Ability of Thigh-Worn Actigraph and activPAL Monitors to Classify Posture and Motion

Purpose: This study compared sitting, standing, and stepping classifications from thigh-worn Actigraph and activPAL monitors under laboratory and free-living conditions.

Methods: Adults wore both monitors on the right thigh while performing six sitting, two standing, nine stepping, one cycling activity, and writing on a whiteboard with intermittent stepping under laboratory observation(n=21), and also during three days in free-living conditions(n=18). Percent time correctly classified was calculated for laboratory conditions. Between-monitor agreement and weighted kappa were calculated for free-living conditions.

Results: In the laboratory, both monitors correctly classified 100% of time standing and >95% of time in four of six sitting postures. Both monitors demonstrated misclassification of laboratory stool sitting time (Actigraph 14%, activPAL 95%).The activPAL misclassified 14% of the time spent sitting with legs outstretched; the Actigraph was 100% accurate. Monitors were >95% accurate for stepping, although the Actigraph was less so for descending stairs (86%), ascending stairs (92%) and running at 2.91 m/s (93%). Monitors classified whiteboard writing differently (Actigraph 83% standing/15% stepping, activPAL 98% standing /2% stepping). ActivPAL classified 93% of cycling time as stepping, while Actigraph classified <1% of it as stepping. During free-living wear, monitors had substantial agreement (86% observed, weighted kappa=0.77). Monitors classified similar amounts of time as sitting (Actigraph 64%, activPAL 62%). There were differences in time recorded as standing (Actigraph 21%, activPAL 27%), and stepping (Actigraph 15%, activPAL 11%).

Conclusions: Differences in data processing algorithms may have resulted in observed disagreement in posture and activity classification between thigh-worn Actigraph and activPAL in specific activity and/or posture types. Despite between monitor agreement in classifying sitting time under free-living conditions, the Actigraph appears more sensitive to free-living upright walking motions than the activPAL.