In most exercise, your muscles cycle through a work phase and a relaxation or recovery phase. I think this is the case for running, swimming, and many other cyclic or rhythmic activities. The recovery phase is important, because it allows the muscles to disengage and extends their ability to keep working. As I understand it, the lower abdominals are supposed to stay engaged and keep the pelvis level all of the time while running, that is, they don’t get any recovery phase. Is this understanding false? If so, when can they relax? If not, how can we expect them to keep working without any rest?
I think this question is also relevant to my previous question of why my back muscles get tired so much faster when I am standing than when I am walking. I conjecture that the reason is because, when walking, there is some phase in the walking cycle when each of these muscles gets to rest, whereas, when standing, they stay engaged all the time. Is there truth in this? If so, one possible way to stand for longer times without getting tired might be to continually shift weight from one side to the other, so that the muscles get a rest during the weight shifting. But again, what about the lower abs? When do they rest?