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*** '''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"/>.
* '''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" />
* '''Weak Quads.''' 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|center|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. However, this belief is not well supported by the evidence<ref name="patellaralignment"/>.
** '''Weak Quads[[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. [[File:Vastus medialis muscle short.jpg|center|thumb|200px|The Vastus Medialis Obliquus or [[VMO]], which is often blamed for mistracking of the kneecap.]] 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"/>. A study suggested ** '''[[VMO]] Delay.''' There is some evidence that people suffering from knee pain (PFPS) has have a delay in the activation of the [[VMO ]] compared with the main quad muscles<ref name="KneeVMODelay"/> and another study noted that people with knee pain had a weaker (atrophied) VMO<ref name="KneeVMOAtrophyKneeVMODelayMeta"/>, which reduced the lateral force by 25%<ref name="KneeVMODelayPower"/>. (Remember Detecting this timing difference is normally requires sophisticated clinical equipment, but there is some suggestion that the weakness could delay can be caused felt directly by placing fingers on the knee pain, with people avoiding exercise belly of the [[VMO]] and stress rather than the weakness causing the knee painVL<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. <br style="clear: both" />
** '''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 was only seen in 6% of knee pain (PFPS) cases<ref name="RunningInjuries"/> and a high Q angle is not associated with biomechanical knee stress<ref name="KneeQAngle"/>. [[File:Q Angle.png|right|thumb|200px|If you draw an imaginary line from the [http://en.wikipedia.org/wiki/Anterior_superior_iliac_spine front of the pelvis] to the kneecap and from the lower leg to the kneecap, the angle between the lines is the Q Angle.]]
* '''NSAIDs'''. Cartilage destruction is a major cause of concern with NSAIDs<ref name="KneeNSAID"/>, but I found no long term studies that link NSAID use with knee problems, so it's not clear if this is a widespread cause of knee pain.
* '''Chondromalacia'''. The term Chondromalacia means soft cartilage and was once thought to be a common cause of knee pain. However, studies have shown that people with advanced Chondromalacia can be pain free<ref name="PPConcepts"/>.
* '''Leg length discrepancy'''. A difference in leg length can be a factor in knee pain<ref name="PFClinical"/>.
=Treatments=
* '''Reduce Knee Stress'''. Stress on the knee should be reduced to prevent further damage<ref name="PFClinical"/>. Personally, I do not believe in complete rest, but prefer reduced exercise, trying to avoid or minimize pain as I believe the exercise promotes healing.
* '''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]]. Electrical muscle stimulation by contrast can directly target The only known way to selectively strengthen the [[VMO while putting relatively little stress on ]] is by the kneecapuse of [[Electrical Muscle Stimulation]]<ref name="PFClinical"/>. See [[Electrical Muscle Stimulation]] for more details.* '''Footwear'''. There are two types of footwear changes that may help with knee pain that are contradictory; minimalist footwear and orthotics. Personally, I am concerned that Orthotics may help with knee pain but cause other problems, and the use of orthotics goes against the evidence for minimalist footwear. However, there is more scientific evidence to support orthotics for resolving knee pain, even though orthotics may increase knee stress. My personal belief is that it is better to cautiously move towards [[Minimalist Running Footwear]] rather than use orthotics, but I want to be clear that there is not clear scientific support for my belief. ** '''Minimalist shoes'''. There is good evidence that running shoes increase the stress on the knee<ref name="ref14"/>, and that a more minimalist approach to footwear may be appropriate. There is plenty of anecdotal evidence<ref name="KneeMinCaseStudies"/> that minimalist footwear helps with knee pain, but only limited science<ref name="KneeMinShoes"/> to back it up. Also, changing too quickly to minimalist footwear may result in [[Too Much Too Soon]] injuries. See '[[Are your running shoes injuring you]]?' for more details. ** '''Orthotics'''. Studies have shown that orthotics reduce knee pain, with a greater benefit shown in those that have greater pronation<ref name="KneeOrthWedge"/><ref name="KneeOrthIm"/><ref name="KneeOrthWAS"/>. However, there are concerns that orthotics may also increase the stress on the knee<ref name="KneeFootware"/>. Personally, I am concerned that Orthotics may help with knee pain but cause other problems, and the use of orthotics goes against the evidence for minimalist footwear
* '''Running form'''. Good running form has many benefits, but changing form tends to be difficult and can easily result in new injuries if done [[Too Much Too Soon|too quickly]]. I would recommend looking at [http://www.chirunning.com/ Chi Running] or the [http://posetech.com/ Pose Method], though personally I don't agree with their approach of pure forefoot running (your heal not touching the ground) and prefer a midfoot strike.
