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Crunch

Draftstrength50%beginner⬤ moderate evidence

View FHIR resourceFHIR R4 ActivityDefinition — the interoperable form of this exercise

Positions & Range of Motion

Start: supine, knees bent ~90°, feet flat on floor, hands lightly behind head or crossed on chest, low back in contact with floor
End: head and shoulder blades curled off the floor, ribs drawn toward the pelvis, low back still in contact
Range of Motion: upper trunk/thoracic flexion, lifting scapulae off the floor (~30° trunk flexion), stopping well short of a full sit-up

How to Perform

Lie supine with knees bent about 90 degrees and feet flat on the floor, hands lightly supporting the head or crossed over the chest. Exhale and curl the trunk, lifting the head and shoulder blades off the floor by flexing the spine — drawing the ribs toward the pelvis — while the low back stays in contact with the floor. Lower back down with control; unlike a full sit-up, the movement stops well short of sitting upright, which limits hip flexor involvement.

Watch on YouTube↗

Opens a YouTube search for “Crunch”

Equipment:noneexercise mat (optional)

Starting position: supine

Coaching Cues

1
Draw your ribs down toward your pelvis as you curl up.
(internal)
2
Peel your shoulder blades off the floor, then lower back down one segment at a time.
(external)
3
Keep a fist-sized gap between your chin and chest — don't pull on your head with your hands.
(safety)
4
Exhale as you curl up, inhale as you lower.
(breathing)

Evidence-Based Dosing & EMG Data

Recommended Dosing

2-3 sets of 10-20 controlled repetitions, 2-3 sessions per week, consistent with general resistance-training guidelines for trunk musculature; progress by adding load or lever length rather than high-repetition volume.

EMG Activation Data

Surface EMG studies consistently show the crunch produces high rectus abdominis activation and moderate external and internal oblique activation, and it compares favorably with — or exceeds — many commercial abdominal devices. Compared with a full sit-up, the shortened range keeps hip flexor (iliopsoas/rectus femoris) activity substantially lower, which is why the crunch is preferred when lumbar shear from hip flexor pull is a concern. Claims of selectively isolating the "upper" versus "lower" rectus abdominis with crunch variants are not well supported; regional differences reported across studies are small and inconsistent. Hands-behind-head performance tends to increase sternocleidomastoid recruitment if the head is pulled, which is a technique fault rather than a training target.

Muscles Involved (7)

Primary Movers

Rectus Abdominis— Prime trunk flexor; drives the rib-cage-toward-pelvis curl throughout the range.

Secondary Movers

External Oblique— Assists trunk flexion bilaterally; activity rises in twisting/oblique crunch variants.

Internal Oblique— Assists trunk flexion and contributes to the posterior pelvic tilt bilaterally.

Stabilizers

Transversus Abdominis— Deep abdominal wall tension and intra-abdominal pressure regulation; produces no flexion torque.

Iliopsoas— Reclassified from synergist. In a short-range crunch the hip angle does not change and the pelvis moves toward posterior tilt, so iliopsoas does not synergize spinal flexion; it provides only low-level pelvic/hip anchoring and is deliberately minimized versus a sit-up by keeping the range short.

Lengthened / Stretched

Erector Spinae Group— Spinal extensors lengthen as the thoracolumbar spine flexes — the stretched antagonist.

Synergists & Assistors

SternocleidomastoidSynergist— Flexes the cervical spine to carry the head with the trunk; cue against over-recruiting it to avoid neck strain.

Easier Variations ↓

Posterior pelvic tilt

Supine with knees bent, flatten the low back into the floor by tilting the pelvis and gently drawing ribs down, without lifting the head or shoulders. Builds the initiating abdominal pattern with minimal load.

Partial-range head-and-shoulder lift

Perform the crunch through a smaller range — lifting just the head and top of the shoulder blades — with hands fully supporting the head to offload the neck.

Harder Variations ↑

Weighted crunch

Hold a plate or dumbbell at the chest (or extended overhead for a longer lever) and perform the same curl-up, progressing load gradually.

Decline or stability-ball crunch

Perform the crunch on a decline bench to increase resistance, or on a stability ball to add range of motion (pre-stretch over the ball) and a balance demand.

Target Movements (3)

Thoracic FlexionThoracic Intervertebral Joints

Lumbar FlexionLumbar Intervertebral Joints

Posterior Pelvic TiltLumbopelvic

Why This Exercise Matters

Everyday activities this exercise helps you do — essential means it directly trains the capacity the task needs:

Getting In and Out of Bed Posture Maintenance

Goals This Helps With

Rehab, performance, prevention, and mobility goals this exercise supports — essential means it's a cornerstone:

Improve Core Strengthperformance

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.