What Causes Your Muscles to Expand When You Work Out?

Muscles swell during a workout, providing a temporary pump.
i Photodisc/Photodisc/Getty Images

Sometimes called muscle “pump” among other things, it’s common for muscles to expand when you work out. In fact, it’s all a part of the muscle growth process. Sure, guys love the effect, and women certainly like the tight and toned feeling, but what’s really happening under your skin? Your muscles are just going through the basic process of building strength and size.

At the Cellular Level

    To really understand what’s going on with your muscles during exercise, you have to look at it from a cellular level. Your muscle tissue is made up of millions of tiny fibers, made by synthesizing protein. The stress of exercise causes some of these fiber structures to break down, basically causing injury to your muscles. As your muscle recovers over a period of days or weeks, the fibers synthesize more protein and heal themselves by adding more mass; this is called hypertrophy. Although your muscles tighten and swell during a workout, this is just a temporary muscle pump -- not hypertrophy -- caused by increased blood flow.

Resistance Training

    Muscles typically don’t expand much during any type of moderate aerobic activity. Resistance training, however, often causes the muscles to swell and harden a bit during the workout. Strenuous activity causes your body to increase blood flow to the muscles that need it. For instance, if you're doing biceps curls, your body will send an increased amount of blood flow to the biceps, causing the muscle to expand. It's only a temporary effect; the muscle will shrink down to normal size once the strenuous exercise is over and blood flow decreases to normal levels.


    A big risk associated with resistance training is overtraining. You now know that resistance training causes injury to your muscle tissue. Your muscles need time to heal and recover from these injuries. If you don’t allow enough healing time, you can actually lose muscle tissue or incur other injuries. To avoid overtraining, always allow at least one day of rest between weight-training workouts. If you’re sore on a workout day, it’s alright to take an extra day of rest. You can progressively increase your workout frequency as you gain strength over the weeks.


    After a tough resistance-training workout, your muscles will feel tight and larger than normal, sometimes for several hours. As your muscles become stronger, your “pump” may not last quite as long postworkout because your muscle tissue is able to recover faster. The key to a good recovery period is to eat a small carbohydrate and protein-rich snack right after the workout, such as a peanut butter and jelly sandwich, and to maintain a sensible diet plan full of vegetables, fruits and lean sources of protein.

the nest