Effects of Fatigue on Six-Degree-of-Freedom Knee Kinematics in Military Personnel Marching on a Treadmill

Background: Knee injuries are common during military training, and fatigue alters lower-limb movement patterns, increasing injury risk. However, little is known about fatigue effects on knee kinematics during military marching. Conventional gait analysis is limited to sagittal plane motion, leaving multi-planar knee function poorly understood. Objective: This study investigated six-degree-of-freedom (6DOF) knee kinematic changes in healthy male military personnel marching at 8 km/h on a treadmill before and after fatigue, focusing on initial contact (IC), stance-phase flexion peak (SPF), and stance-phase extension peak (SPE). Methods: Thirty-eight healthy male soldiers marched on a treadmill at 8 km/h. A motion capture system (infrared stereo cameras + reflective markers) collected 6DOF kinematic data. Fatigue was induced by loaded marching (15 kg vest), with the endpoint defined as Borg RPE ≥ 18 or heart rate ≥ 90% of age-predicted maximum. Post-fatigue data were collected at the same speed. Flexion, rotation, and anterior–posterior displacement at IC, SPF, and SPE were compared pre- vs. post-fatigue. The Holm–Bonferroni correction was applied for multiple comparisons. Results: After Holm–Bonferroni correction, knee flexion angles decreased significantly at SPF (3.64°, p = 0.009, Cohen’s d = 0.52) and SPE (2.75°, p = 0.032, d = 0.41), with a non-significant trend at IC (2.53°, p = 0.086). Internal rotation shifted toward external rotation at IC (2.48°, p = 0.039, d = 0.46) and SPE (2.99°, p = 0.036, d = 0.54), with a non-significant trend at SPF (p = 0.076). No other kinematic changes, including anteroposterior displacement, exceeded the system’s measurement error threshold or remained significant after correction. Conclusions: Fatigue reduces knee flexion during the stance phase at 8 km/h (with the most robust changes at SPF and SPE) and shifts internal–external rotation toward external rotation at IC and SPE. These 6DOF kinematic changes may compromise knee stability and increase ACL and patellofemoral injury risk.