The percentage of male subjects in the LBP and No-LBP groups was 53% and 48% respectively (chi square = 0.36, p = .55). There were no significant differences between LBP and No-LBP groups for age, (44.6 ± 1.8 vs. 41.8 ± 2.3, p = .35), BMI (26.0 ± 0.5 vs. 26.1 ± 0.6, p = .76), and activity levels measured by the Baecke Activity index (8.0 ± 0.3 vs. 7.7 ± 0.5, p = .61). There also were no significant differences between groups for age, BMI and activity level within either males or females. Indices of symptom severity and disability in subjects with LBP are shown in Table .
Indices of symptom severity and disability in subjects with LBP
The following two video clips show examples of thoracolumbar fascia motion during passive trunk flexion in a human subject with No-LBP (Additional file 1
) and a subject with LBP (Additional file 2
). In the subject with No-LBP, the layers within the thoracolumbar fascia can be seen to move independently with some adjacent layers moving in opposite directions. In contrast, in the subject with LBP, there is less apparent differential motion between the adjacent layers.
The next two video clips (Additional file 3
and Additional file 4
) respectively show cumulative lateral displacement and corresponding shear strain maps within the ROI during one flexion cycle of the table. In both movies, red indicates tissue displacement or shear strain toward the right (rostral) and blue indicates tissue displacement or shear strain toward the left (caudal). Figures and respectively show B-scan, cumulative displacement and cumulative shear strain maps at the end of one flexion cycle of the motorized table demonstrating the presence of shear plane deformation within the thoracolumbar fascia as illustrated in Figure .
Figure 5 Cumulative lateral tissue displacement and shear strain maps. A: B-scan ultrasound image ROI. B: Sum of tissue displacement over time (cumulative displacement) during one flexion cycle of the table within the ultrasound image ROI. Red indicates tissue (more ...)
When shear strain was calculated using anatomically defined locations as shown in Figure , average shear strain was 62% (SD = 27.2%) among all subjects tested. On average, thoracolumbar fascia shear strain was 20% lower in subjects with LBP compared with subjects without LBP. For the LBP vs. No-LBP groups, thoracolumbar fascia shear strain was (mean±SE) 56.4% ± 3.1% vs. 70.2% ± 3.6% respectively, p < .01) (Figure ). There was no evidence that this difference was sex-specific (group by sex interaction p = .09) although overall, males had significantly lower shear strain than females (p = .02). There were no significant overall correlations between thoracolumbar fascia shear strain and either age (r = -0.18, p = .06), BMI (r = -0.13, p = .16) or activity level (r = -0.09, p = .34). Additionally, in subjects with LBP, there were no significant correlations between thoracolumbar fascia shear strain and responses to McGill pain questionnaire (r = 0.03, p = .84), pain level (r = 0.03, p = .81), pain intensity on day of testing (r = 0.01, p = .93) or Oswestry disability scale (r = 0.12. p = .34). However, thoracolumbar fascia shear strain was negatively correlated with pain duration in males with LBP (r = -0.46, p < .0004) but not in females (r = -0.07, p = .67).
Thoracolumbar shear strain in human subjects with and without LBP. Thoracolumbar shear strain was ~20% lower in human subjects with chronic LBP compared with No-LBP. *indicates p < .01. N = 121 subjects. Error bars represent standard errors.
Results of testing for perimuscular connective tissue thickness and echogenicity, trunk range of motion and functional measures for male and female subjects are shown in Table . Significant differences were found between the two groups for several outcome measures: flexion range of motion, extension range of motion and Sorrensen's endurance test were decreased in the LBP group while perimuscular connective tissue echogenicity, repeated trunk flexion task duration, repeated sit to stand task duration and 50 foot walk task duration (regular and fast pace) were increased in the LBP group. Some of the outcome measures (perimuscular connective tissue thickness, extension range of motion, repeated sit-to-stand task duration and Sorrensen's endurance test) were gender-specific (see letter superscripts in Table ).
Outcome Measures for Male and Female Subjects with and without Low Back Pain
Significant correlations were found in male subjects between thoracolumbar fascia shear strain and perimuscular connective tissue thickness (r = -0.45, p < .001), echogenicity (r = -0.28, p < .05), trunk flexion range of motion (r = 0.36, p < .01), trunk extension range of motion (r = 0.41, p < .01), repeated forward bend task duration (r = -0.54, p < .0001) and repeated sit-to-stand task duration (r = -0.45, p < .001). No significant correlations were found in females between thoracolumbar fascia shear strain and any of these outcome measures. There were also no significant correlations in either males or females between thoracolumbar fascia shear strain and measures of anxiety, cognitive function, mental health or psychological distress (MOS questionnaire) or kinesiophobia (Tampa questionnaire).