Changes in pressure pain sensitivity in latent myofascial trigger points in the upper trapezius muscle after a cervical spine manipulation in pain-free subjects

Objective: This study analyzed the immediate effects on pressure pain threshold (PPT) in latent myofascial trigger points (MTrPs) in the upper trapezius muscle of a single cervical spine manipulation directed at the C3 through C4 level. Methods: Seventy-two volunteers (27 men and 46 women; mean age, 31 years; SID, 10 years) participated in this study. Subjects underwent a screening process to establish both the presence of MTrPs in the upper trapezius muscle as described by Simons et al (Myofascial pain and dysfunction: the trigger point manual, vol 2. 3rd ed. Baltimore: Williams & Wilkins, 1999. p. 23-34) and the presence of intervertebral joint dysfunction at the C3 through C4 level by the lateral Gliding test for the cervical spine. Subjects were divided randomly into 2 groups: manipulative group, which received a cervical spine manipulation directed at the C3 through C4 level, and a placebo group, which received a sham manual procedure. The outcome measure was the PPT on the MTrP in the upper trapezius muscle ipsilateral to the side of the joint dysfunction, which was assessed pretreatment and 1, 5, and 10 minutes posttreatment by an assessor blinded to the treatment allocation of the subject. Results: The analysis of variance showed a significant effect for time (F = 5.157; P =.02) but not for side (F = 0.234 P -.63). Furthermore, an interaction between group and time was also found (F - 37.240; P <.001). The experimental Group showed a trend toward an increase in PPT levels after the manipulative procedure, whereas the control group showed a trend toward a decrease in PPT. Positive within-group effect sizes ranging from medium to small were found in the manipulative group (0.1 < d < 0.5), whereas negative within-group effect sizes ranging from large to medium were found in the placebo group (0.3 < d < 1). Conclusions: Our results suggest that a cervical spine manipulation directed at the C3 through C4 segment induced changes in pressure pain sensitivity in latent MTrPs in the upper trapezius muscle. Different therapeutic mechanisms, either segmental or central, may be involved at the same time.