Muscle Activation in Specific Regions of the Trapezius During Modified Kendall Manual Muscle Tests

As part of the clinical assessment of the musculoskeletal system, manual muscle tests (MMTs) are used to assess the amount of force a given muscle can produce.¹ For the trapezius muscle, this evaluation is complex, as we often describe the trapezius muscle as comprising 3 regions (upper trapezius region [UTR], middle trapezius region [MTR], and lower trapezius region [LTR]) with distinct mechanical actions. The UTR assists with clavicular elevation and upward rotation of the scapula, the MTR contributes to scapular stabilization and retraction, and the LTR contributes to inferior rotation and depression of the scapula.² Therefore, the trapezius muscle is assessed in multiple positions to evaluate each region separately and selectively recruit associated muscle fibers.¹

The trapezius muscle is often implicated in scapulothoracic and acromioclavicular joint dysfunction because of its role in scapular movement.³ Altered muscle activity of the UTR, MTR, and LTR has been identified in people with shoulder injuries,^4–7 most notably increased UTR excitation and decreased LTR excitation. Alterations in trapezius muscle excitation are associated with altered scapular kinematics, which are thought to increase the risk of shoulder injury.⁸ It is assumed that overactive or underactive regions of the trapezius muscle must be addressed using targeted interventions to alter the activity of these underactive or overactive regions, thereby restoring normal scapular kinematics to reduce the risk of injury.^3,7,9 For example, if we perform an MMT for the LTR when assessing scapulothoracic joint dysfunction, acromioclavicular joint dysfunction, or both, we may conclude that the LTR is underactive or weak if the patient does not produce sufficient force. Conversely, if the patient produces sufficient force, we may assume that the LTR has adequate strength and activity to support proper scapular stabilization and movement. In turn, we use this information in the clinical interpretation and formulation of the patient's treatment plan.

Manual muscle tests are routinely used to assess the scapula, shoulder, and neck.¹⁰ Three tests are conducted when using the Kendall MMTs: one to assess the neuromuscular function of each region.¹¹ During an MMT, we place the patient in a specific position intended to facilitate maximal isometric activation of a muscle or muscle region, with the instruction to meet the manual force applied for approximately 5 seconds. Our subjective evaluations of the force a patient produces in response to an MMT are then used to assess the neuromuscular function of the muscle, including its strength and fatigue resistance. However, despite their routine use, to our knowledge, no researchers have determined if MMTs for the trapezius preferentially recruit these regions of the trapezius. As such, the purpose of our study was to assess excitation of the UTR, MTR, and LTR individually in healthy participants during adapted MMTs. We adapted the MMTs into maximal voluntary isometric contractions (MVICs) for the upper trapezius (UT-MVIC), middle trapezius (MT-MVIC), and lower trapezius (LT-MVIC). Each MVIC was based on a Kendall MMT intended to target the corresponding region of the trapezius. Excitation was evaluated using high-density surface electromyography (HD-sEMG). We hypothesized that, for each region of the trapezius, the corresponding MVIC would produce the highest excitation relative to the other MVICs. For example, the UT-MVIC would produce greater excitation in the UTR than would the MT-MVIC and LT-MVIC.

METHODS

Participants

We recruited a convenience sample of 20 participants (10 men, 9 women; age = 23.9 ± 1.7 years, height = 171.4 ± 9.6 cm, mass = 75.7 ± 11.6 kg) from the general and student population at the University of Manitoba. Volunteers were invited to participate if they were between the ages of 18 and 40 years and self-reported right-hand dominance. Exclusion criteria are presented in Table 1. Participants provided informed consent, and the study was approved by the Education Nursing Research Ethics Board at the University of Manitoba and conformed to the Declaration of Helsinki. All data were collected in a research laboratory.

Table 1 Exclusion Criteria

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Protocol

Participants came to our laboratory for familiarization and experimental sessions, with a minimum of 48 hours between sessions. They completed 3 repetitions each, with a 2-minute rest interval, for 3 MVICs adapted from Kendall MMTs.¹¹ We randomized the order of the MVICs for each participant before data collection. As part of another project, participants performed 5 submaximal contractions at 30% or 60% MVIC, with a 1-minute rest between repetitions and a 5-minute rest between MVIC blocks to reduce fatigue (Figure 1).

