Sit-up

A bodyweight abdominal exercise in which a person raises the torso from a lying position toward the knees and lowers it again.

Rectus abdominis: A paired abdominal muscle that flexes the trunk and helps stabilize the pelvis. It is a principal trunk flexor during the upward phase.
Trunk: The central part of the body, excluding the head, neck, and limbs. The sit-up raises this region while the pelvis and legs provide a base.
Crunch: An abdominal exercise that lifts the head and shoulders while keeping the lower back on the floor. It uses less trunk movement and generally involves less hip flexion.
Bodyweight exercise: A physical exercise that uses the body's own mass as resistance. A sit-up needs no external load or specialized equipment.
Low back pain: Pain located in the lumbar region of the back. Repeated spinal flexion may aggravate symptoms for some people with back pain.
Hip flexors: A group of muscles that flexes the hip, bringing the thigh toward the torso or the torso toward the thigh. They contribute strongly when the torso rises through a full sit-up.
Hip joint: A ball-and-socket joint connecting the femur to the pelvis. Hip flexion helps carry the torso toward the thighs.
Reverse crunch: An abdominal exercise that curls the pelvis toward the rib cage by lifting the hips from the floor. It raises the pelvis rather than bringing the torso toward the knees.
Muscular endurance: The ability of a muscle or muscle group to sustain repeated contractions or maintain force over time. Repeated sit-ups are often used to train or assess this capacity.
Spinal flexion exercise: An exercise that deliberately bends the spine forward under muscular effort. Sit-ups are a common loaded or repeated form of spinal flexion.

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Rectus abdominis muscleRelated: The rectus abdominis contributes to the trunk-flexion phase of the movement.