Knee ups are a bodyweight exercise in which you drive one or both knees toward your chest against gravity, primarily targeting the hip flexors and abdominal muscles. They can be performed standing, hanging from a bar, or lying on your back, and the specific muscles emphasized shift depending on which version you choose and how high you bring your knees. The movement looks simple, but the interplay between the hip flexors and the core underneath it is more nuanced than most gym-goers realize.
What Counts as a Knee Up
The term “knee up” covers a family of exercises rather than one rigid movement. The common thread is hip flexion with the knees bent. In a standing knee up, you lift one knee at a time toward your chest while staying upright, often at a brisk pace that doubles as light cardio. In a hanging knee raise, you grip an overhead bar and curl both knees upward. In the captain’s chair version, you support yourself on padded arm rests and do the same thing. Lying knee raises have you flat on your back, pulling bent knees toward your ribcage. All of these share the same core action: the thigh moves toward the trunk through hip flexion, the abs work to stabilize or curl the pelvis, and the movement resets.
Because the knee stays bent throughout, the quadriceps do not contribute much force. That distinguishes knee ups from straight-leg raises, where the longer lever arm shifts more demand onto the quads and makes the exercise harder for the hip flexors and lower abs. Bending the knee shortens the lever, making the movement more accessible for beginners while still loading the muscles responsible for pulling the thigh upward.
The Hip Flexors Do the Heavy Lifting
The primary movers in any knee up are the hip flexor group, and the most important player in that group is the iliopsoas, a pair of deep muscles (the iliacus and the psoas major) that run from the lower spine and inner pelvis down to the top of the femur. Research on hip flexion exercises found that the iliacus and psoas showed activation levels ranging from roughly 44 to 65 percent and 35 to 67 percent of their maximum voluntary contraction, respectively, during greater degrees of hip flexion.1PubMed Central. Hip Flexor Muscle Activation During Common Rehabilitation and Strength Exercises Those are substantial activation levels, which is why knee ups feel like they burn deep in the hip crease and lower abdomen rather than on the surface of the thigh.
The rectus femoris, the only quadriceps muscle that crosses the hip joint, also contributes to hip flexion during knee ups. However, its role shifts depending on the exercise setup. Studies comparing hip flexion movements found that bilateral leg lifts produced higher activity in the iliacus and sartorius muscles, while sit-up style movements favored the rectus femoris more.2PubMed. Abdominal and hip flexor muscle activation during various training exercises In a knee up, because the knee is bent, the rectus femoris is in a shortened position and contributes less force than it would during a straight-leg raise. The deep hip flexors end up doing a larger share of the work.
The sartorius, a long strap-like muscle that angles across the front of the thigh, also activates during knee ups. It assists in hip flexion and is especially active when the hip flexes while the knee is bent and the leg is slightly rotated outward. You will not feel the sartorius as a distinct burning muscle the way you feel the abs or the quads, but it is part of the team.
How the Abs Get Involved
Knee ups are often marketed as an “ab exercise,” and they are, but the abs do not kick in equally throughout the entire range of motion. During the early part of the lift, when the thigh moves from a hanging or resting position up to about 45 degrees of hip flexion, the hip flexors dominate. As the knees travel higher, the pelvis has to tilt backward (a posterior tilt) to allow more range, and that pelvic rotation is what recruits the rectus abdominis. Research on leg raises found that rectus abdominis activity increased more sharply than rectus femoris activity between 45 and 60 degrees of hip flexion, and the ratio of abdominal to thigh muscle activation kept climbing as the angle increased.3PubMed Central. Muscle activities of the rectus abdominis and rectus femoris and their ratio during leg raises performed by healthy adults – Section: RESULTS
This is the single most useful thing to understand about knee ups: if you only bring your knees to hip height and stop, you are training your hip flexors with modest ab involvement. If you curl your knees all the way up toward your chest and let your pelvis tuck under, you shift the load progressively onto the abs. People who complain that knee raises “only work the hip flexors” are usually stopping the movement too early.
The obliques also play a role, particularly the external obliques. When the pelvis tilts posteriorly and the trunk flexes, external oblique activation increases.4PubMed. Muscle activation during four Pilates core stability exercises in quadruped position Adding a twist to the knee up, bringing the knees toward the opposite shoulder, further increases oblique demand. The internal obliques contribute to trunk stabilization throughout the movement as well, though their activation level depends heavily on the speed and direction of the exercise.
