[quote][cite]kneedraggen said[/cite]Training a muscle when it is sore can be safe, under certain conditions. Any training that causes the muscle soreness to increase should be discontinued.
Light exercise, or exercising other muscle groups, can lead to an increase in blood flow and reduction in overall inflammation.
Light training, with high repetitions, can be done with a muscle that is sore. Aerobic work, or high repetition anaerobic work (such as sprinting, plyometrics, etc) at 75% or less of maximum intensity, may also be safe. Heavy training with a muscle that is not fully recovered may put additional stress on the muscle, and the tendons, and could lead to injury, or at best increase total recovery time
When dealing with muscle soreness it is important to remember that soreness due to intense training is a good thing. It means the muscle has sustained some micro-damage and will repair and be bigger and stronger.
One thing you can do is "flushing" to help smooth out delayed onset soreness. If your legs are sore, ride the bike at low intensity for 5 minutes, or if it's chest, back, or arms, do a couple of sets of light dumbbell in the 25 to 50 ranges, and "flush" the muscle group. Almost always, you'll find a faster recovery.