Introduction 
Force steadiness (FS), the ability to maintain a constant level of force output is known to progressively decrease during prolonged isometric contractions [1,2] which is partly mediated by the level of common synaptic input to motor units (MU). Ischemia, in which occlusion of the limb microvasculature reduces the delivery of oxygen and nutrients may also contribute to performance fatigue and reduced FS.  However, the extent of the influence of each remains undefined. The aim of this study was to determine the effects of vastus lateralis (VL) neural input and microvascular blood flow on FS during a knee extensor fatiguing contraction. 

Methods 
Ten young volunteers (3 females; 30 ± 6 years) completed a 3-minute isometric leg extension at 30% maximum voluntary contraction (MVC), with data collected during the first and final 30-seconds (s) of the contraction. High-density surface electromyography (HD-sEMG) was used to identify individual MU potentials (MUPs) from the VL. FS was defined as the coefficient of variation (CoV) of force and compared between the first and final 30s of the contraction. Estimates of common synaptic inputs were made using the magnitude-squared coherence in the delta bandwidth (0–5 Hz) [3]. Microvascular blood volume (MBV) was determined from the plateau phase of a non-linear regression of echo acoustic intensity (AI) simultaneously recorded using contrast enhanced ultrasound (CEUS)[4]. Between time point data for all parameters was compared using paired t-tests with significance assumed as p<0.05. 

Results
10.4±8.5 (Mean±SD) MUs per person were identified in the first 30s and 10.3±8.1 in the last 30s. CoV force increased from the first to the last 30s of the fatiguing contraction (CoV: 3.13±0.74 vs. 4.97±1.72%; p=0.005) as did MBV (AI: 6.66±0.72 vs. 7.72±1.50 au; p=0.004) but delta coherence (Z-score: 3.40±1.83 vs. 3.40±1.56; p=0.996) did not change. 

Conclusion 
As expected, FS decreased during an isometric contraction of the knee extensors with an increase in MBV also observed. However, no changes were observed in the common synaptic input to MUs. These findings suggest that during an isometric contraction muscle blood flow may have a greater influence on FS at the onset of fatigue than neural properties. These findings can translate into understanding ways to improve athletic performance as well as ameliorating functional impairment in clinical populations therefore, further investigation is needed into the neurophysiological factors involved in maintaining force control during fatigue.