Standing Versus Hanging Versus Lying Versions
The variation you choose changes the difficulty and the muscle emphasis in meaningful ways. Standing knee ups are the lightest version. Gravity acts on the thigh at a relatively short lever arm, and your standing leg and core only need to maintain upright balance. This makes them a good warm-up, a beginner entry point, or a higher-rep cardio drill when done alternating legs at speed. The hip flexors and lower abs still work, but the overall intensity is low.
Lying knee raises on a bench or floor increase the difficulty slightly because you are lifting both legs at once and gravity pulls through a longer effective range. The abs get more involved because you need to prevent your lower back from arching off the surface as you lower the legs. Pressing your lower back into the bench is a common cue for this reason; it forces a posterior pelvic tilt and pre-loads the abs before the legs even move.
Hanging knee raises are the hardest standard version. Grip strength, shoulder stability, and anti-swing control all add layers of demand. Because you are suspended, there is nothing supporting the pelvis, so the abs must do all the work of curling the pelvis upward at the top of each rep. The captain’s chair removes the grip challenge but keeps the core demand high. For most people looking to strengthen both the hip flexors and the abs, the hanging or captain’s chair version offers the best training stimulus per rep.
Why Range of Motion Matters More Than Rep Count
Many people crank out 30 or 40 partial knee raises with their thighs barely reaching parallel and wonder why their abs are not sore the next day. The research pattern is consistent: hip flexor activation and abdominal activation both increase with greater ranges of hip flexion.1PubMed Central. Hip Flexor Muscle Activation During Common Rehabilitation and Strength Exercises Moving through a fuller range, bringing the knees closer to the chest and allowing the pelvis to tuck, is what makes the exercise work for the abs specifically. Ten slow, full-range reps where you pause at the top with your pelvis curled under will typically do more for core development than triple that number of quick half-reps.
The same principle applies to external resistance. Adding ankle weights or holding a light dumbbell between the feet increases activation across both the hip flexors and the abdominals. But adding resistance only helps if the range of motion stays full. Loading up a partial rep just makes the hip flexors work harder without shifting demand to the abs.
Athletic and Functional Benefits Beyond the Gym
Knee ups train the hip flexion pattern that underlies sprinting, climbing stairs, kicking, and any movement where you need to bring your knee up quickly. A study on physically active individuals found that an increase in hip flexion strength of about 12 percent led to improvements in 40-yard sprint times by roughly 3.8 percent and shuttle run times by about 9 percent.5PubMed. Effects of hip flexor training on sprint, shuttle run, and vertical jump performance Those are meaningful numbers for recreational athletes, and the mechanism is straightforward: a stronger hip flexor pulls the recovery leg forward faster during each stride, which shortens ground contact time and increases step frequency.
For older adults, the functional carryover is arguably even more important. Hip flexor weakness makes it harder to clear obstacles while walking, which increases trip and fall risk. The good news is that strength training works well across age groups. Research comparing younger and older adults found that while older participants started about 30 percent weaker, they still achieved isometric strength gains of roughly 30 percent with training, compared to 36 percent in the younger group.6Elsevier / PubMed Central. Adaptations in muscular activation of the knee extensor muscles with strength training in young and older adults The training response does not disappear with age; it just starts from a lower baseline.
Common Mistakes and Spinal Considerations
The most frequent error with knee ups, particularly the lying or hanging versions, is letting the lower back hyperextend during the lowering phase. When your legs descend and your abs cannot maintain a posterior pelvic tilt, the lumbar spine arches sharply, loading the spinal discs in a way that can aggravate existing low back issues. Researchers have raised concerns that bilateral leg raises can compound hip muscle imbalance and both hyperextend and hyperflex the lumbar spine when performed without proper pelvic control.7British Journal of Sports Medicine. Abdominal muscle training in sport The fix is not to avoid the exercise, but to control the lowering phase and never let the legs drop below the point where you can keep your lower back stable. If you are lying on a bench, keep your lower back pressed into the surface. If you are hanging, stop the descent before you start swinging.
Swinging is the other classic issue with the hanging version. People use momentum from their torso to heave the knees up, which turns the exercise into a full-body swing rather than a controlled hip flexion. A useful cue is to pause for a full second at the bottom of each rep to kill momentum before initiating the next lift. If you cannot do that, reduce the rep count or switch to the captain’s chair for more stability.
