Athletic Training and Sports Health Care

Original Research 

The Effect of Attentional Focus on Gluteus Medius Recruitment and Force Production

Molly Taylor, MS, ATC; Grace Golden, PhD, ATC, CSCS

Abstract

Purpose:

To examine the effect of attentional focus on agonist/antagonist muscle activity and force production in an open-chain, early-phase rehabilitation strengthening task applied to isolate the gluteus medius muscle (GMed).

Methods:

Forty-five active college-aged individuals (28 women, 17 men) were randomly assigned to an external focus of attention (EFA) group, an internal focus of attention (IFA) group, or a control group, and given specific movement cues. Co-contraction ratios and force production were determined as percent of maximal voluntary isometric contraction for GMed and adductor longus (AL).

Results:

There was a significant main effect of group for percent change of abductor force production of side-lying GMed hip abduction between the EFA and IFA groups (covariate significant at P < .001, main effect of group significant at P = .034 and EFA greater than IFA at P = .027). No main effect was seen in percent change of co-contraction ratio %GMed:%AL.

Conclusions:

Clinicians should consider employing EFA when assessing isolated muscle strength capabilities in patients and evaluate and strengthen the GMed in 10° of abduction, a position shown to optimize recruitment and force production during open-chain maximal voluntary isometric contraction.

[Athletic Training & Sports Health Care. 2020;12(6):272–282.]

Abstract

Purpose:

To examine the effect of attentional focus on agonist/antagonist muscle activity and force production in an open-chain, early-phase rehabilitation strengthening task applied to isolate the gluteus medius muscle (GMed).

Methods:

Forty-five active college-aged individuals (28 women, 17 men) were randomly assigned to an external focus of attention (EFA) group, an internal focus of attention (IFA) group, or a control group, and given specific movement cues. Co-contraction ratios and force production were determined as percent of maximal voluntary isometric contraction for GMed and adductor longus (AL).

Results:

There was a significant main effect of group for percent change of abductor force production of side-lying GMed hip abduction between the EFA and IFA groups (covariate significant at P < .001, main effect of group significant at P = .034 and EFA greater than IFA at P = .027). No main effect was seen in percent change of co-contraction ratio %GMed:%AL.

Conclusions:

Clinicians should consider employing EFA when assessing isolated muscle strength capabilities in patients and evaluate and strengthen the GMed in 10° of abduction, a position shown to optimize recruitment and force production during open-chain maximal voluntary isometric contraction.

[Athletic Training & Sports Health Care. 2020;12(6):272–282.]

More than 30 years of research demonstrates verbal cueing regarding an individual's focus of attention (FOA) influences motor learning and retention, accuracy, musculoskeletal efficiency, and force production.1 Wulf et al2 were the first to demonstrate differences in motor learning and task performance as a result of internal versus external foci of attention: internal focus of attention (IFA) on one's own body/movements (eg, their muscles) and external focus of attention (EFA) on one's bodily movement effects (eg, how much force is being applied to an external object).2 Investigations have since broadened to include measures of accuracy, muscle activity, motor recruitment, peak force production, hypertrophic muscle adaptation, and aesthetic quality of movement.1,3,4 These assessments have allowed researchers to discover scenario-specific benefits of both IFA and EFA. Recently, IFA cueing has facilitated increased hypertrophy in weight lifting tasks,5 yet most chronicled research emphasizes the benefits of EFA with quantifiable measurements expressing movement efficiency and maximum force production in tasks such as balancing, shooting basketball free throws, throwing darts, jumping, weight lifting, and golfing.2,6,7

The evidence-based benefits of EFA over IFA are attributed to various theories, including the constrained action hypothesis.1 In this case, attempting to consciously control a movement process that otherwise operates through automatic learned motor pathways constrains the efficiency of the system.1,8 In the execution of movement skills, whose fluidity and coordination are controlled by involuntary neural processes, voluntary focus overtly applied to specific aspects of the movement by the individual may lead to constrained and restricted performance. Expanding on the constrained action hypothesis, EFA is proposed to reduce stiffness via reduced motor recruitment, unlock degrees of freedom, and allow for increased variability in movement reactions.9 IFA is proposed to freeze the system, stiffening degrees of freedom and reducing the allowed variability in movements and reactions, thereby reducing the capability to efficiently produce a desired outcome.9,10 Other theories propose voluntary movements are exclusively planned motor behaviors represented as higher-level programmed tasks. This suggests movements are automated based on their outcomes, further implying an internal focus disrupts the pre-programmed outcome for that movement.9 With this perspective, necessary muscle recruitment happens automatically.11

