When the Japanese samurai wanted to knuckle down and make a serious effort at something, they would say, Hara o kukuru: "Tie the guts."
In doing sit-ups, we show the same resolve. The act of sitting up from flat on our back builds spiritual strength — pulling oneself up off the ground through a force of will. "Seven times down, eight times up," goes the Japanese adage. Whatever slips we make on the path, we have to get up and carry on. Sit-ups help build that kind of perseverance and character.
— Philip Toshio Sudo, Zen 24/7