According to Dr. Ira Evans III, a successful Massachusetts orthopedic surgeon--and a mountain biker, most of that is due to training errors, poor biomechanics, overuse, poor bike fit or inherent anatomical variations. Let's get more into the causes behind knee pain and what you can do about it.
"The knee consists of bone, ligaments and cartilage which work in unison to provide a complex roll and glide motion resulting in a stable joint with a wide range of flexion and extension," explains Dr. Evans.
Technically, your knee is the joint that joins the femur (thigh bone) and the tibia (shin bone). The patella floats in front of the two bones and acts as a pulley to give leverage to, and increase the power of, the four quadriceps muscles which straighten the knee. Your quads come together at the patella and a single tendon extends from the patella to the tibia.
The three hamstring muscles in the back of the thigh handle just about all of the knee-bending that occurs at the joint, although sometimes other muscles do help. Strong ligaments on both sides of the knee (the LCL and MCL) keep it from flexing out to either side, and a pair of ligaments that criss-cross inside the joint (the ACL and PCL) also help with stability. Two large pads of cartilage (medial and lateral meniscus) sit atop the tibia and act as shock absorbers, and the whole joint is wrapped in a membrane called a synovial joint capsule.
"It is the complexity of this anatomy, and its inner workings, that put the knee at risk from even very simple disturbances," says Evans.
Figuring Out What's WrongInjuries that result from crashes or other obvious problems are often easy to diagnose and recover from. But what about that random nagging pain that seems to only show up after an hour of riding? Or that grinding that you feel the day after a race? These problems might be harder to pinpoint.
"When assessing a rider with a knee injury," says Evan, "it is important to perform a thorough knee exam, but it is even more important to look for subtle predisposing factors." He adds that your limb alignment, limb rotation in both the femur and tibia, foot rotation and leg lengths should also be evaluated.
"Cyclists with excessive rotation in the tibia (external tibial torsion), varus alignment (bowlegs), flat feet (hyperpronation), and/or significant leg length discrepancies may be predisposed to problems," explains Evans, "and these problems should be addressed at the same time that the knee injury is being managed."
Other problems can arise from differences in strength and/or flexibility in the muscles that control the knee. If an area is tight, it can pull the joint out of alignment, causing excessive wear. Women, in particular, can be afflicted with problems that affect the way that their patella tracks in its groove.