Hip posture affects the firing properties of motor units in the tibialis anterior muscle

In this study, we contend the firing properties of motor units change due to nonphysiological sources. We specifically ask whether changes in the fibular nerve length, without a concurrent change in tibialis anterior architecture, affect motor unit firing and recruitment strategies. We tested this hypothesis based on high-density surface electromyograms (EMGs) collected from the tibialis anterior of 18 healthy young adults for two hip postures, flexed and extended. To control for changes in peripheral nerve length, conduction time between electrical stimulation and generation of compound action potentials in extensor digitorum brevis was measured for the two hip postures during rest. Motor units were decomposed from EMGs obtained during sustained isometric dorsiflexion at 10% of the maximal voluntary contraction (MVC), and during ramp isometric contractions up to 20% MVC. Individual motor unit firings were identified and tracked between the two postures. Nerve conduction time was significantly shorter in hip flexed than in hip extended posture (P < 0.01), suggesting that peripheral nerve was stretched in the flexed hip posture. MVC torque was not different between flexed and extended postures (P = 0.254). Motor unit firing rates during sustained contraction at 10% of MVC, and during ramp-up contraction to 20% of MVC were significantly lower during flexed hip posture than during extended hip posture (P < 0.05). Hip flexion posture, which likely result in a stretching of the fibular nerve, was observed to reduce the average firing rate of active motor units during relatively low contractions.NEW & NOTEWORTHY Peripheral nerve condition can affect motor unit activations. Sciatic and fibular nerves are stretched by ankle dorsiflexion, knee extension, and hip flexion. Hip flexion posture, which likely result in a stretching of the fibular nerve, was observed to reduce the average firing rate of active motor units during relatively low contraction. Proximal joint posture, which does not directly influence muscle architecture, should be considered to interpret neural input properties.