This study investigated the task dependency effect of two Sorensen test variants on paraspinal and hip extensor muscle fatigue. An important finding was that EMG variables had low between-test correlations. A modification of approximately 45° of hip sagittal orientation influenced the EMG indices during a similar isometric testing procedure with the trunk placed in a weight-dependent horizontal position. These results support the hypothesis of a task-dependency effect on lumbo-pelvic muscle fatigue. We initially hypothesized that the difference in hip sagittal orientation between Sorensen variants would induce a task-dependency effect on the hip extensor muscle EMG fatigue indices. We observed that the hip extensor muscles tended to fatigue simultaneously with the paraspinal muscles during both Sorensen variants. However, only L5 level EMG fatigue indices showed a task-dependency effect between S1 and S2. It appears that the hip extensor muscles contribute to load sharing of the upper body mass during the two Sorensen variants, but to a different extent because L5 level fatigue differs between Sorensen variants. The hip extensor muscles have biomechanical and anatomical links to the thoracolumbar fascia, and their relative elongation (contraction or stretching) can influence tension of the posterior thoracolumbar fascia layer [27
]. Because the thoracolumbar fascia can commit to the lumbar extension moment, an increase of hip extensor muscle stretch may result in a higher contribution of the thoracolumbar fascia to the lumbar extensor moment. Consequently, a lower paraspinal muscle effort at L5 could be necessary during S2 to maintain the unsupported load of the upper body. Other authors have documented hip extensor muscle involvement in load bearing of the trunk during a Sorensen test like S1 [13
]. Arendt-Nielsen et al. [1
] hypothesized that the contribution of passive tissues as well as the energy required creating and maintaining a cross-bridge is lower for the stretched muscle. During S2, the hip extensors were stretched to a greater extent that during S1 because the hips were flexed at a 45° angle. This could explain the significantly lower rate of paraspinal muscle fatigue during S2.
Our results reveal that endurance-holding times between the Sorensen variants are moderately correlated. We also found a significant difference between Sorensen variants for endurance time, indicating that the subjects systematically achieved greater holding time during S2. In the present experiments, greater than previously reported endurance times were observed in S1 [3
] with slightly lower endurance times in S2 [8
]. An important factor that could explain these results is selection of the test cessation criteria. In our study, the test was ended when the subjects could no longer reposition their upper body in the horizontal position after verbal feedbacks provided by the investigator. McKeon et al. [23
] gave only one chance to their test subjects to reposition their upper body during the test and obtained a time duration of 124.4 s, whereas Dedering et al. [8
] (385 s) used a light-induced sensor and discontinued the test when relative displacement of the torso was over 2 cm. To reduce day-to-day variability and to standardize the isometric horizontal position procedure, propioceptive or tactile feedback appears to be important.
Variability of the lumbar curvature is another factor to consider during the assessment of back extensor fatigue. Coorevits et al. [3
] evaluated lumbar curvature during the Sorensen protocol, and the fatigue indices were calculated with a fixed threshold value of variance of the lordosis angle. Tveit et al. [26
] reported a significant effect of lumbar lordosis curvature on lever arm length of the back extensor muscles. A longer lever arm of the paraspinal muscles could lead to a mechanical advantage in creating back extensor moments. During our experiments, we carefully instructed the subjects to maintain a normal lumbar curvature during the tests. Further studies should be conducted with kinematic analysis to ascertain the relationship between lordosis curvature and muscle fatigue during different Sorensen protocols and CLBP subjects.
Some authors, comparing EMG fatigue indices of the back muscles between CLBP and healthy participants, observed that CLBP had a lower rate of back muscle fatigue [11
], but others did not find significant differences [5
]. A hypothesis that could explain these conflicting results is that CLBP subjects adopt alternative neuromuscular strategies that could modulate EMG fatigue indices of the back extensor muscles. One of them could be a reweighed contribution of the lumbo-pelvic extensor muscles in bearing the upper body load [14
]. CLBP subjects showed a greater rate of hip extensor muscle fatigue when compared to healthy subjects. These results indicate that LBP participants might increase the contribution of their hip extensor muscles during back endurance tests. Our study highlights the task-dependency effect on the lumbo-pelvic muscles that could contribute to discrepancies in the results of these previous investigations.
The test subjects’ motivation to sustain isometric contraction as long as possible could have influenced the endurance time variable. Some authors have proposed a high correlation between hold time and EMG fatigue indices as a sign of subject motivation [2
]. We detected a task-dependency effect on this relationship. Only the GM showed a significant correlation in both S1 and S2 testing positions, explaining around 40% of the variance, whereas the T10 and L5 paraspinal levels presented a significant relationship only between hold time and EMG fatigue indices during S1 (R2
≈ 0.20). During both test variants, we provided the subjects with verbal encouragement. Thus, it appears that factors other than motivation could also contribute to the endurance time difference between tests.
We applied a linear regression model to calculate the MPF/time slope. Interestingly, higher R2
values were found for L5 level paraspinal muscles, showing that MPF data better fit the simple linear regression model than hip extensor muscles. However, a recent study has demonstrated that more complex statistical models are not valuable on fatigue-related EMG indices [3