Trunk muscle fatigue during a lateral isometric hold test: what are we evaluating?

BACKGROUND Side bridge endurance protocols have been suggested to evaluate lateral trunk flexor and/or spine stabilizer muscles. To date, no study has investigated muscle recruitment and fatigability during these protocols. Therefore the purpose of our study was to quantify fatigue parameters in various trunk muscles during a modified side bridge endurance task (i.e. a lateral isometric hold test on a 45° roman chair apparatus) and determine which primary trunk muscles get fatigued during this task. It was hypothesized that the ipsilateral external oblique and lumbar erector spinae muscles will exhibit the highest fatigue indices. METHODS Twenty-two healthy subjects participated in this study. The experimental session included left and right lateral isometric hold tasks preceded and followed by 3 maximal voluntary contractions in the same position. Surface electromyography (EMG) recordings were obtained bilaterally from the external oblique, rectus abdominis, and L2 and L5 erector spinae. Statistical analysis were conducted to compare the right and left maximal voluntary contractions (MVC), surface EMG activities, right vs. left holding times and decay rate of the median frequency as the percent change from the initial value (NMFslope). RESULTS No significant left and right lateral isometric hold tests differences were observed neither for holding times (97.2 ± 21.5 sec and 96.7 ± 24.9 sec respectively) nor for pre and post fatigue root mean square during MVCs. However, participants showed significant decreases of MVCs between pre and post fatigue measurements for both the left and right lateral isometric hold tests. Statistical analysis showed that a significantly NMFslope of the ipsilateral external oblique during both conditions, and a NMFslope of the contralateral L5 erector spinae during the left lateral isometric hold test were steeper than those of the other side's respective muscles. Although some participants presented positive NMFslope for some muscles, each muscle presented a mean negative NMFslope significantly different from 0. CONCLUSIONS Although the fatigue indices suggest that the ipsilateral external oblique and contralateral L5 erector spinae show signs of muscle fatigue, this task seems to recruit a large group of trunk muscles. Clinicians should not view this task as evaluating specifically lateral trunk flexors, but rather as providing an indication of the general endurance and stabilisation capacity of the trunk.