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Definition of Acute injury: https://farmingdalephysicaltherapywest.com/how-are-acute-injuries-caused-treated-and-prevented/#:~:text=An%20acute%20injury%20is%20a,body%20will%20experience%20a%20change.

I was trying to understand how concentric and eccentric contraction can induce injury or muscle growth, and how an injured muscle responds to those contractions.

What I got is that concentric contractions create microtears, but muscle strains come from eccentric contraction. I understand how lengthening can cause strain, but I do not understand why shortening creates tears. Can somebody explain this process in detail? Also, is the overuse injury referring to too many microtears?

If a muscle is injured from a strain, that means it's torn in an eccentric contraction. Does this mean that we would always be testing for an acute injury with eccentric contractions? When would concentric contractions with resistance be useful for testing?

Appreciate your help, Jingxuan

frank guo
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    Where did you get the idea from that a tearing muscle (more precisely: muscle fibres) is limited to concentric contraction? Sounds like a meaningless play of words. A torn muscle fibre is a torn muscle fibre and speaks for an external force higher than the current loading capacity. – Philip Klöcking Oct 08 '23 at 12:08
  • What I meant is that concentric contraction, which is lengthening, creates microtears which can lead only to overuse injury. Eccentric contractions are the ones that lead to acute injuries. This leads me to the conclusion that concentric contraction is useless for testing acute injuries as they are caused only by eccentric contractions. – frank guo Oct 08 '23 at 16:06
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    Where did you get that from? Firstly, it is eccentric contraction that involves lengthening. Secondly, torn muscle fibres (as well as myosin heads ripped off, micro-injuries) can occur in both and are acute injuries. I do not even understand how this dichotomy should make sense. Yes, micro-tears are more likely in eccentric movements because the number of motor units needed for the force have to be predicted but that does not mean that concentric contraction can't lead to them as well. – Philip Klöcking Oct 08 '23 at 16:52
  • I made a mistake with the first one. As for the second one, I never said that concentric contraction doesn't cause micro-tear. But Micro-tear isn't acute injury. A torn muscle group is an acute injury. Please explain how concentric exercise can cause injury greater than micro-tear if that is what you meant. – frank guo Oct 08 '23 at 18:28
  • @frankguo - A micro tear can be an acute injury. Acute simply means happened suddenly. Any workout that you do where you overload the muscle can produce microtears. This is acute, and is part of what leads to muscle growth. The key is that you recover before you overload the muscle again. If you don't, then you can develop chronic (long lasting) injuries from the same microtears. And yes, this means that body building is essentially damaging the muscle to make the fibers bigger and thicker as they heal. – JohnP Oct 10 '23 at 15:25

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