The Biomechanics of Runner’s Knee

knee runningA patient came into the clinic the other day with a year’s history of bilateral knee pain from increasing her running training both in distance and frequency. As always, I ask what eases a condition and she replied with –

bum exercises

She proceeded to show me what she did. The patient had been shown them by a friend but had no idea why she was performing these exercises for her knee pain, she just knew that they worked.

I’ve had an increased number of runners visit the clinic over the last few years with the form of exercise becoming more popular and most patients give us the same perplexed looks when we talk about strengthening their hips to alleviate their knee pain. Hip dysfunction is just one biomechanical factor that leads to patellofemoral pain or, as it is more commonly called, “runner’s knee”. It has been shown that no single biomechanical factor has been identified as a primary cause of patellofemoral pain.



The symptoms of patellofemoral pain are generally a general discomfort of the medial side (inner front) of the knee, aggravated by activity (running, jumping, climbing or descending stairs are just a few examples) or by prolonged sitting with knees in a bent position. Commonly, pain can be felt on either side of the patella, superior or inferior to the patella (not to be confused with patella tendon problems), behind the patella or even behind the knee. Some patients may also have a vague sense of “tightness” or “fullness” in the knee area due to swelling and, occasionally, the associated loss of quadriceps muscle strength due to pain inhibition, may cause the leg to give way. The vastus medialis oblique weakens due to pain and swelling (pain inhibition) and may not be functioning optimally to support the knee. Remember everyone’s symptoms are different!

Patella (kneecap) biomechanics

The patella sits at the front of our knee to increase the functional lever arm of our quadriceps for efficiency. Did you know that functional deficits owing to patellectomy (removal of the patella) include decreased knee extension power, extension lag, instability, chronic swelling and decreased flexion(bending) range? (Clin Orthop Relat Res. 1979 Oct;(144):51-4. Patellar biomechanics. Kaufer H.)

The patella articulates with the patellofemoral groove in the femur (thigh bone). Several forces act on the patella to provide stability and keep it tracking properly. The patella not only moves moves in an up-and-down direction, it also tilts and rotates, so there are various points of contact between the undersurface of the patella and the femur. Repetitive contact at any of these areas is the likely mechanism of patellofemoral pain syndrome. The repetitive contact results in swelling and pain.


Causes of patellofemoral pain

In general, the literature suggests that the etiology of patellofemoral pain syndrome is multifactorial. The causes include biomechanical and muscular (intrinsic) and overuse/overload (training error or extrinsic) theories. As patellofemoral pain is a HUGE topic, in this post we will discuss the biomechanical and muscular causes only.

Muscle imbalance between the quadricep muscles

You may read that in order to centre the patella in the trochlear groove, the inner knee muscle, otherwise known as VMO (vastus medialis oblique) needs to be strengthened. Controversy remains regarding the extent to which the individual muscle groups making up the quadriceps can selectively be strengthened. Usually, the lateral forces of the vastus lateralis (outer knee muscle) need to be countered better by the vastus medialis. This goal is accomplished best by strengthening all of the quadriceps.

A study compared muscle activity and timing of gait phases during functional activities in 13 subjects with patellofemoral pain and in 11 subjects with healthy knees. The vastus medialis oblique and vastus lateralis had similar patterns during all activities. Subjects with patellofemoral pain had significantly increased activity in the vastus medialis oblique and vastus lateralis compared with the healthy subjects during the most demanding phases of the gait cycle, suggesting a generalised quadriceps weakness in the patients with patellofemoral pain. These data reflect a generalised quadriceps muscle weakness, rather than the prevailing theory of quadriceps muscle imbalance as an etiology of patellofemoral pain. Therefore, the practice of strengthening the entire quadriceps muscle group, rather than attempting to specifically target the vastus medialis oblique, was supported.(Clin Orthop Relat Res. 2003 Oct;(415):261-71. Electromyography of the quadriceps in patellofemoral pain with patellar subluxation. Mohr KJ1, Kvitne RS, Pink MM, Fideler B, Perry J.).

But before you hit the gym to load up on those lunges and squats, read on…

Flat feet

“Flat footed-ness”, or pes planus, has been shown to contribute to patellofemoral (and tibiofemoral, or knee joint) pathology. During most weight bearing activities, the posture and motion of the foot and knee are coupled within a closed kinematic chain. Closed chain coupling may link excessively flat feet to excessive internal rotation of the lower limb. The consequences of this rotation are unknown, but it may have effects on mechanical stress across the knee, possibly resulting in increased rotational stress on the load bearing tissues of the tibiofemoral compartments and increased contact between the articulating surfaces of the lateral patella and femur bone.


