Abstract: BACKGROUND:  Systemic fatigue increases injury risk in individuals with functional ankle instability, while stop-jump cutting is a high risk for ankle injuries. The biomechanical mechanisms underlying non-anticipated stop-jump cutting during systemic exercise fatigue in this population remain unclear.
OBJECTIVE: To quantify differences in kinematic and kinetic characteristics during non-anticipated stop-jump cutting before and after exercise fatigue between individuals with functional ankle instability and healthy controls, revealing the impact of exercise fatigue on stop-jump cutting in individuals with functional ankle instability.
METHODS: Fifteen male participants with unilateral functional ankle instability and 15 healthy male controls were recruited. Kinematic (ankle dorsiflexion, plantar flexion, inversion; knee flexion, varus, valgus; hip flexion; and hip abduction angles) and kinetic parameters (hip, knee, and ankle joint stiffness) during stop-jump cutting before and after exercise fatigue. Statistical analysis of peak joint angles and joint stiffness was performed using two-way repeated measures analysis of variance. Additionally, statistical parametric mapping was used to further analyze the effects of exercise fatigue on ankle joint angles and time series of ground reaction forces in both groups.
RESULTS AND CONCLUSION: (1) Kinematic characteristics: Two-way repeated measures analysis of variance revealed significant main effects of fatigue and significant interaction effects between groups for peak ankle inversion angle, peak knee flexion angle, peak knee valgus angle, and peak hip flexion angle (P < 0.05). Peak ankle inversion angle was significantly increased in the functional ankle instability group post-exercise fatigue compared with pre-exercise fatigue (P < 0.05). Peak knee flexion angle was significantly increased in both groups post-exercise fatigue (P < 0.05), while peak hip flexion angle was significantly increased in the healthy control group post-exercise fatigue (P < 0.05). After exercise fatigue, the peak ankle inversion angle and peak hip flexion angle in the functional ankle instability group were significantly decreased compared with those in the healthy control group (P < 0.05), while the peak knee valgus angle and peak knee flexion angle were significantly increased compared with those in the healthy control group (P < 0.05). Statistical parametric mapping analysis revealed that the peak ankle inversion/eversion angles during the 4%-18% phase of lateral cutting movements were greater in the functional ankle instability group post-exercise fatigue compared with pre-exercise fatigue (P < 0.05). (2) Kinetic characteristics: two-way repeated measures analysis of variance revealed significant main effects of fatigue and group interaction effects on hip, knee, and ankle joint stiffness (P < 0.05). Compared with pre-exercise fatigue, hip and ankle joint stiffness was significantly decreased in the healthy control group post-exercise fatigue (P < 0.05). Knee and ankle joint stiffness was significantly decreased in the functional ankle instability group post-exercise fatigue (P < 0.05). Statistical parametric mapping analysis revealed that in the functional ankle instability group, the vertical ground reaction force during the 5%-16% phase of the cutting phase was greater post-exercise fatigue than pre-exercise fatigue (P < 0.05), and the medio-lateral ground reaction force during the 35%-49% phase of the cutting phase was greater post-exercise fatigue than pre-exercise fatigue (P < 0.05). (3) These findings suggest that exercise fatigue alters the kinematic and kinetic characteristics during stop-jump cutting in individuals with functional ankle instability, particularly affecting knee and ankle joint stability and shock absorption capacity. Fatigue reduces joint stiffness and control force while increasing injury risk, especially during the initial and transitional phases of stop-jump cutting.

Key words: functional ankle instability, exercise fatigue, non-anticipated stop-jump cutting, kinematics, kinetics, statistical parametric mapping, neuromuscular control, sports injury prevention