Abstract
Objective
The purpose of this study was to determine the intra- and interexaminer reliability, concurrent validity, and responsiveness of the applied kinesiology manual muscle test (AK-MMT) to discriminate gluteus medius muscle strength and latency.
Methods
A cross-sectional and methodological study was conducted in 38 participants using electromyography, electrogoniometry, and hand-held dynamometry to measure latency, angular displacement, and muscle force during the assessment of the gluteus medius by AK-MMT. Inter- and intrarater reliability of 2 examiners with different levels of experience were obtained using the intraclass correlation coefficient. Muscle force, latency, and joint angular displacement were compared between groups (facilitated vs inhibited). Latency and angular displacement also were compared within groups by using the Wilcoxon paired test. For the concurrent validity of the AK-MMT in classifying an inhibited muscle as weak, the receiver operating characteristic curve was conducted.
Results
Intra- and interexaminer reliability for the facilitated vs inhibited classifications based on AK-MMT presented good results, with intraclass correlation coefficient > 0.86. For the inhibited group, force and peak force were significantly lower and joint displacement significantly greater. The receiver operating characteristic curve showed an area under the curve of 0.743, demonstrating that the test has concurrent validity (P = .001) to discriminate muscle force. The Wilcoxon paired test showed a significant delay in latency of the inhibited gluteus medius group (0.10 s vs 0.18 s, P = .007) when compared with the facilitated one.
Conclusion
In this study, we found good intra- and interexaminer reliability and concurrent validity for the AK-MMT to determine differences in gluteus medius muscle force. Although the paired data showed a different latency time between groups, the hypothesis of prolonged latency in muscles classified as inhibited by AK-MMT still needs further investigation.
Key Indexing Terms: Kinesiology, Applied; Electromyography; Muscle Strength Dynamometer; Reproducibility of Results; Neural Inhibition
Abbreviations: MMT, Manual muscle test
Introduction
The manual muscle test (MMT) is a clinical examination tool used to evaluate muscle strength in patients with neurological, orthopedic, and rheumatic pathologies.1,2,3 In the 1960s, George Goodheart developed applied kinesiology (AK),4 a diagnostic and therapeutic system in which it is hypothesized that MMT is used to evaluate the neural control of muscle function.5,6 Goodheart developed this hypothesis from the observation that the motor response varies with the introduction of sensorial stimuli during the MMT.7 In AK, muscles are tested before and after applying sensorial challenges or therapeutic techniques, and clinical judgments are made based on transient changes in neuromuscular status.8,9 It has been estimated that 1 million health professionals, including chiropractors, osteopaths, physiotherapists, and several others, use AK-MMT, suggesting the necessity to undertake rigorous research to explore its clinimetric properties.10
The AK-MMT operational definition is similar to the traditional “break test” described by Kendall et al,11 but instead of the 6-level strength scale, it employs a binary system, where the muscles are classified by the tester as “facilitated” or “inhibited.”11, 12, 13 The hypothesis of the AK-MMT is to assess the ability of the nervous system to adapt the intensity of muscle contraction in response to a force variation applied by the examiner.6 When the participant makes this adjustment, keeping the test position stable, the muscle is subjectively classified by the examiner as “facilitated.” When the examiner manages to “break” the muscular contraction, displacing the tested limb, the muscle is classified as “inhibited.”14 Some AK practitioners may refer to facilitated muscles as “strong,” corresponding to grade 5 in the Kendall scale, and to inhibited muscles as “weak,” corresponding to grade 4 or less.13,15 However, other AK professionals describe their perception of muscular inhibition as a delay in the adjustment of the muscular contraction against the changing applied force.5,9 This description can lead to the hypothesis that the inhibited muscles present a longer latency or a delay in the activation of the muscle during motor response.
