Knee ups, often called knee raises, are a fundamental core strengthening exercise that targets the abdominal muscles and hip flexors. This movement involves drawing the knees toward the chest against resistance, making it an effective method for developing stability and muscular endurance. For many, this exercise is a progression toward more advanced movements like the straight-leg raise, helping to build foundational strength.
Defining the Exercise and Proper Starting Position
A knee up is a closed-chain movement where the lower body is brought toward the torso, shortening the distance between the rib cage and the pelvis. The most demanding version is the Hanging Knee Up, which requires a pull-up bar. To begin, assume a dead hang position with a secure, overhand grip slightly wider than shoulder-width.
The body should hang fully extended from the bar, with arms straight and feet clear of the floor. Before initiating the movement, depress and retract the shoulder blades to transition from a passive to an active hang. This engages the upper back and shoulders for greater stability and helps minimize unwanted swinging.
Step-by-Step Execution and Technique
Execution begins by bracing the core and initiating the concentric, or lifting, portion of the movement. Exhale sharply while pulling the knees upward toward the chest. The primary focus is achieving pelvic tilt and spinal flexion, which curls the lower back slightly and ensures the rectus abdominis is the primary mover, not just the hip flexors.
Raise the knees as high as possible, ideally until the thighs are parallel to the floor or slightly higher, pausing briefly at the peak of the contraction. The eccentric, or lowering, phase must be executed with deliberate control. The legs are slowly lowered back to the starting position, resisting the pull of gravity and momentum. This controlled descent increases the time the muscles are under tension.
Common Errors
A frequent mistake is using momentum to swing the body and generate lift, rather than relying on core strength. Swinging drastically reduces abdominal muscle activation. Another common error is failing to achieve spinal flexion, which occurs when the knees are only lifted to a 90-degree angle, primarily engaging the hip flexors. To correct this, the exerciser must consciously curl the pelvis toward the ribs as the knees rise. Allowing the lower back to arch excessively during the lowering phase is also a fault that can place undue stress on the lumbar spine.
Muscles Engaged and Core Stabilization
Knee ups target multiple muscle groups throughout the anterior core and hip region. The iliopsoas, which is the main hip flexor complex, is a primary mover responsible for drawing the femur toward the torso.
The rectus abdominis acts as a primary stabilizer and is responsible for spinal flexion. Simply lifting the legs (hip flexion) will engage the hip flexors, but to maximize core engagement, the rectus abdominis must contract to curl the spine and bring the knees higher toward the chest. The obliques and transverse abdominis also play an important role, working isometrically to stabilize the trunk and prevent lateral swinging or rotation during the movement.
Common Variations and Modifications
For individuals new to core training, a Seated Knee Tuck or Lying Knee Raise offers a supported modification. The seated version involves sitting on the edge of a bench or the floor and leaning back slightly, pulling the knees toward the chest while using the hands for balance, thus removing the demanding grip strength requirement of the hanging version. The lying version, performed on the floor, allows the lower back to remain in contact with the ground, which helps to isolate the abdominal muscles.
To increase the exercise’s difficulty, more advanced variations can be introduced. Weighted Knee Ups involve holding a dumbbell between the feet or wearing ankle weights to increase resistance against the core and hip flexors. The Alternating Knee Up requires one leg to be raised at a time, challenging unilateral core stability. The most significant progression is the Hanging Straight-Leg Raise, where the knees remain extended, greatly increasing the lever arm and placing a higher demand on the abdominal muscles.