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<jats:title>Purpose:</jats:title>
<jats:p>We evaluated the feasibility of quantification of physical activity (PA) and sedentary behaviors (SB) using actigraphy during an entire intensive care unit (ICU) length of stay.</jats:p>
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<jats:title>Methods:</jats:title>
<jats:p>A prospective study was performed in a 12-bed ICU. Triaxial accelerometers were fitted on the right ankle of mechanically ventilated adults. Twenty accelerometers were available to guarantee uninterrupted actigraphy recording 24 hours/day. Data were analyzed: (1) between awakening and ICU discharge to quantify daytime PA/SB and (2) between admission and ICU discharge to quantify day/nighttime inactivity. Secondarily, we assessed the relationship between inactivity/SB and clinical variables.</jats:p>
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<jats:title>Results:</jats:title>
<jats:p>Thirty patients were enrolled, obtaining 5477 recording hours. No patient reported discomfort or injury. The median (min-max) delay time between admission and accelerometer installation was 2.1 (0.0-11.9) hours. Actigraphy recording duration was 5.4 (2.2-34.4) days. The time spent in SB and PA (percentage of minutes per hour) was 94.7% and 5.3%, respectively. PA was stratified by light, moderate, and vigorous levels equating to 91.8%, 7.7%, and 0.5%, respectively. Inactivity time (<jats:italic toggle="yes">r</jats:italic> = 0.991, <jats:italic toggle="yes">P</jats:italic> ≤ .001) and SB (<jats:italic toggle="yes">r</jats:italic> = 0.859, <jats:italic toggle="yes">P</jats:italic> ≤ .001) were strongly correlated with ICU length of stay.</jats:p>
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<jats:title>Conclusions:</jats:title>
<jats:p>Quantifying PA levels with continuous monitoring through actigraphy is feasible, demonstrating prolonged periods of inactivity/SB. This study highlights that uninterrupted actigraphy could contribute to pursuing the optimal dose and the intervention fidelity of the ICU mobilization in the subsequent clinical trials.</jats:p>
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