Sustained isometric muscle contraction (fatiguing contraction) recruits segmental and/or extrasegmental descending inhibition in healthy subjects but not in fibromyalgia (FM). We hypothesized that fatiguing contraction may shift descending pain modulation from inhibition towards facilitation and that the effect of descending pain modulation be dependent on peripheral muscle pain sensitivity.
Pressure pain thresholds (PPT) were measured from 13 points bilaterally in the upper trapezius muscle and from the mid-point bilaterally in the tibialis anterior before-, immediately after-, and 20 min after fatiguing contraction of shoulder abduction in 22 FM patients and 22 matched healthy controls. Rate of fatigue, pain intensity, and the duration of fatiguing contraction were recorded.
The duration of fatiguing contraction was significantly shorter in FM (132.4 ± 25.2 s) than healthy control groups (286.2 ± 24.1 s) (P < 0.05), pain intensity was significantly higher in FM (8.25 ± 0.8 cm) than in healthy controls (5.1 ± 0.65 cm) (P < 0.01), whereas both groups reported similar fatigue intensity (P > 0.05). Following the contraction, PPTs were increased significantly and heterogeneously in the upper trapezius over time, but not, in the tibialis anterior muscle in healthy controls. However, PPT were significantly decreased over time in the tibialis anterior (P < 0.05), but not, in the upper trapezius in FM.
Descending pain modulation shifts from descending inhibition towards descending facilitation following muscle nociception in FM. Peripheral mechanical hyperalgesia and descending facilitation counterbalance the effect of descending inhibition in FM.