Purpose] The aim of this study was to identify how the lumbar stabilizer muscles respond to increased lifting loads. Twenty-four healthy subjects (10 males, 14 females) participated in this study. [Subjects and Methods] The thicknesses of the internal oblique (IO), transverse abdominis (TrA), and lumbar multifidus (LM) muscle were measured by ultrasonography during lifting of loads 10%, 20%, and 30% of body weight. The data was analyzed measured by one-way repeated measures analysis of variance

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... (ANOVA). [Results] There were statistically significant increases in thicknesses of the TrA and LM muscles when lifting a load of 20% of subject's body weight. The thickness of IO was not significantly different at different loads. [Conclusion] The findings of this study suggest that TrA and LM play important roles as lumbar spine stabilizers during lifting activities of less than 20% of body weight. Further study is needed to find the mechanisms of lumbar stability during stoop lifting of loads greater than 20% of body weight.