Sit-Up

Move · Resistance Training · Movement, Typed Supps

The muscles that bend your hip attach to your lower spine, which is why the second half of a sit-up feels different from the first.

Stage by stage

Set-up (Feet free or anchored): Anchoring the feet allows a far stronger hip flexor contribution and correspondingly more lumbar loading.
During the set (Spine flexes segmentally): The opening range is rectus abdominis and obliques shortening the distance between ribcage and pelvis.
Immediately after (Hip flexors finish the rep): Once trunk flexion runs out, iliopsoas pulls the torso up on the pelvis, loading its lumbar attachments.
Weeks later (Full-range flexion strength): Controlled reps build genuine strength through the whole range; whether repeated flexion harms a healthy spine remains unsettled.

Muscles trained

Primary: abs
Secondary: hip flexors

Intensity

2.8 METs (Calisthenics (e.g., situps, abdominal crunches), light effort).

Time to effect

Weeks: You can complete more reps at a controlled tempo before your hip flexors or lower back start to take over the movement.
Months: Your abs get genuinely stronger through a full range of flexion, provided the reps stay controlled rather than turning into momentum-driven yanks.

Common myth

"Sit-ups and crunches work the exact same muscles, so it does not matter which one you do." In reality: Both flex the spine, but a sit-up's larger range of motion recruits the hip flexors substantially more than a crunch does, since lifting the torso all the way to sitting requires flexing at the hip as well as the spine. That is not automatically bad, hip flexors are muscles worth training too, but it is the real, well-documented reason some coaches prefer crunches or reverse crunches for people managing lower-back or hip flexor irritation.