Despite its importance for the validation of the AK-MMT, no targeted investigation has been conducted to explore the hypothesis of longer latency in muscles classified as inhibited. This line of investigation may be relevant, since studies have shown that people with chronic pain present changes in motor control characterized by delayed activation of the muscles involved in the functional stabilization in affected regions.16, 17, 18 In these studies, change in neuromuscular control was demonstrated using electromyography (EMG), ultrasound images, and transcranial magnetic stimulation of the motor cortex.19, 20, 21 These instruments are expensive and difficult to handle in clinical practice. If AK-MMT is sensitive to latency delay, its use as a clinical tool for identifying changes in motor control could be considered. Therefore, the purpose of this study was to determine the intra- and interexaminer reliability, concurrent validity, and responsiveness of the AK-MMT to discriminate gluteus medius muscle strength and latency in individuals that were classified as inhibited by the AK-MMT.
Methods
Study Design and Sample Size
We identified the prevalence of inhibited gluteus medius muscle, classified as such by the AK-MMT, in a group of 38 individuals. Then, a methodological study was conducted to determine the intra- and interexaminer reliability, concurrent validity, and responsiveness of the AK-MMT to discriminate muscle strength and latency.
Individuals of both sexes between the ages of 18 and 70 years and capable of abducting the hip joint against gravity were invited to participate in the study. The participants were recruited in the clinic CinSis Fisioterapia (Belo Horizonte, Brazil) between January and September 2019. Healthy individuals and individuals with chronic pain were included. Exclusion criteria were acute or subacute pain or musculoskeletal disorder that would prevent abduction of the hip joint by at least 10°. The participants were informed of the inclusion and exclusion criteria before their visit so that they would not have to travel to the study site for nothing.
Ethics
This study was approved by the Research Ethics Committee of the Faculdade Ciências Médicas de Minas Gerais (protocol number: 2.870.008), and all the participants signed the informed consent form.
Sample Size
The sample size was calculated according to Walter et al22 to assess the reliability of the method. A power of 80% was considered, as well as 2 replicas of measure, 2 examiners, and a significance level of 0.05 to detect a reliability of Kripperndorff's alpha above 0.8.23 Based on that, in this case, a minimum of 68 measurements would be necessary. Because the tests were made bilaterally, the minimal sample was 34 participants.
Measurement Instruments
Applied Kinesiology's Manual Muscle Test
To classify the gluteus medius muscle as inhibited or facilitated, the AK-MMT was performed following the operational definitions described by Walther5 and Cuthbert and Goodheart.6 Owing to the characteristics of this study, the test procedure was technically divided into 3 steps: “positioning,” “locking,” and “impulse.” Initially, the participant was positioned in the supine position, with a lower limb placed at 10° of hip abduction. In the locking phase, the examiner placed his hand just above the participant's ankle and applied a pressure in the opposite direction of the gluteus medius function, with moderate force. The participant was instructed to resist, performing an isometric contraction to adapt the muscle contraction to the examiner's applied force and lock the limb in the test position. In the impulse phase, the examiner introduced a gradual and constant increase in the force applied, with the intention of breaking the isometric contraction and producing an angular displacement of the tested limb in the direction of hip adduction. This last phase has been standardized to last about 3 seconds and to be executed with submaximal force (Fig 1A). To synchronize the impulse timing with the participant's motor response, just before applying the pressure change, the examiner gave a verbal signal by asking, “May I test?” The participant was instructed to answer audibly “Yes” and resist, trying to stabilize the limb in the test position. If the participant managed to maintain the isometric contraction during the test (3 seconds), the force applied was reduced gradually until it stopped, and the test ended. If they did not manage to do so, their limb was displaced until the end of the test time or at 0° adduction.
Fig 1.
(A) Applied kinesiology manual muscle test of the gluteus medius muscle. (B) The gluteus medius traditional “break test” with handheld dynamometry. (C) The handheld dynamometer Lafayette Manual Muscle Test System (model 01165). (D) Placement of the superficial electromyography electrodes and end blocks of the goniometer. (Color version of figure is available online.)