Similarly, the effects of IFA are not limited to interruption of programmed task behavior, and possibly invoke self-evaluation and behavior monitoring and trigger “choking” under performance pressure.1,9,12 “Choking” is often generalized to performance or competitive sports, but is also suggested to influence simplified movement tasks by causing individuals to overanalyze or worry about their physical actions, thus disrupting outcomes.13 The fluidity of a skill being performed by someone who is highly experienced exhibits the same type of neuromuscular automaticity promoted by EFA.14 As FOA gains attention as an influential aspect of motor learning and performance, many investigations focus on efficiency, accuracy, and maximum force production. Researchers report increased force production and accuracy capabilities with decreased agonist/antagonist co-contraction in goal-oriented tasks, suggesting movement efficiency via decreased energy expenditure.1,8

To date, the promising potential of EFA to influence performance is primarily centered on optimizing functional movement outcomes; however, there is a lack of inquiry regarding the potential influence of EFA on early-phase strengthening exercises aimed to isolate specific muscles as part of the rehabilitation process. Specifically, researchers have not explicitly explored the influence of verbal instruction regarding FOA on hip muscle activity with isolated, low intensity gluteus medius muscle (GMed) therapeutic exercises. These findings could influence the way clinicians cue their patients not only when training both isolated or functional movements, but also in manual muscle testing assessment of force production capabilities. The GMed is the primary hip abductor and also a key stabilizer of the pelvis on the femur, allowing for control of potentially injurious dynamic valgus knee positions. Dynamic valgus has been defined as multi-joint misalignment in single-leg stance characterized by an inferior drop of the contra-lateral pelvis, femoral adduction and internal rotation, tibial internal rotation, and hyperpronation, and is often associated with anterior cruciate ligament tears and chronic patellofemoral syndromes.13,15 In the stance phase of walking and running gait, the GMed generates the most stabilization torque in a lengthened position, at 10° of adduction in the weight-bearing limb, making this its prime position associated with pelvic stability.16

Numerous researchers have explored the exercises and positions that most effectively activate the GMed to determine effective prescriptive rehabilitation exercises. To determine GMed muscle activity, surface electromyography (sEMG) is often used to compare muscle activation levels.17 Normalized sEMG of the GMed during side-lying hip abduction during conditions of maximal voluntary isometric contraction (MVIC) has been reported to have excellent reliability (intraclass correlation coefficient = 0.93)18 and validity when compared to intramuscular EMG.19 Additionally, although side-lying assessment of hip abduction MVIC appears to be the most reliable,20 the position of hip abduction in which MVIC of the GMed is established is inconsistent across studies: standardized abduction position without reported range of motion of position,21 end-range abduction,22 25°,23,24 35°,25 50% of hip abduction range of motion,26 10°,20 and 0°.27 The large range of GMed activation levels across different investigations and associated exercises could be due in part to the position of hip abduction during MVIC assessment, and in particular the ranges of %MVIC of GMed activation measured during the side-lying hip abduction exercise (39% ± 17%22; 62%21; 81% ± 42%23; 42% ± 23%24; 79% ± 29%25; 63%26). Additionally, the existence or type of verbal cueing and encouragement during the MVIC assessment of the GMed varies across investigations, ranging from not reported25,26 to strong verbal encouragement as the participant performed MVIC24 or standard verbal encouragement.23,27 In each case, the prescribed FOA during the MVIC assessment is not clear. Additionally, although Ayotte et al27 measured GMed activation during MVIC at 0°, the level of GMed activation, ranging from 36% to 52% MVIC, was investigated during weight-bearing exercises. Therefore, it is unclear whether the lack of agreement in GMed activation across investigations is due to subtle differences in the exercises (eg, single-limb squat: 82% MVIC,21 64% MVIC,23 52% MVIC27) or the position in which GMed MVIC was established. For this reason, further assessment of the level of GMed activation during MVIC at lesser hip abduction range of motion and therefore closer to the position associated with the GMed role in providing pelvic stability during single-leg stance is warranted. In this position, it is unknown whether GMed recruitment and force production will change if measured at lesser hip abduction instead of 25° to end range (45°).28