* '''Check for ITBS'''. ITBS produces pain to the outside of the knee, rather than under the kneecap. It's possible to have both kneecap pain and ITBS, so read [[Iliotibial band syndrome]].
* '''[[VMO]] Re-timing.''' If the firing of your [[VMO]] is delayed, it may be possible to correct this. Ideally, the correction uses a combination of training and down in EMG biofeedback device. These devices can be quite expensive, but it may be possible to detect the difference in timing between the [[VMO]] and the VL using your fingertips<ref name="PFClinical"/>. The goal should be retraining control rather than strengthening, with 200 repetitions per day (20x 10 reps) being required<ref name="PFClinical"/>. (I did find instructions for a [http://www.instructables.com/id/EMG-Biofeedback/ DIY EMG].)In addition, it may be possible to mitigate the [[VMO]] timing problem with taping<ref name="KneeLongTerm"/><ref name="KneeTapeMcConnel"/> (see below).
* Knee brace?
* Supplements? * '''Taping'''. The use of tape has been shown to be effective at reducing knee pain<ref name="KneeLongTerm"/><ref name="KneeTapeBrock"/><ref name="KneeTapeMcConnel"/>, by over 90% in one study<ref name="KneeTapeEMG"/>. One study used tape to pull the knee cap towards the midline of the body<ref name="KneeTapeMedial"/>, others relied on evaluation to define the correct direction<ref name="KneeTapeBrock"/>. The most common method mentioned is 'McConnell taping'<ref name="KneeTapeMcConnel"/><ref name="KneeTapeBrock"/>, and instructions can be found by Googling 'McConnell taping'.
=Anti-treatments=
The following 'treatments' are not recommended.
* '''Quad strengthening'''. A common recommendation is to strengthen the quad muscles to improve the tracking of the kneecap, especially the [[VMO]]. However, resistance training of the quads puts extra strain on the kneecap and can cause a worsening of the symptoms<ref name="PPConcepts"/>.
* '''Surgery to correct mistracking'''. A study of knee pain indicated that this type of surgery has the second highest failure rate<ref name="PPConcepts"/>.
* '''NSAIDs'''. Using NSAIDs does not generally improve healing, can mask symptoms and is considered a cause of cartilage damage<ref name="KneeNSAID"/>. An animal study showed that Asprin resulted in greater cartilage degeneration<ref name="KneeAsprin"/>. More at [[NSAIDs and Running]].
<ref name="KneeVMODelay"> http://rsi.aip.org/resource/1/rsinak/v82/i10/p105101_s1?isAuthorized=no </ref>
<ref name="KneeVMOAtrophy">Vastus Medialis Obliquus Atrophy: Does It Exist in Patellofemoral Pain Syndrome? http://ajs.sagepub.com/content/early/2011/04/12/0363546511401183 </ref>
<ref name="PFClinical"> Clinical classification of patellofemoral pain syndrome- guidelines for non-operative treatment, Erik Witvrouw, S. Werner, C. Mikkelsen, D. Van Tiggelen, L. Vanden Berghe, G. Cerulli </ref>
<ref name="KneeLongTerm">The management of chondromalacia patellae: a long term solution
http://svc019.wic048p.server-web.com/ajp/vol_32/4/AustJPhysiotherv32i4McConnell.pdf</ref>
<ref name="KneeMinShoes">Joint loading decreased by inexpensive and minimalist footwear in elderly women with knee osteoarthritis during stair descent - Sacco - 2012 - Arthritis Care & Research - Wiley Online Library http://onlinelibrary.wiley.com/doi/10.1002/acr.20690/abstract </ref>
<ref name="KneeMinCaseStudies">Minimalism Case Studies | Running Times Magazine http://runningtimes.com/Article.aspx?ArticleID=22192 </ref>
<ref name="KneeVMODelayMeta">The relative timing of [[VMO]] and VL in the aetiology of anterior knee pain: a systematic review and meta-analysis http://www.biomedcentral.com/1471-2474/9/64 </ref>
<ref name="KneeVMODelayPower">The quadriceps function in patellofe... [Arch Orthop Trauma Surg. 1987] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/3606363 </ref>
<ref name="KneeTapeMedial">Taping the patella medially: a new treatment for osteoarthritis of the knee joint? | BMJ http://www.bmj.com/content/308/6931/753.full </ref>
<ref name="KneeTapeEMG">Effect of taping the pat... [Knee Surg Sports Traumatol Arthrosc. 1993] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/8536023 </ref>
<ref name="KneeTapeBrock">Effects of patella taping on patella po... [Med Sci Sports Exerc. 1993] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/8231783 </ref>
<ref name="KneeTapeMcConnel">The effect of patellar taping on the onset of vast... [Phys Ther. 1998] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/9442193 </ref>
</references>