Reliability of ultrasound assessment of the rectus femoris muscle thickness: intra-rater, inter-rater, and inter-day analysis accounting for age and sex.

Background: The rectus femoris muscle plays an important role in maintaining lower limb stability and biomechanical control during basic physical activities. Both reduced quadriceps strength and decreased muscle thickness of the rectus femoris, as measured by ultrasound, are associated with an increased risk of falls in older adults. We estimated the relative and absolute intra-rater, inter-rater, and inter-day reliability of the procedure.

Methods: A female biologist and a male physical educator, both holding a master's degree in human movement and rehabilitation, captured ultrasound images of the right rectus femoris muscle in 106 healthy participants (58 females and 48 males), aged 18 to 73 years. Each rater captured three images per participant during each visit, with two visits 7 to 10 days apart. A third person subsequently measured the muscle thickness. We calculated the Intraclass Correlation Coefficients (ICC) using a two-way random model and determined the 95% minimal detectable difference (MDD).

Results: The mean muscle thickness was 2.12 cm. The reliability based on single measurements was as follows: the intra-rater ICC for raters A and B was 0.998 at both visits (MDDs: 0.074-0.082 cm). The inter-rater ICC was 0.976 at visit 1 and 0.977 at visit 2 (MDDs: 0.269-0.270 cm). The inter-day ICC was 0.973 for rater A and 0.972 for rater B (MDDs: 0.286-0.291 cm). Sensitivity analyses accounting for age, sex, and the use of mean measurements produced similar results. The results were homoscedastic.

Conclusions: The rectus femoris muscle thickness was measured with good reliability using ultrasonography across all the settings.