Estimates of tibialis anterior persistent inward current are reduced by heel pressure in a supine position

Persistent inward currents (PICs) amplify and sustain motor neuron (MN) output. PICs rely on descending monoaminergic input and are reduced by afferent inhibition. Because PICs are thought to be important for postural maintenance, and there is coupling between plantar cutaneous sensory input and lower limb MNs, our purpose was to assess posture-dependent plantar cutaneous sensory modulation of tibialis anterior (TA) MNs. We hypothesized that cutaneous input, via either heel pressure (HP) or electrical stimulation (Stim) would decrease ΔF estimates of PICs derived from paired motor unit recordings, and that ΔF would be lower in a supine posture. Twenty participants (10 female) performed isometric dorsiflexion contractions with (HP) and without (noHP) heel pressure in seated (SE) and supine (SUP) postures. Cutaneous stimulation (5x1ms pulses, 333Hz, 2x perceptual threshold) was applied to the heel during half the contractions. ΔF was lower with HP (3.77 ± 1.65 Hz) than noHP (4.87 ± 1.25 Hz), but only in SUP (p = 0.027). ∆F in the seated posture was not different between HP (4.28 ± 1.29 Hz) and noHP (4.46 ± 1.59 Hz) conditions. However, ΔF values were not altered by electrical stimulation (Stim: 4.25 ± 1.44 Hz, no Stim: 4.45 ± 1.55 Hz). This suggests that plantar cutaneous pressure shapes MN excitability via modulation of PICs, in a posture-dependent manner. This study adds to our understanding of reflex control of lower limb PICs in tasks associated with plantar cutaneous pressures, such as locomotion or postural stability where task performance depends upon MN input-output properties.