Correlation of Physical Activity Level with Muscle Strength and Size During One Week of Knee Joint Immobilization

<b>Background</b>: Knee joint immobilization is common after surgery or injury. Whether remaining physically active during immobilization preserves muscle strength and size has not been studied. <b>Objectives</b>: This observational study examined correlations between muscle strength, size, and physical activity (PA) levels during one week of knee joint immobilization. <b>Methods</b>: Nine healthy adults (five males, four females) immobilized their left knee and ambulated with crutches for one week. Ankle accelerometers monitored compliance and tracked PA. Isometric and concentric isokinetic peak torque at 30°/s and 180°/s and vastus lateralis (VL) cross-sectional area (CSA) were assessed before and after immobilization. Bivariate correlations were used to examine relationships between time spent in sedentary, light, moderate, and vigorous PA, and changes in isometric and concentric isokinetic peak torque, as well as VL CSA. <b>Results</b>: After immobilization, isometric strength declined by 17.1%. Concentric isokinetic peak torque declined by 5.5% at 30°/s and 2.3% at 180°/s. VL CSA declined by 6.7%. There were weak correlations between strength measures and PA levels (<i>r</i> = -0.497-0.574; <i>p</i> = 0.106-0.709). For CSA, an unexpected pattern was found in which greater sedentary time was correlated with decreased atrophy (<i>r</i> = 0.701; <i>p</i> = 0.035), but light (<i>r</i> = -0.673; <i>p</i> = 0.047) and moderate (<i>r</i> = -0.738; <i>p</i> = 0.023) PA levels were correlated with increased atrophy. Vigorous PA had weak correlations with CSA (<i>r</i> = -0.321; <i>p</i> = 0.399). <b>Conclusions</b>: Contrary to our hypothesis, increased PA levels were not correlated with the preservation of strength and were correlated with greater declines in CSA during knee joint immobilization.