Biomechanical analysis of spinal loading in infantry personnel during multi-day carriage of combat equipment exceeding 35 kg

Low back pain represents the leading musculoskeletal disorder among military personnel, with load carriage training considered a primary etiological factor. The aim of this study was to examine biomechanical changes in the lumbar segment of the spinal column among infantry unit members during multi-day carriage of combat equipment exceeding 35 kg, with emphasis on cumulative effects of intervertebral disc compression, changes in lumbar lordosis, and paravertebral musculature activity. Thirty-two active infantry members participated in the study (mean age 24.7 ± 3.2 years, body mass 78.4 ± 6.8 kg), who underwent a five-day field march with combat equipment averaging 38.6 kg. Measurements included magnetic resonance imaging of the lumbar spine before and after the protocol, surface electromyography of paraspinal muscles, and three-dimensional motion analysis. Results demonstrated statistically significant reduction in intervertebral disc height at levels L4-L5 (0.87 ± 0.23 mm, p < 0.01) and L5-S1 (1.12 ± 0.31 mm, p < 0.001), reduction of the lumbar lordotic angle by 8.4° ± 2.7° (p < 0.01), and significant increase in musculus erector spinae activation measured as a 34.7% increase in integrated EMG amplitude (p < 0.001). The original contribution of this research represents the development of a predictive model of cumulative spinal loading demonstrating that compressive force on the L5-S1 segment reaches a critical value of 5.23 times body weight after 72 hours of continuous load carriage, thereby defining a temporal threshold for mandatory spinal recovery periods. Results suggest that multi-day carriage of combat equipment exceeding 35 kg induces progressive biomechanical changes in the lumbar segment of the spinal column that require implementation of structured rest protocols and preventive interventions in military operations.