Inter-Muscle, -Limb, and -Day, Effects on Motor Unit Discharge Rates of the Superficial Quadriceps Femoris Muscles

Despite the quadriceps being a large muscle group with distinct heads, no study has compared motor unit discharge rates (MUDRs) of the three superficial portions in the same participants. Further, day-to-day variability of quadriceps MUDRs and the influence of limb dominance have not been evaluated. Thus, during submaximal contractions, we assessed MUDRs of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) of the dominant and non-dominant limbs on two separate days. We hypothesized that MUDRs of the RF would be faster than VM and VL but not different between the VM and VL. Additionally, MUDRs would not be different between limbs, or days. During two identical sessions (≥72h apart), 16 healthy, young adults (8 females, 27±7y) performed repeated (7-14 per limb) 15-s isometric contractions at 25% of maximal knee extensor force. Motor unit trains (≥24 per muscle) were recorded from concentric needle electrodes inserted separately into the VL, RF, and VM, and analyzed using decomposition-based quantitative electromyography software. Muscle order was pseudorandomized, though consistent for each limb within a participant. A total of 5391 motor unit trains were included for analysis (26.9±5.5 per muscle per participant). Independent of limb dominance and day (main effects and interactions all P ≥0.115), the mean MUDR for RF (12.6±1.8Hz) was faster than VL (10.2±1.5Hz) and VM (10.9±1.7Hz), and VM was faster than VL (all P ≤0.001). The results indicate the three superficial quadriceps muscles have unique motor unit rate modulation, possibly reflecting different relative contributions to knee extension force production.