Effects of supine rest duration on ultrasound measures of the vastus lateralis.

Introduction: Due to the potential for intramuscular fluid shifts from changing body position, researchers often utilize a 10- to 15-min period of supine rest as a standardizing procedure prior to ultrasound assessment of the lower limbs. However, no previous research has observed the changes in muscle morphological characteristics via ultrasonography of the lower limbs depending on the length of time of supine rest to determine whether 10-15 min of supine rest is necessary. The aim of this study was to examine changes in muscle morphology of the vastus lateralis (VL) at various time-points over the course of 15 min of supine rest.

Methods: Muscle thickness (MT), cross-sectional area (CSA) and echo intensity (EI) of the VL were assessed in 24 adults at four time-points including the following: immediately upon moving from a standing to supine position (T0), after 5 (T5), 10 (T10) and 15 min (T15) of lying in a supine position.

Results: CSA significantly decreased from T0 to T10 (P = 0·001) and T15 (P<0·001), with no difference between any other time-points (P = 0·055-0·666). However, the reported changes in CSA did not exceed the standard error of the measurement for this procedure. No significant differences between any time-points for MT (P = 0·726-0·966) or EI (P = 0·061-0·783) were observed.

Discussion: These findings suggest extended periods of supine rest may not be needed to obtain consistent muscle morphological measurements of the VL using ultrasonography.