Runners certainly suffer their fair share of injuries. Diagnosing and treating running injuries is one thing, but most runners would like to know why they’re injured. One question that often surfaces goes something like this: “Why did this just happen to one leg and not the other? After all, I take the same number of steps with my right as I do with my left.” A simple question without a simple answer.
It is logical reasoning though. There must be some side to side difference in order for one side to be injured and the other side spared. These side to side differences could broadly be split into a few categories:
Left to right asymmetry in:
- Mobility and/or joint angles during running (aka kinematics)
- Forces generated while running (aka kinetics)
1) Left to right asymmetry in anatomy:
This category would include one leg being shorter than the other, a flatter foot on one side, a larger Q angle, etc. We know that minor variations in anatomy have very little to do with running injuries. For example, the leg length discrepancy argument has been investigated over and over again and has nothing to to with running injuries, unless the difference is greater than 2cm which is extremely large and also quite rare to happen naturally (acquired leg length differences due to a hip replacement or leg fracture is a different story).
2) Left to right asymmetry in mobility and/or joint angles during running
Measurement of mobility, limb positioning and joint angles is referred to as “kinematics”. This may include knee flexion angle, hip internal rotation angle, etc. Side to side asymmetry in kinematics has been shown to not be associated with running injuries (1,2).
3) Left to right variability in forces generated while running
Measurement of forces generated in movement is called “kinetics”. This may include loading rate, impact peak, joint moments etc. Side to side asymmetry in kinetics has been shown to not be associated with running injuries (1,2,3,4,5). In fact, LESS asymmetry is usually found in those who are already injured or those who eventually get injured
My thoughts: We must keep in mind that injuries occur because the load you are applying to the body is exceeding the ability of the body to adapt to those loads. In other words, we don’t give enough time for the tissues to adapt to the load we apply to them. These can be broadly categorized as training errors. Usually, injury has occurred because something has changed in your training; whether it’s adding mileage too quickly, adding speed work too quickly, too many miles in a new pair of shoes, a change in terrain, diet, stress, hormones, etc. Basically, you’ve changed something too quickly.
Along with training errors, we know there are issues with running technique that are associated with certain running injuries such as cadence, loading rate and rearfoot and forefoot striking. While these can be associated with running injuries, there isn’t any greater left to right asymmetry in those runners than in non-injured runners. In fact, the asymmetry is usually LESS in the injured runners.
So why then does the injury happen only on one side? The answer to that is beyond our capability for the time being. Obviously there has to be some movement or anatomical variability in order for one side to become painful. Unfortunately, there are so many confounding factors, it is virtually impossible to reveal an easy answer, such as “one leg is too short.” There is likely a constellation of factors that contribute to injury. These left to right asymmetries may theoretically be detectable, but for now, the research tools that are available for measuring these have been unable to provide any answers