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the physics of situps



 
 
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  #1  
Old November 14th 09, 10:37 PM posted to sci.physics,misc.fitness.misc,rec.martial-arts,alt.martial-arts.karate
Sam Wormley
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Default the physics of situps

RichD wrote:
Long ago, in my wasted youth, I participated in
a weekend of sadomasochism, conducted
annually by my college karate club, called
'special training'. One part of this consisted
of a ton of situps.

We paired off, with locked legs, done by count.
It was impossible to do them all with good form,
so I came up with a trick: throw my arms forward
as hard as I could, on each one, to raise myself.

But Newton's physics tells us an isolated body
cannot generate net momentum; action = reaction.
But I swear I couldn't have completed that segment
without the arm swing. So the question is, did this
trick really work as a cheat, physically? Or was
it just an illusion?

--
Rich


Are you taking into account the differences in
static and kinetic friction?

  #2  
Old May 10th 10, 07:53 AM
alfanzo alfanzo is offline
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First recorded activity by FitnessBanter: May 2010
Posts: 2
Default

The difference between a sit up and crunch is range of motion. With a sit up, you raise all the way up and will actually have no tension on your abs at the top. With a crunch, you don't raise up so far and constant tension is kept on the ab muscles.

As far as how you weight them, I don't think it matters. Move the weight around till you find what works for you. I set the plate on my chest for crunches. But I only use weighted crunches as my finisher. The two ab exercises I like to hit hardest are cable crunches and weighted knee raises.

I kneel and crunch to the floor on cable crunches. Sitting on a bench didn't seem to work as well for me. I go as heavy as I can go for 15 clean reps. I usually have to reduce weight as I go. I think those exercises focus on the abs much better with less stress to my back than crunches. But that's just me.
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