There is a  point of complete muscular fatigue which occurs at the biological and chemical level of the muscles and this is called muscular failure.
Rich is very familiar with this feeling from having worked with me over the past year. When we trained, we would do various exercises. I would try to get him to do as many reps as he could with correct, proper form. The point at which  his form started to deteriorate was the cue to cease that particular exercise and muscle group and move on to the next muscle group. So indeed there is a chemical reaction in the body when muscles begin to produce too much lactic acid and the body cannot process all this waste product ( called lactic acid) from the activity the muscles are producing. Muscle fatigue quickly sets in. The muscles cannot keep working and the end result is muscular failure.
A good analogy for students to think about is try to do as many pushups as you can. Maybe you can do 3 or 5 or 20 but eventually you will begin to feel the muscles in your shoulders, arms and chest starting to feel heavier and the exercise will become more difficult. This is the point when the lactic acid in your body is beginning to accumulate. Very soon thereafter you will not be able to do any more pushups. As hard as you try you simply won’t be able to continue or if you do, your form will not be correct and you will risk getting injured because you are recruiting other muscles groups incorrectly. So how will Rich be able to keep sailing when he surely runs into this issue while out in the middle of the ocean?
 Because he is very well trained and used to this uncomfortable feeling, he will simply take a quick break.  He will allow his muscles to recover and then he will have the proper energy and recuperation to begin working hard again.
Athletes who train consistently and with intensity like Rich, have the ability to produce large amounts of lactic acid and work longer than non- athletes before their muscles shut down. They also have a much faster recovery time and can rest for a short time and then begin working again. This is why Rich trained so hard over the past few years. The end result of his training at the biological and chemical level is that his muscles  can tolerate large workloads  and also recover faster.
Question submitted by Rich, on Great American IV