Some investigators suggest that IFA may not always be detrimental. IFA may help build the individual's perceived relationship between movements of body parts during novel skill practice where developing movement coordination is the primary goal.29 Improved movement coordination may be important in a range of task complexities and for early-phase rehabilitation, during isolated muscle exercises closely associated with neuromuscular recruitment. Clinically, improvement of muscular strength, through a combination of enhanced neuromuscular recruitment and muscle hypertrophy, is one of the key goals30 and neuromuscular research suggests positive strength gains require muscle exertion to at least 40% of MVIC, with optimal strength adaptations occurring with more than 60% of MVIC.23,30,31 If the rehabilitation goal is to increase muscle recruitment beyond 40% to 60% of MVIC of an isolated muscle, decreased co-contraction of the isolated agonist muscle in combination with its antagonist, as seen in EFA conditions, may not be ideal.23,30,31 In many postoperative rehabilitation settings, the goal of some early-phase exercises or movements can be maximizing activity of an isolated muscle for strength or reeducation following atrophy, such as the quadriceps following anterior cruciate ligament reconstruction.32 This counters the idea of efficient movement (ie, reduced EMG activity) to always be the optimal goal, allowing for necessary exploration of the influence of FOA on potential strength adaptations in an isolated muscle. Many clinicians and researchers obtain meaningful information from simplified task performance assessment that could be influenced by the use of FOA instructions. Potential to increase agonist activation and force production with concomitant lesser antagonist recruitment due to apt FOA considerations could benefit the clinical goal of improved muscular strength in specific muscles. Although antagonistic muscle activity to the GMed is resourced from several hip adductors, findings by Lovell et al33 demonstrated consistent and amplified adductor longus muscle (AL) EMG output compared to other hip adductors in multiple testing positions and therefore appears to be most reasonable to assess as the primary antagonist to the GMed. The comparisons of GMed to AL activation afford the ability to assess co-contraction levels during a side-lying MVIC hip abduction task and whether these ratios are influenced by FOA conditions.

To explore these parameters in the current study, we performed sEMG analysis and handheld dynamometry (HHD) in an open-chain isometric force production task. We investigated side-lying hip abduction at 10° abduction to approach a position of adduction without disregarding previous literature's abducted testing positions. We used three instruction conditions: control, EFA, and IFA. Appreciating the proven benefits of EFA on functional movements and motor learning, the goal of this study was to test the relevance of attentional focus cues on an early-phase, easy to perform, open-chain rehabilitation exercise. Using findings from Neumann,16 demonstrating optimal GMed torque in stance phase of gait in pure adduction, we anticipated placing the hip closer to a position of adduction (in 10° of abduction) would lengthen and enhance GMed recruitment and be clinically feasible to measure. Additionally, we hypothesized an EFA would result in greater GMed force production and reduced co-contraction of agonist/antagonist muscles (GMed/AL) during hip abduction. Finally, we hypothesized an IFA would result in greater agonist sEMG activity and greater variance in the ratio of agonist:antagonist sEMG activity during the hip abduction task.

Methods

Study Design

This study was a laboratory-based observational cross-sectional investigation in which participants were randomly assigned to one of three experimental conditions for a single testing session for the comparison of baseline and FOA effects on measures of muscle force and muscle activity. Independent variables included three FOA conditions (control, IFA, or EFA). Dependent variables included sEMG of the GMed and AL (normalized to %MVIC), GMed MVIC force production (normalized to body mass), and the ratio of co-contraction of the GMed and AL during side-lying hip abduction in a position of 10°.

Participants

Data were collected from a convenient sample of 45 healthy, physically active young adults aged 18 to 28 years with a mean age of 20.6 ± 2.0 years (28 women, mean age 20.5 ± 2.0 years and 17 men, mean age 20.8 ± 2.0 years). Participants were inexperienced with FOA cueing and reported no lower extremity injury in the past 6 months or lower extremity surgery in the past 3 years. Participants were recruited via association with the institution. Institutional review board approval was obtained via the University of Oregon Protocol Number 10172016.019 on November 8, 2016, prior to all participant data collection. Informed consent was obtained from all participants before beginning the experiment session.

