Flutter kicks: how to do them, what studies of straight-leg lifts found, and easier and harder versions

Flutter kicks are a floor exercise. You lie on your back, lift your straight legs a few inches, and beat them up and down in turn, small and quick. The name comes from the swimming kick, but this page is about the floor exercise. Your legs move; your midsection's job is to keep your lower back from arching off the floor, the same job as in the dead bug and the hollow body hold.

This page covers the setup, what the studies of similar leg lifts measured, how flutter kicks differ from scissor kicks, the easier and harder versions, the faults, and the sets. It fits the core workout's "stop the arch" slot, beside the plank or on alternate sessions.

How to do flutter kicks

1. Lie on your back with your hands flat under your hips, palms down, or by your sides.
2. Press your lower back into the floor and tuck your hips under slightly, as if pulling your belt buckle toward your ribs.
3. Straighten your legs and lift them so your heels are a few inches off the floor. Higher is easier; lower is harder.
4. Kick: lift one leg a little higher while the other lowers, then switch, in small, quick beats like a swimmer's kick. Keep your head down, chin slightly tucked.
5. Breathe steadily through the set, and end it the moment your lower back lifts off the floor.

These are coaching cues, not study findings. The rule that makes it work is the same as the hollow hold's: the lower back stays down.

What the studies found

No study of the floor flutter kick turned up in a PubMed search for this page; every hit was about swimming, diving or treading water. Studies of straight legs held, lowered or lifted from the floor give a picture. All of them measured muscle activity during an exercise, not growth or strength over weeks:

• Lowering both legs against a curl, with the pelvis held. In 15 men, with the pelvis kept in the same tucked position by sensor feedback, lowering both straight legs produced significantly more abdominal muscle activity than a curl.¹
• The tuck matters. In 16 men holding both straight legs about 5 centimeters above a board, activity in the upper and lower front abs was higher with the pelvis tilted back, tucked under, than tilted forward. Neutral wasn't significantly different from tucked, and the external obliques and the rectus femoris, a hip flexor, showed no differences between the three positions.²
• A curl-up beat a leg lowering in a small pilot. In eight healthy adults, three curl-up versions produced significantly higher activity in the upper and lower rectus abdominis than a leg lowering, a reverse curl-up and a ball roll-out. The abstract doesn't say the pelvis was held.³
• A leg raise against a plank. In 18 healthy, physically active women, an isometric bilateral leg raise produced significantly more rectus abdominis activity than a plank.⁴
• The hip flexors work hard in leg lifts. A 2024 systematic review of nine electromyography studies, covering 109 healthy adults aged 20 to 40, reported more than 60% of a maximal contraction in the iliopsoas, a deep hip flexor, in the active straight leg raise at around 60 degrees of hip flexion (a much higher lift than the few inches used here) and in supine hip flexion and leg lifts.⁵

Read together: flutter kicks are leg lifts, so your hip flexors do much of the lifting, and the abs' job is to hold the pelvis and lower back still while they do. Whether straight-leg work beats a curl for the abs may depend on how the pelvis is held, which is why the first rule is that the lower back stays down. These studies were small, and none trained anyone with flutter kicks.

Flutter kicks or scissor kicks?

In this page's working definition, flutter kicks beat the legs up and down in turn, and scissor kicks open the straight legs apart and cross them over each other. Both keep your back down while straight legs move, so the cues here apply to both. No study compared the two.

Easier and harder versions

Each step below is a practical ladder, not a measured one. Move up only when the step before keeps your lower back down for the whole set:

1. The dead bug first, if your lower back lifts on every attempt here.
2. Legs high: straight legs well off the floor, tiny beats, hands under your hips.
3. Legs low: heels a few inches above the floor, still with your hands under your hips.
4. Hands off: arms by your sides or crossed over your chest.
5. Head and shoulders up, as in the full hollow body hold, then the kicks.
6. Slower, bigger beats, or a pause with the legs low between sets of kicks.

No study of ankle weights or any other loaded flutter kick turned up, so this page doesn't add them.

The common faults, and a fix for each

• Lower back arching off the floor. The main one. Raise your legs, bend your knees or go back a step on the ladder.
• Legs too low, too soon. Lowering them is the hard part; raise them until your back stays down.
• Wild, big kicks. Small and quick; if your hips rock, the beats are too big.
• Neck straining. Keep your head down and your chin slightly tucked, or rest your head between rounds.
• Holding your breath. Short, steady breaths.

Sets, rounds and where they go

• Three rounds of 20 to 30 seconds with 30 to 60 seconds of rest, each round stopped when your lower back lifts. These numbers are practical, not from a study.
• At the end of a session, after your main lifts, in the plank's slot on alternate sessions or beside it, with the rest of the core block.
• In a plan, it can fill the core slot of the full-body workout plan.
• Progress one thing at a time: legs lower, then hands off, then head up. (Progressive overload covers the options.)

Common questions

Do flutter kicks work the abs or the hip flexors?

Probably both, going by studies of similar leg lifts (none measured flutter kicks). The leg-lift studies measured the hip flexors working hard,⁵ and the abs' job of holding your pelvis and lower back still while they do is a description, not a measurement.

What if something hurts?

If you have pain, an injury or a medical condition, a doctor or a physical therapist should set the limits before you train; this page is general guidance for healthy adults.

If you'd rather have the week planned

Deciding how much core work your week needs, and which of it should be harder, is the part a page can't do. Coaching is the version where your workouts live in Trainerize, built around the days you actually have, and a weekly call looks at the week you just had and sets the next one. Here's how coaching works, and apply is the first step. Otherwise: at the end of your next session, do three rounds of 20 seconds of flutter kicks with your hands under your hips, and stop each round when your lower back lifts.