Mobility for the Aero Position: Hips, Hamstrings and Holding It for 180 km

On a long-course bike leg, the aero position is the biggest free speed most triathletes have. TriForward's "Free Speed, Honestly Priced" shows why body position comes before almost any equipment purchase. The problem comes later. A position that feels fine for 20 minutes on the turbo can become unbearable at 90 minutes, and by the second half of an Ironman bike many athletes are sitting up, shifting on the saddle or riding on the base bar.

A common explanation is mobility: tight hamstrings and hip flexors stop the pelvis rotating forward, so the back rounds, the hip angle closes and you can't stay down. The fix offered is stretching. This article looks at how much of that story the evidence supports.

It deliberately leaves out ground covered elsewhere. TriForward's "Aero Until It Hurts" covers how aero positions cost power and how that cost grows with time. Here the question is narrower: does your range of motion limit your aero position, and can mobility work change it?

The short answer is that the evidence is thin and almost entirely from male road and time-trial cyclists, not triathletes.

What the aero position asks of your body

In an aero position, you lower your torso to reduce frontal area. Something has to give. Either the pelvis rotates forward, the spine rounds, or the angle between your torso and thigh at the top of the pedal stroke closes. Usually it's a mix of all three.

Three areas of mobility are often blamed when this goes wrong:

• Hamstrings, which attach to the pelvis and resist forward rotation of the pelvis when the knee is straightened at the bottom of the pedal stroke.
• Hip flexion range, which limits how closed the hip angle can get at the top of the stroke.
• Thoracic and lumbar spine, which determine whether you can flatten the back or have to round it.

There's a mechanical logic to this. But logic isn't evidence that mobility limits real riders, or that training it helps.

What a low position does to force

A 2022 study by Jongerius, Wainwright, Walker and Bissas in the Journal of Biomechanics looked at what happens to pedalling mechanics as the trunk is lowered. They tested trained male cyclists: 19 time-trial specialists and 36 road cyclists (the abstract and text differ by one rider). Each rode in their preferred position and in three standardised trunk angles: 40 degrees (upright), 30 degrees (drops) and 24 degrees (aero).

The main findings:

• Mechanical effectiveness fell as the trunk was lowered. In other words, less of the force on the pedals was directed usefully.
• The drop was smaller in the time-trial group. The authors reported that the decline was significant only in the road group.
• Moving from upright to aero increased anterior pelvic tilt by about 8.5 to 8.9 degrees and peak hip flexion by about 6.5 to 7.0 degrees. The pelvis rotated forward more than the hip angle closed.
• The hip moved forward as the trunk was lowered, and a more forward hip position was associated with better mechanical effectiveness in the aero positions.

The authors suggested that moving the saddle so the hip can move forward could offset some of the loss. They tested one power output and cadence over a self-paced effort, studied only men, and couldn't show cause and effect.

Inference: the time-trial riders, who had trained in that position for at least two years, coped with low positions better. That could reflect fit, adaptation, flexibility or self-selection. The study didn't measure flexibility, so it can't say which.

Does flexibility predict how riders set up their bikes?

The most direct evidence comes from two 2021 papers by Wendy Holliday and Jeroen Swart at the University of Cape Town, based on 50 well-trained male cyclists riding their own road bikes.

In the first, in Sports Medicine and Health Science, they measured anthropometry, flexibility and training history and used them to predict each rider's self-selected bike set-up. They found:

• Hamstring flexibility predicted handlebar drop (p = 0.01). Riders with more flexible hamstrings set their bars lower.
• Saddle height was predicted by leg length, a knee extension flexibility test and a lumbar flexibility test.
• The average hip angle on the bike was 77 ± 5 degrees.

In the second, in the Journal of Science and Medicine in Sport, they related the same factors to lab performance (peak power output and VO2max). They found:

• Performance correlated with hamstring flexibility, handlebar drop, saddle setback and ankle plantarflexion.
• A more anteriorly rotated pelvis went with better hamstring flexibility, a greater hip flexion angle and a larger handlebar drop.
• Greater lumbar flexibility was inversely related to relative VO2max.

