In the last decades, ever more employees spend most of their working time at video screens, frequently assuming ergonomically incorrect postures. This condition often leads to the occurrence of musculoskeletal disorders. Among others, low back pain is the most widespread and socially expensive disease since it causes a reduction of work productivity and use of public healthcare. In this context, monitoring back postures to prevent the occurrence of such a condition and to support clinicians during postural treatments can be useful. To achieve this aim, many tools based on different technologies have been developed for the monitoring of the spinal range of motions. Recently, the increased use of fiber optic sensors led to the creation of ever more light and comfortable devices. In this work, a new custom wearable system based on Fiber Bragg Gratings technology for monitoring lumbar flexion-extension (F/ E) movements is presented. The sensitivity to strain (i.e., Sε) was assessed, as well as its capability of detecting lumbar F/E in the sagittal plane. Results show a high value of Sε and ascertain the wearable system ability to monitor back lumbar F/E over time.