**4.5 Suggestions with respect to future studies**

The fact that most studies of chronic trapezius myalgia, chronic WAD, and temporomandibular pain disorders included women is expected, as the prevalence of these conditions are higher in women. Future studies should also include groups of men with chronic muscle pain conditions. In addition, it is important to describe the patient group in detail with respect to clinically relevant examination parameters. Furthermore, systematic descriptions of the patient groups are needed that reflect pain intensity and psychological distress as well as consequences such as work participation and sick leave. A systematic description will allow a more accurate characterization of pain severity in a broad context. Most studies rely on bivariate correlations between pain descriptors such as pain intensity or pressure pain thresholds and the concentration of a certain substance. Multivariate correlation analyses and regression analyses are methods that can be used to investigate how groups of clinical examination variables, several simultaneous symptoms, and the concentrations of several biochemical substances intercorrelate. To better understand the potentially complex biochemical situation of the muscle in chronic pain conditions, it is necessary to also investigate the multivariate interrelationships between the concentrations of the investigated substances. According to ICD, the clinically used pain diagnoses are symptom diagnoses based on temporal and anatomical characteristics (e.g., chronic lumbago). Hence a certain diagnosis may include patients with different activated pathophysiological mechanisms. To identify subgroups of patients with identical pathophysiological mechanisms, it is important to use large patient groups and appropriate statistical methods (e.g., cluster analysis and principal component analysis).
