Patellofemoral pain (PFP) is the most frequently diagnosed condition in patients with knee complaints. It is a common overuse injury which affects young physically active individuals. Studies revealed that one third of individuals with PFP suffer from persistent complaints, indicating that current treatments fail to prevent the chronicity of symptoms. Contributing muscular factors to PFP, such as weakness, delayed onset of the vasti muscles or flexibility deficits have attained widespread acceptance. However, the failing long-term outcomes reflect the need to provide an update on the evidence of underlying muscular dysfunctional factors in PFP. A review was conducted from research databases: Pubmed, Cochrane, CINAHL, SPORTDiscus, and Web of Science. In total 63 studies with a sample size of 1419 individuals with PFP and 1657 healthy controls, were included. The majority of studies analysed muscle strength by quantifying maximal voluntary contraction (MVC) or investigated muscular activity, by using surface EMG. Muscular dysfunctional factors, such as atrophy, muscular inhibition, fatigue, and flexibility remained understudied and not sufficiently addressed in patients with PFP. Particularly muscular inhibition (MI) is an important underlying mechanism of weakness and EMG alterations, as weakness might not only be caused by a reduced voluntary contraction but also by MI. MI is induced by an inhibitory signal, which is sent by the central nervous system to the muscle resulting in an involuntary inability to recruit all necessary motor units. The treatment approach of an inhibited or weak muscle are entirely different and studies revealed that MI needs to be eliminated before improvements in muscle strength can be achieved. Although MI, fatigue, flexibility and atrophy seem to be important underlying factors in patients with PFP, they remain understudied and require future research in this area.
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