Ultrasonic Assessment of Quadriceps Muscle Quality Does Not Reflect Muscle Function in Healthy Individuals

Abstract

Muscle quality assessed by ultrasound imaging represents the composition of the muscle. Poor muscle quality (i.e. greater echo intensity) indicates greater intramuscular fat and connective tissue, and is associated with lesser functional ability and strength in older adults. If validated as a robust indicator of muscle function, this measure could be used to evaluate patient populations who experience muscle dysfunction and are unable to produce maximal muscle contraction due to pain and inhibition (e.g. knee osteoarthritis, anterior cruciate ligament injury). However, the relationships between muscle quality and muscle function are unknown. PURPOSE: To evaluate relationships between quadriceps muscle quality and isometric peak torque, isokinetic peak torque, and voluntary activation. METHODS: Quadriceps (vastus lateralis) muscle quality was assessed from the gray-scale echo intensity obtained from transverse plane ultrasound images. Quadriceps function was assessed via maximal isometric peak torque, isokinetic peak torque at 60 and 240 °/s, and central activation ratio (CAR) in 23 healthy individuals (20 females, 3 males; Age = 21±1 yr; Height = 1.70±0.01m; Mass = 65.6±9.8kg). CAR refers to the ratio of peak voluntary torque to the torque resulting from an electrical stimulus superimposed on the quadriceps during maximal isometric contraction. Relationships between muscle quality and each index of muscle function were calculated via Pearson correlations. RESULTS: Quadriceps muscle quality was not significantly correlated with isometric peak torque (r = 0.324, p = 0.152), isokinetic peak torque at 60°/s (r = 0.013, p = 0.955) or 240°/s (r = −0.071, p = 0.749), or CAR (r = 0.366, p = 0.103). CONCLUSION: Muscle quality does not appear to be an indicator of muscle function in healthy individuals. These findings suggest that muscle quality is not likely a surrogate for muscle function in individuals with joint pathologies who experience muscle dysfunction. The disagreement between our results in healthy, young individuals and those from previous investigations involving older adults may suggest that muscle quality reflects undefined factors associated with sarcopenia. Future research is necessary to evaluate relationships between muscle quality and function in pathological populations.