Handheld Dynamometry
Measurements of muscle force were obtained from both sides of all the participants using the Lafayette Manual Muscle Test System model 01165 (Lafayette Instrument, Lafayette, Indiana), an ergonomic handheld dynamometer.24 The force evaluation of the gluteus medius muscle muscle was conducted against gravity, according to the “break test” technique described by Kendall et al.11 This test style was chosen because it is the one that most resembles the AK-MMT. The participants were in the side lying position with the accessed lower limb at 10° of abduction. The dynamometer was placed right above the lateral malleolus in a way that the applied pressure was transmitted through the device's padded stirrup (Fig 1B). In each test the examiner applied a manual force toward the floor, which the participant had to respond with the greatest possible force in the opposite direction, producing an eccentric contraction of the hip abductors. The dynamometer captured and registered the force data 40 times per second and, with the MMT Download Tool Software (Lafayette Instrument), the peak force and average force were analyzed (Fig 1C). Data were normalized using the technique described by Bazett-Jones et al25 that considers body mass, sex, and an allometric scaling, which is a normalization technique that divides the strength measured by the dynamometer by body mass raised to a power (6-hip muscle average allometric value, or bavg) that diminishes body size effects.
Electromyography
The latency data were obtained with the New Miotool Wireless (Miotec, Porto Alegre, Rio Grande do Sul, Brazil), an 8-channel EMG device. This EMG device has its own software called Miograph (Miotec) that registers, processes and analyzes the EMG data. The New Miotool sensors amplify the signal in 63 times and apply the Butterworth filter. Data were collected with a frequency of 1000 Hz and the digitalization was conducted with 16 bits resolution. A gain compensation of 350 times, a 10 Hz low pass filter, a 500 Hz high pass filter, and a 60 Hz Notch filter were configured in the software.26 The procedures for locating and placing the sensors were carried out according to the recommendations of the European project EMG for noninvasive assessment of muscles (SENIAM).27 Superficial AG/AgCl bipolar electrodes were fixed in the lateral region of the hip, in the middle of a line going from the iliac crest to the major trochanter of the femur, parallel to the gluteus medius fibers (Fig 1D). A reference electrode was fixed on the top of the lateral epicondyle of the humerus.
The temporal muscle activity was collected during the of AK-MMT test. The moment the impulse phase started, a mark (trigger) was introduced in the EMG register and the 3 seconds of the AK-MMT impulse phase were analyzed. The raw EMG data was transferred to a Microsoft Excel spreadsheet (Microsoft Corporation, Redmond, Washington) to calculate the latency and processed as follows: the signal was rectified and cut with a low-pass filter at 6 Hz frequency generating the linear envelope.28 Then the maximum and minimum magnitude of the electromyographic signal were identified. The muscle was considered active when the signal magnitude surpassed 2 standard deviations of the minimal magnitude of the average signal per individual.28,29 Once the activation value was determined, the time in seconds to reach that activation was recorded as the latency of the muscle.
Electrogoniometry
Angular displacement of the hip joint data was collected simultaneously with the EMG register Twin Axis Goniometer SG 150 (Biometrics Ltd, Ynysddu, United Kingdom) during the AK-MMT.30 The end blocks of the goniometer were fixed with double-sided medical adhesive tapes and elastic bands so that the flexible axis crosses over the center of the hip joint. The device was connected to one of the New Miotool channels through a customized transducer, allowing the recording of joint displacement and EMG signal simultaneously by the software Miograph (Fig 1D). The angular displacement data were transferred to a Microsoft Excel spreadsheet and analyzed at 0.5-second intervals, for the 3 seconds of AK-MMT impulse phase.
