To explore the differences in biomechanical load of the lumbar vertebrae-lumbar muscle system and their association mechanism with pain score (PS) in patients with low back pain under 2 postures: upright and bending at 15° with a load of 20 kg.

A total of 1 400 patients diagnosed with low back pain at the Dongnan Hospital Affiliated to Xiamen University from May 2018 to May 2021 were selected and stratified and grouped according to age (21-30, 31-40, 41-50, 51-60, 61-70, 71-80, 81-90 years old) and gender. Based on the principle of biomechanical circular stress, a mathematical and physical analytical stress model was constructed to analyze the stress PS distribution of L₃-L₄ and L₄-L₅ intervertebral discs under different genders, ages and postures (upright/bending at 15° with a load of 20 kg). Independent sample t-test was used to compare the PS differences between male and female groups. Pearson correlation analysis was used to analyze the correlation between PS and age. Linear trend test was used to test the linear trend between PS and age. Two-factor analysis of variance was used to compare the differences in the PS ratio of intervertebral disc stress between L₄-L₅ and L₃-L₄ in each age group between males and females.

Under the posture of bending 15° with a load of 20 kg, the stress PS of intervertebral discs at L₃-L₄ and L₄-L₅ in females were higher than those in males, and the differences were statistically significant (L₃-L₄: t=297.51, 58.77, 70.40, 36.57, 54.39, 73.86, 31.40, all P<0.01. L₄-L₅: t=60.00, 49.19, 31.28, 64.42, 180.81, 38.69, 64.35, all P<0.01) . In the upright position, the stress PS of the L₃-L₄ intervertebral discs in females were higher than those in males (t=78.31, 28.29, 71.04, 26.55, 24.40, 23.87, 28.38), but the stress PS of the L₄-L₅ intervertebral discs were lower than those in males (t=43.24, 40.98, 29.14, 4.61, 10.20, 67.22, 12.62), and the differences were statistically significant (all P<0.01). In the upright position, female age was correlated with the stress PS of intervertebral discs at L₃-L₄ and L₄-L₅, and there were significant linear trends (r=0.914, r²=0.835; r=0.939, r²=0.882; all P<0.01). However, for males in the upright position and the state of bending at 15°with a load of 20 kg, age was correlated with the stress PS of L₃-L₄ intervertebral discs, and there were significant linear trends (r=0.883, r²=0.780; r=0.973, r²=0.947; all P<0.01). The comparison of the PS ratio of intervertebral disc stress between L₄-L₅ and L₃-L₄ in the upright position between male and female genders (1.410±0.004, 1.180±0.004) showed a statistically significant difference (F=1.265, P<0.01).

Regardless of gender, the stress PS of L₃-L₄ and L₄-L₅ intervertebral discs is significantly positively correlated with age, suggesting that the biomechanical load tolerance capacity of the lumbar vertebrae-lumbar muscle system gradually decreases with age.