Effect of interelectrode distance on surface electromyographic signals of vastus intermedius muscle in women and men

Abstract

We previously developed a novel technique to record surface electromyography (EMG) of the vastus intermedius (VI) in men. The purpose of the present study was to assess whether this technique can be applied to women in the same way. We measured the subcutaneous fat thickness at the site of electrode placement on VI using ultrasonography. Nine men and ten women performed isometric knee extensions at 25%, 50%, 75%, and 100% of the maximal voluntary contraction. During the tasks, surface EMG signals were recorded from the superficial region of VI with interelectrode distances (IEDs) of 10 mm (IED-10) and 20 mm (IED-20). The subcutaneous fat thickness in women was significantly greater than in men (women: 8.7 ± 2.1 mm; men: 5.6 ± 1.6 mm, p < 0.01). However, the amplitude and frequency of the EMG signal of VI at the different force levels were not affected by IEDs in either sex. These results suggest that surface EMG recording of VI with both IED-10 and IED-20 would be applicable to relatively lean women with a similar sensitivity to that in men.

Introduction

The quadriceps femoris (QF) muscle group, consisting of one deep muscle, the vastus intermedius (VI), and three superficial muscles, the vastus lateralis (VL), vastus medialis (VM), and rectus femoris, plays an important role in human movements such as standing, walking, and running. Various previous studies on the QF muscle showed differences in the neuromuscular properties, muscle control strategy in a force matching task, and muscle fatigue characteristics between women and men (Clark et al., 2005, Gehring et al., 2009, Hunter, 2009). Regarding the functional properties of VI, its maximal contribution to isometric knee extensor torque reached over 50% of the force production among individual QF muscles during submaximal contraction (Zhang et al., 2003). From an anatomical point of view, VI comprises approximately 30% of the QF muscle volume (Akima et al., 2007). This suggests that it makes a critical contribution during knee extension movements.

Watanabe and Akima (2009) showed a superficial region of VI located in the distal region of the thigh using magnetic resonance imaging and ultrasonography. They successfully recorded EMG signals of VI with a negligible amount of cross-talk from the adjacent VL, and found that the surface EMG signal of VI was closely associated with its needle EMG signal (r = 0.92–0.99) (Watanabe and Akima, 2011a). We developed a technique to measure the neuromuscular activation of VI using surface EMG, and showed unique aspects of its neuromuscular activation pattern during static and dynamic knee extension tasks (Watanabe and Akima, 2011b, Akima and Saito, 2013). However, all subjects in these previous studies were men; therefore, neuromuscular activation of VI during knee extension remains unclear in women.

The interelectrode distance (IED) is an important factor affecting the quality and quantity of EMG signals (Fuglevand et al., 1992, De Luca, 1997, Minetto et al., 2013) when comparing individuals with differences in the subcutaneous fat thickness, such as men vs. women, because it has been reported that women generally show a greater subcutaneous fat thickness than men (Hattori et al., 1991). Minetto et al. (2013) recorded surface EMG of VL and VM with IEDs of 5–25 mm in obese patients, and showed that the EMG–force relationship was affected by obesity. Pinvicero et al. (2001) suggested that a larger skinfold thickness was associated with a lower EMG frequency. Furthermore, in VL and VM, it was clarified that for each contraction level from 10% to 80% of the maximal voluntary contraction (MVC), the median frequency (MdF) increased as IED decreased (Rodriguez-Falces et al., 2015). Therefore, it is speculated that the greater subcutaneous fat thickness could influence surface EMG activity of VI with a small IED. In a previous study, IED of 10 mm was used to record surface EMG activity of VI in men (Watanabe and Akima, 2009). In the present study, we assessed the effect of IEDs on EMG signals of VI in women by comparing them with those in men.

Therefore, the purposes of this study were to determine if the neuromuscular activation of VI can be investigated in women, and to examine the effect of IEDs on EMG activities of VI between women and men during isometric knee extension at difference force levels. Based on the results of Minetto et al. (2013), there is a significant difference in normalized EMG with a 15-mm IED as the cutoff for obese subjects, and we hypothesized that EMG indices, i.e., RMS and MdF, of VI and VL differ between IEDs of 10 and 20 mm due to the greater subcutaneous fat thickness in women. For a similar reason, we also hypothesized that the EMG indices of VI and VL with respect to differences in force levels also differ between women and men.