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Familiarization Session

Based on the randomization order (Research Randomizer version 4.0; Social Psychology Network), participants began the familiarization session positioned to perform an MVIC for the UTR (UT-MVIC), MTR (MT-MVIC), or LTR (LT-MVIC). For the UT-MVIC, participants sat upright on a custom apparatus and used their right hand to hold an adjustable handle, which was affixed to a force transducer (model PY6-1000; BERTEC Corp) and amplifier (model AM6501; BERTEC Corp). The force transducer has an accuracy of 99.5% and a sensitivity of 2 mV/N.¹² Handle length was adjusted for participants so they could hold the handle directly at their side with the elbow fully extended and while seated with a neutral spinal position. We instructed participants to isometrically pull the handle toward their ear, with their head rotated away from their shoulder toward a computer screen (Figure 2).

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For the MT-MVIC, participants were positioned prone on an examination table. They held the handle in their right hand at 90° to the torso (verified using a manual goniometer) in abduction and external rotation, with the handle positioned directly over the force transducer. Handle length was adjusted so that the participant's right arm was directly at his or her side in the frontal plane with minimal flexion or extension while performing the MVIC. We instructed participants to isometrically extend the shoulder and retract the scapula by attempting to pull the handle away from the floor (Figure 3).

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The LT-MVIC was performed in a similar fashion to the MT-MVIC; however, participants held the handle over the force transducer with the shoulder in 120° of glenohumeral abduction (Figure 4).

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For all MVICs, we asked participants to pull the handle as hard as possible, to focus on using the intended muscle for each MVIC, and to try to refrain from using other muscle groups (eg, quadriceps to push through the floor). They performed three 5-second contractions for each MVIC, during which we provided oral encouragement. The MVICs were separated by 2 minutes of rest, and we provided participants with a 5-second countdown before initializing each MVIC. After completing the first set of MVICs (UT-MVIC, MT-MVIC, or LT-MVIC), participants were given 5 minutes of rest before performing submaximal isometric contractions in the same position, followed by another 5-minute rest. Completion of the MVICs and submaximal contractions for the second and third movements followed, with procedures mirroring those of the first (Figure 1).

Experimental Session

The experimental session mirrored the familiarization session, with the addition of the collection of HD-sEMG measurements from the UTR, MTR, and LTR during the MVICs.

RESULTS

Descriptive statistics for excitation are presented in Table 2. Mean ± SD peak force production for the UT-MVIC was 435.5 ± 172.0 N, MT-MVIC was 53.7 ± 25.2 N, and LT-MVIC was 50.3 ± 27.7 N.

Table 2 Descriptive Statistics for the Upper, Middle, and Lower Trapezius Excitation During Maximal Voluntary Isometric Contractions