Procedures
There were 2 examiners in this study: a physical therapist with more than 30 years of experience with the AK-MMT (examiner 1) and a student in his last year of his physical therapy program, previously trained in the execution of the AK-MMT (examiner 2). In the first visit, demographic and clinical data of the participants were collected by examiner 2, and at the moment, examiner 1 was blinded to these data. These data included sex, age, weight, height, pain, and use of medication. Then, examiner 1 taught the participants the steps of the AK-MMT, and then executed the AK-MMT of the gluteus medius muscle bilaterally. After that, examiner 2 executed the same test. Examiner 2 was blinded for the results obtained by examiner 1. The next step was the handheld dynamometry, executed by examiner 2 as a maximal strength break test. The last procedure was the placement of sensors and the collection of EMG and angular displacement data during the AK-MMT. In this last step, while examiner 1 performed the AK-MMT with the participant, examiner 2 assisted in the operation of the software and data recording. In the second visit, only the initial procedures of the AK-MMT and dynamometry were repeated. The AK-MMT results obtained by the most experienced examiner (examiner 1) were used to classify the participants as “facilitated” or “inhibited,” and the results obtained by examiner 2 were used only to assess interexaminer reliability.
Intra- and interexaminer reliability data and hip abductor force data were collected bilaterally in visits 1 and 2, totalizing 152 assessments. The average difference between visits 1 and 2 was 6 days, 1 day being the minimum and the maximum being 12 days. As for the latency and displacement, data were collected bilaterally but only on visit 1, totalizing 76 assessments.
Statistical Analysis
Sample data were descripted using central tendency and dispersion measures. To evaluate intra- and interexaminer reliability, the intraclass correlation coefficient with the 2-way random-effects model was used to compare the classifications of gluteus medius status (inhibited or facilitated) based on the AK-MMT. The intraclass correlation coefficient was interpreted as poor (<0.50), moderate (0.50-0.75), good (0.76-0.90), and excellent (>0.90).31
Force data were compared between groups (inhibited vs facilitated) in visits 1 and 2, considering the total sample, using the Mann-Whitney U test and the 95% confidence interval. To analyze latency and angular displacement, the groups were compared using total sample in visit 1, and also only between volunteers, which presented the gluteus medius facilitated on one side and inhibited in the other. In this case, all data was analyzed using the nonparametric Wilcoxon test and the paired t test, respectively. We also conducted a Spearman correlation test between strength and latency, considering the hypothesis that the higher the force and the peak force, the lower the latency.32
For the concurrent validity we used as the variable response the dichotomic classification of the muscle as inhibited and facilitated, according to the examiner 1, and as the test variable, the strength test obtained with the dynamometer. For this analysis, the receiver operating characteristic curve (ROC curve) was applied. The point chosen to determine the optimal turning point between facilitated and inhibited was the one closest to the superior left corner (which means the cutoff that optimizes the sensibility and specificity of the test), and the area under the curve was chosen as the measure of accuracy. An area under the curve >0.7 was considered sufficient as a discrimination factor.33 All data was analyzed with a significance level of 0.05 and using the IBM SPSS statistic package (IBM, Armonk, NY).
Results
This study included 38 participants with a mean age of 41.1 years (standard deviation = 14.8) and a body mass index of 23.5 kg/m2 (standard deviation = 3.3). Of the total, 25 (65.8%) were female and 21 (55.3%) reported chronic musculoskeletal pain (Table 1). None of the participants were excluded, and all of them attended both assessment visits. The gluteus medius muscle was classified as inhibited on one side of the body (by examiner 1) in 20 participants (52.6%) at visit 1 and in 19 (50%) at visit 2. Of the 20 participants with gluteus medius inhibition at visit 1, 18 (90%) reported pain symptoms predominantly on the same side of inhibition. The lumbar spine, hip, and knee being the most affected areas. Of the 18 with bilaterally facilitated gluteus medius, only 3 (16.7%) reported pain. None of the participants reported pain during the MMT or AK-MMT, and none of them were using pain relieving medications in the days they were assessed.
Table 1.
