Background: It is well known that loss of muscle mass (quantitative change) is a major change that occurs with aging. Qualitative changes in skeletal muscle, such as increased intramuscular fat, also occur as one ages. Enhanced echo intensity (EI) on ultrasonography images of skeletal muscle is believed to reflect muscle quality. Recent studies evaluating the quality of skeletal muscle using computer-aided gray scale analysis showed that EI is associated with muscle strength independently of age or muscle size in middle-aged and elderly women. The aim of the present study was to investigate whether muscle quality based on EI is associated with muscle strength independently of muscle size for elderly men.

Methods: A total of 184 elderly men (65-91 years) living independently in Kyoto, Japan, participated in this study. The EI, muscle thickness (MT), and subcutaneous fat thickness (FT) of the anterior compartment of the right thigh were determined by assessing ultrasonography images. The maximum isometric torque of knee extension at a knee angle of 90° was measured.

Results: The EI showed a significant negative correlation with muscle strength (r = -0.333, P < 0.001). Multivariate regression analysis revealed that the MT and EI of the knee extensor muscle were independently associated with maximum isometric knee extension strength. Even when partial correlation analysis was performed with age, height, weight, and FT as control variables, EI was still significantly correlated with muscle strength.

Conclusion: The results of this study indicate that aging-related changes in muscle quality contribute to diminishing muscle strength. Ultrasonography is a low-cost, easily accessible, and safe method suitable for the assessment of EI as an index of muscle quality.