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Let's do some exercise(duh!)

Ok so lie on the bed. Lift your left leg. Put it down. Lift your right leg.Put it down. Easy?

Now lift your left leg and without putting it down, lift your right leg. Easy ? No.

Now try to lift both the legs together. Easy ? No.

My question : What is the anatomical reason ( arrangement of muscles around the bones etc.) which makes it difficult ?

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I don't know if this is relevant, but with one leg down, we actually push against the bed while raising the other to increase the upward normal force and to shift the normal force toe-wards to maintain stability. This cannot be done if both legs are in air. – Satwik Pasani Mar 7 '14 at 17:47
@SatwikPasani If your reasoning is correct, it would be interesting to know whether people with only one leg are able to raise their leg easily. – biogirl Mar 8 '14 at 14:38
up vote 4 down vote accepted

I suspect it is just physics. Each of your legs makes up about 1/6 of body mass, leaving about 2/3 of your mass superior to your hips.

Raising one leg requires action of the hip flexors (i.e., iliopsoas complex) but also activity in the anterior abdominal wall muscles (rectus abdominis, obliques) and back muscles to stabilize your abdomen. To raise the other leg simultaneously, you recruit the ipsilateral hip flexors. But, importantly, you probably have already maximally recruited most of your abdominal and back muscles.

Imagine the body as a lever system. With one leg raised, you have 1/6 of body mass balanced against 2/3. With both legs up, you have 1/3 vs. 2/3 and with potentially increased torque, because your legs are long (a force acting at a larger distance). The muscle forces to stabilize that must be larger, but the additional muscles are not there.

I can raise both legs with difficulty (admittedly, $n=1$).

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