Your body is flexible because it needs to be able to move and assume myriads of positions. This means that it is not very good at bearing a load without moving

Your body is flexible because it needs to be able to move and assume myriads of positions. This means that it is not very good at bearing a load without moving; it has to work hard to keep those flexible muscles, tendons and ligaments still. Maybe that's why planks world records are set by people in their 60s and 70s. Because most of their flexibility has been lost due to aging.