Sex differences in conditioned pain modulation effects and its associations with autonomic nervous system activities in healthy, younger individuals: a pilot study.

Introduction: Sex differences in conditioned pain modulation (CPM) have not been sufficiently explored. Objectives: This pilot study aimed to examine sex differences in CPM effects and associations between autonomic activities and CPM effects in healthy, younger individuals. Methods: University students were recruited from February to March 2021 and divided by sex. They remained seated for 10 minutes as a rest period, then immersed their right hands in cold water for 2 minutes as a cold period. The pressure pain threshold (PPT) was measured after each period, presenting the CPM index (%) using the formula: (PPT Results: Thirty-two participants were analyzed (14 women and 18 men; aged 21.1 ± 0.6 and 20.9 ± 0.3 years, respectively). Conditioned pain modulation effects were not different at 127.0 ± 19.1% in women and 124.0 ± 18.7% in men. The LF/HF index, LF normalized unit (nu) index (LFnu), and HFnu index had significant predictor variables for the CPM index across overall samples. The LF/HF index and LFnu index were significant predictor variables for the CPM index for women but not for men. Conclusions: Conditioned pain modulation effects between groups seem to be similar. The LF/HF and LFnu indices in women were significant, indicating that descending pain modulations in women might be more associated with autonomic activities than those in men.