Knee Pain and Cycling
by Joshua Cohen, PT MS
Have you noticed how some cyclists pedal a little... differently?
Some riders allow their knees to swivel inwards during each pedal stroke, narrowly avoiding painful collisions with the top tube. Others splay their knees far apart like an adult riding on a toddler's tricycle. These kinds of excess motion rob the power that should be delivered to the pedals to drive the bicycle forward. Ideally, the knees should stay aligned directly over the pedals, transferring all the force of the leg into the cranks like two perfectly efficient pistons.
Of course, our bodies are not designed to work like perfectly machined pistons. Even with ideal positioning on our bikes, a small amount of side-to-side knee motion is acceptable. But excessive 'non-driving' knee movements (that is, those movements which do not directly drive the pedals) may significantly increase the risk of developing painful knee symptoms.
One of the most common of these conditions is chondromalacia patella. This affects the rear of the kneecap. In healthy knees, the rear of the kneecap is smooth in order to allow it to glide across the other bones which make up the knee joint. In chondromalacia, excessive contact pressure between the kneecap and the knee joint over time causes this smooth surface to break down. As the condition progresses there is often an increase in knee pain, especially in forceful extensions of the knee during activities like climbing stairs--or pushing big gears at low cadence.
Gearing choices can become an important variable in preventing the progression of chondromalacia patella. When the leg is forcefully extended, as during a cyclist's power stoke, the knee cap acts as a pulley to transfer the large forces of the quads across the knee joint to the tibia (one of the lower leg bones). As a result, a component of the tendon forces pulls the knee towards the knee joint, greatly increasing the pressure on the back of the kneecap. This pressure can wear down the joint surfaces. Normally, a thin layer of slippery fluid lubricates the joint and prevents wear. However, during slow, forceful extensions (think big chainring grinding uphill) this fluid becomes displaced and the joint loses much of its lubrication. This is not good for your knees or pistons!
Keeping a higher cadence will help to reduce the pressure of the knee cap against the knee joint, allowing the lubricating fluid to remain between the joint surfaces where it belongs, and so lessening the risk of joint surface breakdown. Cadences above 80rpm are generally better for your joints than knee-mashing 60 rpm cadences. Remember to choose your gear inches wisely and take into consideration the terrain and your ability to power your gear to maintain a relatively high cadence.
Since slow cadences alone can provoke injury, you might expect that the combination of excessive non-driving knee motions and slow cadence would greatly increase the risk of injury. You'd be right. A study performed in the Department of Mechanical Engineering at the University of California investigated the affects of common non-driving knee motions on the location and magnitude of contact pressures within the knee joint. The experimenters concluded that that common excessive knee motions during cycling can increase the contact area and force within the knee complex by as much as 29%! These results strongly suggest that 'non-driving' knee movements may accelerate the development of chondromalacia patella.
Non-driving knee motions occur for many different reasons, among them hip weaknesses, poor forefoot positioning, pronated or supinated feet, and poor coordination of the leg muscles during the spinning motion. The good news is that there are a number of ways to decrease excessive knee motion, either through simple modifications to the shoe/pedal interface or specific training to target weak muscle groups or poor muscular coordination. Such treatments are specific to different causes of excessive knee motion and ultimately to the requirements of the individual cyclist.
So, next time you're out on the road, take a look around at other riders, and maybe even glance down at your own legs of steel. If you make sure your pistons are running straight, you can maximize your performance and minimize your risk of knee pain.