Demographic and Clinical Characteristics of the Participants in Visit 1 (n = 38)
| Variable | Participants n = 38 | Facilitated n = 18 | Inhibited n = 20 |
|---|---|---|---|
| Demographic | |||
| Female sex, n (%) | 25 (65.8) | 14 (77.8) | 11 (55.0) |
| Age (y), mean (SD) | 41.2 (14.8) | 37.4 (11.3) | 44.5 (14.9) |
| Weight (kg), mean (SD) | 64.82 (11.1) | 63.8 (11.3) | 65.8 (11.1) |
| Height (m), mean (SD) | 1.66 (0.1) | 1.67 (0.1) | 1.65 (0.1) |
| BMI (kg/m2), mean (SD) | 23.5 (3.3) | 22.7 (2.3) | 24.2 (3.9) |
| Clinical | |||
| Pain | |||
| Yes, n (%) | 21 (55.3) | 3 (16.7) | 18 (90.0) |
| No, n (%) | 17 (44.7) | 15 (83.3) | 2 (10.0) |
BMI, body mass index.
The results of the intra- and interexaminer reliability are shown in Table 2. All reliability values were significant and above 0.86, demonstrating good intra- and interexaminer reliability in the classification of the gluteus medius based on the AK-MMT.
Table 2.
Intra- and Interexaminer Correlation Coefficient of the AK-MMT of the Gluteus Medius Muscle During Visits 1 and 2 (N = 76)
| Intraexaminer |
Interexaminer |
||
| Examiner 1 | Examiner 2 | Examiner 1 vs Examiner 2 |
|
| Visit 1 × Visit 2 | Visit 1 | Visit 2 | |
| 0.864 P < .001 | 0.893 P < .001 | 0.897 P < .001 | 0.862 P < .001 |
The Spearman correlation was significant at P < .05.
To assess strength, the muscles were divided into 2 groups (facilitated and inhibited) based on only the classification of examiner 1. On visit 1, from the 76 muscles assessed, 56 were classified as facilitated and 20 as inhibited, and on visit 2, 57 and 19, respectively (Table 3). Mann-Whitney U test showed that both the average normalized force of the gluteus medius (U = 298.0, P = .002) and the average normalized peak force (U = 347.0, P = .012) on visit 1 were different between facilitated and inhibited groups. On visit 2, we observed no significant difference in the normalized force (U = 382.5, P = .056) and normalized peak force (U = 420.0, P = .145). Considering all data, we found a significant difference between the 2 groups in the average normalized force (U = 1369.5, P = .001) and normalized peak force (U = 1546.0, P = .006). In these comparisons, the facilitated group presented greatest averages of normalized force and peak force. Table 3 shows the average values and the 95% confidence interval of the difference between visits and groups. In relation to the concurrent validity, the ROC curve (Fig 2) showed an area under the curve of 0.743 (P = .001), demonstrating that the classification of examiner 1 could discriminate the force between facilitated and inhibited muscles, indicating that the test has concurrent validity.
Table 3.
Mean, Standard Deviation, and 95% Confidence Interval of the Gluteus Medius Muscle Normalized Force and Peak Force Between Facilitated and Inhibited Groups During Visits 1 and 2, and Total Sample
| Visit 1 |
Visit 2 |
Total |
||||
|---|---|---|---|---|---|---|
| Variable | Facilitated n = 56 | Inhibited n = 20 | Facilitated n = 57 | Inhibited n = 19 | Facilitated n = 113 | Inhibited n = 39 |
| Force (kg/BMˆbavg), mean (SD) | 0.9 (0.6) | 0.5 (0.3) | 0.8 (0.4) | 0.6 (0.4) | 0.8 (0.5) | 0.6 (0.3) |
| P value; 95% CI | .002; 0.1-0.5 | .056; -.04 to 0.4 | .001; 0.1-0.4 | |||
| Peak force (kg/ BMˆbavg), mean (SD) | 2.8 (1.0) | 2.2 (0.9) | 2.6 (1.0) | 2.3 (0.9) | 2.7 (1.0) | 2.2 (0.9) |
| P value; 95% CI | .012; 0.1-1.1 | .145; -0.2 to 0.9 | .006; 0.1-0.8 | |||
BM, body mass; bavg, 6-hip muscle average allometric value;CI, confidence interval; SD, standard deviation.
Significant at P < .025.
Fig 2.
