When you go for a hard run, are you building or damaging muscles? The answer may not be obvious because we know our muscles are working hard to propel us down the road or track. And we know that our legs look more toned after months of serious run training. But the reality is that after a really hard session, say 10 x 400m with 45 to 60 seconds recovery, you've actually caused micro-tears in your muscles. These tears occur at the cellular level, and if you were to go to the doctor and give a urine sample several hours after a hard workout, you'd see an increase of protein in your urine.
At the extreme end of this continuum is the condition called rhabdomyolysis, where the amount of protein in the blood is so high that it impairs kidney function. However, most runners won't have that level of cellular muscle damage after a hard workout, yet the soreness that runners feel in the days after a hard workout is mostly due to the mechanical tearing of the muscles' cells.
Delayed Onset Muscle Soreness (DOMS) describes the phenomenon that occurs at some point after the physical activity when an athlete experiences the greatest amount of muscle soreness. My personal experience with DOMS would happen after a Wednesday run over Flagstaff Mountain in Boulder, Colorado. My legs would feel very sore for the Friday workout. Why? The "eccentric bias" of downhill running causes more muscle damage than running flat or running uphill, which is interesting because even though you feel your legs burning on the uphill, it's the downhill that causes the most cellular damage.
Running hard is basically a catabolic activity, meaning simply that it breaks you down—micro-tears in your muscles are an example of this. We all know that you can build muscle; this process is call anabolism. You've no doubt heard of anabolic steroids, but you've never heard of catabolic steroids. Why? Because the hormones in your body build tissue, called anabolism, rather than tear it down, called catabolism.
Now you're probably thinking, "Great. I know what catabolic and anabolic mean, but how does this relate to my running?" Good question.