Researchers from La Trobe University in Australia performed a meta analysis (10 studies were reviewed). Lachlan et al., concluded that,
Meta-analyses for imaging measurements of quadriceps muscle size demonstrated that quadriceps atrophy is present in the involved limb of individuals with patellofemoral pain (PFP), when compared to their asymptomatic limb and to a comparison group. When using girth measurements, no quadriceps atrophy was noted. The finding of quadriceps atrophy in the involved side of individuals with PFP may provide a rationale for the efficacy of strengthening exercises in reducing pain and improving function in this population, which suggests that assessing quadriceps strength may be a worthwhile clinical parameter . . . these findings support the rationale for use of quadriceps strengthening as part of a rehabilitation program for PFP.
So, your knee hurts . . . are you performing exercises to strengthen your quads?
Giles, LS, Webster, KE, McClelland, JA, Cook. Does Quadriceps Atrophy Exist in Individuals With Patellofemoral Pain? A Systematic Literature Review With Meta-analysis. J Orthop Sports Phys Ther 2013;43(11):766–776.