Intramuscular Fat Negatively Impacts Quadriceps Function In Individuals With Knee Osteoarthritis

Knee osteoarthritis (OA) is associated with profound quadriceps dysfunction. The root causes for this are debated but in part are due to poor muscle quality. Intramuscular fat is often implicated as affecting muscle quality but its effects on quadriceps function has yet to be fully elucidated. Defining the role of intramuscular fat on quadriceps strength in OA would help to direct future exercise interventions. PURPOSE: To determine if intramuscular fat is related to quadriceps strength in individuals with knee OA. METHODS: Quadriceps strength was assessed on 15 individuals with knee OA on an isokinetic dynamometer during maximal isometric and isokinetic contractions. Rate of Torque Development (RTD) was calculated at various time intervals (early RTD 0-100 ms, late RTD 100-200 ms, and RTD between 20-80% of the first 200 ms). Dixon magnetic resonance imaging was used to assess intramuscular fat. The quadricep muscles were manually segmented on three consecutive 3 mm thick slices centered over the muscle belly, and intramuscular fat was calculated as the average fat fraction (%) across all muscles. Pearson correlations were used to assess the association between quadriceps strength (peak and RTD) and intramuscular fat. RESULTS: Average quadriceps intramuscular fat was 8.7 ± 2.6 %. Intramuscular fat was associated with isokinetic quadriceps strength (Panel A) and late RTD (Panel B). There was no association between intramuscular fat and isometric strength (B = -2.39, R2 = .23, p = 0.082), early RTD (B = -0.43, R2 = .15, p = 0.171), or RTD between 20-80% (B = -0.64, R2 = .26, p = 0.061). CONCLUSION: Greater intramuscular fat resulted in significant reductions in quadriceps function. Potentially, greater intramuscular fat impacts contractile function by increasing muscle stiffness. Future research is needed to understand the mechanisms of how fat interferes with contractile function and determine how different modes of exercise may mitigate intramuscular fat development.