4Study Design: Exploratory. Objectives: To explore whether hand dominance, gender, and body mass index (BMI) influence the thickness of the lateral abdominal muscles as measured by ultrasound imaging. To document the extent of improvement in response stability when an average of multiple measures was utilized. Background: Ultrasound imaging is a relatively new tool used to assess the lateral abdominal muscles. A better understanding of how these muscles contract in a healthy population can provide a reference for comparison to patients with low back pain (LBP). Methods and Measures: Thirty-two healthy participants (17 males, 15 females) aged 18 to 45 years (mean ± SD, 31.9 ± 7.8 years) were studied. Measurements of muscular thickness of the lateral abdominal muscles were obtained bilaterally while the subjects were at rest, and while they performed the abdominal drawing-in maneuver. To determine the possible influence of hand dominance and gender on muscle thickness, t tests were used. Correlation coefficients were used to assess the relationship between BMI and muscle thickness. Standard error of the measurement was used to assess response stability of the ultrasound imaging technique. Results: No differences in the thicknesses of the transversus abdominis (TrA) muscle were measured during rest or while contracted, based on hand dominance (P Ն .73). Men had greater muscular thickness (P Ͻ .01), while the TrA in women represented a greater proportion of the total lateral abdominal muscles (P Ͻ .01). BMI was positively associated with muscle thickness (r Ն .66). Compared to a singular measurement, response stability improved by greater than 50% when an average of 3 measurements was used. Conclusions: Future researchers should assess the need to control for gender and BMI as potential covariates in ultrasound imaging studies of the lateral abdominal muscles. Asymmetry in the lateral abdominal muscles in those with LBP would be in direct contrast to the bilateral symmetry measured in those without LBP.