The last great unknown, in terms of physiological training, is the optimum length of a piece. Is three minutes enough? Is ten minutes too much? No one knows. Perhaps someday the question will be answered-we'll find out that thirteen minutes is the perfect length for a training piece when preparing for a 2000 meter race. Until then, coaches will continue exploring the whole scale, up and down, from thirty seconds to sixty minutes and more, in hopes of capturing the optimum time.