Is the Side Bridge Test Valid and Reliable for Assessing Trunk Lateral Flexor Endurance in Recreational Female Athletes?

SIMPLE SUMMARY: Although the side bridge test has been widely used for assessing trunk lateral flexor endurance in sport, clinical, and scientific settings, to the best of the authors’ knowledge, no study has analyzed its validity and reliability in an only female population. The surface electromyography of eight abdominal, back, shoulder and hip muscles was measured during the test to analyze its validity. A one-week test-retest design was performed to evaluate its reliability. No significant differences were found between the trunk lateral flexors and the deltoids. The study data showed that the test performance could be significantly predicted by external oblique and deltoid normalized median frequency slopes and by body mass and trunk height. Based on the results of this study, the shoulder muscle activation and fatigue and the individuals’ anthropometric characteristics, especially the mass, played an important role in the side bridge test performance, which questions the validity of this multi-joint test to specifically assess trunk lateral flexor endurance. In addition, although the side bridge test showed a good data consistency, its intra-subject variability was high, which reduces its utility when small intra-subject changes in muscle endurance are important (e.g., elite sport). ABSTRACT: The side bridge test (SBT) is one of the most popular tests to assess isometric trunk lateral flexor endurance. The aim of this study was to assess the validity and reliability of the SBT in healthy females. Twenty-four (24.58 ± 3.92 years) physically active (1–2 h of moderate physical activity, 2–3 times a week) females voluntarily participated in this study. The surface electromyography (EMG) of eight abdominal, back, shoulder and hip muscles was measured during the SBT. Normalized median frequency slopes (NMF(slope)) were calculated to analyze the muscle fatigue. The EMG amplitudes were normalized to maximum EMG values to assess muscle activity intensity. A one-week test-retest design was performed to evaluate the SBT reliability through the ICC(3,1) and typical error. Higher NMF(slopes) and normalized EMG amplitudes were found in deltoids, abdominal obliques, rectus abdominis, and erector spinae in comparison to latissimus dorsi, gluteus medius, and rectus femoris. However, no significant differences were found between the trunk lateral flexors and the deltoids. Linear regression analysis showed that SBT performance could be significantly predicted by external oblique and deltoid NMF(slope) (adjusted R(2) = 0.673) and by body mass and trunk height (adjusted R(2) = 0.223). Consistency analysis showed a high intraclass correlation coefficient (0.81) and a relatively high typical error (10.95 s). Despite the good relative reliability of the SBT, its absolute reliability was low and its validity questionable, as the shoulder muscle activation and fatigue and the individuals’ anthropometric characteristics played an important role in SBT performance.