There are no direct recordings of obturator internus muscle activity in humans because of difficult access for electromyography (EMG) electrodes. Functions attributed to this muscle are based on speculation and include hip external rotation/abduction, and a role in stabilization as an "adjustable ligament" of the hip. Here we present (1) a technique to insert intramuscular EMG electrodes into obturator internus plus (2) the results of an investigation of obturator internus activity relative to that of nearby hip muscles during voluntary hip efforts in two hip positions and a weight-bearing task. Fine-wire electrodes were inserted with ultrasound guidance into obturator internus, gluteus maximus, piriformis and quadratus femoris in ten participants. Participants performed ramped and maximal isometric hip efforts (open kinetic chain) into flexion/extension, abduction/adduction, and internal/external rotation, and hip rotation to end range in standing. Analysis of the relationship between activity of the obturator internus and the other hip muscles provided evidence of limited contamination of the recordings with crosstalk. Obturator internus EMG amplitude was greatest during hip extension, then external rotation then abduction, with minimal to no activation in other directions. Obturator internus EMG was more commonly the first muscle active during abduction and external rotation than other muscles. This study describes a viable and valid technique to record obturator internus EMG and provides the first evidence of its activation during simple functions. The observation of specificity of activation to certain force directions questions the hypothesis of a general role in hip stabilisation regardless of force direction.
Keywords: Electromyography; Hip external rotation; Hip stability; Obturator internus; Ultrasound guidance.
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