Changes in deep abdominal muscle thickness during common trunk-strengthening exercises using ultrasound imaging.

STUDY DESIGN: Cross-sectional study design. OBJECTIVES: To characterize changes in muscle thickness in the transversus abdominis (TrA) and internal oblique (IO) muscles during common trunk-strengthening exercises, and to determine whether these changes differ based on age. BACKGROUND: Although trunk-strengthening exercises have been found to be useful in treating those with low back pain (LBP), our understanding of the relative responses of the TrA and IO muscles during different exercises is limited. METHODS AND MEASURES: Six commonly prescribed trunk-strengthening exercises were performed by 120 subjects (40 subjects per age group: 18-30, 31-40, and 41-50 years). Ultrasound imaging was used to measure the thickness of the TrA and IO during the resting and contracted state of each exercise. The average thickness of the muscles while in the contracted position was divided by the thickness values in the resting position for each exercise, based on 2 performances of each exercise. Two 3-by-6 repeated-measures analyses of variance were used to determine significant changes in muscle thickness of the TrA and IO, based on age group and exercise performed. RESULTS: For both muscles, the trunk exercise-by-age interaction effect (TrA, P = .358; IO, P = .217) and the main effect for age (TrA, P = .615; IO, P = .219) were not significant. A significant main effect for trunk exercise for both muscles (P<.001) was found. The horizontal side-support (mean +/- SD contracted-rest thickness ratio: TrA, 1.95 +/- 0.69; IO, 1.88 +/- 0.52) and the abdominal crunch (mean +/- SD contracted-rest thickness ratio: TrA, 1.74 +/- 0.48; IO, 1.63 +/- 0.41) exercises resulted in the greatest change in muscle thickness for both muscles. The abdominal drawing-in maneuver (mean +/- SD contracted-rest thickness ratio: TrA, 1.73 +/- 0.36; IO, 1.14 +/- 0.33) and quadruped opposite upper and lower extremity lift (mean +/- SD contracted-rest thickness ratio: TrA, 1.59 +/- 0.49; IO, 1.25 +/- 0.36) exercises resulted in changes in TrA muscle thickness with minimal changes in IO muscle thickness. CONCLUSION: Changes in TrA and IO muscle thickness differed across 6 commonly prescribed trunk-strengthening exercises among healthy subjects without LBP. These differences did not vary by age. This information may be useful for informing exercise prescription.