Altered Passive Knee Kinematics in Type 2 Diabetes Mellitus Revealed by Pendulum Test and Model-Derived Viscoelastic Parameters

Background: Type 2 diabetes mellitus (T2DM) is associated with musculoskeletal alterations that may affect joint mechanics, yet quantitative characterization of passive knee behavior remains limited. This study investigated passive knee mechanics in individuals with T2DM using an instrumented pendulum test combining kinematics with biomechanical modeling. Methods: Twenty matched participants (10 with T2DM, 10 controls) underwent ten pendulum test trials of the right lower limb under electromyographic monitoring. Primary kinematic outcomes included flexion and extension amplitudes, angular velocities, relaxation indices, and oscillation duration, whereas model-derived stiffness and viscosity were secondary outcomes. Participant-level kinematic outcomes were compared using Wilcoxon rank-sum tests with Benjamini–Hochberg false discovery rate correction. Stiffness and viscosity across successive flexion–extension half-cycles were analyzed using aligned rank transform ANOVA models including Group, flexion-extension Half-Cycle, and their interaction. Results: Compared with controls, participants with T2DM showed reduced normalized first flexion and extension amplitudes and reduced extension velocity, whereas first flexion velocity was not significantly changed. Plateau amplitude and oscillation duration also differed between groups, while relaxation indices did not. Model-derived knee stiffness and viscosity showed no significant overall Group effect but varied across half-cycles and exhibited significant Group × Half Cycle interactions. However, post hoc comparisons did not identify significant between-group differences at corresponding individual Half-Cycles after correction for multiple comparisons. Conclusions: Individuals with T2DM exhibited differences in passive knee behavior, primarily characterized by reduced oscillatory excursions and altered temporal characteristics. Exploratory analyses of model-derived stiffness and viscosity indicated differences in their phase-dependent patterns across the pendular response, although no specific half-cycle showed a significant between-group difference. These exploratory findings require confirmation in larger cohorts. Instrumented pendulum testing may provide complementary, non-invasive information on system-level passive knee kinematics beyond conventional clinical assessment.