We can see from the above picture that the arch drop in the foot results in a malalignment of the patellofemoral joint and hence can contribute to patellofemoral pain.

This means that proper footwear is very important for individuals with patellofemoral pain if pes planus is assessed as a contributing factor to the problem. Our physiotherapists can evaluate the patient’s biomechanics and recommend proper shoes and orthotics, which in turn can lessen the pain. Orthotics are often of benefit in returning the subtalar joint to a nearly neutral position; this reduces foot pronation, thereby decreasing the above mentioned rotational forces in the tibia that affect tracking of the patella during walking.


A study compared the effectiveness of off-the-shelf orthotics in the treatment of patellofemoral pain
with that of either flat inserts or physiotherapy; the report also investigated whether the combined use of orthotics and physiotherapy is more effective than physiotherapy alone. The prospective, single-blind, randomised trial utilised 179 patients between ages 18 and 40 years. By six weeks, patients using orthotics had experienced greater improvement than had people using flat inserts, but the orthotic group had experienced no significant difference in improvement over patients treated with physiotherapy or with a combination of orthotics and physiotherapy. This study proves that patellofemoral pain involves a multimodal approach.

Is that it!? Is it as easy as getting my feet right and quadriceps strong? We’re not done yet, read on…

Weak hip muscles

Studies have shown that weak muscles around the proximal (top) end of the femur may alter the forces acting on the knee and lead to compensatory stress at the distal aspect of the femur and the patellofemoral joint. Results of the studies have also shown that a relationship exists between knee pain and weak gluteal musculature and interventions focused on restoring gluteal weakness may be warranted. The gluteus maximus is one of the muscles thought to play a role in controlling alignment of the lower limb and is identified as an important muscle to consider when treating patellofemoral pain. The gluteus maximus posteriorly rotates the pelvis and controls limb activities during rotational
movements so the gluteus maximus could, therefore, alter the rotational forces on the femur during activities and possibly affect the patellofemoral joint. Another study (Ireland M L, Willson JD, Ballantyne BT, et al. Hip strength in females with and without patellofemoral pain. J Ortho Sports Phys Ther. 2003;33:671–676) found that subjects with patellofemoral pain demonstrated 26% less hip abduction strength and 36% less hip external rotation strength than similar age-matched controls. Dysfunction of the hip external rotators results in compensatory foot pronation, which we know adds to patellofemoral stress.

On a side note, as the VMO originates on the adductor magnus tendon, this is the anatomic basis for including adductor strengthening as a possibility as well.


So this is why my patient performs her “bum exercises”!

But before you spend your day working on your bum clenches, remember the mutimodal approach! Read on…

Q angle


Some investigators believe that a “larger” Q angle is a predisposing factor for patellofemoral pain, others question this claim. One study found similar Q angles in symptomatic and non-symptomatic patients (Caylor D, Fites R, Worrell TW. The relationship between quadriceps angle and anterior knee pain syndrome. J Orthop Sports Phys Ther. 1993;17:11–6). In this study, we shall focus on the aspects on which we can evoke change.

Tight hamstrings, iliotibial bands and calves

A tight iliotibial band places excessive lateral force on the patella and can also externally rotate the tibia, upsetting the balance of the patellofemoral joint. This problem can lead to excessive lateral tracking of the patella.

The hamstring muscles bend the knee. Tight hamstrings place more posterior force on the knee, causing pressure between the patella and femur to increase. Tight calves can lead to compensatory foot pronation and, like tight hamstrings, can increase the posterior force on the knee.


A good friend of mine rang me from San Francisco the other week, complaining that her knee pain was worsening as she was preparing for her first half ironman. She was prescribed squats and lunges and was performing these religiously. Her quadriceps weren’t the problem, she was super strong in these muscles. Her feet were as flat as pancakes and her gluteals needed work. The approach to treatment was altered and she completed the half ironman successfully over the weekend.

In order to optimise treatment, it is important to pursue a full biomechanical assessment with your
physiotherapist to address these intrinsic issues. Modification to your activity will also be recommended to address the extrinsic contributing factors to your symptoms.


Lauren May

Lauren is the Principal Physiotherapist and co-owner of South Perth Physiotherapy. As a keen runner herself, Lauren enjoys treating and preventing knee injuries in people of all ages and with all levels of fitness. Whether it’s a half-ironman, a local fun run or the set of stairs to your apartment, let Lauren and the physiotherapists at South Perth Physiotherapy help you conquer your challenges, whatever they may be.



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