A systematic review on quantifiable physical risk factors for non-specific adolescent low back pain

Sitting height emerged as a risk factor for adolescent LBP from one cross-sectional [30] and one prospective [40] study, although its importance was minor [40]. For sitting posture, contradictory results were reported. A weak association between greater slump degree and LBP that was observed in one cross-sectional study [27] was contradicted by another study that found differences in sitting posture only when the adolescents were sub-classified [29]. Posture alignment in stance was concordantly stated to be a risk factor for LBP by three studies. Non-neutral alignment emerged from one prospective study [43] and an increased pelvic displacement angle [44] and sway back posture [45] were demonstrated in two cross-sectional studies as risk factors for LBP in adolescence for boys, but not in girls [46]. No association of local spinopelvic parameters, such as vertebral level of the lumbar apex, pelvic tilt or sacral inclination [44], and of general motor competence, assessed by a test for neuromuscular development [28], with adolescent LBP were found.

4. Discussion

The aim of this systematic review was to summarize the current knowledge about quantifiable physical risk factors for LBP in adolescents between 10 and 18 years. Prospective, cross-sectional or retrospective studies that used quantifiable measures beyond questionnaires were included. There is wide consensus in the literature that psychosocial factors are associated with LBP in adolescents [10, 18, 47, 48]. The role of mechanical factors was reported to be minor, although many studies restricted mechanical factors to body weight, school bag weight or physical activity [18]. However, there is some evidence that the importance of physical risk factors might have been underestimated [19]. Indeed, this review revealed that there is conclusive evidence that some quantifiable physical parameters might put adolescents at risk for LBP. This accounts particularly for back muscle endurance concordantly reduced in adolescents with LBP, which was in line with a recent review by Lardon et al. [49]. There was the exception of one study that reported both reduced and enhanced back muscle endurance to be associated with LBP, but only in girls [28]. This study reported, however, small associations of all investigated factors to LBP and was the only one that did not use the Biering-Sorensen test as assessment for back muscle endurance. Similarly, reduced endurance of the abdominal muscles was, with the exception of one low-quality study, associated with LBP in three cross-sectional studies, two of which focused on adolescents with recurrent or continuous LBP. However, as no prospective study was found in this context, no conclusion about causality can be drawn. The relevance of trunk muscle strength became less clear. Only one cross-sectional study found some association between LBP and the ratio of flexion and extension peak torque in boys [26]. Thus, in accordance with the review of Lardon et al. [49], who reported no association between back muscle strength and adolescent LBP, the measurement of trunk muscle strength seems to be of minor importance. Accordingly, the results regarding spinal mobility were rather diverse as were the measurement methods that were used (Schober test, fingertip-floor distance, flexicurve). In a recent prospective study by Aartun et al. [20], both methods, the fingertip-floor distance as well as the Schober test (with dichotomized results), could not predict future LBP. Interestingly, one prospective study of high quality reported a combined measure of strength and mobility, more precisely a high lumbar mobility-extension strength ratio to be predictive for adolescent LBP [37]. However, in fact, the authors conducted a modification of the Sorensen test, which means that the proposed measure is rather a combination of back muscle endurance and spinal mobility. Thus, it can be hypothesized that the decrease in back muscle endurance is a risk for adolescent LBP, which becomes more accentuated when spinal mobility is enhanced. The results of the study by Salminen et al. [34] seem to further support this hypothesis as they found decreased endurance of back muscles and abdominal muscles along with increased flexion mobility in the adolescents with recurrent/continuous LBP. Unfortunately, they did not combine these measures. Trunk asymmetry, assessed by forward bending, was consistently associated with LBP in this review, but did not have a predictive value in the study by Aartun et al. [20]. This test is broadly used in scoliosis screening, and was reported even in this context to have limited positive predictive and sensitivity values [50]. Thus, its application in the field on non-specific adolescent LBP seems of minor value.

Lower limb performance was associated with LBP in three cross-sectional studies. Accordingly, knee extensor strength was significantly smaller in adult runners with chronic LBP when compared to control runners [51]. Furthermore, adult patients with chronic LBP who reported psychologic distress and a high pain level showed reduced quadriceps muscle torque, probably as a result of increased inhibition of muscle activity [52]. Given these findings, a prospective study in adolescents that includes quadriceps strength might be of interest. The situation in terms of hip mobility and hamstrings flexibility seems more complex as the measuring methods differed substantially. Jones et al. [31] tested hip flexion with the knee extended, which is comparable to the passive straight leg test used by Salminen et al. [34] for assessing hamstrings flexibility. Both studies dealt with a subpopulation of adolescents with more severe LBP and found a positive association between reduced flexibility and recurrent LBP. However, it was shown via ultrasonography that the straight leg raise score is not determined by hamstring elongation [53], but rather by altered neurodynamics such as sliding capacity of the nerve roots and the spinal cord in the thoracolumbar region [54]. Thus, it might rather be impaired neural sliding than hamstrings tightness that could be a risk factor for adolescent LBP. The limited importance of hamstrings flexibility is underlined in the present review of two studies by Sjölie [41] and Harreby et al. [42] that measured hamstrings flexibility via knee extension deficit in 90 degrees of hip flexion and found contradicting results. Furthermore, in the study done by Perry et al. the use of sit-and-reach tests for assessing hamstrings flexibility was not recommended [55] or only with reservations [56].

Anthropometric measures and sitting posture seem to play, at most, a minor role in the context of adolescent LBP, while alignment in stance seems more important. In the prospective study by Smith et al. [43], any deviation from the neutral posture (sway, flat, hyperlordotic) was a risk for back pain. The three cross-sectional studies by Dolphens et al. [44, 45, 46] reported sway-back posture to be associated with LBP in boys, while the investigated local spinopelvic parameters, such as apex of lumbar lordosis and pelvic orientation in the sagittal plane did not show a relationship. In contrast, in a recent study of this group, higher lumbar lordotic apex and an increase in pelvis retroversion were the two physical factors with the highest odds for LBP [19]. This corresponds well with findings in adults with chronic LBP, where a greater proportion of patients with a low sacral slope and a long but small lumbar lordosis were reported when compared to healthy controls [57].

5. Conclusions

This review demonstrated 1) correlations between LBP and back and abdominal muscle endurance, and to a lesser extent, with sagittal spinal mobility, sagittal postural alignment and neurodynamics; and 2) a need for prospective studies of high quality that assess pos- sible causality between physical risk factors and adolescent LBP. Based on the results of this study, it is recommended that such studies focus on back and abdominal muscle endurance, possibly in combination with sagittal spinal mobility, sagittal postural alignment and neurodynamics as possible modifiable risk factors for the development of LBP in adolescence.