How Your Aerobar Height Impacts Your Heart Rate — Triathlete

In the pursuit of aerodynamic efficiency, most triathletes aim to minimize stack height, or the vertical distance from the center of the bottom bracket to the top of the head tube – that is, lowering their aerobars. While an upright position may be more comfortable, a lower torso angle is the primary driver of drag reduction.

Yet this raises a fundamental question: How low can a rider go before the physiological cost begins to compromise performance, and what exactly is the metabolic price of such an aggressive position?

A social media experiment on stack height and heart rate

Former professional cyclist, cycling coach, and bike builder Arne Peters recently put that question to the test in an Instagram video, where he used a specially equipped road bike that could quickly raise and lower the front end between a low and a high stack height. While the rider did not use aerobars, he maintained an aerodynamic position, with his hands on the brake levers and forearms flush against the bars.

Multiple five-minute trials were held in each position, at different levels of functional threshold power (FTP), measured objectively using a power meter. Using heart rate (HR) as a marker for how hard the body was working showed that at each intensity, average and maximal HRs were higher in the low stack height position than in the high stack height condition.

Peters concluded that the body’s cardiovascular system has to work physiologically harder in low stack height positions when the bars are far down.

How valid were the conclusions?

Let’s take a look at the results of this experiment more closely. At 85% FTP, the mean HRs were higher in the low stack height position, but the differences were small (less than 2 beats/min at most) and not statistically significant.

At the higher intensity (100 and 120% FTP), the differences were larger (3-4 beats/min) and did reach or approach statistical significance.

At 100-120% FTP, the body is unlikely to remain stable, meaning that the HR would be expected to continually increase, with a lag in the first trial until the body “catches up” on the difference between oxygen demand and delivery. Once “primed,” the body responds more quickly, thus leading to higher HRs on subsequent reps. Indeed, if we omit the HR data from the first rep, the differences in HR between the positions lessen and no longer reach statistical significance.

But, even with the potential shortcomings in study design, does Peters’ point here on stack height influencing heart rate add up?

What the actual science says about stack height and heart rate

While Peters’ adjustable configuration was a specialized tool for demonstration, the physiological consequences of transitioning between the hoods, drops, and aerobars (each of which modulates torso angle like stack height) have been extensively scrutinized in laboratory settings.

So, does a more aggressive posture truly drive a significant increase in heart rate?

Though heart rate has plenty of affecting variables, things like hydration, thermal conditions, and caffeine intake, it provides a critical window into the interplay between internal workload (oxygen consumption) and external workload (power output). By serving as a quick n’ dirty marker for metabolic efficiency, we get an answer: a lower heart rate maintained at a consistent power output shows that your body is working efficiently.

Body position does influence top-end performance, as cyclists are usually able to generate greater max power, VO2 max, and max HRs in upright versus aerodynamic positions – most of us have felt this when climbing in our base bars (though there’s also some differences in leverage positions there too). Changing the trunk and hip angles alters length/tension relationships of muscles, leading to changes in activation and efficiency.

Lower torso positions can restrict the diaphragm and chest, possibly leading to changes in breathing and lung function. Blood flow to and from the heart can also be altered with position, as preload (blood return) to the heart and stroke volume (the amount of blood pumped with each heartbeat) could differ.

These physiological changes all have the potential to impact oxygen needs and delivery (and therefore heart rate), so how does that hold up in studies?

Muscle activation

Compared to upright cycling, riding in the aero position with increased trunk and hip angles (similar to lowering stack height in Peters’ video) results in less force production at the top of the pedal stroke, but greater force production at the bottom half of the downstroke.

This is related to greater (and later) activation of the gluteus maximus and quad muscles that cross the knee joint only, with decreased activation in the rectus femoris. More weight is shifted onto the upper body.

The side effect? If you’re unfamiliar with that lower position, you could feel yourself working harder as your muscles adjust – this could impact heart rate.

Blood flow

One study found that blood flow in the aorta, heart rate, and stroke volume showed no differences between riding in the uprights, drops, or aerobars at increasing intensities.

In another small study, despite no difference in heart rate between the three positions at 70% VO2 max power, perceived exertion and ventilation were increased in the drops compared to the upright position.

Breathing frequency, work of breathing, minute ventilation, and oxygen consumption have been seen to be higher in the aerobars and at lower torso angles. A 3% increase in oxygen consumption and a five beats-per-minute increase in heart rate when moving from an upright position into an aero position was found in elite cyclists. Heart rates during a 12-minute TT were also about five beats per minute higher when the test was performed in the aerobars vs. uprights.

Time to exhaustion as the intensity ramped from 80 to 95% VO2 max in one study was about 70 minutes upright vs. about 60 minutes in the aerobars. This was deemed not statistically significant, but practically, 10 minutes makes a difference.

