Page 1 2

If you have ever competed in a duathlon or triathlon or even carried out a 'brick' workout, in which you shift quickly from biking to running, you will know how challenging the bike-run transition can be. Once you have left the relative comfort of your bike seat, those first running steps can feel awesomely difficult; your running pace will certainly be below par - and you may well wonder whether you will be able to finish the run portion of your competition or workout with any kind of quality at all.

Why do your legs feel so leaden when you try to run after strenuous cycling? If you happen to be competing in an Olympic, middle- or long-distance triathlon, your leg muscles may well be somewhat glycogen-depleted after 40, 80, or 180k of strenuous cycling respectively. This glycogen depletion can cause significant fatigue, making your legs 'heavy' and unresponsive. When your leg muscles are low on glycogen, they simply don't have enough of the 'high-octane fuel' needed for fast running.



If you haven't stoked up on fluids properly during the bike stage, dehydration may also be taking its toll. But even when glycogen level and fluid intake are fine, there is another complication which can make you struggle: as you start the run phase of your bike-run transition, your nervous system is still geared towards controlling the mechanical movements required for cycling. It takes time for the brain and spinal cord to adjust to the completely new patterns of neuromuscular coordination needed for running, and during this adjustment period running feels sluggish and uncoordinated. This is an attractive explanation for the troubles experienced during the first few minutes of running after biking, especially since the muscles often begin to feel less fatigued after a few minutes, which would not happen if glycogen depletion or dehydration were the source of the fatigue.

There's also another intriguing explanation for the difficulties of running after cycling: the change from one activity to the other induces a redistribution of blood flow to the propulsive force-producing muscles in the legs(1). Thus, muscles that are activated more for running than for cycling may have to wait for a short period of time to receive augmented flows of blood. During this waiting period, they may be somewhat short of the oxygen (or fuel) required for high-intensity effort.
How bad are the drop-offs in capacity during the initial portion of the running stage of a triathlon? Recently French researchers found that 70% of national-level triathletes remained up to 10% below their average 10k running velocity over the first 500-1000m of the run phase, losing much valuable time in the process(2).

Why the bike-run transition is
such a critical period
Such findings make an important point - and raise a couple of key questions. First, it's clear that the bike-to-run transition is a critical time during the triathlon, when much time can be gained or lost. Athletes who can preserve running ability during the transition will have a significant advantage over the majority of competitors, who have significant troubles with this stage of the race. The key questions then are:
a. How did some triathletes (up to 30%) manage to hang on to their usual 10k speeds during the run portion of the bike-run transition?
b. What training and racing strategies are best for enhancing the ability to manage the transition effectively?

To find out more about the problems associated with the bike-run transition and to develop techniques which might optimise performance during this key portion of a triathlon, British sports scientists Millet and Vleck studied the physiology and biomechanics of the transition in both junior and elite triathletes(3). They defined the 'cycle-to-run transition' as the period from the beginning of the last kilometre of the cycle section to the end of the first kilometre of the run, and the 'transition area' as the clearly demarcated area in which athletes dismount their cycles and begin running.
They begin their analysis by pointing out an obvious transition strategy - to reach the transition area at the head of a group, rather than in the middle or at the end of it, thus avoiding collisions and jams within the transition area. In fact, most experienced triathletes increase their speed during the final kilometre with this goal in mind - and often find, in doing so, that they have 'something left' at this stage. There is some evidence, though, that significant speed-ups over the last kilometre of the cycling stage may have a negative impact on performance during the first part of the running stage - on which more in a moment.
Although it represents just a small fraction of the total time required to complete a triathlon, time spent in the transition area is actually a fairly good predictor of finishing position and overall triathlon ability. For example, during the 1997 and 1998 Triathlon World Championship competitions, elite senior triathletes took an average of 56 seconds to traverse the transition area (less than 1% of the total time required to finish an 'Olympic' triathlon), while junior-level athletes took 83 seconds (1.1% of total time). Amazingly, the very best triathletes require less than eight seconds to rack their cycles, take off their helmets and put on their running shoes (and should be put to work immediately at nursery schools, teaching young children how to tie their shoes laces in fewer than the usual 20 minutes!) Millet and Vleck were able to show that the higher a triathlete is placed in the field toward the end of the cycling section, the greater the importance of transition area time to his/her final finishing position. Since top 10 finishing positions are gained or lost by seconds and even fractions of seconds, transition area skills are extremely important.

Transition-phase running is harder
Millet and Vleck summarised other studies documenting the physiological uniqueness of the bike-run transition - investigations which showed that transition-phase running really is harder than running on its own. Oxygen consumption, respiratory frequency, ventilation rate, and heart rate all tend to be higher during transition-phase running than during routine running at the same speed. In one study of 13 female duathletes and triathletes, running economy (the rate at which oxygen is used at a specific speed) was measured at running speeds of 169, 177, 196, and 215m per minute during control running (without prior biking) and also after 45 minutes of cycling at the rather modest intensity of 70% VO2max. In each case, running economy was worse (ie oxygen consumption rate was higher) after the biking(4). This increase in oxygen consumption appears to range from about 1% in some athletes to as much as 12% in others! In other words, in some triathletes it 'costs' 12% more to run at race pace after the bike portion of a triathlon than it does just to run. Small wonder that running feels tough after biking!
Significantly, this rise in oxygen cost (or decrease in economy) at the beginning of the run portion of a triathlon is inversely related to triathlon ability level: the greater the rise (compared to running without previous cycling), the poorer the performance(5). Thus, it makes sense for triathletes to train in a way that reduces this drop in economy (more on this in a moment). Interestingly enough, in this study the researchers were able to show that athletes who best preserved economy tended to be the ones who maintained normal leg stiffness from the outset of their runs - ie their legs were better controlled, more stable and with roughly the same resilience associated with usual running. To put it another way, their legs were less fatigued and better controlled by their nervous systems.
What causes the oxygen squandering which is so typical of the bike-run transition? The mechanical changes mentioned above undoubtedly play a role, and glycogen depletion may be an additional interrelated factor. If triathletes start their runs with low muscle-glycogen concentrations because of glycogen depletion during the bike phase, their leg muscles might turn increasingly to fat to supply the energy needed to continue. For a given power output (running speed), the utilisation of fat rather than carbohydrate for energy causes a significant rise in the rate of oxygen consumption. Thus, it is critically important for triathletes to maximally load their muscles with glycogen before the event - and to keep carbohydrate flowing into the body amply and steadily during the competition itself. At least five regular swallows of sports drink should be taken in every 15 minutes during the bike portion of a triathlon, and a 10oz 'slug' of sports drink should probably be taken right at the beginning of the biking. Plain water should not be ingested along with the sports drink, as this would dilute it in the stomach and lower the absorption rate of carbs. This consumption pattern should also help control rises in body temperature and limit the risk of dehydration, both of which can impair economy.
In an exploration of bike-run transition problems, which attempted to mimic an Olympic-style triathlon, five triathletes ran 10k after either a normal warm-up or a strenuous 40k cycle ride. Average running speed was significantly slower after the cycle ride and there was also a trend toward decreased stride lengths(6).

Continued »