Prolonged sitting is a major risk factor for lumbar spine disorders, significantly affecting both physical and mental health. However, conventional clinical diagnosis primarily relies on imaging evaluations conducted after symptom onset, often missing opportunities for early intervention and allowing for disease progression. To address this, this paper presents a diagnostic method based on electromyography (EMG) using an adaptive flexible electromyography sensor (FES). The FES consists of a thermo-responsive in situ gelation hydrogel and flexible mesh electrode patch. The hydrogel undergoes a sol–gel transition at body temperature, enabling conformal skin contact and strong adhesion. As a result, the adhesion of the FES is 15 times stronger than that of conventional EMG electrodes. Consequently, the contact impedance is significantly reduced to 40 kΩ/cm2 at 10 Hz, and a high signal-to-noise ratio of 23.28 dB is achieved, allowing for the effective monitoring of subtle electrophysiological signals during prolonged sitting. Overall, this research provides a foundation for the early-stage diagnosis of lumbar disorders, facilitating the transition of lumbar disease management from reactive treatment to proactive prevention.