Muscle strength, not age, explains unique variance in echo intensity

Echo intensity (EI) is being increasingly utilized by investigators as an index of skeletal muscle quality. Previous studies have reported independent associations between EI versus both age and muscle strength. Purpose: We sought to determine whether EI is more strongly associated with age or strength. Methods: Twenty-eight younger adults (13 men, 15 women; mean age = 22 years) and 25 older adults (10 men, 15 women; age = 71 years) participated. B-mode ultrasonography was utilized to acquire images of the vastus lateralis and rectus femoris. ImageJ software was used to quantify corrected EI and muscle cross-sectional area (CSA). Each participant performed 40 maximal concentric isokinetic muscle actions of the knee extensors (velocity = 180°·s⁻¹). The mean peak torque of the best three attempts was used to quantify muscle strength. Specific torque was calculated as strength relative to CSA. Fatigability was also quantified. Statistical analyses included independent samples t-tests and stepwise regression. Results: There were large differences between age groups for strength (p < .001, d = 1.831) and CSA (p = .003, d = 0.872). When corrected for subcutaneous tissue thickness, the difference in EI between age groups was small (p = .184, d = 0.371). Stepwise regression revealed that muscle strength was the single best predictor of EI (R² = 0.206), with age, CSA, specific torque, and fatigability explaining no unique variance. Conclusion: Concentric isokinetic muscle strength is a better predictor of EI than age.