Hip position and sex differences in motor unit firing patterns of the vastus medialis and vastus medialis oblique in healthy individuals

Abstract

Weakness of the vastus medialis oblique (VMO) has been proposed to explain the high prevalence of knee pain in female subjects. Clinicians commonly use exercises in an attempt to preferentially activate the VMO. Recently, our group found evidence to support clinical theory that the VMO is neurologically distinct from the vastus medialis (VM). However, the ability to voluntarily activate these muscle subsections is still disputed. The aim of this study was to determine if VM and VMO activation varies between sexes and if control of the two muscles is different between rehabilitation exercises. Thirteen men and 13 women performed isometric straight leg raises in two hip positions, neutral hip rotation and 30 degrees lateral hip rotation. Bipolar intramuscular fine-wire electrodes were inserted into the VM and VMO to obtain motor unit recruitment thresholds and initial firing rates at recruitment. Linear mixed models and Tukey post hoc tests were used to assess significant differences in 654 motor units. Women demonstrated faster motor unit firing rate at recruitment, 1.18 ± 0.56 Hz higher than men. Motor units fired 0.47 ± 0.19 Hz faster during neutral hip rotation compared with lateral hip rotation. The VMO motor units were recruited 2.92 ± 1.28% earlier than the VM. All motor units were recruited 3.74 ± 1.27% earlier during neutral hip rotation than lateral hip rotation. Thus the VM and the VMO can be activated differentially, and their motor unit recruitment properties are affected by sex and hip position. NEW & NOTEWORTHY This is the first study to reveal differential activation of the vastus medialis oblique from the vastus medialis in clinical exercise protocols. Our research group used fine-wire electrodes to examine EMG signals of the vastus medialis oblique and vastus medialis to avoid possible cross talk. We also consider the effect of sex on motor unit firing patterns because of higher prevalence of knee pain in women, and yet few studies evaluating the sex differences in neuromuscular control.

Optimal Noise Level for Imperceptible Vibrotactile Stimulation during a Force Stability Task

“…In recent decades, vibrotactile noise stimulation has received attention as a possible intervention to improve somatosensory function and supplement postural control (Priplata et al, 2002;Galica et al, 2009;Magalhaes & Kohn, 2011;Lipsitz et al, 2015). Although the mechanism by which this intervention is effective is still uncertain, it has been theorized that mechanical noise stimulation elicits a stochastic resonance-like effect on the somatosensory system (Manjarrez et al, 2002;Collins et al, 2003;Moss & Milton, 2003).…”

“…In other words, the application of vibrotactile noise may sharpen one's control of body position by providing additional sensory information and increasing the body's ability to detect and respond to changes in body orientation. Vibratory noise stimulation has shown to be effective in reducing postural sway among healthy young subjects as well as elderly subjects and patients with degraded somatosensory function due to diabetes and stroke (Collins et al, 2003;Priplata et al, 2002Priplata et al, & 2006Magalhaes & Kohn, 2011). Additional work has demonstrated that postural sway may also be improved with other modes of noise stimulation.…”

“…Developing interventions which utilize imperceptible noise stimulation to aid with postural stability has been of interest for many years (Priplata et al, 2002;Collins et al, 2003;Magalhaes & Kohn, 2011;Lipsitz et al, 2015). Currently, however, more research is needed regarding the signal content and amplitude that elicits the greatest benefits.…”

“…Currently, however, more research is needed regarding the signal content and amplitude that elicits the greatest benefits. In some cases, research groups have elicited postural stability improvements by applying imperceptible vibrotactile noise stimulation as a prescribed percentage (70-90%) of an individual's sub-sensory threshold level (Priplata et al, 2002;Galica et al, 2009;Lipsitz et al, 2015). Despite observed stability improvements, uncertainties still exist regarding the mechanism responsible for these improvements.…”

Effect of McKenzie Techniques on Muscle Strengthening in Anterior Knee Pain

“…Garrick et al believed that strengthening of the vastus medialis is an important goal in the treatment of anterior knee pain [12]. The vastus medialis and the vastus medialis oblique can be activated differentially and their motor unit recruitment properties are affected by sex and hip position [13]. The appropriate treatment for this syndrome is still unknown, but the most commonly accepted therapeutic intervention is exercise therapy [12,[14][15][16][17].…”