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Excitation of the UTR

The Mauchly test indicated that the assumption of sphericity was not met (\(\def\upalpha{\unicode[Times]{x3B1}}\)\(\def\upbeta{\unicode[Times]{x3B2}}\)\(\def\upgamma{\unicode[Times]{x3B3}}\)\(\def\updelta{\unicode[Times]{x3B4}}\)\(\def\upvarepsilon{\unicode[Times]{x3B5}}\)\(\def\upzeta{\unicode[Times]{x3B6}}\)\(\def\upeta{\unicode[Times]{x3B7}}\)\(\def\uptheta{\unicode[Times]{x3B8}}\)\(\def\upiota{\unicode[Times]{x3B9}}\)\(\def\upkappa{\unicode[Times]{x3BA}}\)\(\def\uplambda{\unicode[Times]{x3BB}}\)\(\def\upmu{\unicode[Times]{x3BC}}\)\(\def\upnu{\unicode[Times]{x3BD}}\)\(\def\upxi{\unicode[Times]{x3BE}}\)\(\def\upomicron{\unicode[Times]{x3BF}}\)\(\def\uppi{\unicode[Times]{x3C0}}\)\(\def\uprho{\unicode[Times]{x3C1}}\)\(\def\upsigma{\unicode[Times]{x3C3}}\)\(\def\uptau{\unicode[Times]{x3C4}}\)\(\def\upupsilon{\unicode[Times]{x3C5}}\)\(\def\upphi{\unicode[Times]{x3C6}}\)\(\def\upchi{\unicode[Times]{x3C7}}\)\(\def\uppsy{\unicode[Times]{x3C8}}\)\(\def\upomega{\unicode[Times]{x3C9}}\)\(\def\bialpha{\boldsymbol{\alpha}}\)\(\def\bibeta{\boldsymbol{\beta}}\)\(\def\bigamma{\boldsymbol{\gamma}}\)\(\def\bidelta{\boldsymbol{\delta}}\)\(\def\bivarepsilon{\boldsymbol{\varepsilon}}\)\(\def\bizeta{\boldsymbol{\zeta}}\)\(\def\bieta{\boldsymbol{\eta}}\)\(\def\bitheta{\boldsymbol{\theta}}\)\(\def\biiota{\boldsymbol{\iota}}\)\(\def\bikappa{\boldsymbol{\kappa}}\)\(\def\bilambda{\boldsymbol{\lambda}}\)\(\def\bimu{\boldsymbol{\mu}}\)\(\def\binu{\boldsymbol{\nu}}\)\(\def\bixi{\boldsymbol{\xi}}\)\(\def\biomicron{\boldsymbol{\micron}}\)\(\def\bipi{\boldsymbol{\pi}}\)\(\def\birho{\boldsymbol{\rho}}\)\(\def\bisigma{\boldsymbol{\sigma}}\)\(\def\bitau{\boldsymbol{\tau}}\)\(\def\biupsilon{\boldsymbol{\upsilon}}\)\(\def\biphi{\boldsymbol{\phi}}\)\(\def\bichi{\boldsymbol{\chi}}\)\(\def\bipsy{\boldsymbol{\psy}}\)\(\def\biomega{\boldsymbol{\omega}}\)\(\def\bupalpha{\bf{\alpha}}\)\(\def\bupbeta{\bf{\beta}}\)\(\def\bupgamma{\bf{\gamma}}\)\(\def\bupdelta{\bf{\delta}}\)\(\def\bupvarepsilon{\bf{\varepsilon}}\)\(\def\bupzeta{\bf{\zeta}}\)\(\def\bupeta{\bf{\eta}}\)\(\def\buptheta{\bf{\theta}}\)\(\def\bupiota{\bf{\iota}}\)\(\def\bupkappa{\bf{\kappa}}\)\(\def\buplambda{\bf{\lambda}}\)\(\def\bupmu{\bf{\mu}}\)\(\def\bupnu{\bf{\nu}}\)\(\def\bupxi{\bf{\xi}}\)\(\def\bupomicron{\bf{\micron}}\)\(\def\buppi{\bf{\pi}}\)\(\def\buprho{\bf{\rho}}\)\(\def\bupsigma{\bf{\sigma}}\)\(\def\buptau{\bf{\tau}}\)\(\def\bupupsilon{\bf{\upsilon}}\)\(\def\bupphi{\bf{\phi}}\)\(\def\bupchi{\bf{\chi}}\)\(\def\buppsy{\bf{\psy}}\)\(\def\bupomega{\bf{\omega}}\)\(\def\bGamma{\bf{\Gamma}}\)\(\def\bDelta{\bf{\Delta}}\)\(\def\bTheta{\bf{\Theta}}\)\(\def\bLambda{\bf{\Lambda}}\)\(\def\bXi{\bf{\Xi}}\)\(\def\bPi{\bf{\Pi}}\)\(\def\bSigma{\bf{\Sigma}}\)\(\def\bPhi{\bf{\Phi}}\)\(\def\bPsi{\bf{\Psi}}\)\(\def\bOmega{\bf{\Omega}}\)\({\rm{\chi }}_2^2\) = 14.085, P = .001). As such, a Greenhouse-Geisser correction was applied. We observed a difference across MVICs, with a large effect size for UTR excitation (F_1.161,13.937 = 10.567, P = .005, partial η² = 0.468). Post hoc analysis indicated an increase in UTR excitation during the LT-MVIC compared with the UT-MVIC (0.048 V [95% CI = 0.009, 0.088 V]; P = .016) and MT-MVIC (0.03 V [95% CI = 0.025, 0.053 V]; P < .001; Figure 5).

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Intraclass Correlation Coefficient

The ICC values are presented in Table 3. Coefficient values for both the MT- MVIC and LT-MVIC were > 0.9.

