Muscle Tenderness in Women

Sammanfattning: Longstanding pain experienced from the musculoskeletal system is reported by one third of the Swedish population. The underlying mechanisms are not known in the majority of cases. In the present studies, the changes in pressure pain thresholds (PPTs) over the trapezius and deltoid muscles after a unilateral static contraction of maximal duration of one arm was measured in healthy females (n=25) and in a group of female cleaners with longstanding shoulder pain (n=17). Simultaneous measurements of EMG-related muscular fatigue were performed over both muscles, evaluating the fatigue in the trapezius muscle by an increased amplitude (root mean square) and in the deltoid by a decreased mean power frequency of the EMG signals. A paradoxical bilateral and longstanding (10-20 min) increase of PPTs was observed, while the EMG-related muscular fatigue was shortlasting with half recovery within 15 s after the end of contraction. The findings indicate that central sensory changes, maybe through descending inhibition, are important for the mechanonociceptive thresholds in muscle. In a separate study on 24 healthy females, the test-retest reliability of PPT measurements on the trapezius and deltoid muscles bilaterally was examined with an electronic algometer (Somedic) at four experimental sessions, one day and four weeks apart, each including four consecutive measurements at 10 min intervals. Two examiners participated. The intraindividual test-retest repeatability for each point in all sessions was high, intraclass correlation coefficient (ICC): 0.70 - 0.94, mean max abs. difference 53 - 102 kiloPascal (kPa), and between points in each muscle, ICC: 0.59 - 0.90, mean max abs. difference 65 - 111 kPa. Side to side differences were small. The interindividual variation of PPTs was fivefold (trapezius 88 - 574 kPa; deltoid 91 - 529 kPa), necessitating normalization in group level studies. Inter-rater repeatability was high with no significant differences. The effect of repeated (30) mechanocociceptive stimuli (1.5 times pain threshold; 0.4 Hz) over one trapezius muscle on PPTs in the same point was explored in a study on 27 healthy females. Two reaction types were found, either a PPT decrease of 34% (n=12) or a PPT increase of 23% (n=15), both significant and lasting 5 - 10 min. On repeated PPT measurements of nine subjects with a "decrease" reaction a strong correlation was found between sequential observations (r= 0.54; p< 0.001; Pearson). The "increase" reaction was significantly associated with the experience of childbirths. In conclusion, central changes in sensory transmission are important for pressure pain (tenderness) sensitivity in human muscle.