With global aging accelerating, effective health assessment tools for the elderly are increasingly needed. Lower limb flexibility is a key determinant of functional independence and quality of life, while reduced flexibility heightens fall risk and mobility impairments. Existing measurement tools are often complex, costly, and difficult for elderly individuals to use independently, limiting their accessibility in home and community settings. This study develops a simple, portable, and user-friendly tool to assess lower limb flexibility based on human factors engineering principles. The tool accurately measures joint range of motion (ROM) in non-professional settings, enabling independent use by elderly individuals. A literature review was conducted to identify limitations in existing methods, and the hip, knee, and ankle joints were selected as primary reference points based on skeletal anatomy. This approach ensures measurement precision and standardization. Additionally, the tool integrates Digital Human Modeling (DHM) applications, allowing motion data to support ergonomic design, rehabilitation strategies, and personalized healthcare solutions. This fusion of physical measurement and computational modeling enhances flexibility assessments, making them more data-driven and standardized. This study advances elderly health monitoring by providing an accessible, scientifically grounded solution for flexibility assessment. The tool simplifies the measurement process while ensuring accuracy, supporting elderly health management, rehabilitation planning, and fall prevention. Its ergonomic design and DHM integration enable widespread adoption in home, clinical, and community environments.