Bilateral asymmetry: are you unbalanced and underperforming?

How symmetrical is your riding style, how does it affect performance and what can you do to improve your riding symmetry? Andrew Hamilton looks at what the science says

One of the most appealing aspects of cycling is that it is a sport that can be widely enjoyed appeal right across the age spectrum and into older age. Unlike many other sports such as running, the motions involved in cycling are smooth and impact free, which makes life much easier on ageing joints. Another plus is that by sitting on a bike with your feet fixed on the pedals, your patterns of muscle and joint movement are precisely dictated by the motion of the spinning pedals – ie controlled and symmetrical on each side of the body. This is another big bonus for those seeking to avoid injury, and contrasts with other sports (eg running, swimming etc where the movement of the limbs has to be coordinated by the brain and neuromuscular system) where asymmetries can easily develop over time, leading to muscular imbalances and overuse injuries.

How symmetrical is the pedaling motion?

The symmetry of muscle and joint involvement when pedaling might sound like a given. But just how symmetrical are most cyclists and are more experienced cyclists more symmetrical than their younger, less-experienced counterparts? Contrary to what you might expects, some research by French scientists on older and very experienced cyclists has suggested that a symmetrical riding position and movement does not necessarily equate to symmetrical biomechanics in older cyclists⁽¹⁾. This in turn could mean an increased risk of overuse injuries and premature fatigue during longer or harder rides.

In the study, eleven masters cyclists who performed regular training and long-distance cycling events were each analysed for their pedaling biomechanics during a incremental cycling test. In particular, the researchers wanted to see whether the cyclists typically pushed harder on one leg than the other – ie tended to apply more torque to either the left or right crank. The cyclists, whose average age was 53 years, were very experienced (riding between 4,500 and 14,000kms per year) and considered themselves to have good technique. Also, none of them had sustained injuries prior to the study or were aware of any reason why they might have asymmetric pedaling biomechanics.

The 18-minute incremental test consisted of: ten minutes at 100 watts, three minutes at 150 watts, three minutes at 200 watts and two minutes at 250 watts. The test was conducted on a road bike equipped with an SRM crank system to very accurately measure applied force and power as the cranks rotated. The rear wheel of the bike was mounted in a Tacx ergometer, and the bike was adjusted so that each rider could ride in his optimum position. During the test, the researchers recorded the average force applied by the riders at each point of the crank revolution for both left and right sides.

Asymmetry is everywhere

When the data was number crunched, an index of asymmetry was produced for each rider, where a score of 0% asymmetry equated to perfect symmetrical biomechanics (ie the forces applied to the left and right cranks as they rotate are exactly the same at all points through the 360 degrees of circular motion). For a ‘well-balanced’ cyclist, you would expect an asymmetry score of less than 10%. However, except for one subject, the asymmetry scores were all significantly higher, regardless of the power level. Across the group as a whole, the average asymmetry score measured 30% at 100 watts while at 250 watts, it averaged 23%. In one rider, the highest value of asymmetry was nearly 60%!

Implications for cyclists

The degree of asymmetry between the left and right legs of the cyclists surprised the researchers, not only because it was apparent at all power outputs and affected all but one of the group, but also because of the size of the left/right discrepancy in some of the subjects. Putting aside the increased risk of injury, a large imbalance in force production between left and right legs is a recipe for premature fatigue; having one leg working harder to compensate for the other is always going to be a less efficient way of producing sustainable power than if both legs work equally hard.

These results on asymmetry in cyclists are consistent with other findings. For example, a study by Brazilian scientists looked at sub-elite competitive male cyclists who pedaled a SRM Training Systems cycle ergometer throughout a simulated 40-km time trial⁽²⁾. The time trial was analysed as four 10km stages to see how asymmetry changed through the event. It found that as the time trial progressed, pedaling asymmetry increased so that by the third 10km segment, it rose to over 17%. Interesting however, in the final stages, when the cyclists increased their efforts and pedaled flat out, asymmetry dropped.

Another very recent study has looked at movement asymmetries in professional racing cyclists. But rather than measuring generated forces, the researchers used ‘3D motion capture’ to observe different in joint movements, both in terms of angles and timing. It too found a significant degree of limb movement and timing asymmetry (up to 18%) – even in these super efficient cyclists. Although the feet were anchored to the pedals, analysis showed that the dominant leg (usually the right leg) was mostly ahead of time with respect to the left leg, a trend that was accentuated with an increase in power output.

Practical advice

You might assume that your pedaling action is fairly balanced and symmetrical. However, the research suggests that even highly-trained and experienced cyclists may display significant pedaling asymmetry. Given these finding, it could be a good investment of time to check your pedaling biomechanics.

One way to do this is to seek out the use of a ‘Wattbike’ stationary bike, which provides real time torque analysis of your pedal stroke for both left and right legs. If there is a significant asymmetry, the shape or size of the displayed ‘pedaling circle will be fairly obviously different, which will enable you to identify your dominant leg.

Once this has been established, you can undertake some unilateral leg strength training, where the left and right legs are trained separately. Good examples of these include:

• Bulgarian split squat

also provides some additional examples of good single leg exercises. In all these exercises, you should work your less dominant leg first. Make a mental note of how many reps were achieved and then follow up with the dominant leg, but ensuring you perform only the same number of reps (not more). Over time, your leg strength will even out, which will hopefully reduce your asymmetry!

References

1. J Sci Cycling 2012. Vol. 1(2), 42-46
2. J Sports Med Phys Fitness. 2007 Mar;47(1):51-7
3. J Sports Sci. 2018 Oct;36(19):2155-2163