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I understand how motor neurons work, but I would like clarification on what exactly is going on when a muscle contracts. For the sake of simplicity, let's just use the bicep as an example. If I do a bicep curl with a weight that is the maximum weight my bicep can lift, all the muscle fibers in my bicep will be activated and contract(more or less). If I remove the weight and do the same bicep curl, now only a small percentage of the muscle fibers in my bicep will be activated and contract, because that extra force isn't needed. But, the whole bicep muscle is still contracting! It must be, otherwise wouldn't the other muscle fibers staying at regular length prevent the bicep from contracting?

So the only thing I could think of is that the unactivated muscle fibers are pliant and DO contract, they just don't initiate the contraction, but instead go along for the ride. So if you looked at the level of sarcomeres, in the activated muscle fibers, the myosin heads pull on the actin, causing contraction. In the unactivated muscle fibers, myosin heads are NOT pulling on the actin, but the sarcomeres are still contracting, due to the forces from the activated muscle fibers. Is this a correct interpretation of what's going on or am I wrong?

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I don't think the unused sacromeres will contract though because every time you contract, it requires ATP and so it will be really expensive if you contract when you don't need to. I believe the unused muscle fibre will just be compressed or bent, since cells have a certain degree of compliance. I am guessing that is also why you won't feel your muscle as hard if you are not doing MVC

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  • $\begingroup$ If you define contraction, as the myosin heads pulling on the actin and engaging in the cross-bridge cycle, that requires ATP, so yes, that can't be happening. I agree with you and that's not what I was saying. Rather, I was talking about contraction as simply the shortening of the muscle fiber. I was describing would be what you said, compression. So if you were able to see the sarcomeres during contraction, the actin relative to the center of the sarcomeres would be in the same position in both the activated and unactivated muscle fibers. But... $\endgroup$
    – Steven
    Aug 7, 2016 at 23:44
  • $\begingroup$ It would be due to the cross-bridge cycle in the activated fibers, and due to compressive forces in the unactivated fibers. Still, this is speculation on both our parts. I would like a reference that can verify this. $\endgroup$
    – Steven
    Aug 7, 2016 at 23:46
  • $\begingroup$ References please! To support your answer $\endgroup$
    – Polisetty
    Aug 14, 2016 at 11:25

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