A new systematic review examined only trials where the strengthening intervention was intense enough to meet the American College of Sports Medicine's definition of true progressive resistance training: a load of 8 to 12 repetitions maximum (better known as 12RM) for at least 2 sets with progressive increases of the load.
11 studies involving 370 participants were included in this systematic review. For each trial that measured multiple muscle groups, a preliminary pooled analysis of the multiple muscle groups was performed in order to generate one standardised mean difference (SMD) for each study.
This required an adjustment to account for the fact that multiple data from the same participants were being used. In order to reduce the sample size of each trial to its ‘effective sample size’, the sample size of each group was divided by the ‘design effect’. This simply meant that the review didn't need to discard relevant data by choosing just one muscle group. Neat, huh?