Crunch

Move · Resistance Training · Movement, Typed Supps

The short range is the design — past about thirty degrees of curl, the hip flexors take over and the abdominals stop being the ones working.

Stage by stage

Set-up (Knees flexed, neck neutral): Bending the knees reduces psoas pre-tension, and letting the head rest avoids the neck strain crunches are blamed for.
During the set (Thirty degrees of trunk flexion): The rectus abdominis exhausts its range early; rising further is hip flexion by the iliopsoas, not abdominal work.
Immediately after (Endurance-range stimulus): Resistance is capped at trunk mass, so high repetitions are the norm and strength progression is slow.
Weeks later (Definition tracks body fat): The muscle thickens with training, but whether it is visible depends on the fat layer over it.

Muscles trained

Primary: abs
Secondary: obliques

Intensity

3.5 METs (Resistance (weight) training, multiple exercises, 8-15 repetitions at varied resistance).

Time to effect

Weeks: The ability to isolate the abs without the hip flexors taking over tends to improve quickly with practice, since the short range makes cheating with momentum harder than in a sit-up.
Months: Ab endurance builds steadily, though visible definition depends far more on overall body fat than on how many crunches are done — a well-known limit of any single ab exercise.

Common myth

"Crunches will give you a six-pack." In reality: Crunches strengthen the rectus abdominis, but visible muscle definition is mostly determined by body fat percentage — the muscle can be well developed and still invisible under enough fat.