Table 3 Intraclass Correlation Coefficients Across Trials for Excitation of the Upper, Middle, and Lower Trapezius During Maximal Voluntary Isometric Contractions

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DISCUSSION

The purpose of our study was to assess the excitation of the UTR, MTR, and LTR individually during 3 MVICs, which were adapted from the Kendall MMTs and designed to target the respective regions of the trapezius. The MMTs are a common tool used by clinicians to assess the strength of individual muscles through specific MVICs.¹⁰ Generally, a patient is placed in the appropriate position and instructed to meet the manual force applied by the clinician for approximately 5 seconds. These testing procedures allow us to subjectively evaluate the patient's response specific to the tested muscle, including strength and fatigue resistance. To assess the scapula, shoulder, or neck, individual MMTs are performed for the 3 regions of the trapezius; each is thought to preferentially test and recruit the respective region of the trapezius.¹¹ We hypothesized that, for each region of muscle fibers, the MVIC that was designed to target it would elicit the greatest excitation relative to the 2 other MVICs. However, based on our electromyographic findings, the UT-MVIC and MT-MVIC did not maximally recruit the targeted regions of the trapezius more than the other tests did. In agreement with our hypothesis, the LT-MVIC was the only MVIC that produced greater excitation for the targeted region (ie, LTR) compared with the other MVICs. Interestingly, the LT-MVIC produced the greatest excitation in all regions of the trapezius.

Although widely used clinically and often described as producing maximal contractions,¹⁸ trapezius MMTs for the UT and MT may not produce maximal excitation of the intended muscles in the shoulder complex. Using the Kendall MMTs to normalize their EMG data, Ekstrom et al¹⁸ noted that participants who performed a dynamic shrug without neck rotation produced a normalized mean of 119% ± 23% MVIC in the UTR. In a follow-up study, Ekstrom et al¹⁹ found that, indeed, an MMT involving scapular elevation with neck extension and rotation away from the shoulder produced a mean of only 82% ± 16% MVIC in the UTR when normalized to the maximal EMG values produced by the other MMTs studied. In contrast, shoulder flexion- and shoulder abduction-based MMTs produced means of 83% ± 13% MVIC and 92% ± 9% MVIC in the UTR, respectively.

In more recent work, Cibulka et al²⁰ investigated the potential for assessing the trapezius as a whole using 1 whole-muscle MMT. The EMG data obtained during the Kendall UTR MMT (scapular elevation with side flexion and rotation away from the shoulder) were used to normalize the EMG signals. Their proposed whole-muscle MMT, involving resisted shoulder abduction and internal rotation, produced a mean of 160.84% ± 58.78% MVIC in the UTR. Similar to the reports of Ekstrom et al,^18,19 these findings suggested that shoulder abduction produced substantial excitation in the UTR. In addition, these data implied that the Kendall UTR MMTs did not produce a maximal contraction in the UTR, as the whole trapezius MMT produced approximately 61% more EMG excitation than did the Kendall UTR MMT. These results are consistent with ours, as the MVIC adapted from the Kendall UTR MMT produced the least amount of excitation in the UTR across the 3 MVICs examined (Figure 5).

The clinical utility of MMTs is inherent to their ability to test strength and muscle function of specific muscles. Based on the aforementioned studies, if an MMT is unable to elicit maximal contractions in the intended muscles of healthy individuals, then it will provide limited clinical information regarding specific muscle function or strength. Rather, interpreting the amount of force produced by an MMT that did not produce maximal contractions within a given muscle would only supply general information regarding muscle functioning of the synergists and coactivators for the movement. The interpretation of MMTs as producing maximal contractions in a specific muscle, therefore, may misdirect the clinical interpretation.

We observed greater UTR excitation during the LT-MVIC than the UT-MVIC and MT-MVIC. Despite applying resistance in the sagittal plane (ie, making the shoulder resist extension while positioned in 120° of abduction), the LT-MVIC was able to produce the greatest excitation of the 3 MVICs. Shoulder abduction to 90° with side flexion of the neck and rotation to the opposite side with resistance applied toward the ground produced the greatest UTR excitation in the Ekstrom et al¹⁹ study. In addition, as previously mentioned, Cibulka et al²⁰ found that resisted shoulder abduction produced substantial UTR excitation in comparison with the Kendall UTR MMT. Collectively, it would appear that placing the shoulder in abduction, regardless of whether resistance is applied in the plane in which the muscle acts, helps to elicit greater excitation of the UTR during MMTs or MVICs compared with MMTs or MVICs involving scapular elevation exclusively.