These are cross-sectional associations in male road cyclists. Fitter riders may simply have been more experienced, more flexible and set up lower, without one causing another. The authors suggested that riders wanting a lower front end might benefit from improving hamstring flexibility, but they didn't test that.

The study that complicates the story

A 2011 study by Muyor, Alacid and López-Miñarro in the Journal of Human Kinetics tested 96 highly trained cyclists (mean age 30; sex not reported) who trained two to four hours a day. They grouped riders by hamstring extensibility using a straight-leg raise:

• Reduced extensibility (below 80 degrees): 30 riders.
• Moderate (80 to 90 degrees): 35 riders.
• High (above 90 degrees): 31 riders.

In a sit-and-reach test, the groups differed clearly in pelvic tilt and thoracic posture, as you'd expect. But on the bike, with hands on the drops, there was no significant difference in pelvic tilt between groups: about 33.5, 35.4 and 35.7 degrees respectively (p = 0.382). Lumbar posture didn't differ either.

Inference: in this group, how flexible a rider was on a mat didn't change how they sat on the bike in the drops. A road position in the drops isn't a time-trial position, and a tighter position might expose differences that this one didn't. But it is a warning against assuming that a stretching test result tells you what happens on your bike.

Can you change hip and hamstring range?

Yes. A 2024 meta-analysis by Konrad and colleagues in the Journal of Sport and Health Science pooled 77 studies with 3,870 healthy participants (mean age 27) and found that regular stretching produced a moderate overall increase in range of motion. Gains at the hamstrings and hip flexors were among the largest, with effect sizes of about 1.2 and 1.1. Static and contract-relax stretching worked better than dynamic stretching. TriForward's "The Ten-Minute Morning Mobility Routine" covers that evidence in more detail.

The gap is between range on a test and position on a bike. No trial has stretched cyclists or triathletes and then measured their aero position, frontal area, power in position or how long they could hold it.

What the wider position research says

A 2024 systematic review by Husband and colleagues in the Journal of Sports Sciences gathered the evidence on bike position changes. It included 47 studies with 724 participants: 454 men, 90 women and 180 whose sex wasn't reported. Most studies were rated poor quality, and none was rated good.

• Four studies found higher power output upright than in the drops or aero bars.
• Effects on oxygen cost were mixed.
• Regular training in the aero position may offset some of its functional disadvantages, according to the review's conclusions.
• Saddle fore-aft position changes both hip angle and trunk angle, and moving the hip forward can open the hip angle.
• The review identified no studies of flexibility or mobility interventions, and called for research on how riders adapt to positions over time.

Inference: if time in position helps, part of what mobility work is credited with may actually come from simply riding in the bars more. The two aren't mutually exclusive.

Holding it for 180 km

Even if mobility limits how low you can go, it may not be what limits how long you can stay there. As "Aero Until It Hurts" explains, no laboratory study has tested an aero position for longer than a 40 km time trial. Over five or six hours, other factors are likely to matter:

• Saddle comfort and pressure, which often force riders to sit up long before muscles fatigue.
• Neck, shoulder and trunk endurance, to hold the head and torso still.
• Fatigue in the hip extensors, which work at a closed hip angle in a low position.
• Heat, since sitting up can feel cooler.

Inference: flexibility is one factor among several, and for many age-groupers it may not be the one that gives out first.

Who these studies tested

It's worth being blunt about the population gap, because it is larger than in most TriForward topics. Every primary study above was in trained cyclists, and every one with a reported sex split was entirely or mostly men. The Jongerius and Holliday studies were all male. The Muyor study didn't report sex. Across the 47 studies in the Husband review, only 90 of 724 participants were women.

None of the studies was in triathletes. That matters for three reasons:

• Triathletes ride with a different aim. A position has to leave the legs able to run afterwards, which a time-trial cyclist doesn't need to consider.
• Many age-group triathletes come from running or swimming, with less experience in any bike position, let alone a low one.
• Triathlon positions are held for much longer than the efforts tested in the lab.

