Take It Easy
The martial arts, traditionally, is a very physical pursuit. Truly a labor of love, training in the martial arts taxes the human body like few things can. Energy (ki) flows most efficiently through an open channel, much like water flows through an unobstructed garden hose. Any impediment will not only block the delivery of energy, but will actually create a much greater demand for energy than is actually needed for the task. In other words, the ability to relax creates a state which utilizes martial arts techniques with premium efficiency. Speed and power are born of the relaxed state. Relaxation starts in the mind. Anxiety, rage and doubt - all fear based behaviors - are analogous to bending a garden hose that is supplying the water. Most negative psychological states create unconscious muscular contractions which interfere with proper execution of technique. Throughout the move, stay loose and relaxed; only at the end of the strike or block should the muscles contract - then immediately relax again. This describes the concept of snap. Snapping techniques actually involve staying loose through about 99% of the move.
While being in shape is a natural by-product of dedicated training, the inability to relax will definitely produce rapid fatigue. A performance of kata by a master practitioner clearly illustrates how important it is to loosen up in certain sections, then explode into a fusillade of punches and kicks in others. The "breaks" in the kata are very important; otherwise it would be one frantic volley of offensive tactics that would leave the practitioner exhausted. Proper breathing - which has been proven to produce a calming effect on the central nervous system, is yet another factor to consider. The ability to relax conserves energy, allows clear-minded awareness, and prepares the practitioner to strike with devastating power.