Effect of breathing 7% carbon dioxide on the human soleus Hoffmann-reflex recruitment curve

ObjectiveTo examine the effect of inhaling 7% carbon dioxide on the recruitment curve of the soleus H-reflex.MethodsFive males and five females (apparently healthy; 21–40 yr) with surface EMG electrodes over the soleus muscle randomly breathed room air or a 7% CO2, 21% O2, balance N2 mix for 10 min. The tibial nerve was stimulated to elicit the H-reflex recruitment curve. H-reflex threshold, motor threshold, slope of ascending H-reflex curve, Hmax:Mmax ratio, and latency of Hmax were compared.ResultsBreathing 7% carbon dioxide increased the H-reflex threshold (as a percent of motor threshold) from 73% (SD 0.09) to 84% (SD 0.12; p < 0.05), decreased the Hmax:Mmax ratio from 0.504 (SD 0.290) to 0.403 (SD 0.253; p < 0.05), and increased the H wave latency (in msec) from 32.8 (SD 1.6) to 34.6 (SD 2.6; p < 0.05). Slope of ascending H-reflex curve (room air: 125 (SD 89); CO2: 135 (SD 92); p > 0.05), Mmax (room air: 3.70 mV (SD 1.57); CO2: 3.69 mV (SD 1.53); p > 0.05), and motor threshold (p > 0.05) remained unchanged.ConclusionsBreathing 7% carbon dioxide reduces afferent synaptic transmission in the soleus muscle by slowing transmission, increasing threshold stimulus, and reducing H wave size.SignificanceClinicians and scientists should consider the possibility that carbon dioxide can reduce afferent synaptic transmission.