Instrumentation

Hip abduction (10°) was determined using a bubble inclinometer (Baseline Bubble inclinometer; Medline Medical; accuracy = ± 1°, hip range of motion intra-rater reliability ICC = 0.61 to 0.90, standard error of the mean = 2.0° to 5.0°34). Participants' dominant limb was held in the position of hip abduction by a towel bolster made by the principal investigator (MT) and a rigid belt fastened to the underside of table for isometric contraction trials. An HHD was used to acquire force production data (MicroFET 2; Hoggan Scientific LLC). Muscle activation was measured using Noraxon EMG self-adhesive bipolar surface electrodes (Ag/AgCl, conductive area diameter 1 cm, inter-electrode distance 2 cm; Noraxon).

Procedure

Participants completed an intake questionnaire; read and signed informed consent; confirmed inclusion/exclusion criteria for eligibility; and self-reported height (inches), weight (lbs), and age (years) and current level of physical activity. Participants were then randomly assigned to one of the three conditions (slips drawn by blinded principal investigator or research assistant) and performed pretesting warm-up and stretching consisting of 5 minutes on a stationary bicycle at a self-selected pace followed by 5 minutes of dynamic stretches directed by the principal investigator (Figure 1) with three repetitions of 3-second holds of alternating limb walking stretches including knee flexion, hip flexion, ankle dorsiflexion, and hip abduction and external rotation followed by five standing leg swings on each limb in both hip abduction/adduction and hip flexion/extension directions.

Dynamic stretches for warm-up: three repetitions of 3-second holds of alternating limb walking stretches including (A) knee flexion, (B) hip flexion, (C) ankle dorsiflexion, and (D) hip abduction and external rotation followed by five standing leg swings on each limb in both (E) hip abduction/adduction and (F) hip flexion/extension directions.

Figure 1.

Dynamic stretches for warm-up: three repetitions of 3-second holds of alternating limb walking stretches including (A) knee flexion, (B) hip flexion, (C) ankle dorsiflexion, and (D) hip abduction and external rotation followed by five standing leg swings on each limb in both (E) hip abduction/adduction and (F) hip flexion/extension directions.

To determine which limb hip would be used for assessment for strength and muscle activity, an assessment of limb dominance was conducted, as described by Hoffman et al.35 The participant completed each of the three tasks one time: stepping up onto a short box (20 cm), reaction to a perturbation (light push from behind to the mid-thorax by the principal investigator) to observe the stepping limb, and kicking a ball. The preferred limb for each of these tasks was noted, and dominance ascribed to the limb used for the majority of the tasks.

For sEMG sensor placement, the skin was wiped clean with an alcohol swab and shaved of excess hair if necessary. Self-adhesive bipolar surface electrodes (Ag/AgCl, conductive area diameter 1 cm, inter-electrode distance 2 cm; Noraxon) were placed as follows: (1) AL: medially on the thigh of the dominant limb, one-third the distance from the pubic tubercle to the insertion of the AL on the lower two-thirds of the linea aspera of the femur (Figure 2A1)36,37; (2) GMed: laterally on the thigh of the dominant limb, one-third the distance from the iliac crest to the greater trochanter of the femur (Figure 2B2)23,38; and (3) reference electrode was placed on the anterior superior iliac spine of the same hip (Figure 2B3).22

Surface electromyography (sEMG) electrode placement. (A1) Adductor longus electrode (*origin and insertion). (B2) Gluteus medius electrode (*origin and insertion). (B3) Anterior superior iliac spine (reference electrode).

Figure 2.

Surface electromyography (sEMG) electrode placement. (A1) Adductor longus electrode (*origin and insertion). (B2) Gluteus medius electrode (*origin and insertion). (B3) Anterior superior iliac spine (reference electrode).

The EMG sensors were connected to wire leads from the Noraxon Myotrace 400 (Noraxon) handheld unit, which was connected via hardwire to a laptop via Noraxon surface EMG collection software. Data files were exported as ASCII files in preparation for analysis with LabVIEW software (version 24; National Instruments). Raw sEMG data were corrected for DC bias, band pass filtered (4th order Butterworth of 20 to 350 Hz), then rectified and smoothed via a root mean square algorithm time constant of 20 samples per 20 Hz. The onset of the MVIC trial was determined by sampling the initial baseline level of muscle activity and a level of activity exceeding 2× baseline was designated as the onset of the contraction. From the onset of muscle activity, a 200-ms sliding window was used and the window with the greatest average peak sEMG for the GMed was extracted, as well as the corresponding average peak sEMG for the AL during the same trial.23 All sEMG from the GMed MVIC trials was normalized to the highest of the three baseline trials to determine %MVIC. The same procedure was used to measure the greatest average 200-ms window for the AL during the baseline measures of AL MVIC. All sEMG from the AL was normalized to the highest of the three baseline AL trials to determine AL %MVIC. Normalized GMed sEMG was averaged across trials 1 to 3 (representing baseline) and trials 4 to 6 (representing the FOA condition for the corresponding groups).

