on the right track to maintain the level of muscle size, strength, and definition you desire.
I urge you to continue strength training to maintain a healthy level of muscle function and bone density. This is especially important for young women; bone mass peaks around age 30; with regular strength training, you’ll optimize your bone density and reduce your risk of osteoporosis and fracture later in life. Because muscle tissue is metabolically very active and burns calories even at rest, maintaining muscle tissue via strength training also helps with long-term weight control. The benefits of strength training can’t be overstated; making it a permanent part of your fitness routine will pay off in big ways — today and with every passing year.