Muscular compatibility in triathlon
Triathlon is an endurance sport requiring an excellent physical preparation for the three sports: swimming, biking and running.
This sport uses strongly the cardiovascular system and a lot of muscles. As the "slow" fibers must be the main part of muscles, short time intermittent exercises are not suggested because they act on the "fast" fibers.
Further more, considered separately, each sport doesn't require the whole muscles used in triathlon. The cardiovascular system is even used differently in swimming, cycling or running.
Some muscles required in one sport become a handicap in an other. To agree with that, it's sufficient to compare the morphologies required in each sport to understand all the difficulty of the physical preparation for the triathlon.
However, the experience shows that running is the most important sport. Then, it is important to focus the preparation on this sport without disregarding the others.