Time course of perceived knee stiffness following prolonged sitting in healthy adults.

Background: Knee stiffness is a common complaint experienced during prolonged periods of sedentary behaviour, including following prolonged sitting, in both healthy and people with musculoskeletal conditions. Reduction of self-reported knee stiffness is important to patient-centred outcomes. Our aim was to obtain pilot data to determine the timeframe for knee stiffness onset in healthy populations. Methods: Forty-two participants (20-76 years) with no diagnosed chronic musculoskeletal condition were recruited. After pilot testing, perceived knee joint stiffness and pain (Visual Analogue Scale) were measured while sitting (at 15, 30, 60, and 90 ​min) with the knee joint held at 90°. Correlations between knee stiffness after 90 ​min of sitting and baseline demographic data, and knee range of motion were analysed. Results: Greater levels of self-reported stiffness were observed (mean ​± ​SD) after sitting still for 30 (1.8 ​± ​1.6; p ​< ​0.001), 60 (3.4 ​± ​2.4; p ​< ​0.001) and 90 ​min (4.1 ​± ​2.6; p ​< ​0.001) compared to baseline (n ​= ​31). Increase of self-reported pain occurred by 60 and 90 ​min of sitting (p ​< ​0.001). Minimal clinical important difference (MCID) (±2 ​cm) in stiffness occurred by 60 ​min of sitting (+2.6 ​cm) with pain not reaching MCID by 90 ​min. Self-reported stiffness at 90 ​min of sitting was negatively correlated with participant age (r ​= ​-0.433, p ​= ​0.004). Conclusions: Healthy adults begin to experience knee stiffness when sitting still after 30 ​min, though this was only clinically relevant by 60 ​min. Moving the joint, even whilst still sitting, helped to alleviate joint stiffness and pain, which may assist in providing advice to all adults on sedentary behaviours.