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I've just started a HIIT training routine and after two workouts my arms (triceps & biceps) grew 1/4 inch. I went to the gym this morning and after I got home 12 hours later measured my arms and they'd shrunk by 1/8th of an inch.

I know that muscles grow when I rest and sleep. But will the gain return in the morning? And is the shrinkage a sign of possible over training? I haven't trained to failure in any of my muscle groups in the last week.

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Did you take your first measurement after you had worked out? Right after a workout the muscles are filled with blood (we call this the pump). This is a temporary thing and will go down after you train. Try to always take your measurements either directly after a workout, or before - be consistant. –  ElvisLikeBear Nov 27 '12 at 2:49

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Several things can affect the size and makeup of your muscles:

  • What you eat. Eating carbs post workout can help increase the size of the muscles for a time. Also, more fat in your muscle tissue adds to the bulk. If you burn that fat, the muscle appears to shrink even though the strength is unaffected.
  • How you train. Long endurance work makes muscles that are smaller in size. Even differences in how you do weight lifting can affect the size and strength of the muscle.
  • Stress. Stress is catabolic, and too much stress over a prolonged period of time will induce overtraining and loss of muscle.
  • Lack of training. Muscles can atrophy if they aren't used. However, this takes weeks and months.

All that said, you will have some variance in muscle size due to the fact that they are living tissue, and the fact that there is always a margin of error in measurement. I've noticed that my calves and thighs can vary fairly widely (.5 inch) from week to week. I'm mostly interested in strength, so for me this is simply anecdotal.

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It turned out that my muscle did return to it's new size, but after a few days of recovery. –  Salsero69 Apr 5 '12 at 20:10

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