# Lying Knee Flexion And Extension On Exercise Ball - Canonical: https://fitwill.app/exercise/10061/lying-knee-flexion-and-extension-on-exercise-ball/ - Media ID: 10061 - Primary muscle: Hamstrings - Body part: Thighs - Equipment: Stability Ball - Video: https://fitwill.app/videos/10061/lying-knee-flexion-and-extension-on-exercise-ball.mp4 ## Description Lying Knee Flexion And Extension On Exercise Ball is a supine hamstring exercise where you rest your lower legs on a stability ball and curl the ball toward your hips by bending your knees, then slowly extend back out. Because your body weight is supported on the floor and only your heels or calves contact the ball, the hamstrings do the work of both pulling the ball in and controlling it on the way out, which makes the lowering phase just as demanding as the pull. This movement is a solid alternative to machine leg curls for anyone training at home or without access to a gym. It also carries a stability demand that machine curls don't have: the ball shifts slightly under you, so your core, glutes, and lower back have to brace continuously to keep your hips from sagging or twisting. That combination of hamstring work and trunk control makes it a common pick in posterior-chain circuits, rehab progressions for knee flexion strength, and warm-ups before heavier deadlift or hinge work. Setup matters more than most people expect. If the ball is too far away, your hips drop and the hamstrings lose tension; if it's too close, the range gets shortened and the movement turns into a hip lift rather than a knee curl. Aim for a position where your legs are nearly straight at the start, heels resting on top of the ball, arms flat on the floor beside you for balance. To execute it well, think about dragging the ball toward your glutes with your heels rather than just bending your knees. Keep your hips lifted and level the whole time so the hamstrings stay loaded through the full arc. Pause briefly at the top when your knees are bent and the ball is close to your hips, then extend back out under control, resisting the ball's tendency to roll away from you. A practical coaching point: the floor-based support makes this friendlier to the lower back than bridging variations where the shoulders are elevated, but form still matters. Avoid pushing your hips high into an arch at the top, and don't let your lower back take over when fatigue sets in. Two to four sets of eight to twelve controlled reps works well for most people, and slowing the return to three or four seconds is an easy way to progress without adding load. ## Instructions 1. Lie on your back on an exercise mat with your arms flat on the floor at your sides, palms down for balance. 2. Place your heels and lower calves on top of a stability ball and straighten your legs so your body forms one long line from shoulders to feet. 3. Press your shoulder blades into the floor and lift your hips slightly so your weight sits on your upper back, glutes, and the ball. 4. Brace your core and squeeze your glutes before moving to stop your hips from sagging toward the floor. 5. Bend your knees and pull the ball toward your hips by dragging your heels along the surface of the ball. 6. Continue until your knees are fully bent and the ball is close to your glutes, keeping your hips lifted and level throughout. 7. Pause for a beat at the top, then slowly straighten your knees and roll the ball back out to the starting position, resisting the rollout. 8. Breathe out as you curl the ball in and breathe in as you extend back out. 9. Complete all reps with your hips held in the same elevated position, then lower your hips and step or slide your legs off the ball to reset. ## Tips & Tricks - The most common mistake is letting the hips sink toward the floor as the ball rolls in, which shortens the hamstring work and shifts strain to the lower back — re-squeeze the glutes before every rep. - If the ball keeps sliding away from you, you're likely extending too fast at the end; slow the rollout and stop just short of full lockout so your heels maintain traction. - Point your toes slightly up (dorsiflex) to drive through the heels; curling with slack or pointed feet reduces how much the hamstrings and calves contribute. - Use a ball that lets your legs sit nearly straight at the start with your heels on the top surface — a ball that's too small turns this into a short, jerky pull. - On a smooth floor, place the ball against a corner or use a mat underneath it until you've learned the control; a runaway ball mid-set usually ends the set. - If you feel it mostly in your lower back, reduce the hip lift — you only need enough height to keep a straight line, not a full bridge. - To make the exercise harder, perform the lowering phase over three to five seconds or try single-leg variations once two-leg reps feel easy. - Keep your arms relaxed on the floor; pressing hard through your hands to lift your hips is a sign your glutes and hamstrings aren't doing their job. - Move your feet slightly lower on the ball (toward the top of the calves) for less challenge, or keep only your heels in contact for more. ## Frequently Asked Questions ### What muscles does Lying Knee Flexion And Extension On Exercise Ball work? The hamstrings are the primary movers, since they bend the knee and control the ball rolling back out. Your calves, glutes, and core work to stabilize your hips and keep the ball tracking straight. ### Is this exercise a good substitute for a leg curl machine? Yes, for most purposes it trains the same knee-flexion pattern without equipment beyond a stability ball. The main difference is the added balance demand, which some lifters will find harder at first and others will find limits how much load they can apply. ### Why do my hips drop when I curl the ball in on this exercise? This usually means the glutes aren't bracing or the ball is being pulled too quickly. Squeeze your glutes before each rep and control the pull so your hips stay elevated in a straight line with your shoulders and knees. ### Should my heels or calves be on the ball? Heels on top of the ball is the standard position and gives the hamstrings the best leverage. Resting the calves on the ball shortens the leverage slightly and makes the movement a bit easier, which is useful for beginners. ### Can beginners do Lying Knee Flexion And Extension On Exercise Ball? Yes. Start with the calves resting lower on the ball, keep the hip lift modest, and move slowly. Once eight to twelve reps feel controlled, progress to heel-only contact or a slower lowering phase. ### How is this different from a stability ball hamstring curl with hips in a bridge? In the bridging version your shoulders are elevated on a bench and your hips carry more load, making it harder. In this floor-based version your upper back stays on the ground, so it's gentler on the lower back and easier to learn first. ### What size stability ball should I use? A ball sized so your legs sit almost straight with heels on top when you lie down — generally 55 to 65 cm for most adults. Too small and the range is cramped; too large and your hips lift excessively at the start. ### How many sets and reps should I do? Two to four sets of eight to twelve slow, controlled reps is a solid range. Because the resistance comes from your body weight, progressing by slowing the tempo or adding a pause at the top works better than chasing high reps. ### Is it safe for my knees and lower back? For most people, yes — the movement follows the knee's natural bending arc and your weight is supported on the floor. Stop if you feel pinching at the knee or sharp lower back discomfort, and check that your hips aren't sagging or over-arching.