Sensitivity and specificity. The receiver operating characteristic curve cutoff point value is ≥1.95 of the average strength. Sensitivity = 0.956; specificity = 0.615. Area under the curve = 0.743. (Color version of figure is available online.)
The latency results (Table 4) showed no significant difference between the facilitated and inhibited groups (U = 485.0, P = .376) when the entire sample was considered (N = 76). However, considering only the data from the participants that presented the gluteus medius facilitated on one side and inhibited on the other (n = 40), the Wilcoxon paired test showed a significant difference between muscles (P = .007). The average latency of the facilitated group was 0.10 seconds (0.11 seconds) and of the inhibited group was 0.18 seconds (0.17 seconds) (example in Fig 3). No significant correlation was found between latency and average force (r = -0.013, P = .908), but we found a weak but significant and positive correlation between latency and peak force (r = .276, P = .016).
Table 4.
Mean and Standard Deviation of the Gluteus Medius Latency Between Facilitated and Inhibited Groups on Visit 1 (N = 76) and Between Participants Who Presented Inhibition on One Side and Facilitation on the Opposite Side (n = 40)
| Groups Visit 1 |
|||
|---|---|---|---|
| Variable Latency (s) | Facilitated (n = 56) | Inhibited (n = 20) | P Value |
| Mean | 0.13 | 0.19 | .376a |
| Standard deviation | 0.13 | 0.17 | |
| Paired group |
|||
| (n = 20) | (n = 20) | ||
| Mean | 0.10 | 0.18 | .007b |
| Standard deviation | 0.11 | 0.17 | |
P < .05 was considered significant.
Mann-Whitney U test.
Wilcoxon paired test.
Fig 3.
Example of raw electromyography and linear envelope showing the latency difference in an individual with inhibited gluteus medius on one side and facilitated on the other. Arrows mark the beginning of muscle activation. (Color version of figure is available online.)
Angular displacement data are shown in Table 5. Considering the total sample (N = 76), the inhibited group showed a significantly greater angular displacement at 1.5 seconds. When we matched the 20 participants who presented inhibition on one side and facilitation on the opposite side (n = 40), the displacement difference was significant at 1 second of the test.
Table 5.
Comparison Between Minimum and Maximum Hip Joint Angular Displacement in Increments of 0.5 Seconds Between Facilitated and Inhibited Groups on Visit 1 (N = 76) and Between Participants Who Presented Inhibition on One Side and Facilitation on the Opposite Side (n = 40)
| Visit 1 |
|||
|---|---|---|---|
| Angular displacement phase (s) | Facilitated n = 56 Mean (SD) | Inhibited n = 20 Mean (SD) | P Value |
| 0.5 | 4.1 (1.1) | 4.0 (1.1) | .884a |
| 1.0 | 5.5 (1.5) | 6.5 (1.7) | .014a |
| 1.5 | 5.9 (1.8) | 11.4 (5.3) | <.001a |
| 2.0 | 6.2 (1.8) | 15.9 (4.8) | <.001a |
| 2.5 | 6.4 (2.0) | 17.3 (4.7) | <.001a |
| Paired group |
|||
| n = 20 Mean (SD) |
(n = 20) Mean (SD) |
||
| 0.5 | 4.1 (1.0) | 4.0 (1.1) | .850b |
| 1.0 | 5.4 (1.2) | 6.5 (1.7) | .005b |
| 1.5 | 6.0 (1.4) | 11.4 (5.3) | <.001b |
| 2.0 | 6.4 (1.4) | 15.9 (4.8) | <.001b |
| 2.5 | 6.8 (1.4) | 17.6(4.8) | <.001b |
P < .01 was considered significant.
Mann-Whitney U test.
Paired t test.
Discussion
The main goals of this study were to evaluate the clinimetric properties of the AK-MMT and investigate the hypothesis of a different latency between muscles classified as facilitated and inhibited by the AK-MMT. To our knowledge, this is one of the first studies to investigate whether there is a latency delay in muscles classified as inhibited. We identified the prevalence of inhibited muscles during AK-MMT of hip abductors and the frequency of pain symptoms in a group of 38 individuals, followed by a methodological study to determine intra- and interexaminer reliability and concurrent validity of AK-MMT in the discrimination of muscle strength and latency.
