Association between quadriceps muscle strength and ultrasound-derived echogenicity in individuals with patellofemoral pain syndrome: A cross-sectional study

Background Patellofemoral pain syndrome (PFPS) is commonly associated with quadriceps weakness and altered knee mechanics. While muscle quantity is frequently assessed, the role of muscle quality, indirectly represented by ultrasound-derived echogenicity, remains unclear in this population. This study aimed to compare quadriceps strength between individuals with PFPS and healthy controls (CON) and to explore whether quadriceps strength is associated with rectus femoris echogenicity, age, BMI, and knee function. Methods A cross-sectional study was conducted with 55 participants with PFPS and 43 healthy controls. Rehabilitative ultrasound imaging was used to measure rectus femoris echogenicity at rest and during contraction at 5% body weight. Quadriceps strength was assessed using hand-held dynamometry, and knee function was evaluated using the Knee Injury and Osteoarthritis Outcome Score (KOOS). Results The PFPS group demonstrated significantly lower quadriceps strength than the CON group ( P = 0.045). In the PFPS group, quadriceps strength was significantly and inversely correlated with resting echogenicity ( r = −0.284, P = 0.035), contraction echogenicity ( r = −0.296, P = 0.028), and age ( r = −0.381, P = 0.004). Quadriceps strength was positively correlated with KOOS scores ( r = 0.434, P = 0.001). However, after controlling for age and BMI, the associations between quadriceps strength and resting echogenicity ( r = −0.167, P = 0.233) and contraction echogenicity ( r = −0.173, P = 0.215) were attenuated and no longer statistically significant. No significant correlations were observed in the CON group. Conclusion Individuals with PFPS demonstrated reduced quadriceps strength. Rectus femoris echogenicity may provide complementary information regarding muscle quality, but its association with strength appears to be partly influenced by age and BMI.