Investigation of task-specific biomechanical control strategies during sit-to-stand-to-sit in patients with knee osteoarthritis

: Knee osteoarthritis (KOA) progressively limits knee function and the ability to perform high-demand activities, such as sit-to-stand-to-sit (STS). STS tests do not allow arm support, limiting clinical applicability. This work evaluates the assistance and phase-dependent effects on knee loading, postural sway, and smoothness. Unassisted (uSTS) and assisted (aSTS) executions were evaluated in eighteen patients with KOA using three-dimensional motion capture and force platforms. Knee moments, centre-of-pressure (CoP) velocity and range in anteroposterior (AP) and mediolateral (ML) directions, and Spectral Arc Length (SPARC) were computed separately for ascent and descent. Differences between phases and modalities were evaluated with the Wilcoxon signed-rank test, and correlation with Spearman's rank correlation. Knee flexion moment (KFM) was higher in uSTS than aSTS, especially during ascent, and within modalities was greater in ascent. AP-CoP velocity and range were greater in uSTS during descent, while within aSTS both were higher in ascent. Smoothness was higher in aSTS across phases. Correlations were task and phase-dependent. In aSTS ascent, KFM correlated with AP-CoP range (ρ = 0.61, p < 0.05), and AP-SPARC with KFM (ρ = 0.66, p < 0.05). In uSTS during ascent, KFM correlated with ML-CoP range (ρ = 0.72, p < 0.05), while V-SPARC correlated negatively with KFM (ρ = - 0.50, p < 0.05). Including both assisted and unassisted STS provides a more complete characterisation of knee loading and control. Unassisted protocols capture a different control strategy than everyday assisted transfers, so assessments should include both modalities to guide patient-specific assessment and rehabilitation.