It's normal for your weight to fluctuate a couple of pounds from day to day, and if you're engaging in resistance training, newly-acquired muscle may reflect a slight gain when you weigh yourself. But you may also notice weight gain due to fluid retention. There are two primary causes of water retention related to strength training: microscopic muscle tears and dehydration.
Muscle Tears and Fluid Retention
Microscopic tears occur in your muscle fibers when you lift weights. These tears are part of the muscle-building process, and they are often the cause of delayed-onset muscle soreness after intense training sessions. When these tears occur, muscles may swell slightly and retain fluid for a few days after the exercise. This is a normal part of the recovery process.
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Exercise and Dehydration
Your fluid intake needs increase when you begin an exercise program. Because you perspire during workouts, it's necessary to replace lost fluids with extra water. If you don't do this, your body enters a state of dehydration. When you're dehydrated, your body will begin to retain water as a survival mechanism. Therefore, increasing your fluid intake will usually help combat water retention caused by dehydration.