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Using both assisted and unassisted sit-to-stand tests provides a more complete picture of knee loading and movement control in patients with knee osteoarthritis.
Patients with knee osteoarthritis (KOA) often depend on their arms to rise from a chair, yet standard clinical sit-to-stand-to-sit (STS) assessments usually prohibit upper-limb support. A new biomechanical study suggests that this conventional approach may overlook fundamental movement strategies employed during everyday activities, highlighting the value of assessing both assisted and unassisted transfers.
Letizia Mancini and other researchers evaluated 18 patients with KOA performing both unassisted STS (uSTS) and assisted STS (aSTS) using three-dimensional motion capture and force platforms. The ascent and descent phases were analyzed separately to quantify knee flexion moment (KFM), centre-of-pressure (CoP) velocity and excursion in the anteroposterior and mediolateral directions, and movement smoothness using Spectral Arc Length (SPARC). Differences between tasks and phases were assessed via the Wilcoxon signed-rank test, while relationships among biomechanical variables were examined using Spearman's rank correlation.
The analysis showed that unassisted STS generated higher KFM than assisted STS, with the greatest differences noted during the ascent phase. Within both testing conditions, knee loading was consistently higher during ascent than descent, indicating that standing up imposed greater mechanical demands on the knee than sitting down. Postural control also differed between the two task conditions. During unassisted STS, anteroposterior centre-of-pressure velocity and range were greater during the descent phase, suggesting increased postural adjustments while sitting down without arm support.
In contrast, during assisted STS, both anteroposterior CoP velocity and anteroposterior CoP range were higher during the ascent phase. Across both movement phases, assisted STS elicited smoother movements, as reflected by higher SPARC values. Correlation analyses further illustrated that movement strategies varied according to both task type and movement phase. During assisted ascent, greater knee loading was linked with a larger anteroposterior CoP range (ρ = 0.61, p < 0.05), while anteroposterior-SPARC positively correlated with KFM (ρ = 0.66, p < 0.05), indicating smoother motion with increasing KFM.
In unassisted ascent, KFM positively correlated with mediolateral CoP range (ρ = 0.72, p < 0.05), whereas vertical SPARC showed a negative correlation with KFM (ρ = −0.50, p < 0.05), reflecting a different biomechanical control strategy when upper-limb support was unavailable. The investigators concluded that assisted and unassisted STS tasks capture different biomechanical control strategies, and relying solely on conventional unassisted testing may not fully reflect patients' functional performance during daily activities.
Incorporating both testing modalities into clinical assessment could offer a more comprehensive evaluation of knee loading, balance control, and movement quality, enabling more individualized rehabilitation strategies for KOA.
Journal of Biomechanics
Investigation of task-specific biomechanical control strategies during sit-to-stand-to-sit in patients with knee osteoarthritis
Letizia Mancini et al.
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