Is the side bridge test valid and reliable for assessing trunk lateral flexor endurance in recreational female athletes?

Key points

1. 01Deltoid muscle activation and fatigue were not significantly different from trunk lateral flexor activity, meaning the shoulder plays as large a role as the trunk in test performance.
2. 02Deltoid and external oblique fatigue together predicted 67.3% of the variance in side bridge test endurance time (adjusted R2 = 0.673).
3. 03Body mass and trunk height predicted an additional 22.3% of endurance time variance, so heavier and taller participants consistently scored lower regardless of trunk strength.
4. 04Relative reliability was good (ICC = 0.81), but absolute reliability was low with a typical error of 10.95 seconds and a coefficient of variation of 30.75%, making it unsuitable for detecting small within-person changes.
5. 05A significant drop in endurance time was observed between session 1 (77.25 s) and session 2 (68.96 s), p = 0.015, suggesting a learning or motivation effect.

How it was conducted

Design

Cross-sectional validity study with one-week test-retest reliability component

Participants

24 physically active women, mean age 24.58 +/- 3.92 years, mean mass 60.90 +/- 2.90 kg

Primary outcome

Side bridge test endurance time (seconds) and EMG-derived normalized median frequency slopes (NMFslope) for 8 muscles

Muscles assessed

External oblique, internal oblique, rectus abdominis, erector spinae, deltoids, gluteus medius, latissimus dorsi, rectus femoris

Reliability metrics

ICC(3,1), typical error with 95% confidence limits, and coefficient of variation

What they found

• Mean endurance time was 77.25 +/- 26.62 s in session 1 and 68.96 +/- 21.21 s in session 2 (significant decrease, t = 2.624, p = 0.015); range 38-132 s.
• Relative reliability was good: ICC(3,1) = 0.81 (95% CI: 0.60-0.91).
• Absolute reliability was low: typical error = 10.95 s (95% CI: 8.51-15.36 s); coefficient of variation = 30.75%.
• Deltoid NMFslope showed the greatest median frequency decline (-0.50%/s) and was significantly greater than latissimus dorsi, gluteus medius, and rectus femoris (p < 0.05); no significant difference was found between deltoid and trunk lateral flexors.
• Normalized EMG amplitudes: external oblique 50.2% MVIC, rectus abdominis 41.9% MVIC, deltoid 36.3% MVIC, erector spinae 28.2% MVIC, gluteus medius 21.2% MVIC, latissimus dorsi 14.0% MVIC, rectus femoris 5.85% MVIC.
• Significant high correlations between endurance time and NMFslope: external oblique r = 0.722 (p < 0.01), erector spinae r = 0.664 (p < 0.01), deltoid r = 0.650 (p < 0.01).
• Multiple regression with NMFslopes: EO and deltoid together predicted endurance time with adjusted R2 = 0.673 (equation: 102.120 + 55.434 x NMFslope_EO + 25.832 x NMFslope_DE).
• Multiple regression with anthropometrics: body mass and trunk height predicted endurance time with adjusted R2 = 0.223 (significant, p < 0.05); body mass correlation r = -0.416 (p < 0.05).

Limitations

• All participants were young, healthy, physically active females; results cannot be generalized to males, sedentary populations, athletes, or clinical patients.
• EMG crosstalk may have affected internal and external oblique signals due to their overlapping anatomical positions.
• High inter-participant variability in EMG amplitude and NMFslope values limits the precision of individual-level conclusions.
• The endurance time drop in session 2 was likely influenced by the extra burden of the EMG protocol (electrode placement, MVIC testing), not a true reliability characteristic.

Why it matters

For patients

People who have been given the side bridge test during rehabilitation or fitness assessment should know that a single score may not accurately reflect their trunk muscle endurance, especially if they have shoulder weakness or are heavier or taller.

For clinicians

Clinicians using the side bridge test should account for body mass as a confounder and recognise that a typical error of nearly 11 seconds (CV 30.75%) means meaningful within-person change requires a large improvement before it can be distinguished from noise.

For readers

This study provides EMG evidence that the side bridge test is a multi-joint test where shoulder fatigue is as limiting as trunk fatigue, which challenges its face validity as a trunk-specific endurance measure.