Tight hip flexors are a common concern, and people sometimes avoid knee ups because they worry the exercise will make tightness worse. In practice, the issue is not that hip flexors are too strong but that they are too weak or poorly coordinated, and the body compensates by keeping them in a shortened position. Strengthening a muscle through a full range of motion generally improves its function and reduces the reflexive tightness that comes from weakness. If you have significant anterior pelvic tilt, pairing knee ups with hip flexor stretching and glute strengthening is a reasonable strategy rather than cutting hip flexion work entirely.
What Happens as You Fatigue
Form degradation during knee ups is not just about willpower. When the primary movers fatigue, the body recruits compensatory muscles and changes the movement pattern. Research on abdominal exercises found that after a fatiguing protocol, activation of both the lower and upper portions of the rectus abdominis increased significantly, while external oblique activation did not change.8Taylor & Francis Online (Ergonomics). Electromyographic investigation of abdominal exercises and the effects of fatigue This suggests that fatigue during core exercises drives the rectus abdominis to work harder to compensate, but the obliques check out. If oblique involvement matters to you, doing your sets before you are deeply fatigued or switching to an oblique-specific movement while still fresh may be more effective.
From a practical standpoint, this also means the last few reps of a high-rep knee up set are qualitatively different from the first few. The hip flexors tire first, the abs try to pick up the slack, and eventually the lumbar spine starts absorbing forces it should not. Keeping sets to a range where you can maintain full control, typically somewhere between 10 and 20 reps depending on the variation and your fitness level, and adding sets rather than grinding out marathon sets usually produces better results with less risk.
Adding Instability for Greater Core Demand
Once standard knee ups feel easy, a common progression is to perform them in less stable conditions. Hanging from a bar is inherently less stable than lying on a bench. Suspension trainers like TRX straps, where you place your feet in the loops and perform knee tucks from a push-up position, add yet another stability challenge. Research on core exercises performed under unstable versus stable conditions found that the unstable versions produced superior improvements in muscle activation after an eight-week training period.9PubMed. The efficacy of an eight-week core stability exercise program in suspended and stable conditions on the muscle activation and functional disability of amateur soccer players with non-specific chronic low back pain
The trade-off is that instability limits how much external load you can add. You cannot hold a heavy dumbbell between your feet while your hands are in suspension straps. For people whose primary goal is ab and hip flexor hypertrophy, progressing with added weight on a stable setup will likely build more muscle. For people whose goal is functional core stability for sports or injury prevention, the unstable variations are worth incorporating. Most training programs benefit from some of both.
How Knee Ups Compare to Other Ab Exercises
An electromyographic study comparing four popular abdominal exercises found no significant difference in mean rectus abdominis activity between them.10PubMed Central. Electromyographic analysis of four popular abdominal exercises This is a finding that has held up across multiple studies and it tells you something important: no single ab exercise is dramatically superior to the others for rectus abdominis activation. Where knee ups stand out is in their hip flexor training effect. Crunches barely involve the hip flexors at all because the legs do not move. Planks train the abs isometrically but again leave the hip flexors mostly out of it. Knee ups train both the abs and the hip flexors in one movement, which makes them a more time-efficient choice if you care about both.
The flip side is that if your only goal is maximum abdominal tension and you do not care about the hip flexors, a well-executed crunch or cable crunch isolates the abs without the hip flexors “stealing” any of the work. But for most people training for general fitness or athletic performance, the combination of hip flexor and abdominal work in a single exercise is a feature, not a bug.
Programming Knee Ups Into a Routine
For most people, knee ups fit naturally at the end of a workout as part of a core circuit or at the beginning as part of an activation warm-up. When used for core strength, two to four sets of 10 to 15 controlled reps with a pause at the top is a reasonable starting point. When used as a warm-up or conditioning drill, standing alternating knee ups at a faster pace for 30 to 60 seconds work well.
Pairing knee ups with an anti-extension exercise like a plank or ab wheel rollout and a rotational exercise like a cable woodchop covers the three main planes of core function: flexion, anti-extension, and rotation. You do not need to do six different ab exercises every session. Two or three that hit different demands will cover more ground.
People with existing low back pain should start with the lying version, where the bench supports the spine, and focus on keeping the lower back pressed down throughout. Gradually progressing to hanging versions as pelvic control improves is safer than jumping straight to the bar. If hanging knee raises cause pain even with good form, the captain’s chair is a reasonable long-term alternative that keeps the spine in a more supported position while preserving the core and hip flexor training effect.