As different as these two models sound, they both have relevance to triathletes regardless of distance. Both models indicated that a more upright position benefitted athletes at speeds less than 19 mph. Over 20 mph, from an energy efficiency standpoint, it's better to get low. You heard correctly: losses in power generation due to increased hip angle are compensated by reduced energy demand. This is mostly because of the reduction in aerodynamic drag.
More: Decreasing Aerodynamic Drag
At this point the two models diverge. Though both models agree that your torso angle should decrease as your speed increases, the power-drag model always favors sitting up a little more. It favors a better hip angle for power production. Understanding why requires us to know a key difference between the two models. The power-energy model assumes a flat course with little to no wind. The power-drag model presumes a windy course with more hills. In cases when the incline and wind combine to produce exceptionally stiff resistance, a lower body position simply won't give you enough benefits. When you can no longer cut down the resistance, you have to generate power. Sitting up does that for you. The power-drag model holds true for sprints as well.
More: 5 Things I Learned in the Wind Tunnel
The two models agree once again when the data is extrapolated to speeds in excess of 27 mph. At that point, they both predict that an athlete encounters so much wind resistance and is so close to his or her maximum power output that it's more beneficial to get low and cut losses than to sit up in an effort to generate more power.
Overall, the Birmingham study makes a significant contribution to our understanding of how bike fit can influence performance. It makes a solid case to be skeptical of the claim that "hip angle trumps all." In fact, some losses in power generation are acceptable and perhaps even favorable if we can reduce the forces opposing us on the bike.
More: The Principles of an Aerodynamic Bike Fit
However, proponents of aggressive bike positions should be equally careful in their treatment of these findings. These are only two methods of observing the problem, but there are others, including how athletes respond to more aggressive positions as a function of muscular flexibility and strength. Many triathletes have abandoned a race as a result of low back pain. The "well trained" athletes in this study were not only accustomed to the cycling position from years of pedaling, but they were also relatively injury-free and did not have any pre-existing conditions before entering the sport. So even the results in this study have to be considered within the confines of the criteria studied by the researchers.
The biggest takeaway for coaches, athletes and fitters is that even professional athletes change their position over time, based on factors ranging from aerodynamics to injuries. In the continual search for improvement, we should look for new information and ways to develop ourselves. The Birmingham study opens the door for exploration of a topic many had thought was a closed case. Scientists will no doubt continue their research. Athletes and fitters shouldn't be afraid to experiment on their own.
More: Improve Your Aero Positioning With These 5 Stretches
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