Differences in estimated persistent inward currents between ankle flexors and extensors in humans.

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Persistent inward currents (PICs) amplify motoneuronal synaptic inputs, crucial for motor activation.
• Delta-F (ΔF) is an indirect measure of PICs derived from motor unit firing patterns.
• Previous studies indicated higher ΔF in upper limb extensors compared to flexors.

Purpose of the Study:

• To investigate and compare PICs (estimated via ΔF) between lower limb extensors (soleus) and flexors (tibialis anterior) during isometric contractions.
• To test the hypothesis that lower limb extensors exhibit higher ΔF than flexors, mirroring upper limb findings.

Main Methods:

• High-density surface array electrodes were used to record EMG signals from soleus and tibialis anterior muscles.
• EMG signals were decomposed into individual motor unit firing patterns using a convolutive blind source separation algorithm.
• ΔF was calculated from motor unit firing rates during isometric sitting and standing at 10-30% maximum voluntary contraction.

Main Results:

• Contrary to the hypothesis, the tibialis anterior (flexor) demonstrated significantly higher ΔF than the soleus (extensor) in both seated and standing positions.
• The tibialis anterior exhibited a higher maximum discharge rate than the soleus, with no significant difference in rate increase.
• Even when comparing unit pairs with similar maximum discharge rates, ΔF remained higher in the tibialis anterior.

Conclusions:

• Lower limb flexor muscles (tibialis anterior) display greater persistent inward currents than extensor muscles (soleus), a finding that contrasts with observations in the upper limb.
• This study highlights a potential difference in motor control strategies between upper and lower limbs regarding PICs.
• Further research is needed to elucidate the functional implications of these observed differences in lower limb muscle PICs.