Baseline strength testing began with three 5-second MVIC trials of the AL. Participants were allowed one to two practice trials to become familiar with the movements for each task. The participant assumed a supine position on an examination table with hips and knees at 0° flexion. The participant pushed both limbs against the HHD, positioned between the knees at the level of the medial femoral condyles (the force plate of the HHD was positioned against the dominant limb, as described by Lovell et al33), for 5 seconds (Figure 3). Force production was initiated on hearing the verbal command “go” and halted on hearing “relax” from the research personnel. Force was measured by recording output display on the device (visually) by the principal investigator and manually recorded on the data collection sheet. sEMG was recorded by starting the Noraxon software simultaneously with MVIC force effort during the task. Participants were allowed 30 seconds of rest between each 5-second trial. The verbal instruction for adductor MVIC trials was “squeeze as hard as you can.”

Adductor longus maximal voluntary isometric contraction (MVIC) trial.

Figure 3.

Adductor longus maximal voluntary isometric contraction (MVIC) trial.

For the remainder of data collection, MVIC and condition testing for the GMed required the participant to assume a side-lying position on the examination table with the dominant leg on top and the lower limb positioned comfortably (hip flexed approximately 45°, knee flexed approximately 120°, as visually determined by the principal investigator) on the table.

The participant was fitted with an adjustable non-elastic belt looped around the underside of the treatment table and over the top limb at the level of the distal one-third of the leg (Figure 4).39 The length of the strap was adjusted to allow the top limb to achieve 10° of hip abduction as measured by bubble inclinometer and slight extension as determined visually by the principal investigator (Figure 5). The position was maintained with a towel bolster fashioned by the principal investigator. The HHD was fitted against the lateral leg, 5 cm proximal to the superior edge of the lateral malleolus underneath the belt (Figure 6).39

Belt placement, distal one-third of leg.

Figure 4.

Belt placement, distal one-third of leg.

Hip abduction (10°) measured by bubble inclinometer.

Figure 5.

Hip abduction (10°) measured by bubble inclinometer.

Handheld dynamometer placement, 5 cm proximal to superior border of lateral malleolus.

Figure 6.

Handheld dynamometer placement, 5 cm proximal to superior border of lateral malleolus.

Procedures were repeated for GMed MVIC. Participants performed three trials pushing their top limb into the HHD affixed to their limb by the belt for 5 seconds. Force production was again initiated on hearing the verbal command “go” and stopped on hearing “relax” from the principal investigator. Force was measured by recording output display on the device (visually) by the principal investigator and manually recorded on the data collection sheet. sEMG was recorded by starting the Noraxon software simultaneously with MVIC force effort during the task (research assistant). Participants were allowed 30 seconds of rest between each 5-second trial. These data recording procedures were identical for baseline and FOA conditions.

Muscle force (N) during all trials was normalized to body mass via the following formula:40 Muscle Force = Recorded Strength (N) / Mass (kg)2/3.

The verbal instructions for the three baseline repetitions of GMed MVIC were “push up as hard as you can.” After these three trials were completed, participants were given a 3-minute rest period before completing the MVIC trials for the particular FOA condition that they were randomly assigned to. The position, time in contraction, and number of repetitions of MVIC trials for FOA conditions were the same as baseline. For control group intervention condition trials 4 to 6, verbal instructions were “Focus on pushing up as hard as you can.” For IFA group intervention condition trials 4 to 6, verbal instructions were “Focus on contracting the muscles of your outer hip as hard as you can.” For EFA group intervention condition trials 4 to 6, verbal instructions were “Focus on pushing up into this pad as hard as you can.”

Data Analysis

Analysis of covariance was used for the “gain” in performance with baseline as the covariate with pairwise comparisons (main effect of group) conducted with Sidak adjustment.41–44 An a priori level of significance of an alpha value of 0.05 or less was used for comparisons. Statistical analysis was conducted with SPSS software (version 24.0; IBM Corporation).

