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* '''Highly cushioned shoes'''. Counterintuitively, the more cushioned your shoes are, the greater the loading force on your knees<ref name="ref9"/><ref name="ref10"/>.
* '''Obesity'''. Obviously being overweight puts more stress on the knees and is linked to knee pain<ref name="KneeObesity"/>.
* '''Bad running form[[Running Form]]'''. Problems with running form [[Running Form]] can increase the landing forces and cause injury.** '''Overstriding. '''. [[Overstriding ]] is''' '''landing with the foot ahead of the hip, can increase the landing forces and cause injury<ref name="KneeOverstride"/>. (See [http://runningtimes.com/Print.aspx?articleID=23217 Are You Overstriding?] and [http://runningtimes.com/Article.aspx?ArticleID=20271 An Analysis of Running Technique] for details on overstriding.)** '''Heel strike. '''Landing with the weight on the heel of the foot creates a far greater peak force than landing so the weight is taken by the midfoot or forefoot <ref name="KneeForefoot"/>. See [[Foot Strike]] for more details.<br style="clear: both" />
==Inactivity==
The human body needs activity to remain healthy, and the knee is a prime example of this. Animal studies have shown than complete immobilization of the knee can result in a 50% reduction in the cartilage thickness within weeks<ref name="KneeInactivity"/>. Thankfully this damage appears to be reversible.<br style="clear: both" />
It is common for people with knee pain to have weak quads<ref name="PFClinical"/>. However, I have found no research to suggest that weak quads are a cause of knee pain, a rather than a result of knee pain. I believe that it is more reasonable to conclude that knee pain results in less exercise as the patient avoids activities that cause pain, and thus the quads become weaker through inactivity.
==Maltracking/malalignment==
[[File:Kneecap.png|right|thumb|200px|A cross section of the kneecap and femor (thigh bone) showing how the kneecap fits in a groove.]] The kneecap moves in a groove in the thigh bone (femur), and it is widely believed<ref name="KneeRW"/> that if the kneecap does not track in this grove it will rub on the sides and cause knee pain. While this belief is not well supported by the evidence<ref name="patellaralignment"/>, the following underlying problems may still cause knee pain through other mechanisms.
* '''Weak [[VMO]].''' The alignment of the kneecap is not related to the overall strength of the quads, but rather an imbalance of the muscles that make up the quads. The quads consist of four muscles, and a relative weakness in a subdivision of one muscle, the Vastus Medialis Obliquus or [[VMO]], has been linked to kneecap alignment<ref name="KneeLinTomo"/> and knee pain<ref name="KneeVMOAtrophy"/>.
* '''[[VMO]] Delay.''' There is some evidence that people suffering from knee pain (PFPS) have a delay in the activation of the [[VMO]] compared with the main quad muscles<ref name="KneeVMODelay"/><ref name="KneeVMODelayMeta"/>, which reduced the lateral force by 25%<ref name="KneeVMODelayPower"/>. Detecting this timing difference is normally requires sophisticated clinical equipment, but there is some suggestion that the delay can be felt directly by placing fingers on the belly of the [[VMO]] and the VL<ref name="PFClinical"/>.
* '''Weak Hips.''' A weakness in the hip muscles has been linked to knee pain<ref name="KneeHip"/>. Weak hip muscles result in the leg rotating so the foot points towards the midline of the body (internal rotation), so when the leg bends extra stress is placed on the knee.
* '''[[Q Angle]]'''. The thigh bone and lower leg are not in a straight line, but form an angle at the knee called the '[[Q Angle]]'. A large [[Q Angle]] is often thought to cause or contribute to knee pain, but a high [[Q angle Angle]] was only seen in 6% of knee pain (PFPS) cases<ref name="RunningInjuries"/> and a high [[Q Angle]] angle is not associated with biomechanical knee stress<ref name="KneeQAngle"/>. * '''Over Pronation'''. Pronation is the natural movement of the foot where the arch flattens to absorb landing forces. If the foot pronates too much, the foot will lean towards the midline of the body and the lower leg and knee will follow this motion, moving towards the midline. Like weak hips, the movement of the knee towards the midline creates extra stress on the knee.
==Arthritis==
Arthritis is an inflammation of the joints and can affect the knee. There are various types of arthritis, and diagnosis requires X-Ray, testing the fluid around the joint or inserting a viewing scope into the joint. Arthritis is outside of the scope of this article.
* '''Ice'''. The use of ice will not remove the underlying cause of knee pain, but it can help with recovery and healing. See [[Cryotherapy]] for more details.
* '''Cadence'''. If you're cadence is too low it can cause various problems, and should be optimized to around 180 steps per minute (90 steps/min for each leg). See [[Cadence]] for more details.
* '''[[Massage]] the [[VMO]]'''. While the evidence for weak quads and [[VMO]] is marginal, massaging the [[VMO]] is easy and reasonably risk free. While the [[Foam Roller]] is good for massaging most of the quads, it tends not to get to the [[VMO]], and if the [[Foam Roller]] is your only quad massage technique then it's possible that your [[VMO]] is suffering from neglect. I would recommend using [[The Stick]] as well as using your elbow on the [[VMO]]. I've also found that using an electronic muscle stimulator on the [[VMO]] can help. See main article on [[Massage]] for more details.
* '''[[Massage]] the glutes.''' Weak glute muscles have been linked to knee pain<ref name="KneeHip"/><ref name="Glutes"/>, so massaging them may help them recover their strength and functionality.
* '''[[Electrical Muscle Stimulation]]'''. Strengthening the quad muscles using normal resistance training tends to put extra stress on the kneecap aggravating the injury. Resistance training the quad muscles tends to equally train all parts, rather than focusing on the [[VMO]]. The only known way to selectively strengthen the [[VMO]] is by the use of [[Electrical Muscle Stimulation]]<ref name="PFClinical"/>. See [[Electrical Muscle Stimulation]] for more details.