In the 6,000-person flexibility dataset covered in TriForward's "Mobility After 50", women were more flexible than men at every age, and in the Konrad meta-analysis they also gained more range from stretching. Inference: if flexibility does limit aero position, it may limit men more often than women, but no study has tested this on the bike.

In short, the evidence that would answer the title's question, a trial of mobility training in triathletes measuring position and how long they hold it, doesn't exist. Everything here is assembled from adjacent findings.

What to do with this

The following is TriForward's inference, not a tested protocol.

1. Start with the fit. A professional fit can change hip angle, saddle position and pad height immediately. Moving the hip forward has some evidence behind it; stretching for position has none.
2. Test whether range is actually your limit. A straight-leg raise and a seated hip flexion test are simple checks. If you already have good range, more stretching is unlikely to help.
3. If range is limiting, stretch specifically. Hamstrings and hip flexors, with static or contract-relax stretches, most days, for at least several weeks.
4. Build time in the bars gradually. Add aero time to long rides in blocks, so the position becomes familiar before race day.
5. Train the trunk and hips. Hip extensor and trunk endurance may help you hold a position longer. TriForward's "Does a Strong Core Make You Faster?" covers how much to expect.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 46-year-old man has a new time-trial bike for his first Ironman. He can hold the position for 40 minutes, then his lower back aches and he sits up. A friend tells him to stretch his hamstrings.

Based on the evidence above, he might:

• Book a fit first and check saddle setback, pad drop and saddle choice, since back ache and sitting up are often fit problems.
• Test his hamstring and hip flexion range. If they're limited, add targeted stretching for six to eight weeks.
• Extend aero time progressively on long rides, rather than trying to hold it for the full ride straight away.
• Accept a slightly higher position if it is one he can hold for 180 km. A position he can't hold isn't faster.

Common mistakes

• Stretching to fix a fit problem. If the saddle is wrong, more hamstring length won't make it right.
• Copying a pro's position. Professionals ride the position for years. Inference: the time-trial riders in the Jongerius study coped better after at least two years of position-specific training.
• Going lower than you can hold. Aero gains vanish when you sit up.
• Changing everything before race day. Position changes need weeks of riding to test.
• Ignoring numbness or pain. These are reasons to see a fitter or clinician, not to stretch harder.

What we don't know

• Whether mobility training changes aero position or how long you can hold it. No trial has tested it.
• Whether results from male road cyclists apply to triathletes, women, or time-trial positions.
• What limits position over 180 km. No study has tested a position for longer than 40 km.
• How much adaptation comes from flexibility and how much from riding in position.

References

• Jongerius N, Wainwright B, Walker J, Bissas A. The biomechanics of maintaining effective force application across cycling positions. Journal of Biomechanics, 2022;138:111103. doi:10.1016/j.jbiomech.2022.111103
• Holliday W, Swart J. Anthropometrics, flexibility and training history as determinants for bicycle configuration. Sports Medicine and Health Science, 2021;3(2):93–100. doi:10.1016/j.smhs.2021.02.007
• Holliday W, Swart J. Performance variables associated with bicycle configuration and flexibility. Journal of Science and Medicine in Sport, 2021;24(3):312–317. doi:10.1016/j.jsams.2020.09.015
• Muyor JM, Alacid F, López-Miñarro PA. Influence of hamstring muscles extensibility on spinal curvatures and pelvic tilt in highly trained cyclists. Journal of Human Kinetics, 2011;29:15–23. doi:10.2478/v10078-011-0035-8
• Husband SP, Wainwright B, Wilson F, Crump D, Mockler D, Carragher P, et al. Cycling position optimisation: a systematic review of the impact of positional changes on biomechanical and physiological factors in cycling. Journal of Sports Sciences, 2024;42(15):1477–1490. doi:10.1080/02640414.2024.2394752
• Konrad A, Alizadeh S, Daneshjoo A, Hadjizadeh Anvar S, Graham A, Zahiri A, et al. Chronic effects of stretching on range of motion with consideration of potential moderating variables: a systematic review with meta-analysis. Journal of Sport and Health Science, 2024;13(2):186–194. doi:10.1016/j.jshs.2023.06.002