Application of 8 hours/16 hours/8 hours/16 hours stretch protocol. (A) Representative morphological images of human static and high mechanical strain (HMS) cultured annulus fibrosus (AF) and nucleus pulposus (NP) cells. Scale bar: 200 μm. (B) Gene expression analysis of both NP and AF cells after the additional 16-hour resting period. IVD, Intervertebral disc; NGF, Neuronal growth factor; TLR, Toll-like receptor; TNF, Tumour necrosis factor. Error bars represent SEM. Three independent experiments were performed. *P < 0.05 and **P < 0.01 by Student’s t-test.