Does the body break other muscles (ex, legs, chest, etc) to get the needed protein to repair the microdamage ?
This is very interesting question that has always been on my head.
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If you have any protein intake, that protein will be used to repair and over-compensate the microdamaged muscle tissue. More protein will make this response stronger.
If you literally don't have any protein intake, you will have many different medical issues, one of them being that you are unable to repair this damage, meaning you will be catabolic.
If it is just one day a week then increasing your protein the following day will leave some extra protein for yesterday underfed muscle to be replenished and repaired efficiently. It is nothing grot worry about as far as you are hitting your weekly goal your body has a way if evenly distributing protein accordingly. The beauty of our body is the mechanics within.
Yep, if you don't eat enough protein it's likely that the body will get protein from other less used muscles, also if you eat enough calories but not enough protein you can still improve your strength but you won't improve your size much(you could even get fatter). Strength improvements are much faster with a bigger protein intake though.