Originally Posted by Unforgiven
You aren't sore any more because you have purged your muscles of lactic acid. That is what causes muscle soreness. When you work out everyday it doesn't build up, so you don't get sore. OK, Pup's done trying to sound semi intelligent.
The lactic acid bit is spot on and while there's probably something in the lower build up idea, I thought the primary reason daily workouts hurt less is that the endorphins (natural pain killers) the exercise causes your body to produce take a little over 24 hours to clear.
That's also why the aches and pains from exercise don't normally kick in until the next day.
And why you can get addicted to exercise.
And why some people conjecture that exercise can be a treatment for depression as the endorphins trigger seratonin production in the brain and depression is thought to be due to lowered seratonin (I think this idea has been discounted but hey: it's not going to hurt to try).