Objectives: To investigate biomechanical changes in lumbar disc herniations. Methods: Patients with lumbar disc herniation verified on a 1.5–3-T magnetic resonance imaging (MRI) scanner were imaged in a weight-bearing 0.25-T MRI scanner in (1) standing position, (2) conventional supine position with relative lumbar flexion, and (3) supine position with a forced lumbar extension by adding a lumbar pillow. The L2-S1 lordosis angle, the disc cross-sectional area, the disc cross-sectional diameter, and the spinal canal cross-sectional diameter were measured for each position. Disc degeneration and nerve root compression were graded, and the pain intensity was reported during each scan position. Results: Forty-three herniated discs in 37 patients (36.7 ± 11.9 years) were analyzed in each position. The L2-S1 lumbar angle increased in the standing position (mean difference [MD]: 5.61°, 95% confidence interval [95% CI]: 3.44 to 7.78) and with the lumbar pillow in the supine position (MD: 14.63°, 95% CI: 11.71 to 17.57), both compared with the conventional supine position. The herniated disc cross-sectional area and diameter increased during standing compared with during conventional supine position. No changes were found in the spinal canal cross-sectional diameter between positions. Higher nerve root compression grades for paracentral herniations were found during standing compared with during conventional supine position. This was neither found with a lumbar pillow nor for central herniations in any position compared with conventional supine. Conclusion: Disc herniations displayed dynamic behavior with morphological changes in the standing position, leading to higher nerve root compression grades for paracentral herniated discs. Key Points: • Lumbar herniated discs increased in size in the axial plane during standing. • Increased nerve root compression grades for paracentral herniated discs were found during standing. • Weight-bearing MRI may increase the diagnostic sensitivity of nerve root compression in lumbar disc herniations.