The Relationship between Echo Intensity and Specific Strength as Measures of Muscle Quality

Abstract

The use of ultrasound (US) echo intensity (EI) as a measure of muscle quality has increased in popularity over recent years. However, in early studies muscle quality was quantified as a function of force per unit of muscle, or specific strength (MQST). Limited data exist that have examined the relationship between these two measures of muscle quality. PURPOSE: The purpose of this study was to examine the relationship between US EI and MQST as measures of muscle quality. METHODS: Thirty-one healthy volunteers (mean ± SD age, 35 ± 7 yrs; Body Mass Index [BMI], 28.66 ± 5.08) participated in this study. Following a familiarization session, participants visited the laboratory on a separate occasion for isometric strength testing and skeletal muscle US imaging. Peak torque (PT) was determined from highest mean value (500 ms) that occurred during three maximal voluntary contractions. Vastus lateralis (VL) cross-sectional area (CSA) and EI were assessed with a B-mode US device. The VL images were taken perpendicular to the longitudinal axis of the muscle at half of the distance between the greater trochanter and articular cleft of the knee. All US images were analyzed in ImageJ using the polygon function to determine the VL CSA that included as much muscle as possible without the surrounding fascia. The same region of interest was used for the EI (gray-scale) analyses and subsequently corrected for subcutaneous fat thickness. MQST was defined as PT per unit of muscle size (VL CSA). Pearson correlation coefficients were used to examine the relationship between VL EI and MQST, while separate partial correlation coefficients were used to examine the influence of BMI and sex. An alpha level of P ≤ 0.05 was used to determine statistical significance. RESULTS: There was a significant relationship between VL EI and MQST (r = 0.409, P = 0.022). In addition, similar relationships existed when controlling for BMI (r = 0.376, P = 0.041) and sex (r = 0.411, P = 0.024). CONCLUSIONS: A significant relationship exists between VL EI and MQST, however a large amount of variance is left unexplained (r2 = 0.167). Although both are considered to reflect muscle quality, each measure may be influenced by unique physiological variables (i.e. muscle activation, hydration status). GRANT FUNDING: National Institute of Occupational Safety and Health (T42OH008673)