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I can only curl with one arm 21 kilograms once, this is my maximum but I can hold at 90° degrees 30 kilograms for for up to 2 minutes then my arm starts to shake too much.

So I'm clearly immensely stronger in isometrics, both in terms of raw power and endurance. Why is it so?

Does the fact that can use heavier weights for more volume make isometric training superior for muscle growth?

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The reason you are stronger in isometrics is simply because less is being asked of the muscle fibers. Instead of them going from a fully stretched position to a fully contracted position (full range of motion), the muscle fibers are simply maintaining a contracted state. On top of ignoring strength curves, you also ignore the strength that it takes to contract muscle fibers to a certain point. Full range of motion will always be more difficult than any singular part of that range of motion, and that's why you are stronger in isometrics.

"Does the fact that can use heavier weights for more volume make isometric training superior for muscle growth?"

No. Your muscles don't necessarily recognize weight, but they do recognize tension. As you've already noticed, the amount of tension placed on a muscle is less in isometrics than it is during regular exercise - even at higher weights. Not only that, but in only doing an isometric you are lacking both a concentric and eccentric component of an exercise. Thereby missing out on the biggest opportunities for muscle growth. You certainly could build muscle with isometrics, but it's going to be more difficult.

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