Results

Participant demographics are listed in Table 1. We report a significant main effect of group for percent change of abductor force production of side-lying GMed hip abduction between the EFA and IFA groups (covariate significant at P < .001, main effect of group significant at P = .034 and EFA greater than IFA at P = .027) as expressed in Table 2. The mean sEMG activity for all participants is provided in Table 3. No main effect was seen in percent change of co-contraction ratio: %GMed:%AL.

Participant Demographics

Table 1:

Participant Demographics

Gluteus Medius Muscle Force (N) Production Results as Normalized to Body Mass (kg)

Table 2:

Gluteus Medius Muscle Force (N) Production Results as Normalized to Body Mass (kg)

sEMG Derived from Focus of Attention Conditions as Percent of MVICa

Table 3:

sEMG Derived from Focus of Attention Conditions as Percent of MVIC

Discussion

The purpose of this study was to evaluate sEMG activity and force production of the GMed in a relatively novel position of assessment at 10° hip abduction. We chose this position to measure GMed activation and force production in a position of lesser hip abduction, more similar to the position of hip abduction necessitated to provide closed chain pelvic stability, while examining the potential influence of FOA on an early-phase rehabilitation exercise. Previous literature confirms that an EFA results in optimal performance results, including increased muscle efficiency, force production, accuracy, and power for functional movements.1,3,6,9 EFA applied during rehabilitation has also been shown to improve motor control, functional skill acquisition, and postural stability via increased intracortical inhibition.32 However, to our knowledge, FOA has yet to be investigated as influential during movement intervention relative to early-phase rehabilitation protocol goals associated with muscle reeducation or low-level strength adaptations.

We report increased force production of the GMed (% change) with an EFA (+8.42%) as compared to an IFA (−8.82%). Our findings support those found in the existing literature. A review by Marchant10 demonstrated greater, more accurate, and better maintained force production capabilities with EFA versus IFA in lifting, jumping, isometric, and isokinetic movements. Halperin et al3 found athletes instructed with EFA cues could produce 9% greater force in an isometric mid-thigh pull than athletes instructed with IFA cues. Similarly, the current study demonstrates that EFA promotes increased force production and IFA may be a possible detriment to performance, leading to a decrease in force production as compared to baseline MVIC.

Contrary to our hypothesis, we found no significant correlation between FOA and agonist/antagonist co-contraction ratios. One potential explanation is that early-phase rehabilitation tasks may be too simple to reap any muscle efficiency benefits from any FOA instruction. Without multi-joint demand of complex functional movements such as with jumping, balancing, or aiming, the simplicity of an isolated movement may not draw on the theorized benefits of EFA via the constrained action hypothesis.1,6,8,14 Additionally, without consideration of bilateral AL input for MVIC testing and the perceived low level activation of the AL in our decreased hip abduction testing angle of 10°, the co-contraction measurement may be considered null. It is within reason to theorize different agonist/antagonist muscle groups may respond differently based on joint angle and associated shortening or lengthening during testing.

Our data indicate that further EMG investigations into simple task performance could result in a significant correlation between FOA instructions and increased sEMG muscle activation variability. Our observation of inconsistent muscle performance with FOA cueing warrants further investigation into muscle recruitment in remedial tasks. Continued inquiry may provide evidence that early-phase, low skill level movement is unnecessarily complicated by the influence of FOA.

Our %MVIC data indicates that GMed activation is higher at 10° of hip abduction than previously tested positions. Compared to testing performed by Ekstrom et al22 at 25°, Distefano et al23 at 30°, and McBeth et al31 at 35°, our EFA group produced 49.5%, 12.4%, and 10.5% greater percent of MVIC, respectively. Bolgla and Uhl,24 testing hip abduction sEMG at end range hip abduction, reported 39% ± 17% of MVIC, equating to 52.5% less than our EFA group at 10° of hip abduction (91.5% ± 14.4%). Based on these comparisons, hip position during assessment of isometric side-lying hip abduction influences force output, making comparison across studies difficult for the clinician as a consumer. If GMed torque production is more critical in gait stance in hip positions closer to neutral and into a pure hip adduction position, as proposed by Neumann,16 we may need to refine our clinical assessment of GMed strength to consider torque production with greater ecological considerations. Additionally, if the goal of early-phase rehabilitation is to facilitate strength gains, the position of 10° hip abduction results in greater than 80% MVIC, regardless of FOA condition, and therefore creates an optimal opportunity for strength adaptations.

