Persistent Inward Currents Increase With The Level Of Voluntary Drive In Plantar Flexor Low-Threshold Motor Units

Aim: To test the hypothesis that estimates of persistent inward currents in the human plantar flexors would increase with the level of voluntary drive. Methods: Twenty-one participants volunteered for this study (29.2{+/-}2.6 years). High-density surface electromyograms were collected from soleus and gastrocnemius medialis during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired motor unit analysis to calculate delta frequency ({Delta}F). Results: Significant increases in PIC were observed from 10% to 20% ({Delta}=0.6 pps; p<0.001) and 20% to 30% ({Delta}=0.5 pps; p<0.001) in soleus, and from 10% to 20% ({Delta}=1.2 pps; p<0.001) but not 20% to 30% ({Delta}=0.09 pps; p=0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% (respectively, {Delta}=1.75 pps, p<0.001; and {Delta}=2.43 pps, p<0.001) and 20% to 30% (respectively, {Delta}=0.80 pps, p<0.017; and {Delta}=0.92 pps, p=002). The repeated-measures correlation method identified associations between {Delta}F and increases in maximal discharge rate for both soleus (r=0.64; p<0.001) and gastrocnemius medialis (r=0.77; p<0.001). Conclusion: An increase in voluntary drive tends to increase PICs strength, which has key implications for the control of force but also for comparisons between muscles or between studies when relative force levels might be different. These data indicate that increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit firing, and thus force output modulation.