Conversely, some research has found no significant difference in ventilation, heart rate, or oxygen consumption between upright, dropped, and aero positions, or between lower aero positions at submaximal or increasing intensities. With that, marked variation between riders exists.

Power and efficiency

While an overall 5% decrease in critical power when moving from an upright to a time trial position was seen, individual responses varied significantly, with athletes testing anywhere from 80W higher in the upright position to 56W higher in the TT position. Training and experience (not surprisingly) matter.

In less trained subjects unfamiliar with riding in aerobars, mechanical efficiency was compromised as compared to upright positions, and the study that brought riders up to 95% VO2 max did find large individual variations in time to exhaustion in the aerobars based upon rider experience, indicating the importance of training in the position.

Other bike fit considerations

Stack height and torso angle aren’t the only variables that could impact heart rate on a bike.

Seat tube angle

The steeper (~75-78 degrees or greater) seat tube angles on TT bikes vs. road bikes (~72-75 degrees) are intended to open up hip angles and allow for greater efficiency in lower positions. Indeed, HR was found to be lower when riding in the aerobars at higher (76+ degree) seat tube angles than at lower (69 degree) angles.

Crank arm length

Crank arm length has gained increasing attention in recent years, as Tour de France champions Tadej Pogacar and Jonas Vingegaard have been spotted riding (and winning with) shorter cranks. Shorter cranks help to open up hip angles and make aerodynamic lower positions more comfortable.

Perceived fatigue has been found to be lower with shorter (165 or 170mm) vs. longer (175mm) cranks, but no differences in heart rate or cycling economy were seen.

Seat height

As for seat height, evidence is mixed, with some studies finding lower oxygen costs at higher saddle heights, some at lower ones, and an overall conclusion that small changes (<4% of leg length) don’t impact oxygen consumption or efficiency. So, yes, that TT bike geometry is advantageous at keeping HR down, but individual comfort likely rules when it comes to crank length and saddle height.

So how does bike fit impact heart rate?

Overall, there’s evidence to prove that Peters was correct: It’s likely that lower positions impact physiology, including heart rate. He’s also correct that aerodynamic benefits need to be balanced with power and energy costs, as rider position is the main contributor to drag.

The studies above largely concluded that the benefits of lower positions prevailed. The metabolic efficiency cost of moving from the uprights to the aerobars and decreasing torso angle has been estimated to be about 2-3%, but frontal area reductions are about 10%, and drag reductions can range anywhere from 15-35%.

What about smaller changes, like adding or subtracting spacers from your aerobars? Dropping aerobars 5cm can reduce frontal area and drag by about 4% without sacrificing metabolic efficiency. What’s the tipping point, though? With experience and practice, riders will physiologically adjust to positions, but, as with everything, to a limit.

Full-on wind tunnel testing remains the gold standard for testing that aerodynamic/efficiency balance, but this isn’t for everyone. So, some at-home testing similar to Peters’ experiment could help dial in stack height.

An at-home experiment for triathletes

Technically, triathletes should be concerned with arm pad stack, or the height of the aerobar elbow pads in relation to the bottom bracket, as this determines torso position in the TT position.

The following test can be performed by adding bar pad spacers, but that can get tedious (particularly mid-ride) with bolts and wrenches, so towel layers can be substituted to raise the elbows. Yes, the towels will compress a bit and won’t be perfect, but they’re practical (just be careful they don’t come loose and get wrapped into your wheel!).

For the test, try the following:

• Start with your aerobar pads in their lowest position and no risers (note that you might have to adjust down initially).
• Warm up for 20 minutes.
• Select a sub-threshold intensity between 75-80% of FTP or critical power (whatever you use).
• Ride 10 minutes at the first stack height. Record average and max HRs over the final 5 minutes of the trial. Note RPE as well.
• Ride 5 minutes easy (40-50% threshold), add a spacer or towel layers of equivalent thickness to your bar pads, and repeat the previous step.
• Continue the 10-minute trial/5-minute recovery protocol for as many heights as you’re interested in testing.
• Repeat the protocol on a different day, but start at the highest height and drop each repetition.

Afterward, evaluate the data, including subjective perceptions. Were heart rate averages relatively similar, or was there a noticeable jump across levels? What about RPE and comfort – did each height feel relatively similar, or were your muscles starting to burn the lower (or higher) you got? Did anything hurt?

Even if heart rates didn’t differ, you’ll still need the muscular endurance to hold that position for hours at a time. Weigh those factors and use a bit of art and science to determine the lowest stack height where heart rate stayed in line, the effort felt manageable, and nothing hurt.

Remember, the body will adjust with training, but significant differences or discomforts might not be worth it. Then, try it out in the real world! Ultimately, speed determines outcomes, so experiment with adding or subtracting spacers before long rides.

Speed on the bike, of course, is highly variable with courses and conditions, but look for overall trends (longer Strava segments can be helpful) with the interplay of power, heart rate, speed, and comfort in order to dial in your best, most efficient position.