Biomechanical phenotypes of unilateral versus bilateral anterior knee pain: evidence from walking kinematics, kinetics, and symmetry indices.

Background: Anterior knee pain (AKP) is a common musculoskeletal condition with heterogeneous presentations, including unilateral and bilateral symptoms. Although altered gait biomechanics have been reported in AKP, it remains unclear whether unilateral and bilateral AKP exhibit distinct biomechanical phenotypes, particularly in terms of interlimb asymmetry. Purpose: To compare lower-limb kinematics and joint moments during level walking between individuals with unilateral and bilateral AKP, and to quantify interlimb asymmetry using symmetry indices. Methods: Seventy-one individuals with AKP (unilateral: n = 36 (16 male and 20 female); bilateral: n = 35 (16 male and 19 female); age range: 11-35) underwent three-dimensional gait analysis at a self-selected speed. Hip, knee, and ankle joint angles (three planes) and normalized joint moments were extracted. The primary kinetic outcome was peak knee extension moment within 0%-60% of the gait cycle. Predefined kinematic and kinetic peak measures for the hip, knee, and ankle joints during defined phases of the stance period were also analyzed. We collected surface electromyographic (sEMG) data from a subset of participants to analyze muscle activation patterns during gait. Interlimb asymmetry was quantified using symmetry indices between more symptomatic (MS) limb for unilateral AKP; and less symptomatic (LS) limb. Between-group comparisons (unilateral vs. bilateral AKP) for baseline characteristics and symmetry indices (SI) were conducted using independent samples t-tests or Mann-Whitney U tests. Within-group differences (more symptomatic vs. less symptomatic limb) for kinematic and kinetic parameters were analyzed using paired t-tests or Wilcoxon signed-rank tests. Results: Both groups demonstrated significantly reduced peak knee flexion angle and knee extension moment in the more symptomatic limb during stance. Distinct distal adaptations were observed: In the unilateral AKP group, the more symptomatic limb showed lower peak ankle plantarflexion moment (0.95 ± 0.21 N·m/kg vs. 1.06 ± 0.22 N·m/kg, p < 0.05) and a greater foot progression angle (10.03° ± 3.93° vs. 7.62° ± 3.34°, p < 0.05) compared to the less symptomatic limb. In the bilateral AKP group, peak ankle dorsiflexion angle was lower on the more symptomatic side (10.89° ± 3.36°) than on the less symptomatic side (12.60° ± 3.86°) (p < 0.05). Symmetry index analysis revealed no significant between-group differences across most variables, indicating comparable overall asymmetry. sEMG findings showed reduced rectus femoris activation in the symptomatic limb. Conclusion: Unilateral and bilateral AKP share a protective gait strategy characterized by reduced sagittal-plane knee loading. However, overall gait asymmetry does not differ significantly between groups, suggesting that bilateral AKP may also involve side-specific compensations or globally conservative movement patterns. These findings highlight that the magnitude of interlimb asymmetry alone may not distinguish between unilateral and bilateral presentations. Instead, the observed differences in distal joint adaptations (e.g., ankle dorsiflexion, plantarflexion moment, foot progression angle) provide more informative biomechanical targets for phenotype-specific rehabilitation strategies.