Pain-related function, muscle stiffness, and muscle activation in adults with patellofemoral pain.

BACKGROUND: Patellofemoral pain (PFP) is a prevalent knee condition affecting adolescents and active adults, characterized by diffuse anterior knee pain exacerbated by weight-bearing activities and knee flexion. The onset and improvement of PFP are believed to be related to the condition of the quadriceps muscles. However, there remains a lack of rapid, convenient, and accurate clinical tools for assessing the characteristics of muscles. Our research aimed to explore muscle stiffness in three states (resting, stretching, and contraction), as well as to evaluate muscle activation, and pain-related function in adults with patellofemoral pain (PFP). And to assess the diagnostic value of shear wave elastography (SWE). METHODS: Eighteen subjects with unilateral PFP (39.8 ± 11.9 years) and 20 control subjects (30.3 ± 8.2 years) underwent muscle stiffness measurements in resting, stretching, and contraction positions. Quadriceps muscle stiffness and surface electromyography (sEMG) data were collected. VMO/VL activation and stiffness ratios were analyzed between groups. Self-report questionnaires (AKPS and KOOS-PF) assessed pain and knee function. RESULTS: At resting, no significant differences were found in the stiffness of the VMO, VL, or RF between groups. During stretching, VMO stiffness was significantly lower in PFP limbs compared to pain-free controls (p = 0.032), while VL stiffness was significantly higher in PFP limbs compared to both asymptomatic limbs (p = 0.019) and controls (p = 0.008). During contraction, VMO stiffness was significantly lower in PFP limbs compared to asymptomatic limbs (p = 0.039). The VMO/VL ratio was significantly reduced across three conditions. Combining VMO/VL ratios during stretching and contraction yielded the highest diagnostic performance, with an area under the curve (AUC) of 0.832, specificity of 80%, and sensitivity of 66.67%. CONCLUSION: Individuals with PFP exhibited significant impairments in the VMO/VL stiffness ratio. Specifically, the VMO stiffness decreased during contraction, while VL stiffness increased during stretching. Integrating VMO/VL stiffness from stretching and contraction positions enhanced diagnostic accuracy for PFP.