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  1. #1
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    The physics of a situp

    Hi. Lately I have been wondering about the physics and biomechanics of doing "full" situps ([url]http://www.jefit.com/exercises/exercise-database/?id=345&exercisename=Sit-Up[/url])

    Obviously, I've noticed that it's much easier to do with something holding the ankles/feet done, but don't understand exactly *why* it is like that, and that without something holding them down, the feet tend to lift from the ground when the upper body rises. Withouth ankle support I'm only able to do a full situp by "throwing" my weight forward, using my arms as a lever and even then the feet lift off the ground. Could someone a bit more physics-minden try to explain what's happening?

    Also, what should I do to be able to do situps without ankle support? I assume just do high weigth/few repetitions with support? And what muscles are involved here?

  2. #2
    More Experienced than a Senior mrwright's Avatar
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    I'd assume having your feet/ankles supported gives you something to like push against you kind of push up with your legs against the support which helps you bend don't really know the whole physics behind it

    And if you can't really do it without the supports try crunches, lifting your legs up and your body, or become creative and use machines, dumbbells/barbells, a chair anything to keep ur feet down
    E N G L A N D!

  3. #3
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    I'm glad I'm not the only person who likes to think about the physics of various exercises! I'm not really an expert but here is my analysis...

    Sit-ups use the abs and hip flexors, which together curl your back and bring your legs towards your chest. So with the feet unsupported there is a tendency for your back and knees to move upwards together, which isn't that effective an exercise particularly as you'll probably lose balance somehow!

    With the feet anchored only the back can move, and you then have the resistance of the weight of your head and upper back (and any weight you may be holding) to overcome.

    The "opposite" exercise is a leg-hip raise ([url]http://www.exrx.net/WeightExercises/RectusAbdominis/BWLyingLegHipRaise.html[/url]) in which the upper back is anchored and the legs and hips move, with the resistance being provided by the weight of your legs (and any weight you hold between your ankles).

    An incline (well, really I guess it should be called decline) sit-up ([url]http://www.exrx.net/WeightExercises/RectusAbdominis/BWInclineSitUp.html[/url]) is harder than a flat sit-up because you're moving the weight of your upper back against gravity through a greater range of motion.

  4. #4
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    Oh, if you don't have a suitable ankle support I've found placing a couple of medium-weight dumbbells at the bottom of a wall (so they can't roll away from you) and jamming your feet under the handles works pretty well.

  5. #5
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    Thanks for the replies!

    That all makes sense. I guess I'll just do more incline situps and weighted situps with ankle support then.

  6. #6
    More Experienced than a Senior mrwright's Avatar
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    I find it easier and harder on the abs, especially the bottom ones if you sit at a lat pulldown machine, hook your feet under the actual machine and do your situps like that, that way you can even choose how much of a decline you want
    E N G L A N D!

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