The impact of skeletal muscle disuse on distinct echo intensity bands: A retrospective analysis

Echo intensity (EI) is a novel tool for assessing muscle quality. El has traditionally been reported as the mean of the pixel histogram, with 0 and 255 arbitrary units (A.U.) representing excellent and poor muscle quality, respectively. Recent work conducted in youth and younger and older adults suggested that analyzing specific El bands, rather than the mean, may provide unique insights into the effectiveness of exercise and rehabilitation interventions. As our previous work showed deterioration of muscle quality after knee joint immobilization, we sought to investigate whether the increase in El following disuse was limited to specific El bands. Thirteen females (age = 21 yrs) underwent two weeks of left knee immobilization and ambulated via crutches. B-mode ultrasonography was utilized to obtain images of the immobilized vastus lateralis. The percentage of the total number of pixels within bands of 0-50, 51-100, 101-150, 151-200, and 201-255 A.U. was examined before and after immobilization. We also sought to determine if further subdividing the histogram into 25 A.U. bands (i.e., 0-25, 26-50, etc.) would be a more sensitive methodological approach. Immobilization resulted in a decrease in the percentage of pixels within the 0-50 A.U. band (-3.11 +/- 3.98%), but an increase in the 101-150 A.U. (2.94 +/- 2.64%) and 151-200 A.U. (0.93 +/- 1.42%) bands. Analyses of variance on the change scores indicated that these differences were large and significant ( %EI0-50VS. %EI101-150: p< .001, d= 1.243); %EI0-50 vs. %EI151-200: P = .043, d = 0.831). The effect size for the %EI51-100 versus %EI(101-150)comparison was medium/large (d= 0.762), but not statistically significant (p = .085). Further analysis of the 25 A.U. bands indicated that the percentage of pixels within the 25-50 A.U. band decreased (-2.97 +/- 3.64%), whereas the 101-125 (1.62 +/- 1.47%) and 126-150 A.U. (1.18 +/- 1.07%) bands increased. Comparison of the 50 A.U. and 25 A.U. band methods found that 25 A.U. bands offer little additional insight. Though studies are needed to ascertain the factors that may influence specific bands, changes in El during muscle disuse are not homogeneous across the pixel histogram. We encourage investigators to think critically about the robustness of data obtained from El histograms, rather than simply reporting the El(mean) value, in muscle quality research.

Automated summary

The Echo intensity (EI) is a novel tool for muscle quality assessment. By analyzing specific EI bands, unique insights into the effectiveness of exercise and rehabilitation interventions can be gained. Changes in muscle quality during disuse are not uniform across the pixel histogram, with significant differences observed between certain EI bands.