Effect of Slope Angle on Lower-Limb Joint Reaction Forces during Human Walking

Walking on inclined and declined surfaces introduces distinct biomechanical challenges that alter lower-limb joint loading and stability. This study aimed to investigate the effects of surface inclination on ankle joint reaction forces and to determine the safest walking angle based on the minimum forces exerted. Ten healthy male participants from Universiti Malaysia Perlis walked at self-selected speeds across slope angles of 0°, ±5°, ±7.5°, and ± 10°. Motion capture data were collected using a Qualisys Track Manager system integrated with embedded force plates, and joint reaction forces were analyzed through Visual3D software using inverse dynamics. Data were filtered with a fourth-order Butterworth low-pass filter at 4 Hz to obtain accurate kinematic and kinetic outcomes. The findings revealed that the mean maximum ankle joint reaction force remained nearly constant between level walking and 5° inclination but decreased noticeably at 7.5° and 10° slopes in both uphill and downhill conditions. Statistical analysis showed no significant difference between level and uphill walking, whereas a significant reduction in joint reaction force was observed at −7.5° and − 10° declinations. The ankle joint consistently exhibited the highest load compared to the knee and hip joints across all slope angles. Decline walking demonstrated lower mechanical demands on the ankle, indicating a safer condition compared to incline walking. The study provides valuable insight into joint behavior on sloped terrains and underscores the importance of biomechanical understanding and clinical awareness to minimize joint stress and potential musculoskeletal injury during locomotion on inclined and declined surfaces.