Aging involves neuromuscular changes such as a decline in ankle push-off power and ankle flexibility, which has also been known to differ depending on sex. This study investigated effects of age and sex on muscle mechanics and gait patterns during incline walking where a larger ankle flexion is required. Healthy young and older males and females walked on at four slope angles in the normal and longer stride length. GM muscle ultrasound showed a shorter operating range for older adults. When walking with a longer stride length, ankle plantarflexion increased at higher slopes. As the stride length increased, young adults increased RF and GMAX muscle activity while older adults decreased GM activity. Our findings suggest a different adaptation strategy to the inclined surfaces when modifying stride length depending on age, which can be used as a guideline during gait training to improve walking performance for individuals with different ankle muscle and joint functions.