Legs with functional length difference

The long and the short of leg length difference in cycling

Leg length difference is something that comes up quite a bit in my line of work.

As an Exercise Physiologist, I’ve had many patients and clients who have been diagnosed by a health professional as having a leg length difference. That is, one leg is shorter than the other and the difference is causing or contributing to dysfunction and pain.

As a bike fitter I have some clients note issues like “one leg feels weaker than the other”, “one leg feels stiffer”, “I can’t reach the pedals as easily on that side” and “I’m experiencing saddle sores more predominantly on one side”.

In this article, I am going to outline the difficulties in diagnosing a leg length difference. I’ll look at how a difference might lead to issues when sitting on a bike. Finally, if leg length difference is actually causing problems, what interventions are available?

Leg length difference

There are 2 main types of leg length difference – structural and functional.

Structural leg length Discrepancy
This is where the bones are longer on one side than the other. Some studies have found that up to 90% of the population have a measurable difference in leg length. Twenty per cent have a difference larger than 9mm!

Functional Leg Length Discrepancy
This is when, rather than an actual difference in bone length, there is some other cause of apparent leg length difference. There can be various contributing factors including one flat foot, medial knee collapse (valgus) or a torsioned pelvis. For example, consistently carrying a heavy bag on one shoulder may result in the pelvis hitching up on one side and giving the appearance of a shorter leg. Similarly, always weight bearing on one leg when standing drinking at the bar could result in a functional leg length difference.

How is it assessed?

There are 3 main ways to clinically measure a limb length difference.

  1. CT Scan. This measures the length of the thigh and shin bones and utilises a single image from the pelvis to feet.  It is highly accurate and much lower in radiation than previous x ray imaging methods (approximately 1/7 of the radiation of a plain x-ray).
  1. Tape Measure: This method involves measuring the length of the femurs and tibias on both sides of the body. Whilst easy to administer and cost-effective, the accuracy and reliability of these measures can be quite low. It can be impacted by rotation of the hips or soft tissue issues.
  1. Heel Raises: By adding a heel raise to the shorter leg when standing, a therapist can monitor if the hips level out. This usually means that there is a leg length difference, however does not distinguish between structural and functional differences.

Hanada et al (2001) showed  tape measurements could be easily reproduced with a high degree of consistency, but held only moderate validity compared to radiological methods (bone scans, X-rays). They concluded that tape measurements were only relevant when measuring large assymetries in right/left pelvis.

Friburg et al (1988)  found the error of tape measures was plus/minus 8.4 mm compared to radiologic measurements.

Interventions and treatment

To compound the measurement issues, Gurney observed clinicians differing in agreement as to what constitutes a significant difference that requires treatment. Some have suggested a 5mm difference warrants intervention whilst others are not concerned until the discrepancy reaches 20-30mm!

In addition, the individual’s height needs to be taken into account. A 5 mm leg length difference for someone who is 190 cm might be less significant than for someone 150cm tall.


A common method used by bike fitters and musculosketeal specialists alike is to insert an insole into the shoe of the shorter leg. This effectively equalises the length difference. The insoles can either be an off-the-shelf model or custom made to the specific requirements of the client.

Several studies have demonstrated that insoles can be beneficial, however the greatest outcomes were shown for differences of 10 mm or less. Given that most clinical assessment have an error rate of 5-10 mm, many researchers have cautioned against providing intervention for differences below 10mm.

The other drawback of insoles and similar structures inside a cyclist’s shoe is that they can take up valuable real estate and squash the soft tissues of the foot. This in turn can lead to pressure pain and numbness. This might be an option if you were planning on purchasing new shoes in the near future and can size up.

The lifespan of insoles is at most 3 years. You might consider trialing generic inserts (often sold in chemist stores) to establish if there is a tangible difference before forking out for more expensive custom ones. At Wining Position, I have various methods to establish whilst riding whether these sort of structures will make a difference before considering more expensive options.

Cleat shims

There are a number of global companies that manufacture plastic wedges and shims that can be added between the cleats and soles of a cycling shoe. For a smaller stack, I have often used 2 varus cleat wedges in opposing directions to create an equal shim height. For larger leg differences, there are 1-5 mm shim widths that can be multiplied. I am cautious to not add too much stack height as the foot stability to pedal can be adversely affected.


A pelvic assymetry when seated on the bike may be due to the rider requiring a new saddle. This could be due to a twisted and torsioned pelvis creating a sagging and wear to one side of the saddle. It is important that the ischipubic rami (the bony ridges extending from the sit bones) are well supported to create optimal pelvic stability.

The saddle height on the bike should aim to accomodate the length of the shorter leg.

Soft tissue therapy

Shoe inserts, insoles, cleat wedges and shims will have only a small effect if the driver of the leg length difference is the posture and orientation of the pelvis.

Manual or soft tissue therapy techniques can be employed to help reduce the overloaded tissues around the pelvis, lower back and legs. The focus of these techniques will depend on the origin of increased tone and tightness that is contributing to the shorter leg. For example a pelvis that is rotated backwards will require treatment of the gluteal and hip flexors. However, if there is restriction around a rider’s ankle joint, a focus on the lower leg and ankle muscles is likely required.

The success and sustainability of these methods will of course depend on the skill and expertise of the clinician and an individual’s ability to “hold” these changes. Home based self-massage treatment and exercises can assist in achieving long term results.

Adjusting daily habits

Often an individual can be contributing to the imbalances and asymmetries of their lower limbs by the behaviours and habits they adopt on a daily basis. Becoming aware of these movement patterns is the first step to addressing them. Do you frequently cross one leg over the other when seated at your work desk? Do you prop your toddler on the same hip each day and shift your weight to the other leg? Do you observe a lateral weight shift when squatting weights at the gym?

These habits could be reinforcing your asymmetries and bike riding is just the means through which we observe the them.  (The bike is great at showing asymmetries as you are fixed in place by the contact points. This is particularly true of a bike in a trainer or a fit bike due to its rigidly vertical orientation.)

It is important to train a variety of movements directions, and loading scenarios. Variety helps break up monotony and prepare the body to encounter various stimuli to cause adaptations. As a movement specialist, I can provide my clients with individualised home exercise programs that create these varying planes of movement.

One final point worth noting is that it’s often really difficult to connect the dots between a potential biomechanical feature and pain or other symptoms. The cause of pain and altered movement patterns is multifactorial, and often biomechanics are simply a small part of that. The body is great at compensating to find the least energy expensive and painful way of doing things.

This is not to say  that there isn’t a connection with biomechanics, but that only chasing a treatment of reducing the potential discrepancy likely won’t fix all of the problems that may be contributing to a cyclist’s pain and dysfunction.