Muscle soreness two days after exercise -- why does it go away?

If I survive a few more months, I'll write about why I started CrossFit at 53.

It's been an interesting experience, not least because of the extreme muscle soreness during the first 3-4 weeks of workouts. About 48 hours after exercise I had a hard time descending stairs. This is known as "delayed onset muscle soreness" or DOMS - also known as "muscle fever". 

No, there's no evidence at all that nutritional supplements make any difference.

I've run into this before of course, typically after playing hockey, but until now I hadn't seriously wondered about the cause. I dimly recall some handwaving explanations in my 1980s med school; something about "muscle tears" and/or lactic acid. The latter was silly, and the former hasn't held up. The current consensus seems to be that it arises from some sort of microscopic injury and healing, but ...

Re-evaluation of sarcolemma injury and muscle swell... [PLoS One. 2013] - PubMed - NCBI

.... results do not support the prevailing hypothesis that eccentric exercise causes an initial sarcolemma injury which leads to subsequent inflammation after eccentric exercise... fibre swelling in the soleus muscle is not directly associated with the symptom of DOMS...

But if it were some kind of injury response, why does DOMS become much less severe over time? After about 4-6 weeks of CrossFit I still have muscle soreness, but it's quite mild -- nothing like my original experience.

After I thought about this a bit my hypothesis was that the fundamental mechanism was apoptosis, or cell death. My old underused muscles probably had a good number of old creaky cells on the edge of apoptosis; perhaps a sudden mitochondrial activity surge pushed them over the edge. Lots of cells die at once, but after a few weeks the marginal ones are gone and I'm back to baseline muscle soreness.

Strangely, because apoptosis was a very hot topic in the 90s and 00s, I could find only one reference with a PubMed search on apoptosis and DOMS. That was in a relatively obscure journal back in 2000:

Gender differences in muscle inflammation aft... [J Appl Physiol. 2000] - PubMed - NCBI

"... exercise may stimulate the expression of proteins involved in apoptosis in skeletal muscle."

Research tends to follow fashions, and apoptosis was overexposed a decade ago. It's time for it to make a comeback though, so I'm looking forward to reading about apoptosis and DOMS in the years to come. Researchers just need to study 50+yo men starting CrossFit.

Update 12/19/2018: Still loving that CrossFit. Reading this I must have started around 4/1/2013. So I’ve been doing it for 5y8m. I’m never as sore as I was those first few weeks …