As I regressed from my regular (almost daily) training last year, while trying to at least climb once a month, usually with a heavy pack, I've started to suffer more frequently from DOMS.
I'm talking about Delayed Onset Muscle Soreness, and not Dirty Old Man Syndrome (though some would contend that I am also afflicted with this). As the name implies, DOMS is a delayed reaction of the muscle tissues from a strenuous workout, usually appearing after 6 to 8 hourse, peaking around 24 to 48 hours. I initially thought that lactic acid build-up was related to DOMS, but upon further reading, scientific findings have rejected this correlation. Sources say that DOMS comes from muscle inflammation caused by micro-tears in the muscle after eccentric (i.e. lengthening) exercises. I don't have the complete grasp yet of the science behind this condition; all I know is that after every extended climb up mountains, I get this, forcing me to do the "penguin walk".