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Cross-training is when an athlete undertakes training in a discipline other than their main sport for the sole purpose of enhancing performance in their main sport.  This form of training became popular in the 1980's with the emergence of the sport of triathlon, and has been widely recommended in recent years as a technique for retaining training adaptations, and even improving performance.  Some world-class athletes, such as Mark Allen, six-time winner of the Hawaii Ironman, believe that a heavy cycling program improves distance running performance, and point out that elite triathletes seldom run more than 100 km/wk in preparation for a marathon.  Paradoxically, the available scientific data generally suggest that the training effects gained in running are more likely to transfer to cycling than vice versa (for review see Tanaka, 1994).

Since both running and cycling utilize the major muscle groups in the lower extremities, it seems logical to speculate that transfer of training effects are likely to occur due to some overlap of the use of the same muscles.  However, a closer examination of the two disciplines show that while the quadriceps muscle group of the upper leg are extensively used in cycling, the smaller plantar flexors of the lower leg are preferentially recruited in running.  This observation is supported by the finding that the muscle enzyme activities of the quadriceps muscles of competitive cyclists are always far higher than those found in endurance-trained runners (Tanaka, 1994).

Purely on the principle of the specificity of training adaptations, the performance benefits to athletes who participate in cross-training must be seriously questioned.  To date, however,  there have been only a few scientific studies which have examined the effects of combined cross-training on endurance performance.

In possibly the first attempt to investigate the possible benefits of cross-training, Pate et al. (1978) examined the effects of either arm training only, or leg training only on the retention of training effects originally induced by leg exercise (cycling).  After 4 wk of this cross-training, the leg-trained athletes had retained or actually increased their cycling capacity.  However, those individuals who trained the upper body had simply detrained.  The results of this study imply that pure cross-training (that is, training another body part from that which is used in the athletes primary activity or event) is useless for retention of training effects.

In the most recent and perhaps well-controlled study, Mutton et al. (1993) investigated the effects of 5 wk of high-intensity cross-training (a combination of cycling and running) versus a equal-intensity running-only training program on 1609-m (one mile) and 5,000-m run performance, as well as the laboratory measure of maximal aerobic power (VO2max).  Twelve moderately-fit men who had been running up to 30 km/wk for the two months prior to the study acted as subjects.  After 5 wk of training, there were improvements in both the run-only and the cross-trained athletes for one mile time (21 vs 18 s), 5000-m performance (1.7 min for both groups) and VO2max (5.2% vs 5.9%).  Although there were no differences in any of the performance measures between the two groups, Mutton et al. (1993) concluded that their results support the use of cross-training as an alternative to increasing performance.   It is difficult to see how such a conclusion was warranted, when the effects of  cross-training on performance never exceeded those induced by specific training.  In addition, the principles of specificity of training are likely to have greater significance for well-conditioned competitive athletes compared to the moderately-trained recreational subjects who participated in this study.

Both scientific evidence and anecdotal reports overwhelmingly indicate that the best way to retain a training effect and improve subsequent performance is to continue using the primary mode of activity in which the athlete wants to participate, or an exercise that is very similar in terms of neuromuscular recruitment.  Observing the world's top athletes show clearly that the best marathon runners do not undertake any of their training in the swimming pool. Conversely, the best swimmers certainly wouldn't be advised to take to the road and start serious running!  Thus, cross-training is somewhat of a misnomer; and for the serious single-event athlete, something best left to those multi-event sports that require competitors to be proficient in more than one discipline.

A common but slightly different example of cross-training is where swimmers undertake dry-land training (usually cycling or running) in addition to their already huge swimming load, in an attempt to control their body fat levels.  Whether this is of benefit to energy balance has not been studied systematically, but the additional fatigue or increased risk of injury associated with the extra training surely makes it a risky practice.

Perhaps the only advantage of cross-training is when an athlete has been forced to stop training for their primary activity (usually because of injury), and there is a need to maintain general fitness.  The injured runner, for example, may choose to replace running with cycling or water running with a flotation vest.  If an athlete has to revert to training by undertaking activity in another discipline, then the closer the range of motion of the substitute training to the primary activity, the better will be the retention of training adaptation. But if the intent of training is to simply to maintain or increase general cardiorespiratory fitness, then any aerobic exercise may be appropriate.  In this case, the advantage of cross-training is that an individual is likely to be able to undertake a greater total volume of training without the risks of over-training or injury.  So why do some athletes cross-train?  Well, it's definitely better than doing nothing, it provides a variety of training stimuli which may increase general fitness, and it may help prevent injury.

References

Hawley, J.A. and Burke, L.M. (1998).  Peak Performance:  Training and Nutritional Strategies for Sport. Sydney:  Allen and Unwin.

Mutton, D.L., Loy, S.F., Rogers, D.M., Holland, G.J., Vincent, W.J, and Heng, M. (1993).  Effect of run vs combined cycle/run training on VO2max and running performance.  Medicine & Science in Sports & Exercise, 25, 1393-1397.

Pate, R.R., Hughes, D., Chandler, J.V.and Ratliffe, L. (1978). Effects of arm training on retention of training effects derived from leg training. Medicine & Science in Sports & Exercise, 10, 71-74.

Tanaka, H. (1994).  Effects of cross training.  Transfer of training effects on VO2max between cycling, running and swimming.  Sports Medicine, 19, 330-339.