This research project is the first to investigate the role ofTrA size, voluntary contraction changes in size, and strength onthe frequency of ETAP. A significant difference in Sahrmann testscores was evident between groups of different frequency of ETAP.Although TrA thickness at rest was significantly different betweengroups, the change in thickness during the abdominal draw inmanoeuvre was not. Age and training volume was also differentbetween the groups but did not alter the significant group differ-ences for TrA relaxed thickness and Sahrmann test score.Participants who never experienced ETAP recorded significantlyhigher scores on the Sahrmann test than participants who expe-rienced ETAP weekly or yearly, suggestive of stronger anteriortrunk muscles in individuals who do not experience pain whilethey run. Additionally, those who never experienced ETAP aver-aged Sahrmann test scores of 2.3 points (or more than three times)higher than that scored by the participants who experience ETAPweekly. While the monthly group were not significantly differentto the never group the effect size for the difference between thesegroups was still moderate (0.55) and may have been due to studylimitations discussed later. As the Sahrmann test is a measure offunctional core stability it is proposed that these results supportthe hypothesis that trunk muscle activation may be an importantfactor for those who do not have pain while they run. High level acti-vation of these muscles may be required to stabilise the abdomen,and this may not be linear, but rather a stepwise relationship, asindicated by our regression analysis (IRR = 1.6).