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As I train various muscle groups, I've noticed that I cannot flex a few of them the way I would biceps or quads. The flex becomes painfully tight verging on a muscle cramp before I finish the range of motion. Potential causes I thought of:

  • The opposing muscle group is over-relaxed, allowing this muscle to overtighten
  • The brain is not familiar with the movement and is allowing a tighter contraction than is safe
  • My musculo-skeletal structure is weird

This has been happening with various movements such as external hip rotation, shrugs behind the back, and torso twists -- small, seldom-used muscles. Should I just keep flexing them so the brain can learn to use them normally?

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I would stretch them out first and try to increase your flexibility. Maybe you are not recovering from your workouts sufficiently. Not knowing you, it is hard to hypothesize, but those are the only issues I can think of outside of an existing injury.

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Sometimes, this does go away if you train the muscle with weight so your nerves and muscle fibers can work better together. (personal experience)

Another possible cause for painfully tight flexing is an overshortened muscle. Muscles that cross two joints enter active insufficiency when both joints are flexed together. For example, if you flex your bicep by your side and then raise your elbow, you can feel the flex in your bicep getting tighter and tighter, because the biceps crosses both the elbow and shoulder joints.

To avoid this (and it's not a good way to train a muscle, because the muscle is weak in this position), stick to exercises with more natural body motions instead of trying to flex muscles in isolation. In natural body motions, muscles that cross two joints always relax at one joint if they're pulling on the other.

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