The butterfly stroke is the most physically demanding of the four competitive swimming strokes. It uses a simultaneous overhead arm pull combined with a wavelike body movement called undulation and a two-legged “dolphin kick” to drive the swimmer forward. It’s the second-fastest stroke after freestyle, burning upward of 800 calories per hour, and it’s one of the most visually striking movements in all of sport.
How the Body Moves Through the Water
What makes butterfly unique is undulation. Rather than staying flat like in freestyle, your entire body moves in a wave pattern that starts at the head, travels through the chest and hips, and finishes with a snap of the feet. This wave is what generates forward momentum and keeps the stroke rhythmic. The chest presses down slightly, the hips rise, and then the hips press down as the chest lifts. It’s often compared to the motion of a dolphin, which is where the kick gets its name.
Butterfly is a highly hip-driven stroke. The power comes from your core, not your knees or feet. Think of your hips as the engine: they initiate the wave, and everything below them follows. Bending the knees too much is one of the most common mistakes beginners make, because it creates drag and breaks the smooth chain of movement. The feet should finish each kick with pointed toes, like a small whip crack at the end of the wave.
The Arm Pull: Three Sweeps Per Stroke
The arm movement breaks into three connected phases, and both arms move together at all times. First, your hands enter the water about shoulder width apart, thumbs leading, with elbows slightly higher than the hands. From there, your hands sweep down and outward to form a Y shape in front of your body. This is the catch, where you grab hold of the water.
Next, you turn your hands back in toward each other while keeping your elbows high. This inward sweep is where most of the propulsive force happens. Finally, your hands sweep backward along your sides, pushing water toward your feet. Once the push is complete, both arms recover aggressively over the water’s surface, stretching forward to re-enter and start the cycle again. That over-the-water recovery, with both arms swinging wide and forward simultaneously, is the signature visual of butterfly.
The Dolphin Kick and Its Timing
Each arm cycle includes two dolphin kicks. The first, smaller kick happens as your hands enter the water. The second, larger kick occurs as your hands push past your hips at the end of the pull. This two-kick rhythm is essential. Without it, the stroke falls apart, because the kicks help lift your upper body for the arm recovery and keep your hips from sinking.
Both legs move together at all times. Competition rules are strict on this point: the legs and feet don’t need to be at exactly the same level, but they cannot alternate like a flutter kick. Breaststroke-style kicks are also not permitted. The movement should feel smooth and continuous, originating from the hips, flowing through slightly bent knees, and finishing through pointed feet.
When and How to Breathe
Breathing in butterfly is all about timing it with the natural rise of your body. As you start pulling water and your shoulders lift, that’s when your head comes forward to grab a breath. By the time your hands finish the pull, your chin should be at the surface. The key is to get your head back down before your arms complete their recovery and re-enter the water. If your head is still up when your hands hit the water, you’ll land flat and lose momentum.
A helpful way to think about it: breathe as your body rises, and lower your head as your body falls. Don’t linger at the surface. The breath should feel quick and natural, not like a deliberate pause in the stroke. Many competitive swimmers breathe every other stroke to maintain speed, though breathing every stroke is perfectly fine for fitness swimming or longer distances.
Muscles Used in Butterfly
Butterfly is essentially a full-body exercise. During the pulling phase, the large chest and back muscles generate the primary power. Smaller muscles around the shoulder blade and rotator cuff control the rotation of the arm and help stabilize the joint. One study using electromyography found that the serratus anterior (a muscle along your ribs that anchors the shoulder blade) and the subscapularis (a deep rotator cuff muscle) stay highly active throughout the entire stroke, making them especially prone to fatigue.
The recovery phase, when the arms swing over the water, relies on the deltoid muscles of the shoulder and the muscles that rotate the arm outward. Meanwhile, your oblique abdominals and spinal muscles contract continuously to maintain the undulating body position. Your glutes, hip flexors, and quads drive the kick. There really isn’t a muscle group that gets to rest.
Why Butterfly Causes More Injuries
Shoulder and lower back injuries are the top two injury sites for competitive swimmers, and butterfly is a major contributor to both. The shoulders are vulnerable because the stroke demands a wide range of motion under load, thousands of times per practice. Shoulder impingement, where tendons get pinched during the overhead recovery, and joint capsule looseness from repetitive stretching are the most common problems.
The lower back takes punishment because the oblique and spinal muscles never fully relax during butterfly. Over time, this can lead to disc issues or chronic low back pain, particularly in swimmers who lack core endurance. Preventive exercises focus on three areas: rotator cuff strengthening (light resistance, high reps), lower trapezius work to stabilize the shoulder blade, and core exercises like front bridges with diagonal limb raises. Regular stretching of the spine and hip muscles after practice also reduces risk.
Competition Rules
World Aquatics (formerly FINA) has specific rules that define a legal butterfly. Your body must stay on the breast (face down) at all times, except during turns. Both hands must touch the wall simultaneously at every turn and at the finish, with hands separated. This two-hand touch rule is one of the most commonly enforced in competitive swimming, and a one-hand touch results in disqualification.
After the start and each turn, you’re allowed to glide underwater with dolphin kicks for up to 15 meters before your head must break the surface. Many elite swimmers use this underwater phase aggressively, since dolphin kicking beneath the surface can be faster than swimming on top of it. Once you surface, you must stay there until the next turn or finish.
Drills for Learning Butterfly
Butterfly’s coordination challenge makes it one of the hardest strokes to learn all at once. Breaking it into pieces with drills is the standard approach. One-arm butterfly is one of the most popular: you swim with just one arm pulling while the other stays extended in front of you, using a flutter kick instead of a dolphin kick. This flattens out your body and lets you focus on the arm path without worrying about full-stroke timing. You can also do this with the non-stroking arm at your side, which makes controlling your body position harder and builds strength faster.
Swimming full butterfly with a flutter kick (instead of dolphin kick) is another useful drill. It strips away the undulation and forces you to stay flat, which helps swimmers who bob too much up and down instead of moving forward. Head-up butterfly, where you keep your face above water the whole time, pushes your hips down deliberately, training you to kick harder and engage your core to compensate. It’s uncomfortable by design, and it makes regular butterfly feel easier by comparison.
Calorie Burn and Fitness Value
Butterfly is the highest calorie-burning swim stroke. A person weighing around 140 pounds or more can burn over 800 calories per hour, or roughly 450 calories in 30 minutes. Heavier swimmers burn more. For context, that’s comparable to running at a fast pace, but with far less joint impact. The catch is that very few people can sustain continuous butterfly for an extended period. Most fitness swimmers incorporate butterfly in shorter intervals, mixing it with other strokes, which still delivers a significant training stimulus to the shoulders, core, and cardiovascular system.