The Relationship between Muscle Activity and Muscle Grade of the Trunk Flexors Using Manual Muscle Testing with Electromyography

[Purpose] To quantitatively investigate whether the muscle grades of the Curl-up (CU) and Double-leg-lowering (DLL) test show any electromyographic changes in the upper and lower rectus abdominis (RA) as trunk flexors. [Methods] The muscle activities of both the upper and lower RA in thirty healthy young males were recorded during each muscle grade task of two tests. The percentages of maximum voluntary contractions (%MVC) of the upper and lower RA were compared between each muscle grade and test. The %MVC ratios of the upper and lower RA were calculated to investigate the contribution of each muscle site in two tests. [Results] Although there were significant electromyographic differences among muscle grades in the CU test, there was no significant difference in the DLL test. The %MVC from both sites in the CU test were significantly larger in all muscle grades than in the DLL test. However, the %MVC ratio was consistently similar to 1.0 in both tests and the contribution of each muscle site did not differ. [Conclusion] These findings suggest that the three grading tasks used in this manual muscle testing procedure may be insufficient to objectively detect muscle weakness in the trunk flexors. The grading criterion for trunk flexors may need remediation.