The comparison of individuals with tension-type headache and headache-free controls on frontal EMG levels: a meta-analysis.

For many years, the dominant theory of tension-type headache has been that individuals who suffer from these headaches have elevations in electromyographic (EMG) activity at rest or during stressful life experiences. This theory has come under increasing attack in recent years. Although the research that is relevant to the controversy has been reviewed previously using box score and voting methods, no one has analyzed the relevant data using a statistical approach. This study used meta-analysis to summarize the studies that have directly compared individuals with tension-type headache to headache-free control subjects on measures of frontal EMG at baseline and during rest. Twenty-three studies were included in the analysis. The overall effect size was .395; however, there was heterogeneity of effect size. Several potential moderator variables were investigated. Perhaps the most interesting moderator variable was whether the studies clearly stated that subjects were headache-free during assessment. This group of studies produced a nonsignificant effect size that had homogeneity of effects. Several interpretations of the results are presented.