To firm your butt and hips, you don’t need a gym membership or archaic seen-on-TV contraption — all you need is a water bottle.

The move is known as the Cook hip lift, named after its creator, physical therapist Gray Cook. It’s a cousin of the basic hip raise — a butt-toning exercise in which you lie on your back, place both feet on the floor, and lift your hips. (If you’re imagining something dirty, you’ve got it right.) But Cook’s variation not only works your core, glutes (butt muscles), and the back of your legs, it’s also kinder to your lower back.

Start on your back with your knees bent about 90 degrees, your feet planted on the floor in line with your shoulders. Grab your right thigh and pull your right knee toward your chest, pinning a water bottle, a tennis ball, or a rolled-up towel in the crease of your hip. Lift your left toes off the floor. Keeping the bottle in place, raise your hips as high as you can, pause, and lower your rear back down.

“Too often during a normal bridge with both feet on the ground, people have a tendency to move more from the lower back and round the low back too much at the top of the exercise,” said sports medicine expert Lee Burton.

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