Comparison of human motor unit firing rates in the vastus intermedius and vastus lateralis during isometric knee extension.

purposeNeuromuscular properties have been well-studied in three of the four muscles of the quadriceps femoris. However, lack of accessibility of the vastus intermedius (VI) due to its deep location has precluded comprehensive insight on motor unit (MU) properties. Thus, the purpose here was to compare the MU firing rates of the VI and vastus lateralis (VL).

methodsIn ten young participants (3 females), ultrasound-guided intramuscular tungsten microelectrodes were used to record MU firing rates from the VI and VL during non-fatiguing contractions at four intensities, including during maximal efforts.

resultsMaximal knee extension torque was 518 ± 113 N with voluntary activation of over 90%. A total of 2741 MU trains were sampled with equal numbers from both muscles. There was a linear relationship between contractile intensity and mean MU firing rate in both the VI and VL (r = 0.79 and 0.81; P < 0.001), and rates ranged from ~ 8 Hz at 10% to ~ 27 Hz at maximum with no difference between the two muscles (P = 0.45) at any force level (P = 0.08).

conclusionDespite, differences in anatomical and muscular architecture allowing for varying contributions to knee extension, MU firing rates were similar between the VI and VL. This indicates at the motor unit level that fundamental neural activation strategies between the two larger torque generating muscles of the quadriceps may be similar based on mean MU firing rates. Any variation in the relative contributions to knee extensor torque from these synergists may be governed more by biomechanical factors rather than apparent motor unit firing rate.