(8) INFLUENCE OF NOXIOUS FATIGUING EXERCISE ON MOTOR UNIT RECRUITMENT AND FIRING RATE PATTERNS IN MALES AND FEMALES

Purpose: This study examined cold pressor test (CPT) effects on motor unit (MU) behavior following volitional failure. Methods: Nine lower-body resistance-trained males (21±2 yrs) and 12 females (21±2 yrs) had a surface electromyographic (EMG) sensor placed over the vastus lateralis during the control (CON) and experimental (CPT) visit. Participants completed 2-3 maximal voluntary isometric contractions (MVC) of the knee extensors, an isometric trapezoidal muscle action at 70% MVC (PRE-70%), repeated 50% MVCs until participants reported a rate of perceived exertion (RPE) of 8, placed their left hand in ice- (CPT) or room-temperature water (CON) and completed 50% MVCs to failure. Participants removed their hand and after 3-5 mins, completed a 70% MVC (POST-70%). Heart rate and blood pressure were recorded at rest and RPE 8 to calculate mean arterial pressure (MAP). EMG during the 70% MVCs was decomposed to extract MU: recruitment thresholds (RT), action potential amplitudes (MUAP_AMPS), and mean firing rates (MFR). EMG amplitude (EMG_RMS) was normalized to MVC (N-EMG_RMS). Y-intercepts (y-ints) and slopes were calculated for the linear MUAP_AMP vs. RT- and a and b terms for the exponential MFR vs. MUAP_AMP-relationships. Change scores (POST-PRE [D]) were calculated for the coefficients from the MU relationships during the 70% MVCs, and for MAP (RPE 8-PRE [DMAP¹]; hand immersion-PRE [DMAP²]). Five 2-way mixed factorial repeated measures (RM) ANOVAs (sex [males vs. females] x condition [CON vs. CPT]) examined the Dslopes and Dy-ints for the MUAP_AMP vs. RT and the Da and Db terms for the MFR vs. MUAP_AMP relationships, and DN-EMG_RMS for the 70% MVCs. One 3-way mixed factorial RM ANOVA (sex [males vs. females] x condition [CON vs. CPT] x time (DMAP¹ vs. DMAP²) examined DMAP. One 3 way mixed factorial RM ANOVA (sex [males vs. females] x condition [CON vs. CPT] x time [RPE 8 vs. FAIL]) examined repetitions (REPS) completed. Results: For the MFR vs. MUAP_AMP relationships, there was no (p>0.05) interaction or main effects for the Da terms, whereas there was an interaction (p=0.027) for the Db terms. For the MUAP_AMP vs. RT relationships, there were interactions for the Dslopes (p=0.026) and Dy-ints (p=0.031). For DN-EMG_RMS and REPS, there were no (p>0.05) interactions or main effects. For DMAP, there was a 2-way interaction (p< 0.001). Refer to table 1 for means±SDs and significant follow-up tests. Conclusions: Females increased their firing rates to a greater degree for the POST-70% MVC during CON than CPT, and compared to males during CON. Although not significant, males nearly (p=0.053) displayed accelerated recruitment for the POST-70% MVC following CPT. Practical Applications:Strength coaches should note that males and females modulated torque differently following fatiguing exercise with painful stimuli. These data may suggest males rely more on recruitment following noxious fatiguing exercise, which could reduce fatigue resistance for subsequent exercise bouts. Acknowledgements: None