We identified 20 individuals with the gluteus medius unilaterally inhibited, and 90% of them had musculoskeletal pain. In comparison, the prevalence of pain symptoms in individuals with facilitated gluteus medius on both sides was only 16.7%. The high prevalence of painful symptoms in individuals with inhibited gluteus medius, predominantly on the same side of the inhibition, indicates a relevant association between muscle inhibition and musculoskeletal pain. However, this correlation was not the focus of this work and it is highly recommended to explore it further in properly designed studies.
The methodological study showed that AK-MMT for the gluteus medius was a reliable test even when performed by an inexperienced examiner. The reliability results in our study were better than those found in previous studies.34,35 Conable et al36 argued that differences in test style and parameters are confounding factors and may explain the inconsistent results in several studies on AK-MMT interexaminer reliability. According to Conable et al, the standardization of the test, with clear parameters of strength, duration, and angular displacement, is the first necessary step for the research and clinical application of AK-MMT.36 Pollard et al37 standardized the force applied during AK-MMT and found good reliability among 2 examiners with different levels of experience. The reliability results of the present study were better, probably because we standardized the duration of the impulse phase of the test. We followed the recommendations of Conable,38 who found better results in intraexaminer reliability with a duration of 3 seconds.
Our results showed that inhibited muscles were weaker than the facilitated ones. Although the AK hypothesis is that muscle inhibition is a transient deficiency in motor control, we found a noticeable difference in strength between the facilitated and inhibited groups. Schmitt and Yanuck8 described the changes of muscular function identified by the AK-MMT as changes in the central integrative state of lower motoneurons. This state is defined as the sum of all the excitatory and inhibitory impulses that arrive simultaneously at a neuron and the probability of this neuron to generate an action potential.39 According to Schmitt and Yanuck,7 the central integrative state of the lower motoneurons modulates the generation and sustain of muscle contraction, affecting the functional strength of a skeletal muscle. Our results support that premise by showing that the peak force and the average force were smaller on the inhibited muscles group.
Our results showed that AK-MMT for the gluteus medius had concurrent validity within this group of participants and the examiner discriminated differences in muscle strength. The ROC curve demonstrated that the more experienced examiner's classification differentiated the strength between the facilitated and inhibited muscles. We feel that the clinical importance of this finding suggests that the AK-MMT was able to discern muscle strength through a pressure variation applied with moderate to submaximal strength, without having to submit the individual to maximum strength tests. These results are consistent with the study of Leisman et al15 that investigated the strength and resistance to fatigue on inhibited and facilitated muscles. Ratings obtained by experienced examiners during the AK-MMT were compared with EMG data and dynamometric measures during tasks of short and long duration. The inhibited muscles presented a lower production of strength and higher electromyographic activation, generating “less efficient” contractions than the facilitated muscles. The authors concluded that the AK-MMT was able to distinguish physiological differences in the muscular function without being necessary to submit the muscles to a demand of maximum strength or the fatigue situation.15
Regarding latency, this study did not find a significant difference between the facilitated and inhibited groups when considering the total sample (N = 76). However, when comparing the muscle groups of individuals who had the gluteus medius inhibited on one side and facilitated on the other (n = 40), this difference was noticeable. As EMG is an examination susceptible to great variability, morphological differences between muscles and individuals can affect the electromyographic signal and lead to inconsistent results.40,41 When using the opposite side of the same individual as a control, the variability was reduced and the difference of the latency could be evidenced (example in Fig 3). Although our data did not show that there was a latency difference between muscles identified as inhibited and facilitated by the AK-MMT, it suggested consistent indication in that direction. For this reason, we believe that more studies should be carried out to confirm, with a greater degree of certainty, the hypothesis that the inhibited muscles present greater latency or delay in the motor response during the AK-MMT.
