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.