Purpose: The study's purpose was to determine whether trunk muscle activity levels are different during spine stability exercises performed in water compared with on land.
Methods: Eleven male participants performed four abdominal trunk exercises on land and in water at the depth of the xiphoid. The exercises were abdominal hollowing, abdominal bracing, and anteroposterior and mediolateral pelvic tilts. During the exercises, surface EMG activity of muscles rectus abdominis (RA), external oblique, lower abdominals, multifidus, and erector spinae (ES) were recorded. EMG data were normalized to a maximal voluntary contraction (MVC), and the subsequent percentage of activity was compared between environments (water and land) with paired t-tests.
Results: Normalized EMG values for muscles RA, external oblique, lower abdominals, multifidus, and ES were significantly greater for all exercises performed on land than in water (P = 0.026-0.001, effect sizes = 0.52-1.61). The only exception was for mediolateral pelvic tilts where muscle ES values were not different between environments (P = 0.098).
Conclusions: When healthy adults perform abdominal hollowing, abdominal bracing, and pelvic tilt exercises in water, most trunk muscles display substantially lower EMG activity when compared with performing the same exercises on land (e.g., abdominal bracing for RA = 20% MVC for land and 10% MVC for water). It is possible that with hydrostatic pressure and buoyancy, trunk muscles play less of a stabilizing role in the aquatic environment, which minimizes their EMG activity levels. Regardless of the mechanism, patients with back pain may find it easier to perform trunk muscle exercises in an aquatic environment first then progress to the land environment because EMG activity may be gradually increased.