Analyzing the difference between the position of the limb at the beginning of the test and its most deviant position we found that the inhibited group showed a greater displacement than the facilitated one. These findings are consistent with Caruso and Leisman,14,35 who compared force, displacement, and time to analyze AK-MMT parameters. Caruso and Leisman found 2 typical displacement graphs for time, depending on whether the muscle being tested is inhibited or facilitated. In our study, displacement and latency data were collected simultaneously, and a similar analysis between groups was conducted. When comparing the difference in angular displacement by time interval considering the total sample (N = 76), the difference in displacement was significant after 1.5 seconds. When we paired the participants with inhibited gluteus medius on one side and facilitated on the opposite (n = 40), the displacement difference was evidenced in a shorter time, with 1 second. Our hypothesis was that this difference occurred because of the variability of the electromechanical delay. During AK-MMT, the force generated by the muscle contraction needs to be transmitted to the examiner's hand. The electromechanical delay is the time it takes for tension to propagate through the serial elastic components (muscle fibers, aponeuroses, and tendons) along the tested limb.42,43 Therefore, when pairing individuals with inhibition on one side and facilitation on the opposite side, the variability of electromechanical delay was reduced and the difference in angular displacement was evidenced with a shorter time.
Limitations
One of the limitations of the present study was that we investigated only 1 muscle, reducing its external validity. Thus, other studies using other muscles may not necessarily have similar findings. A second limitation was that we based the size of the sample on the study of the AK-MMT reliability. Owing to the common difficulty in AK-MMT studies of obtaining a good sample of inhibited muscles and that we had no initial idea of the latency effect, we made the decision to define the sample size in this part of the study.15,38 That may have reduced the power of this research to determine the difference on the latency between groups.
Another limitation of this study was that we had only 2 examiners, both physical therapists, which may have restricted the results. Also, we must consider that the AK-MMT is, ultimately, a subjective test where the interpretation of the muscle response depends on the examiner's perception. This means that, despite leading to an objective result in the form of a binary classification (facilitated vs inhibited), the AK-MMT results can be influenced by the subjective opinion of the examiner, especially as it is related to his previous experience and professional training. The methodology for further studies can be improved by including more examiners from different professions, such as neurologists, orthopedic surgeons, and chiropractors, with different levels of experience.
Conclusion
This study investigated the inter- and intraexaminer reliability and concurrent validity of the gluteus medius AK-MMT in a group of adults. Our results demonstrated that AK-MMT has good clinimetric properties and was sensitive to differences in strength of the gluteus medius muscle. Our results support the hypothesis that the AK-MMT may be sensitive to differences in latency. Although our data indicated a different latency between the paired groups, the hypothesis of prolonged latency in muscles classified as inhibited by the AK-MMT still needs further investigation. Considering this is the first study that has investigated the latency delay on the muscles classified as inhibited by the AK-MMT, we suggest that future investigations use other muscle groups, larger samples, and the combination of EMG with other methods of examination, such as ultrafast ultrasound or mechanomyography, to detect the onset of muscle contraction in a more accurate way.
Funding Sources and Conflicts of Interest
No funding sources or conflicts of interest were reported for this study.
Contributorship Information
Concept development (provided idea for the research): D.G.O.
Design (planned the methods to generate the results): R.N.K.
Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): R.N.K.
Data collection/processing (responsible for experiments, patient management, organization, or reporting data): D.G.O., G.M.O.
Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): R.N.K., G.M.O.
Literature search (performed the literature search): D.G.O.
Writing (responsible for writing a substantive part of the manuscript): D.G.O.
Critical review (revised manuscript for intellectual content, this does not relate to spelling and grammar checking): R.N.K.
Practical Applications.
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The applied kinesiology manual muscle test was reliable even when conducted by an inexperienced examiner.
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Our results show that inhibited muscles were weaker than the facilitated ones.
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Our findings support the hypothesis that inhibited muscles may have a longer latency.
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We conclude that the applied kinesiology manual muscle test is a reliable and valid clinical tool to identify neuromuscular inhibitions.
Alt-text: Unlabelled box
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