📖 Static demo — read-only snapshot
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Sit-Up
Positions & Range of Motion
- Start
- supine hook-lying, knees bent ~90°, feet flat on the floor, arms crossed over chest or hands lightly behind the ears, chin gently tucked
- End
- torso upright and off the floor, spine segmentally flexed and hips flexed to bring the chest toward the thighs
- Range of Motion
- full trunk flexion — segmental spinal curl through the thoracolumbar spine followed by hip flexion to upright, then controlled segmental eccentric lowering to supine
How to Perform
Lie supine with knees bent to about 90 degrees and feet flat on the floor, arms crossed over the chest or hands lightly behind the ears. Curl the trunk by flexing the spine segmentally, then continue rising through hip flexion until the torso is upright, and lower back down with control to the start position.
Opens a YouTube search for “Sit-Up”
Coaching Cues
- 1
Peel your spine off the floor one segment at a time rather than jerking up in one piece.
(internal) - 2
Imagine curling your ribs down toward your pelvis as you rise.
(external) - 3
Keep your chin gently tucked, as if holding a small orange under it, so your neck does not lead the movement.
(internal) - 4
Lower down as slowly as you came up, letting the floor meet your back one segment at a time.
(external)
Evidence-Based Dosing & EMG Data
EMG studies consistently show the sit-up is a two-phase movement: rectus abdominis and the obliques dominate the initial trunk-curl (roughly the first third of range), after which the hip flexors — psoas major, iliacus, and rectus femoris — become the primary torque producers as the trunk rises off the floor. Indwelling-EMG work has demonstrated substantial psoas activation during both bent-knee and straight-leg sit-ups, with bent knees not eliminating hip-flexor involvement as once believed. Anchoring the feet further increases hip-flexor and rectus femoris recruitment. Spine biomechanics research (notably McGill's laboratory) has measured lumbar compression during repeated full sit-ups at levels approaching occupational action limits (on the order of 3000+ N), which is the basis for preferring curl-up variations in low-back pain populations. Trunk-flexor activation in a partial curl-up is comparable to the full sit-up with markedly lower lumbar load, which is why the crunch and McGill curl-up serve as regressions rather than fundamentally different exercises.
Muscles Involved (9)
Primary Movers
Secondary Movers
Stabilizers
Lengthened / Stretched
Synergists & Assistors
Easier Variations ↓
Crunch (partial curl-up)
Curl only the head and shoulder blades off the floor, keeping the low back down. Reduces range, hip-flexor demand, and lumbar load while still training the trunk flexors.
McGill curl-up
With hands under the lumbar spine and one knee bent, lift only the head and shoulders a few centimeters while maintaining a neutral spine. Trains trunk-flexor endurance with minimal spinal flexion, useful for those with low-back sensitivity.
Harder Variations ↑
Decline sit-up
Perform on a decline bench with the head below the hips, increasing the gravitational moment on the trunk and hip flexors through a longer range.
Weighted sit-up
Hold a plate or dumbbell at the chest (or overhead for greater challenge) to add external load and allow progressive overload in lower rep ranges.
Target Movements (4)
Why This Exercise Matters
Everyday activities this exercise helps you do — essential means it directly trains the capacity the task needs:
Goals This Helps With
Rehab, performance, prevention, and mobility goals this exercise supports — essential means it's a cornerstone:
Research Notes
claude-researched 2026-07; muscle/movement links validated against the roster. Adversarial clinical verification pending — do not promote past needs_review without human review.