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At the Gym I use an abdominal machine to do stomach crunches and over time I've increased the weight and made steady progress. However I have just started a Taekwondo class and as part of the warm-up we have to do sit-ups, I cannot even do a single sit-up without someone holding my feet. I can lift my upper back of the floor but not the bottom half of my back. For what it's worth, in case there is a genetic component, neither can my son.

I can do the other exercises including press-ups, I'm surprised I cannot do them at all since I have made progress on the abdominal machine. Will doing them with my feet held strengthen the correct muscles so that over time I can do them unaided, or is there a particular muscle/mechanical issue that will continue to prevent me doing them unaided?

  • Try ankle weights, I'd say it's a compromise between unaided and fully locked down. – Aequitas Dec 17 '15 at 21:04
  • Perhaps you have a relatively long torso compared to your legs. – BKE Dec 19 '15 at 20:29
  • I think I do have long torso, actually if I have my legs flat I can situp but not easily and my feets/legs do leave the floor so I think Im not using my stomach properly, perhaps the ankle weights would help – Paul Taylor Dec 20 '15 at 23:16
  • I can't either, never have been able to. – jon Jan 6 '17 at 7:22
  • situps while holding the legs down and doing situps without holding legs down are two different exercises. I do both. – user28002 Mar 6 '18 at 11:00
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The problem is not that your core muscles are not strong enough to lift your chest up. It looks like you are facing another (bigger problem) : physics.

Actually, it is not weird not to be able to do what you are trying to do. In the position that you are in (supposedly with your feet close to your butt), your center of gravity is located above the waist. So whenever you try to lift up your chest, you will simply more or less roll on your back.

Two ways of solving this :

  • Stick you feet under something heavy (furniture, children, wife, whatever is available)

  • Simply lift your feet like this

Classic crunch

Actually, what you are currently doing will mostly target your upper abs. So you don't need to go all the way.

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  • From laying on my back, I can sit up just fine with my legs flat on the ground. If I go slow I can keep the backs of my heels pinned to the ground, if I go fast, they pop up a bit. But in a knees bent (back of heels at my butt) position, there's no way I can sit up. – Eric Dec 17 '15 at 19:29
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    Well, that is explained easily by understanding how your legs position affect your center of gravity. With you legs flat on the ground, it is lower than with your knees bent. That is why you have a harder time sitting up. – Erhann Dec 17 '15 at 19:58
  • You're not using center of mass correctly here. Lifting your legs as shown in the image will cause the same issue of really only being able to do a crunch rather than a full sit-up. .... – Alex L Dec 17 '15 at 21:42
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    Or just, you know, unbend your legs and stick them out a little further. – Kevin Dec 17 '15 at 21:55
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    Upper and lower abs are a myth. It's one sheet of muscle. – JohnP Dec 18 '15 at 16:23
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It is possible that you're using your leg muscles to push yourself up in the sit-ups. I know I had that problem at one point. If you're feeling tension in your legs while doing the sit-ups, there's a decent change you're engaging the legs. If so, doing them with your feet held will have more limited use since you're diluting your effort with your legs. That said, they're generally still of some use since you have to use your stomach some.

The two ways in which I trained myself into it was a) tapering off the amount of weight on my feet by putting them under progressively smaller chairs (I know... it sounds silly, but it worked by providing me some support, but giving me a warning sign that I was using my legs whenever the chair started tipping up) and b) doing negatives, starting up in the completed position, feet on the floor and unsupported, and trying to lower myself as slowly as possible. Eventually, I got the hang of it.

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  • If I lay flat I still can't do it, so longer centre of gravity problem ? My legs rise in the air does that indicate I'm using my legs, or just that my core muscles are too weak. Does holding my legs, basically mean I can then use my legs to (incorrectly) to do some of the work, and then legs + core is enough to do a sit-up. If so is by best bet to the crunch shown in picture above which does seem to work my abs, strengthening them so strong enough to be able to do a situp – Paul Taylor Dec 24 '15 at 21:25
  • {nods} Can you do negatives without your legs flipping up in the air, starting up and then lowering yourself down? – Sean Duggan Dec 24 '15 at 21:51
  • No I can't do that, I can do the feet in the air abcrunch though, I wonder how many I should be aiming for – Paul Taylor Dec 28 '15 at 19:49
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It is because when you've used the machine at the gym you haven't had to activate the hip flexors, that is the muscle group connecting your legs to pelvis and abdomen. These are activated when you do a classic crunch on the ground and move your upper body all the way up towards your knees.

Really the only way to do crunches without anchoring your feet is to at your own pace try to mentally focus on activating your core and keep your feet down while doing your crunches. Don't anchor your feet under something else like someone suggested, that way the hip flexors are over exaggerated and a resistance is created from where they can pull, and they're not tensed and released for every crunch which is what you want if you want to strengthen them.

There is no advice that will work instantly just because you "do the crunch right", you have to be patient. Over time you will have to strengthen those muscles and you will be able to do crunches without anchoring your feet.

Good luck!

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Sometimes it's not about strength and as mentioned in earlier posts it's just physics.

It's similar to a seesaw, it's all relative to weight and distance from the pivot point. If two equal weights are positioned at each end of a seesaw, at equal distances from the pivot point, then it is perfectly balanced. However if you increase one of the weights or move it further from the pivot point (such as a long torso) then the other end (feet) will lift up.

The only ways to prevent the feet lifting up are; Reduce weight to torso, increase weight to legs or get someone to hold your legs... or any combination of these.

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  • That's odd. I can easily do sit ups with my feet close to my butt but I don't need extra weight at my feet or someone holding me down. My legs and hips are fatter than my torso and are thus probably heavier so maybe that's why? – Caters May 30 '18 at 4:54
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I do pilates and sit-ups, as well as the reverse lowering down from the sit-up position in a slow a controlled way, are major parts of my pilates session. I am able now to do a controlled rollback, however the sit up with legs straight is a struggle. Weirdly if my instructor holds the tips of my fingers without even pulling me up, I have no difficulty doing a sit up legs straight and in a slow controlled way. I was thinking is there a psychological thing going on. Well major breakthrough at yesterdays class, in that another instructor put a small lumber support pillow thing under my lower back and as they say here in New Zealand, land of cows and sheep. I could do sit-ups until the cows come home now. So I'm not crazy maybe some lower back muscle strength needed or it may be physics too in terms of a fulcrum. I fantasise that my legs are long however maybe my torso is longer. Hope this helps you can find lumber support things here https://www.thebodyworkshop.co.nz/products.html. Or maybe just roll up a towel and stick it under your lower back - that's called the number 8 wire approach in NZ.

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