The cycle-run transition in triathlon represents a huge psychological barrier for most triathletes, but the perception that running is much harder after cycling is more than just the mind playing tricks. The cycle phase presents some serious physiological challenges that triathletes have to overcome as they begin their run.
In triathlon, the bike-run (T-run) transition is associated with a number of changes, including an impairment in running economy; specifically, there is a 2-12% decrease in running economy during the early part of the run phase (when compared to running economy in the 'fresh' state). The most obvious external cause for this deterioration is a change in running mechanics, which is often apparent as a slightly forward leaning posture. It is thought that this abnormal posture, and the perception of poor coordination after the T-run transition may be due to the inability of the neurosensory system to adjust quickly to the sudden change of posture from cycling to running. However, there are also some less obvious, and more fundamental physiological changes induced by the cycle phase. The fact that breathing discomfort also seems to be elevated during the early stages of the run phase provides some clues about the origin of these physiological changes.
During triathlon, the lungs are subjected to huge demands, and there have been repeated observations of reductions in post-event lung function . An important deficit is that of the lung diffusing capacity, which is impaired post-event (and presumably also during the latter stages of the race). Other respiratory-related changes are also present; specifically, some breathing pump muscles exhibit evidence of fatigue during and after the event. However, the pattern of the changes is not as you might predict.
Research on swimmers has shown that front-crawl swimming is associated with the highest magnitude of inspiratory muscles fatigue (IMF) yet recorded a 29% deficit in strength after a 200m swim at 90-95% of race pace . In light of this, we might predict that IMF would be present after the swim phase of the triathlon and that it would become progressively more severe after the cycle and run phases.
However, the two studies that have examined the influence of triathlon upon respiratory muscle function have shown little or no IMF after the swim phase. In contrast, both of these studies observed IMF after the cycle and run phases (~25%), but there was no worsening of fatigue between the cycle and run phases. In other words, cycling induced fatigue that was not exacerbated by the subsequent run. In addition, there was no evidence of expiratory muscle fatigue .
It is likely that the absence of IMF following the swim phase is due to triathletes' pacing strategy, and not to the fact that triathletes are more resistant to the IMF induced by swimming than swimmers are. There is some evidence to support this. For example, one study found that the slowest 50% of swimmers were significantly faster in the initial stages of the subsequent cycle phase . Another found that athletes undertaking the swim phase at 80% of their maximal swim trial velocity completed a simulated event faster than when the swim was undertaken at 100% of maximal swim trial velocity. Thus, triathletes probably pace themselves during the swim in the knowledge that not pushing too hard during this phase results in a better overall performance.
It is clear from the data relating to inspiratory muscle fatigue that the cycle phase must represent a particular challenge to the inspiratory muscles, since this phase induces IMF that is not worsened by subsequent running. So what is known about the demands of cycling and the T-run transition?