Cycle to Run Transition
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 cycle-run (CR) transition is associated with a number of changes, including an impairment in running economy; specifically, there is a 2-12% (Chapman et al. 2000) 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 CR 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 (Hamond et al. 1991). An important deficit is that of the lung diffusing capacity, which is impaired post-event (Hue et al. 2001) 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.
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 (Lomax and McConnel 2003). 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 (Hamond et al. 1991 and Sharpe et al. 1996). 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 (Hamond et al. 1991).
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 (Vleck 2006). 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 CR transition?
The cycle-run transition
The CR transition became a particular focus of respiratory research because for many years there was no satisfactory explanation for the increased perception of breathing discomfort that is present during the first minutes of the run phase. As mentioned previously, research has now shown that during the first minute of the run phase following the CR transition, the energetic cost of running is higher. Associated with this is an increase in the ventilatory requirement, and these changes have been ascribed, at least in part, to IMF (Hue et al. 1998 and Hue et al. 1999).
A research group in France has attempted to tease out the independent and combined influences of cycling and running upon IMF and lung diffusing capacity. In one study (Chevrolet et al. 2001) they compared the influence of a 20-minute cycle followed by a 20-minute run (CR), with that of a 20-minute run followed by a 20-minute cycle (RC) (all at 75% of maximal oxygen uptake). They found that the RC combination induced the greatest magnitude of IMF. The explanation for this is that cycling presents the greatest challenge to the inspiratory muscles and that when the run follows the cycle, the inspiratory muscles have time to recover. In contrast, when the cycle follows the run, the full magnitude of the cycle-induced IMF is apparent.
But why should cycling be more challenging for the inspiratory muscles than running? It probably relates to the influence of trunk posture upon breathing mechanics. The crouched body position associated with the use of 'aerobars' has some disadvantages when it comes to breathing. Research suggests that cyclists who are inexperienced in the use of aerobars experience detrimental effects on their breathing and mechanical efficiency compared to cycling in the upright position (Ashe et al. 2003). For example, compared with upright cycling, aerobars resulted in a lower maximal oxygen uptake and lower maximal ventilation. In addition, breathing appeared to be constrained, such that tidal volume was lower and breathing frequency was higher. This is a very inefficient breathing pattern; indeed, the study found that mechanical efficiency was lower when using aerobars, i.e. the same amount of cycling work required more energy.
The explanation for these findings resides in the influence of a crouched body position upon inspiratory muscle mechanics during cycling. Firstly, crouching forward forces the contents of the abdomen (stomach, liver and gut) upward against the diaphragm. This impedes the movement of the diaphragm during inhalation because the abdominal contents are pushed up against the diaphragm causing it to 'work harder' for each breath. Secondly, the higher breathing frequency means that inspiratory flow rate must be higher, which means that the inspiratory muscles must work in a region of their force-velocity relationship where fatigue and effort sensations are greater.
In a follow-up to their RC/CR study, Boussana et al. (2003) tested their hypothesis that the crouched body position of cycling, and its negative impact upon respiratory muscle mechanics, might account for the differences between the effects of CR and RC transitions. Subjects performed either 20 minutes of cycling, 20 minutes of running, or 20 minutes of cycling followed by 20 minutes of running (CR). Interestingly, they noted that cycling and CR induced almost identical IMF, whereas running induced none. This suggests that cycling fatigues the inspiratory muscles in a unique way that is most likely related to the crouched body position.
As mentioned above, changes in lung diffusing capacity have also been noted after CR and RC transitions (Galy et al. 2003). As was the case with IMF, the RC transition generated the greatest deficits in diffusing capacity. The authors speculated that this was due to a reduction in the volume of blood within the lung circulation during RC, which would reduce the proportion of the lung available for oxygen exchange between blood and air. Further, they speculated that the reduced lung blood volume was due to IMF, and secondary to alterations in breathing-induced pressure changes within the chest and thus to the amount of blood returning to the lungs.
Another potential explanation for the change in diffusing capacity relates to the influence of IMF upon blood flow distribution during exercise. During exercise that fatigues the inspiratory muscles, there is a narrowing of blood vessels, including those to working muscles (Dempsey et al. 2006), and possibly also those in the lungs.
