Influence Of The Cold Pressor Test On Pain Threshold, Motor Unit Characteristics, And Input Excitation

The cold pressor test (CPT) is commonly used to induce pain and stress, and activate the sympathetic nervous system (SNS). Although pain can alter neuromuscular behavior, it remains unknown if motor unit (MU) recruitment and firing rate patterns during a high-intensity contraction are affected by the CPT. PURPOSE: To investigate the effects of the CPT on pressure pain threshold (PPT) and MU behavior of the vastus lateralis (VL). METHODS: Ten lower-body resistance-trained males (21 ± 2 yrs, 180.56 ± 8.97 cm, 80.80 ± 12.01 kg) performed isometric maximal voluntary contractions (MVCs) of the right knee extensors, followed by an isometric trapezoidal muscle action at 70% MVC (control [CON]). Following 5 mins of rest, participants placed their left hand in ice water (2 ± 2 ° C) for the CPT. After ≥7 minutes, participants performed another 70% MVC. Surface electromyographic (EMG) signals were recorded from the VL during the muscle actions, and EMG recorded during the 70% MVCs was decomposed for extraction of recruitment thresholds (RT), MU action potential amplitudes (MUAPAMPS), and mean firing rates (MFR). EMG amplitude (EMGRMS) was normalized to MVC (N-EMGRMS). Blood pressure and heart rate (HR) were recorded, and mean arterial pressure (MAP) was calculated prior to the 70% MVC for each condition. Pain pressure threshold (PPT) was assessed over the VL during CPT and CON. Y-intercepts (y-ints) and slopes were calculated for the MUAPAMPS and MFR vs. RT relationships for CPT and CON. Eight paired samples t-tests were used to compare MAP, HR, PPT, y-ints and slopes for the MUAPAMP and MFR vs. RT relationships, and N-EMGRMS during the CPT and CON conditions. RESULTS: There were no significant differences for the y-ints or slopes of the MUAPAMP and MFR vs. RT relationships. However, there were significant differences for MAP (CON = 97.33 ± 9.89 mm Hg, CPT = 109.33 ± 9.66 mm Hg; p = 0.001), HR (CON = 80.20 ± 15.22 bpm, CPT = 88.50 ± 17.53 bpm; p = 0.049), PPT (CON = 7.54 ± 2.08 kg/cm2, CPT = 9.17 ± 2.01 kg/cm2; p = 0.001), and N-EMGRMS (CON = 86.49 ± 34.01%, CPT = 101.14 ± 39.73%; p = 0.031). CONCLUSION: CPT increased MAP, HR, PPT, and N-EMGRMS. Additionally, the greater N-EMGRMS during CPT suggests an upward shift in the input excitation-torque relationship, indicating more excitation to the motoneuron pool was necessary when producing the same absolute torque.