The current study had several limitations. The use of surface EMG is convenient and non-invasive, but reliability can be inconsistent and influenced by electrode type and placement, shifting movement patterns and compensations, “cross-talk” from nearby muscles, and surface impedence.38 The bolster and rigid belt set-up to hold participants in an abducted position occasionally allowed mild anterior-posterior shifting/swinging while participants were applying maximum upward force, potentially influencing dynamometer output, despite attempts to control for proper positioning by the principal investigator. Although the HHD is considered a reliable handheld measurement tool without consideration of counter force applied from the administering clinician, our study failed to take contralateral adductor force production and muscle activity into account when collecting baseline AL MVIC data. Our baseline mean normalized AL sEMG activation (83.7% ± 8.0%), which was used to calculate AL activation during the hip abduction task and our co-contraction ratios, was 13% different compared to the findings of Lovell et al33 (73.4 ± 17.4%) as measured in the same hip position during MVIC assessment. This difference could be due in part to our assessment of the AL on the dominant limb, whereas Lovell et al33 compared injured and uninjured limbs. Therefore, our co-contraction ratios lacked significance but may have been influenced by our unilateral data recording.33,39 Additionally, the generalizability of our findings are specific to relatively young healthy adults and therefore future investigations are needed to evaluate the effect of FOA on a simple early-phase rehabilitation exercise such as sidelying hip abduction, to determine whether or not EFA or IFA influence force production in patients with existing lower extremity injury or pathology.

Implications for Clinical Practice

An EFA is recommended for cueing or training efficient, functional movements. Our findings indicate that clinicians and researchers investigating early-phase, isolated movements or performing manual muscle testing may also want to consider introducing an EFA for patients to generate true force production capabilities.

Clinicians and researchers seeking to maximally recruit the GMed should consider positioning their participants closer to our testing position of 10° abduction, reducing shortening of the muscle and approaching the position of optimal stabilization torque of single-limb stance phase for improved progression to functional gait activities. Furthermore, when aiming for GMed strength gains, clinicians should prescribe open-chain exercises challenging the muscle in this lengthened position.

References

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Participant Demographics

ParameterMenWomenOverall
Age (years)20.8 ± 2.020.5 ± 2.020.6 ± 2.0
Weight (kg)85 ± 17.463.8 ± 6.571.8 ± 15.6
Height (cm)181.5 ± 9.9167 ± 6.9172.5 ± 10.7
Activity level (hrs/wk)10.18 ± 4.211.39 ± 4.910.9 ± 4.6
Limb dominance2 left, 15 right3 left, 25 right5 left, 40 right

Gluteus Medius Muscle Force (N) Production Results as Normalized to Body Mass (kg)

ParameterControlExternal FOAInternal FOA
Baselinea5.14 (1.87)5.46 (2.04)5.68 (1.78)
Condition of FOAb5.53 (1.87)5.95 (2.00)5.41 (1.38)
Adjusted condition of FOA5.78 (0.19)5.92 (0.19)c5.19 (0.18)
Percent change (%)+12.45+ 8.42− 8.82

sEMG Derived from Focus of Attention Conditions as Percent of MVICa

ParameterControlExternal FOAInternal FOAOverall
Baseline EMG GMed (trials 1–3)85 (6.4)87.3 (5.2)89.7 (6.8)87.3 (6.3)
Condition of FOA EMG GMed (trials 4–6)87.4 (11.3)91.5 (14.4)90.6 (21.7)89.8 (16.1)
AL EMG% change31.1 (43.2)39.7 (72.4)38.0 (57.4)36.3 (57.6)
GMed EMG% change2.8 (10.4)4.8 (15.8)0.46 (21.1)2.7 (16.1)
Authors

From the Department of Human Physiology, University of Oregon, Eugene, Oregon.

The authors have no financial or proprietary interest in the materials presented herein.

The authors thank Sam Johnson PhD, ATC, CSCS, for his expertise and assistance with data analysis; Jamie Destache, MS, ATC, and Brandon Jaeger for their roles in pilot data collection and photography; and research assistants Brian Greene, MS, ATC, Dan Drewry, Bri Dixon, Jason Agtarap, and Meagan Barnes for assisting with data collection.

Correspondence: Molly Taylor, MS, ATC, Department of Human Physiology, University of Oregon, East 13th Avenue, 181 Esslinger Hall, Eugene, OR 97403. Email: mollyjdtaylor@gmail.com

Received: May 11, 2020
Accepted: September 10, 2020

10.3928/19425864-20200922-01

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