Gluteus medius strengthening and electromyography

Abstract

The gluteus medius has been a muscle of research interest given its reported association with a variety of lower extremity disorders and its role as a stabilizer of the hip in both the frontal and transverse plane. The authors of this review have provided a timely review of studies examining gluteus medius muscle activation during therapeutic exercises as measured by electromyography (EMG). While acknowledging the difficulties in comparing EMG studies, the authors offer no definite conclusions on the benefit regarding incorporating these exercises into a progressive gluteus medius strengthening program. In support of the authors’ conclusions, many variables make comparisons of EMG studies difficult. The authors have outlined several of these challenges including the speed of contraction, type of muscle contraction, and joint angle of the recorded muscle activity. Several additional factors influence recorded EMG activity including the specifics of the exercises themselves. As an example, trunk lean in the frontal plane influences muscle activity during a single limb squatting exercise. Thus, if not specifically controlled or described comparison of what appears to be similar exercises across studies is compromised. Another factor is placement of the electrodes themselves. The gluteus medius is a muscle capable of contributing to abduction as well as internal and external rotation at the hip. The specific electrode placement and specific hip position where recording takes place influences recorded muscle activity with a specific functional exercise. Another confounding variable is the reference standard used to determine the magnitude of EMG activity. The 15 studies reviewed all presented EMG activity as a percentage of a maximal voluntary isometric contraction (MVIC). To establish the MVIC, a variety of methods were used including sidelying hip abduction, standing hip abduction, and use of a Biodex. The MVIC frequently is established via a manual muscle test. Thus, MVIC results will differ based on the specifics of the manual muscle test including lever arm used, position of hip abduction, and the type of test utilized (make or break test), and the variation between different examiners performing the same test. In sum, as EMG activity is expressed as a percentage of the MVIC, if the MVIC reference varies across studies, comparison is further compromised. While the focus of the review is on functional therapeutic exercises for strengthening the gluteus medius, one must recognize the limitations of EMG in this regard. EMG is a means to quantify muscle activity. The authors report that %MVIC levels of 40–60% represent a threshold. With these levels used as a reference, many of the exercises produced EMG levels below levels necessary for strength gains. Hence, conclusions for the review were limited due to the focus on strengthening. This does not mean the exercises in these studies do not have merit in a progressive therapeutic exercise program. It simply means that they may not be optimal exercises if strengthening the gluteus medius is a goal. For example, lack of control at the hip may lead to adduction and internal rotation of the femur during weight bearing activities contributing to conditions such as patellofemoral pain. While this abnormal movement pattern may be due to a strength deficit, it may also be due to lack of dynamic control. It is the clinician’s challenge to determine the reason for the observed movement pattern. If a strength deficit is noted and strengthening is required, exercises that adequately challenge the gluteus medius are indicated. Examples cited in the review include sidelying hip abduction, sidebridging, and lateral step downs from a 30-cm step, all of which were reported as having high % MVICs. If strength is determined adequate, abnormal lower extremity movement patterns may be due to factors such as endurance or dynamic control. A focus on the relative EMG activity of various weight bearing and non-weight bearing functional activities as outlined in the review then becomes valuable to the clinician as a guide in designing a progressively challenging program aimed at improving movement patterns associated with functional positions and activities. Many of the studies reviewed looked at the EMG activity of the gluteus medius in normal subjects performing therapeutic functional activities. The challenge to the researcher is to further investigate comparison of EMG activity in normals to subjects with movement abnormalities or clinical conditions. As strengthening of the gluteus medius is considered to be an important management strategy for lower extremity disorders, and many of the exercises utilized in studies demonstrated low activation levels, future research is necessary to report outcomes utilizing these exercises in therapeutic programs. David Krause, Mayo Clinic, Rochester, MN, USA. Email: krause.david@ mayo.edu Krause Commentaries