A Question

Symmetry is beauty and beauty improves the likelihood that your genes will be recombined and transmitted to the next generation. That is the conclusion that I came to after reading Nancy Etcoff’s Survival of the Prettiest (a book I highly recommend). While facial right/left symmetry is the strongest predictor of attractiveness, asymmetries in motor control allow for more time efficient specialization. Throwing or writing with my left hand reminds me of the time and effort that went into learning those skills on my right side. Just as handedness is often determined by the hand people choose to write with, footedness is often determined by the foot people choose to kick with.

Asymmetry seems fine, perhaps even desirable when the task is asymmetrical, but what about when the task is symmetrical? Running and cycling are two symmetrical activities often continued for hours on end. With each leg taking 5,400 strides and 6,000 pedal strokes per hour it seems like any little asymmetry in limb size, strength or usage could cause problems. Could limb asymmetry in a symmetrical task reduce performance or cause injury?

Some Data

I found the following article to answer this question: “On the bilateral asymmetry during running and cycling: A review considering leg preference. This article was published in Physical Therapy in Sport in June 2010 and authored by Felipe Carpas, Carlos Mota and IrvinFaria. Fifty-seven articles were included in the review and spanned the topics of joint angles, ground reaction forces and timing parameters during running. Analogous measures were used in the cycling (pedaling) studies. Other measures of symmetry included in the review were limb length, limb circumference and strength.

Here are their conclusions on running asymmetry: 1) The quality and quantity of research on asymmetry during running is limited and of poor quality.  2) Both injured and non-injured runners have similar levels of asymmetry.  3) There is some evidence showing that elite runners have slightly less asymmetry than amateur runners. 4) Asymmetry decreases as speed increases. 5) Leg muscle strength asymetries did not correlate with movement asymmetry during running.

Here are their conclusions on pedaling asymmetry: 1) The quality and quantity of research on asymmetry during pedalingis limited and of poor quality.2) There is insufficient evidence to make a comparison of pedaling asymmetry between injured and non-injured or amateur and elite cyclists .3)Asymmetry decreases as workload increases.

My Application

Frankly this review didn’t  provide much guidance as the evidence is too immature.I was expecting to find moderate evidence supporting a link between asymmetry and injury, but instead I found there wasn’t enough evidence to say one way or the other. This leaves the field open to my opinions (for better or worse).

I think asymmetry during symmetrical task can certainly cause injury if the asymmetry is great enough and the task is repeated long and often enough. The difficulty of course is in determining the threshold, and with no good evidence to go on this will be an educated guess. When I find an asymmetry that I think is related to an injury someone has I given them exercises to try to reduce it.

But I am wary of trying to reduce minor asymmetry in an athlete who doesn’t yet have any problems. Preventative medicine is only as good as the evidence supporting it and when it comes to running and cycling, it seems to me that I could cause an injury by fixing something that isn’t broken.

I found the relationship between an increase in speed or workload and a reduction of asymmetry to be interesting. The authors of the review think this is due to increased common bilateral neural input due to fatigue at higher workloads. In a practical sense, if I am measuring asymmetry by measuring angles on video or someone running I should be aware of the speed they are running can be a major influence. As for those of you wondering what this means to you, just go out for a run or a bike ride and don’t worry too much about the fact that your right leg is working a little harder. Oh yeah, and run and pedal faster! More on that later…