Figure 1.

Forest plots of the mean differences in heat and pressure pain sensitivities between dark and light skin human cohorts. Study name is defined as the last name of the first author and all studies are listed in Table S1. (a) Heat pain intensity (% of visual analogue scale) in the forearm to 48 or 49 °C stimulation is higher in dark skin cohort than light skin cohort (6 studies). Heterogeneity among the studies was moderate (I ² = 44, P < 0.11). (b) There is no difference in heat pain threshold (the first painful temperature) in the forearm between the two cohorts (10 studies). However, there is a trend toward lower heat pain thresholds in the dark skin cohort. Heterogeneity among the studies was moderate (I ² = 46, P < 0.06). (c) Heat pain tolerance (intolerance temperature) in the forearm is lower in dark skin cohort than light skin cohort (7 studies). There is no heterogeneity among the studies (I ² = 40, P < 0.98). (d) There is no difference in the pain pressure threshold in trapezius muscle (4 studies). However, there is a trend toward lower pressure threshold of the dark skin cohort than that of the light skin cohort (P = 0.05). Heterogeneity among the studies is moderate (I ² = 46, P < 0.13). (e) There was no difference in the pain pressure threshold in the masseter muscle (3 studies). There is no heterogeneity among the studies (I ² = 40, P < 0.62).