Ultrasound-Derived External Oblique Muscle Thickness Was Not Associated with Surface EMG Amplitude or Shear Modulus During Progressive Isometric Upper-Limb Loading: An Experimental Study

The aim of this study was to assess whether external oblique muscle (OE) thickness was associated with surface electromyography (sEMG) amplitude and shear modulus under progressively increasing load and to examine the effect of load on these three measures. Thickness and shear modulus of the OE were assessed using an Aixplorer ultrasound system in shear wave elastography mode, while sEMG amplitude was recorded using the eMotion system. Measurements were obtained in a semi-supine position at rest, in the unloaded starting position, and during isometric upper-limb contractions with external loads corresponding to 3%, 6%, 9%, and 12% of body mass. Forty-two healthy adults (21 women and 21 men; age 23.1 ± 1.4 years) participated in the study. Load effects were assessed using Friedman tests, whereas associations were examined using linear mixed-effects models after natural-log transformation of sEMG amplitude and shear modulus. Progressive loading significantly increased OE sEMG amplitude and shear modulus (both p < 0.001). Changes in OE thickness were small despite a significant overall effect of load (p = 0.04; Kendall’s W = 0.06). No significant load × OE thickness interaction was found for either sEMG amplitude (p = 0.417) or shear modulus (p = 0.740). In the additive mixed-effects models, load significantly predicted both outcomes (p < 0.001 for both), whereas OE thickness was not associated with either sEMG amplitude (p = 0.814) or shear modulus (p = 0.475). These findings indicate that ultrasound-derived changes in OE thickness should not be interpreted as direct indicators of muscle activity in the semi-supine position and during the isometric upper-limb loading task examined in this study.