Author(s): FREIRE, R., LUCAS, L., HAMIRICK, D., WADE, J., SIEGLER, J., Institution: ARIZONA STATE UNIVERSITY, Country: UNITED STATES, Abstract-ID: 202

INTRODUCTION:
Group III and IV afferent fibers are sensitive to muscle contraction-induced metabolites (e.g., H+) and may play a role in the feedback mechanisms related to effort perception and voluntary muscle drive. Inducing a state of metabolic alkalosis has been shown to influence voluntary muscle activation, but the effect on the perception of effort remains unknown. Therefore, this study investigated a) the effect of sodium bicarbonate (SB) on perception of cycling effort as determined by self-selected power outputs along a spectrum of exercise intensities, and b) whether SB supplementation had an effect on feedback mechanisms related to effort perception during the cycling task.
METHODS:
Thirteen subjects (two females) participated in this randomized, crossover study comprising four visits. Visits 1 and 2 were dedicated to a VO2max test and familiarization with the experimental protocol. In visits 3 and 4, participants ingested either 0.3g/kg SB (MaurtenTM) or a placebo control (PLA) one hour prior to arrival at the lab. Bicarbonate was assessed prior to cycling. Participants were asked to ride a cycle ergometer at a clamped rating of perceived exertion (RPE) (RPE 9, 12, and 15) for 10 min each, then repeated after a 3-minute break. The second series of efforts also included a single bout at RPE 17 lasting 5 min. Participants were blinded to all feedback, except cycling cadence. Power output (PO), heart rate (HR), muscle activation (rms-EMG), ventilation (VE), and muscle deoxyhemoglobin (m-HHb) were recorded continuously throughout the protocol. A post-exercise acid-base measurement was also collected.
RESULTS:
Alkalosis was confirmed in the SB condition before (SB: 28.5 vs PLA: 25.5 mmol/L, p < 0.001) and after (SB: 23.9 vs PLA: 20.2 mmol/L, p < 0.001) exercise. SB supplementation did not influence perception of effort, as there were no differences between conditions for PO (p = 0.99). VE was not different between conditions (p = 0.21); m-HHb (p < 0.001) and rmsEMG (p = 0.01) were higher in the PLA condition compared to the SB condition.
CONCLUSION:
Higher muscle firing rates were observed with SB, despite changes in power output, suggesting that metabolic alkalosis altered neuromuscular activation strategies. A plausible explanation is that SB, by attenuating intramuscular acidosis, reduced metabolite-sensitive group III/IV afferent feedback and thereby released some of the inhibitory constraint on spinal motoneurons. M-HHb data provide insight into the exercising muscle milieu. SB required less oxygen extraction to maintain the same external work. It seems that there is a compensatory effect between muscle activation and oxygen utilization under the alkalotic state, allowing for the maintenance of the same external work. SB supplementation does not affect perception of cycling effort across different exercise intensities. However, SB does appear to influence local oxygen delivery and neural recruitment across exercise intensities.