Maarten van Egmond

71 The association of muscle strength with muscle mass 4 We described standardized operating procedures (SOPs) of all measurements in order to guarantee uniformity and accuracy in operationalization as well as prerequisites for inter-rater reliability. Trained and experienced physiotherapists executed the standardized measurement protocol. All physiotherapists received an in-depth training of the study protocol, the standardized measures as well as data registration. Statistical analysis We entered and analyzed all data in the Statistical Package for the Social Sciences (SPSS Inc., Chicago, IL), version 21.0. We checked data for completeness and skewness with the Shapiro-Wilk test. We summarized baseline characteristics with descriptive statistics, where discrete variables were expressed as counts with percentages, ordinal variables as median and interquartile ranges (P25-75) and continuous variables as mean and standard deviation and in case of a skewed distribution as median and interquartile range. For evaluating the relation between muscle mass and muscle strength, we compared data using Pearson correlation coefficients ( r ), interpreted according to Zou et al 34 (rating r = 0.5 as moderate and r = 0.8 as strong). We built linear regression models on skeletal muscle area at L3 with each aspect of muscle strength as an exploratory variable, adjusted for age, gender, height, and weight. Consecutively, we forced all variables in one multiple linear regression model and with backward elimination we created a final regression model. We tested all hypotheses 2-tailed with a significance level set to .050 and we considered regression coefficients significantly different if the 95% confidence intervals (CI) were not overlapping.