Aero Positioning: How Low is Too Low?
TOOLBOX: Time trialing has a massive focus on aerodynamics, both to the bike and the rider. But where does the balance lie between aerodynamics, power output, metabolic economy, and thermoregulation? How low should you go?
A Game of Limbo
Anyone who has ever ridden a time trial know that, besides a willingness to suffer, the two dominant ingredients are generating a huge amount of watts while at the same time being as aerodynamic as possible. The former pushes you forward, while the latter decreases the amount of air resistance pushing you back.
The interplay between power and aerodynamics has swung like a pendulum over the years, especially after the introduction of aerobars. It’s fine to be really low and aerodynamic, but if you can’t generate power in that aero position, then it becomes a net negative.
Furthermore, the ability to generate power is also a complex interplay. On the one hand is the biomechanical and neuromuscular aspect, whether you can actually generate a particular force or power. On the other is your metabolic economy, as in whether a particular position requires more energy to generate the same force. For example, it may be that more muscles need to be recruited to ride at the exact same wattage of 300 W if you are in a really low aero position.
And throwing my environmental physiology hat into the ring, what about thermal considerations? Theoretically, a more aero position means less airflow over your body for cooling. Would that also impair your ability to dissipate heat and put you at a thermoregulatory disadvantage?
Faulkner & Jobling 2020
Balancing all of the above arguments was the purpose of a study by my friend Steve Faulkner & Philippa Jobling at Nottingham Trent University in the UK (Faulkner and Jobling 2020). Faulkner was the sport scientist working with the Huub-Wattbike Team Pursuit squad, so he definitely draws from a deep well of applied sport science experience.
- 11 well-trained male cyclists, with 5+ years of TT/triathlon experience.
- After max testing and familiarization, 5 trials were performed a work-based TT (75% Wmax for 20 min or 321.4 kJ) with different hip angles by adjusting upper body position with lower body position maintained: control (self-selected TT position, ~15°), 12°, 16°, 20°, and 24° hip angle.
- Electromyography, oxygen uptake, skin and core temperatures were measured throughout.
- Testing occurred in a 18.6°C chamber with fan speed relatively low at 9.0 km/h.
- Digital imaging was used to calculate Cd (coefficient of drag) and A (frontal surface area).
Need for Speed
The first key finding is that there were no statistical differences in TT completion time across the 5 conditions. When compared to the smallest worthwhile change in performance of >1.5% (~17 s), the 12° position may be classified as “possibly harmful.”
- There was indeed a difference in performance based on position. Namely, compared to the control position, the lowest position (12°) had a greater metabolic cost. Total energy cost was higher than with control, and also the power at which 4.0 mmol lactate was reached was lower than with 20° and 24°.
- The authors then calculated a measure they called “aero economy” that integrated power output (W), aerodynamic efficiency (CdA) with each hip angle, and oxygen cost (L/min). Namely, W·CdA·L/min.
- As seen in the graph, there was a main statistical difference across all 5 conditions, with the 12° angle trending towards the most economical and the 24° (most upright) position less economical compared to control.
- No thermoregulatory differences were seen among the 5 different positions. Personally, this was not surprising given the relatively brief TT duration, moderate temperatures, and minimal airflow.
- No differences were seen in EMG activity among the 5 different positions. Again, not overly surprising given the relatively messy measure of a large muscle mass throughout the lower body.
The study ultimately found, at best, a minimal difference in TT performance across the different hip angles. To me, this somewhat blunts the impact of a deeper analysis, especially with the 12° position being the best in terms of aero efficiency yet being potentially the slowest in real life performance. Faulkner & Jobling summarize that aerodynamic benefits outweigh any energetics impairment, but I’m not certain that I agree given the lack of real difference in TT performance.
What I would take away from this study is the complex interplay between aerodynamics, power output, and the metabolic cost of generating that power output. Overall, the study is interesting in being one of the first to integrate multiple factors behind TT performance into a single variable, and I think that this “aero efficiency” unit can be used to model many aspects of cycling, including sprinting positions.
By the way, Steve Faulkner is a strong triathlete and cancer survivor, and he’s taking part in the Geoff Thomas Foundation’s ride for leukemia, tackling each stage of the 2021 Tour a day ahead. Best of luck!
Ride fast and have fun!
Faulkner SH, Jobling P (2020) The Effect of Upper-Body Positioning on the Aerodynamic–Physiological Economy of Time-Trial Cycling. Int J Sports Physiol Perform 16:51–58. https://doi.org/10.1123/ijspp.2019-0547