A recent study into the influence of inspiratory muscle training (IMT) on exercise performance under conditions of simulated high altitude (low oxygen) using a PowerBreathe training device found that oxygen diffusing capacity and arterial oxygen saturation were increased during exercise after IMT, compared to before IMT. This may indicate that after IMT, a vasoconstrictor influence upon the blood circulation to the lungs has been removed, causing blood volume and diffusing area to be increased. Thus, the impairment of diffusing capacity at the CR transition of the triathlon may be a manifestation of pulmonary vasoconstriction that has been induced by inspiratory muscle fatigue.
It seems likely therefore that the respiratory impairments induced by cycling carry over into the run, causing run performance to also be impaired. The greater IMF induced by the RC combination most likely occurs because in the CR combination, the inspiratory muscles can recover slightly from the impairments induced by the preceding cycle; whereas in the RC combination, there is no opportunity for recovery. It appears that the mechanical constraints of cycling, which restrict rib cage and diaphragm movement, induce impairments in both inspiratory muscle function and lung diffusing capacity, both of which can impair performance.
The obvious question is what can be done to minimise these effects? Since studies appear to show that the aerobar position has fewer detrimental effects in cyclists who have used them for a prolonged period (Ashe et al. 2003), it appears likely that the inspiratory muscles adapt to the increased demands imposed by aerobars. Clearly this adaptation does not appear to abolish the IMF, since there are numerous studies showing that even very highly trained triathletes and cyclists still experience IMF. However, an intervention that has been shown to abolish IMF is specific resistance training of the inspiratory muscles (Romer et al. 2002).
The benefits of inspiratory muscle training
The data described above creates a fairly compelling argument in favour of specific inspiratory muscle training (IMT) in order to minimise the detrimental influence of the mechanical constraints to breathing that are imposed by cycling. Unfortunately, there are so far no published studies evaluating the benefits of IMT for triathlon performance. However, we can infer the likely benefits by considering the following facts, as well as data from studies of IMT in cyclists, all of which suggest that good breathing and avoiding IMF are central to success:
Finally, in considering the merits of adding IMT to an already time-consuming training schedule, it is worth considering some facts regarding the time efficiency of IMT compared other training adjuncts. A typical IMT programme requires less, around 4 minutes per day, and can produce a 4.6% improvement in 40km cycling time trial performance. So, let us consider what else could be added to a training schedule in order to achieve a similar magnitude of benefit.
Very few studies have examined the influence of adding a different type of training to the endurance regimens of already highly trained endurance athletes. Fortunately, one of the few studies to have undertaken such an appraisal utilised a 40km cycling time trial as an outcome measure, making it possible to compare their data directly with those obtained using IMT.
The authors examined the effect of a number of interval training regimens, one of which produced an improvement in 40km time trial performance of ~5% over the 4-week period of their training intervention (Laursen et al. 2002). The intensity of training was very high, being set at the power output that elicited maximal oxygen uptake (VO2 max) during an incremental exercise test. Athletes were required to undertake eight intervals of ~2.4 minutes duration interspersed with recovery periods of ~4.8 minutes. Athletes trained twice per week and the duration of each session was ~53mins.
Over the 4 weeks of the intervention, the total duration of high-intensity interval training required to elicit a 5% increase in 40km time trial performance was 7hrs. Compare this to the total time required to attain a 4.6% improvement in performance following 6 weeks of IMT, which is around l.Shrs! Another salient point is the intensity and duration of each training session (53 minutes at VO2 max, 2 minutes at moderate inspiratory muscle load), as well as the fact that IMT can be undertaken anywhere; there is no need for a bike, or even to break into a sweat! The choice is yours...
In this article we have considered the unique challenge to breathing posed by the triathlon, as well as the rationale for training the breathing pump muscles. Inspiratory muscle fatigue appears to contribute to the discomfort and physiological challenge of the run phase of triathlon. However, IMT reduces this fatigue and improves cycle time trial performance, indicating a strong likelihood that it will also ease the CR transition in triathlon, thereby enhancing performance. We have also considered what else could be added to a training programme to obtain the same performance improvement that has been demonstrated in response to IMT. The numbers speak for themselves and make the argument in favour of IMT a complete 'no brainer'. It is quick, it is easy, it is convenient, and you do not need to flog your guts out. Just add 4 minutes per day of relatively easy exercise to your normal training to achieve a 4.6% gain in your 40km time trial performance!
The information on this page is adapted from McConnell (2007) with the kind permission of Electric Word plc.
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