Neuromuscular Activation Of Vastus Intermedius Muscle During Isometric Knee Flexion At Different Knee Joint Angles

Abstract

Activation pattern of vastus intermedius (VI), which locates at deeper region of the quadriceps femoris (QF), is poorly understood during extension as well as flexion of the knee joint in humans. Montgomery et al. (1994) found that electromyographic (EMG) signal by insertion of a wire electrode directly to VI reached to 20% of maximum effort of knee extension in the initial swing phase of running. They suggested that activation of VI could control knee flexion of the knee joint in running. PURPOSE: The purpose of this study is to investigate activity of VI during isometric knee flexion using a newly developed technique to record surface EMG activity of VI (Watanabe and Akima 2009). The main hypothesis was that the VI would activate during isometric contraction of the knee flexion. METHODS: Ten healthy men (24.2±8.1 years old, 174.7±5.9 cm, 67.4±12.5 kg) performed 25%, 50%, 75% and 100% of maximal voluntary contraction (MVC) of knee flexion at 90°, 120° and 150° of knee joint angles (180° = fully extended). Surface EMG of four individual muscles, i.e. VI, vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) of the QF was recorded using the same technique to our previous studies (Watanabe & Akima 2009, 2010). Ultrasonography was used to identify the superficial region of VI for placement of a surface EMG electrode. Root mean square (RMS) of individual muscles of QF was normalized by RMS of MVC during isometric knee extension. RESULTS: At three tested knee joint angles, normalized RMS of VI was significantly higher than that of VL, VM and RF at all force levels (all p<0.05). Comparison of MVC among three different knee joint angles, normalized RMS of VI at 150° (63.0±29.7%) was significantly higher than that of 90° (29.2±19.3%) and 120° (40.4±22.7%) of knee joint angle. Overall, the highest RMS of VI was found at 150° and the lowest RMS of VI was found at 90° when comparing with the same force levels. CONCLUSIONS: An important evidence of this study is that surprisingly higher activation was found in VI, which is well known as a monoarticular muscle of knee extension, during isometric "knee flexion", and that the degree of action of VI was increased with extension of the knee joint. These results suggest that VI may contribute somehow to isometric knee flexion exercise.