# body-mechanical Q on legs as counterweight to levering up one's upper body

**URL:** <https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918>\
**Category:** Factual Questions\
**Created:** [November 25, 2009, 4:36pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918 "2009-11-25T16:36:07Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Mops](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mops/32/16494_2.png) [@Mops](https://boards.straightdope.com/u/Mops)\
**Post date:** [November 25, 2009, 4:36pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918/1 "2009-11-25T16:36:07Z")

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An observation that is contrary to my understanding of mechanics:

Consider me lying on my back. Trying to lever up my upper body will lift my legs up

```auto

_._

\./

```

(the dot being my butt, as the fulcrum of movement)

Now if I bend my knees I can lift my upper body without my feet leaving the floor

```auto

/\._   

/\./

```

But… shouldn’t it be the other way round? With bent knees the leverage that the weight of my legs can exert as a counterweight is less, so it should be in the bent-leg position rather than the extended-legs position that my feet leave the grounds.

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**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\
**Post date:** [November 25, 2009, 4:42pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918/2 "2009-11-25T16:42:31Z")

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I can lift my upper body up while leaving my legs out straight on the ground. Maybe your premise is flawed.

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**Author:** ![Mops](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mops/32/16494_2.png) [@Mops](https://boards.straightdope.com/u/Mops)\
**Post date:** [November 25, 2009, 4:59pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918/3 "2009-11-25T16:59:21Z")

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That’s probably to do with different builds (more weight in the belly, for me)

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**Author:** ![Blut\_Aus\_Nord](https://avatars.discourse-cdn.com/v4/letter/b/9e8a1a/32.png) [@Blut\_Aus\_Nord](https://boards.straightdope.com/u/Blut_Aus_Nord)\
**Post date:** [November 25, 2009, 5:22pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918/4 "2009-11-25T17:22:28Z")

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The muscles are all outstretched when the legs are outstretched. The muscles are compressed when the legs are bent, making it easier to sit up.

My guess.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [November 25, 2009, 5:44pm UTC](https://boards.straightdope.com/t/body-mechanical-q-on-legs-as-counterweight-to-levering-up-ones-upper-body/518918/5 "2009-11-25T17:44:01Z")

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You don’t bend your knees and then sit up, you sit up while bending your knees. It’s the acceleration of your legs that matters here, not their position.