Characterizing Motor Unit Firing Rates In The Vastus Intermedius In Humans

Neuromuscular properties have been well-studied in three of the four muscles of the quadriceps femoris. Lack of accessibility of the vastus intermedius (VI) due to its deep location likely has precluded comprehensive insight on motor unit (MU) properties. The VI is a major component of the quadriceps comprising ~50% of its muscle volume and potentially contributing ~50% of the torque during knee extension. However, it is unknown whether the VI has similar MU properties to the more comprehensively studied other two main knee extensors – the vastus lateralis (VL) and vastus medialis (VM). Because of the greater slow twitch fibre percentage in the VI (~60%) compared with the VL and VM (~50%), MU properties may be unique. PURPOSE: Thus, the purpose here was to compare specifically the MU firing rates (MUFR) of the VI and VL during varying intensities of knee extension. METHODS: Participants (3 males; 1 female) aged 20-29y were recruited. In a Humac Norm dynamometer knee extension torque was measured from brief (~5 s) isometric maximum voluntary contraction (MVC) levels, and at 10%, 25%, and 50% of MVC. Percutaneous electric stimulation was used to assess voluntary activation (VA) using the interpolated twitch method. Tungsten needles (ultrasound guided) were used to record MUFR from the VI and VL during repeated non-fatiguing contractions at each of the four contraction levels. RESULTS: Maximal knee extension torque was 492.5 ± 232.4 N with VA of over 90%. A total of 235 MUs were sampled with equal numbers from both muscles. At 10% MVC, MUFR ranged from 5-9 Hz (mean 7.26 ± 1.12 Hz) in the VL and 4-8 Hz in the VI (mean 6.69 ± 1.55 Hz). At 25% VL ranged from 4-12 Hz (mean 8.03 ± 1.68) and VI ranged from 5-11 Hz (mean 8.91 ± 1.54 Hz). At 50% VL ranged from 9-13 Hz (mean 11.3 ± 1.22 Hz) and VI from 8-15 Hz (mean 11.7 ± 2.08 Hz). At MVC rates ranged from 15-24 Hz in the VL (mean 19.3 ± 2.79 Hz) and 15-26 Hz in the VI (mean 19.5 ± 2.91 Hz). There were no mean differences at any force level (p > 0.05). CONCLUSION: Despite the VI having a greater slow twitch composition compared to the other quadriceps muscles and contributing a significant portion of torque generation during knee extension, MUFR are not different than the VL. In addition to characterizing the VI, we demonstrate the accessibility of the VI with intramuscular recordings. Supported by NSERC. NSERC