Like the decathlon and the heptathlon, the triathlon represents an ultimate cross training, multidisciplinary challenge for the athlete. The individual triathlon segments of swimming, cycling, and running each present distinct training issues: swimming requires strength, endurance, and an adherence to proper, efficient technique; cycling also demands efficient form and stamina; and running training will be directed to optimal stride, running form, and cardiovascular fitness. Integrated with these macro-training features will be triathlon-specific race training, such as interval training in each discipline, and hill workouts necessary for cycling and running success.
It is the nature of the sport that every part of the musculoskeletal system must be trained if the athlete is to be a successful triathlete. In the Olympic distance, the most popular of triathlon competitions, the 1.5-km (1 mile) swim, 40-km (24 mile) cycle, and the 10-km (6.2 mile) run are relatively equal in their relative importance to overall race success. The training time devoted to each event at a base level should also be equal, subject to the preexistence of a greater level of ability or training in one or more of the events.
The demands of the three triathlon events creates the ultimate training requirement to develop each of the foundation parts of total fitness: speed, power, endurance, flexibility, and strength. Exercises that integrate more than one of these components will be an asset to the triathlete.
Much of triathlon training will be directed to the three principle activities. The training exercises must support the athlete with respect to the development of the basic parts within each event, but in the advancement of the athlete's overall abilities. A fundamental aspect of the training in each event segment is that of overload, the universal sports training theory that an athlete must do more training than he or she may expect to face in competition. The cycling training is an example of how overload principles apply to the triathlon. Triathletes will not train to finish 40-km on the cycle portion; they will train to finish the 40-km course in a physical condition that they may easily move to the next segment, without undue fatigue. The overload can be accomplished in two ways: the triathlete will organize a training program where distances in excess of 40-km are easily covered in single training sessions, coupled with weekly or monthly mileage totals that build significantly greater endurance over time.
The second of the fundamental approaches to triathlon exercises will be the development of maximum flexibility and corresponding range of motion in each joint. The triathlete can utilize specific exercises drawn from basic calisthenics stretches to ensure the proper extension of the working tissues, coupled with flexibility-enhancing stretch routines common to yoga and gymnastics. The shoulder is an example of the importance to enhanced joint flexibility in triathlon. In the swim, the triathlete will be using the shoulder in a rotational motion to power through the water. During the cycling portion, the shoulder will assume a relatively rigid position, absorbing the forces of the road through the handlebars and upper body. In the 10-km run, the shoulders are part of the balancing mechanism needed to offset the power of the legs driving forward. Exercises that put the shoulder through the full range of those anticipated motions will assist in triathlon performance.
The third fundamental is the development of training intensity. Intensity will be a factor in both the event training as well as general strength, training, and flexibility sessions. Interval training, which can occur in swimming, cycling and running training, is an excellent intensity developer. It is impossible for an athlete in any sport, particularly one as demanding as the triathlon, to continually train at a lesser intensity level and compete at a higher intensity level. The enhancement of physiological qualities such as oxygen uptake, the ability of the body to process oxygen efficiently (expressed as VO2max), require periods of maximum training intensity.
Much of the event training in the triathlon is geared to the overall development of the triathlete's aerobic fitness. However, there are a number of instances in a triathlon where anaerobic fitness, in support of the ability to surge or sprint, is of critical importance. The start of the swim, often in a mass start, with hundreds of swimmers attempting to get away quickly, is an example of where sprinting speed is important. To be able to sprint up hills on the bicycle or to finish the run section with a kick are also examples of anaerobic fitness requirements. Hill-training on both foot and bicycle and the swimming of 100 m to 200 m repeats both develop this capability in each of the three competition segments.
Weight training or other resistance training is an essential component to a well-rounded triathlon program. While the swim portion requires a measure of upper body strength, it is the maintenance of an optimal strength to weight ratio that will best assist the athlete, the balancing of the desired amount of mass versus the requisite power and necessary energy to move the body efficiently. In both cycling and running, the development of the athlete's core strength, the capacity of the interconnected muscles of the gluteal (buttocks), abdomen, groin, and upper legs will assist in the provision of both balance and stability in movement. Circuit weight training—emphasizing sets comprising a high number of repetitions using lighter weights—tends to achieve the desired balance, as does vigorous Swiss ball workouts, which combine the resistance of the ball and stretches of these muscle groups through exercises such as abdominal crunches and extensions.
A training exercise unique to triathlon competitions is that of transition, where the triathlete practices the steps necessary to transform from swimmer to cyclist, and from cyclist to runner. Each of these transitions requires the triathlete to change or remove clothing and footwear in a designated area known as the transition area. The competitor's time to complete a triathlon begins when the swim race portion commences, and ends when the triathlete crosses the run course finish line; all time in transition is counted, and the faster the competitor can move through the transition stages, the more efficient the athlete shall be. Triathletes will specifically rehearse how they will move from one event to another, with all requisite equipment carefully organized at the transition station for ease of access. Modern triathlon clothing such as wetsuits and cycling clothing has been designed to be removed with greater ease for this reason.