This Is Why Your Muscles Look Bigger Right After You Work Out

“A pump is the fix and rush of bodybuilding. Some bodybuilders will tell you that it feels better than coming. Whether that is true or not, it is one of the finest and most complicated physical sensations you can have. The part of the body being pumped feels like one of those fast-frame films of flowers blooming or seeds ripening; the muscles seem actually to go from pod to blossom in seconds under the skin.”

That’s how Charles Gaines and George Butler—who helped launch the career of Arnold Schwarzenegger with their film Pumping Iron—describe the physical sensation known as the pump.

Everyone likes getting a pump. Your muscles swell up and feel full. Your skin feels tight. You look more muscular than you really are, especially if you’re standing in front of your favorite mirror where the lighting is just right. It feels good. In fact, Austrian psychoanalyst Wilhelm Reich noted that all pleasurable sensations are characterized by the flow of blood from the center of an organism to its periphery.

But why does it happen? What’s going on inside your muscles to give you a pump? And is getting one a sign that growth is sure to follow?

Let’s say you pick up a dumbbell and start training your biceps. As you curl the weight, the individual fibers inside your biceps expand. This compresses the veins, taking blood out of the muscle. Because blood is being pumped into your biceps faster than it can leave (when you work out, there's an increase in blood flow), the muscle becomes temporarily engorged with blood. With each rep, compounds known as metabolites—some of which are responsible for that burning sensation you get toward the end of a tough set—build up inside your biceps. This has the effect of drawing additional fluid into the muscle.

It’s the accumulation of blood and fluid inside the muscle that’s responsible for giving you that pumped sensation. When you finish the set, your biceps look and feel a little bigger because they are. That’s why bodybuilders spend time “pumping up” by performing a series of high-rep sets immediately before they step on stage. Doing so makes their muscles look bigger, which gives them a better chance of winning the contest. Unfortunately, the pump is relatively short-lived, and your muscles will have begun the journey back to their normal size by the time you’ve had a shower and left the gym.

However, not every form of resistance exercise will make your muscles look bigger immediately afterward. Training programs geared toward making you stronger—which typically involve the use of heavy weights, low reps, and long rest periods between sets—don't usually lead to much of a pump.

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Why not? Fewer reps means less accumulation of metabolites, and less time for your muscles to become engorged with blood. Taking several minutes of rest between each set also gives your body plenty of time to transport those metabolites out of your muscles before you start lifting again.

Another thing people with the goal of gaining mass might ask is whether getting a pump leads to muscle growth. Some trainers say that getting a pump is a temporary cosmetic effect that means nothing—that your muscles might expand for a few hours, but that doesn’t mean they’re going to grow any faster. Others will tell you that getting a pump is essential for muscle growth and that a pump-less workout is a wasted one. They swear that getting a pump is a cast-iron guarantee that growth has been stimulated.

There’s plenty of research to show that training with lighter weights and higher reps—the type of training that gives you a pump—is an effective way to build muscle in the long term. However, your muscles can also be made to grow with training that is geared towards building strength—lifting heavy weights for a handful of reps, and taking several minutes of rest between each set. This technique will get you a bit of a pump, but it’s nowhere near the type you’d normally get with lighter weights, higher reps, and shorter rest periods.

In a 2015 study, a team of US researchers compared two different training programs over an eight-week period. Lifters in the first group did four sets of 10 to 12 reps, taking one minute of rest between each set. Subjects in group two did the same exercises and number of sets, But they used a much heavier weight that limited them to 3 to 5 reps, and rested for around three minutes between sets. Everything else, including the exercises and number of weekly training days, was identical.

If getting a pump was essential for growth, you’d expect to see the higher-rep lifters building the most muscle. But that isn’t what happened. In fact, none of the differences in body composition between the groups were statistically different—both the “high pump” and “low pump” training programs had a similar effect on muscle growth.

Still, the researchers did spot a clear trend toward greater gains in the group lifting heavier weights. (There are multiple reasons a result might not cross the statistically significant threshold but still be physiologically relevant). Men in the lower-rep, higher-weight group were the ones to build the most muscle.

So, getting a pump isn’t essential for muscle growth. It might feel good while you’re in the gym lifting weights, but it’s not a critical part of the recipe for long-term results. However, the type of training that leads to a pump does provide a muscle-building stimulus, and may well work in a different way to heavy lifting.

If you want to maximize your gains, do a bit of both. Combining heavy lifting with lighter, higher-rep training will take advantage of the multiple pathways involved in muscle growth, as well as giving you the “fix and rush” of getting a pump.

Christian Finn is an exercise scientist and former "trainer to the trainers" based in Northamptonshire, England.