Muscle activity generates both physiological electrical and mechanical signals, the monitoring of which is crucial in rehabilitation and sports medicine, underpinning effective diagnosis, treatment, and rehabilitation processes. Advances in flexible electronics enable force myography (FMG) and surface electromyography (sEMG) signals for muscle activation monitoring, but the multi-sensor integration and physiological mechanisms underlying FMG signals remain poorly studied, limiting the accuracy of muscle function assessments and underutilizes the high sensitivity of the flexible sensors. This study introduces a novel thin-film iontronic force-electromyography (iFEMG) sensor, integrating a high-sensitivity iontronic pressure sensor and sEMG electrodes for high-fidelity muscle physiological signal acquisition. Based on ultrasound imaging and statistical analysis, the relationship between muscle force, muscle geometric features, and FMG signals is established, providing evidence for elucidating the physiological mechanisms of FMG signals. Based on these findings, an effective and highly adaptable method is proposed for precise muscle force prediction. The iFEMG system is successfully applied to assess motor nerve and muscle function in patients, demonstrating its clinical utility. This system holds significant potential for broader applications, such as rehabilitation training and early diagnosis of musculoskeletal disorders, paving the way for advanced personalized healthcare solutions.