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Geometrically speaking, a muscle contraction should exert a force between the two attachment sites. However, the actual effect of the force on the overall geometry of the body near the point of attachment may not match the direction the force pulls. For example, although the bicep contraction in principle would pull the forearm closer to the humerus, instead during eccentric lengthening the arm is actually extending due to how the rest of the system is configured. This is an example of where the direction of force exerted by the muscle doesn't coincide with the movement of the

My Question:

Can a muscle's net effect on the geometry of the system it acts on change mid-way through contraction?

For example, if my arm is eccentrically extending due to too heavy a weight in my hand and suddenly I drop the weight and now the bicep can contract. That's a simple example but I'm wondering how common it is for a the effect of a muscle's action to change midway through.

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    $\begingroup$ A muscle's action is one way, and not two-way as you seem to imply if I understand the question correctly. The biceps bends the arm by contracting. Upon bending the triceps relaxes. The reverse happens when the arm is flexed, i.e. the triceps contracts and biceps relaxes. Biceps and triceps are known as antagonists. Many striated (skeletal) muscles work this way, i.e., with antagonistically paired muscles. $\endgroup$ – AliceD Feb 28 '15 at 8:22
  • $\begingroup$ These folks would seem to disagree: biology.stackexchange.com/questions/29655/… $\endgroup$ – Stan Shunpike Feb 28 '15 at 8:47
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    $\begingroup$ They don't disagree with me and I not with them as far as I can see, it's kind of a huge post. Be specific in who disagrees with what and why. $\endgroup$ – AliceD Feb 28 '15 at 10:02
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    $\begingroup$ It is not clear to me what you are asking. $\endgroup$ – Memming Feb 28 '15 at 15:13
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    $\begingroup$ The question in the title and the body are asking two different things. $\endgroup$ – Luigi Feb 28 '15 at 15:20
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Yes, because net force is the only thing that matters. To address @Chris's comment, in an eccentric contraction and a concentric contraction, the muscle is applying force in the same direction. However, in an eccentric contraction, the muscle is not applying enough force to actually lift the load. A biceps curl is a good model:

sportscience.com

SportScience

In the second image, the muscle is contracting but not exerting enough force to lift the weight--the net force causes a slow lowering of the weight.

Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load. - Muscle contraction (Wikipedia)

It may be clearer with an actual force diagram:

force diagram

University of Guelph

The only factor influencing which way the lever (your arm) moves is the net force. So yes, if you change the load (force F in the above image), the muscle could begin pulling your arm in the opposite direction. The muscle isn't really "changing action", it's exerting the same force, but now there's less force resisting it and the overall movement changes direction.

The question in the title and in the body are not really the same question, but the force is still exerted in the same direction as the muscle. You don't "see" movement in that direction because the forearm on which the force is being exerted is fixed at the elbow joint.

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