reckonerMachine crunch
The ab machine is the least fashionable way to train the abs and one of the most useful, because it lets you load a crunch precisely and add to it week after week. What it demands is that the spine does the moving, which the seat and the pad will happily do for you if you set them wrong.
Curl the ribs toward the hips.
- Pattern
- Core flexion
- Trains
- Abs
- Kit
- Machine ab crunch
- Loading
- A weight you choose
- Reckoner prescribes
- 3 sets of 10 to 15 reps at RPE 8
- Progression step
- 5 kg once the top of the range is clean
- Skill
- Simple to learn
- Recovery cost
- Very low
- Not prescribed if you have declared
- Lower back, neck
Muscles worked
How to do it
Set the seat so the pivot matches your spine
Adjust the height until the machine's hinge sits roughly level with the middle of your torso rather than at your hips. If the machine folds where your hip joint is, it will train your hip flexors and you will wonder why.
Take a grip that lets the arms rest
Hands on the handles or the pad, elbows relaxed. Your arms are there to keep contact, not to pull. If you can feel your arms working, the load is going through them and not through your abs.
Curl the ribs down toward the hips
Round the spine and shorten the front of your torso. Chin stays off the chest and the neck stays long. The pad comes down because your back rounded.
Come back until the abs are stretched
Return slowly to a long spine without letting the stack slam or the weight pull you past a comfortable position. Then go again.
What goes wrong
Hauling on the handles
The arms are right there and pulling with them makes the number go up. It is the commonest fault on this machine and the easiest to spot: if the weight moves while your torso does not, that is what happened.
Setting the pivot at the hips
A seat too high or too low turns the movement into a hip fold. It still feels like something and it will not be your abs, because the spine never rounded.
Bouncing out of the bottom
Letting the stack pull you back and reversing on the rebound gets more reps done. It also removes the part of the movement where the abs are stretched, which is not the part to skip.
How Reckoner progresses this lift
Six weeks of this lift, from 30 kg, run through the real engine. Nothing below was typed in: the prescription, the decision and the sentence beside it all come from the same code that writes your week.
| Week | Prescribed | What it did |
|---|---|---|
| 1 | 3 x 10 at 30 kg | More reps 10 → 11 repsYou finished every set at RPE 7, with room left in the range, so the reps climb before the load does. |
| 2 | 3 x 11 at 30 kg | More reps 11 → 12 repsYou finished every set at RPE 7, with room left in the range, so the reps climb before the load does. |
| 3 | 3 x 12 at 30 kg | More reps 12 → 13 repsYou finished every set at RPE 7, with room left in the range, so the reps climb before the load does. |
| 4 | 3 x 13 at 30 kg | More reps 13 → 14 repsYou finished every set at RPE 7, with room left in the range, so the reps climb before the load does. |
| 5 | 3 x 14 at 30 kg | Deload 40% of sets offFive weeks of building, then one week lighter. About 40% of the sets come off. The load holds and your reps stay where you earned them, so you keep the strength and lose the fatigue. |
| 6 | 2 x 14 at 30 kg | More reps 14 → 15 repsYou finished every set at RPE 7, with room left in the range, so the reps climb before the load does. The lighter week is over, so this is back to three sets. |
Real engine output, not an illustration. Every row was produced by running the engine itself when this page was generated.
The full rule, and why the reps move before the load does, is in how Reckoner decides to add weight.
Questions people ask
Is a machine worse than free weight ab work?
For this job it is one of the better options. The crunch pattern already measures among the highest for both halves of the rectus abdominis and for both obliques while keeping the hip flexors quieter than a sit-up1, and a machine is simply the most controllable way of adding weight to it.
How much weight should I be using?
Enough that the last reps are genuinely hard while your spine still does the moving. The moment you have to yank with the arms or fold at the hips to shift it, the machine has stopped training what you set it up for.
How many sets a week?
More weekly sets keep buying more size and more strength, with diminishing returns as the count rises, and how many days you spread them over matters far less for size than the total2. Reckoner works the number out from your plan and what you logged.
Similar movements
Reckoner eases the load first when this lift stalls, and offers one of these only if it stalls again after that. A lift already as light as your kit goes has no load left to ease, so the offer comes straight away. Each of them has a page.
Sources
- Escamilla and colleagues, Journal of Orthopaedic and Sports Physical Therapy, 2006. An electromyographic analysis of commercial and common abdominal exercises: implications for rehabilitation and training fourteen participants across nine abdominal exercises, with the crunch among the highest for upper and lower rectus abdominis and for both obliques, and with rectus femoris activity greatest in the bent-knee sit-up, which the authors say may be a problem for people with low back pathology
- Pelland and colleagues, Sports Medicine, 2025. The resistance training dose response: meta-regressions exploring the effects of weekly volume and frequency on muscle hypertrophy and strength gains sixty-seven studies and 2058 participants, with size and strength both rising as weekly set volume rises and both showing diminishing returns, and weekly frequency compatible with a negligible effect on size once volume is accounted for