Whereas the inability of the UT-MVIC to produce the greatest excitation in the UTR was unexpected, a few explanations, including the recruitment of secondary and accessory muscles, are probable. Hintermeister et al²¹ recorded EMG for multiple muscles in the shoulder complex while participants performed 7 common shoulder rehabilitative exercises using resistance bands, including a shrug, which was similar to our UT-MVIC. During the elevation phase of the shrug exercise, the shoulder muscles other than the trapezius were substantially excited, with the subscapularis being the most excited muscle based on the MVIC %. In turn, this may suggest that force from secondary muscles contributed substantially to the UT-MVIC in our investigation. This is speculative, however, as we did not record EMG from muscles other than the trapezius.

In addition to secondary muscle contributions, individual variability may also have affected our results. Individual differences in motor-recruitment strategies have been demonstrated in the trapezius muscle.²² Furthermore, multiple authors^23,24 determined that the absolute maximum excitation may not be generated for each muscle in every participant when examining the trapezius and shoulder muscles. As such, depending on the MMT or MVIC, it is possible that the level of excitation in each region of the trapezius varies significantly based on the large SDs present (Table 2). Indeed, some participants did produce a higher level of UTR excitation from the UT-MVIC; however, this was not the general trend of our data.

Interestingly, the LT-MVIC descriptively produced the greatest excitation of all regions. Although we expected that the LT-MVIC would produce the greatest excitation in the LTR, the greater excitation produced in the MTR from the LT-MVIC compared with the MT-MVIC may pose concerns for MTR MMT validity. Similar to us, Ekstrom et al¹⁸ found that raising the shoulder overhead in line with the LTR fibers, a movement indicated for preferentially recruiting the LTR,¹¹ produced the greatest MTR excitation (101% ± 32% MVIC). Yet previous researchers¹⁸ proposed that horizontal abduction with external rotation, rather than other MMTs, provided the greatest MTR excitation. As such, across these studies, it is unclear which MMT is more appropriate for assessing the MTR fibers. Alternatively, given that the UTR is involved in scapular elevation,² our results may suggest that participants were using abduction to resist the movement rather than the intended extension. Our experimental setup did not restrict motion in the frontal plane, so we do not know if this was the case.

Although the UT-MVIC did not produce the greatest excitation in the UT, it may still serve as a useful test for EMG normalization. Across the 3 trials for each MVIC, the ICC for the peak excitation of the corresponding regions was > 0.9, indicating very high reliability at least within sessions. In turn, from a reliability perspective, our UT-MVICs, MT-MVICs, and LT-MVIC data could be useful for normalizing EMG data.

Given that participants pulled against a handle rather than pushing against a clinician's force, the MVICs used are modified from their original form as described by Kendall et al.¹¹ Thus, our findings and interpretations of these data are limited to the MVICs used in our study. Other MVICs or MMTs may result in greater peak muscle excitation. Furthermore, MMTs are normally collected as part of the assessment of injured shoulders, and our population was healthy. Therefore, it is possible that these MVICs may produce different results in the intended population.

In our work, 1 researcher performed the MVIC positioning and supplied instructions to participants for consistency. Because participants were applying maximal force to a fixed object through the pull of the handle, the amount of force they would have had to push against did not differ. They were instructed to pull the handle as hard as they possibly could, but they were not strapped to the table or apparatus. It is possible that participants were not exerting maximal effort or were using additional muscles when performing the MVICs. Additional “noise” may have been added to the data by the submaximal contractions that participants completed between MVICs, although they were permitted substantial rest to reduce the potential effect of fatigue. Furthermore, MVIC and submaximal contraction intensity were randomized and counterbalanced, which should have minimized any effect on the data.

CONCLUSIONS

We assessed excitation of the UTR, MTR, and LTR individually during 3 MVICs. Only the LT-MVIC demonstrated the greatest peak excitation in the targeted region. Therefore, the adapted MVICs for the UTR and MTR may not efficiently excite the corresponding regions of the trapezius. Given our small sample size and the likelihood of large interindividual variability, more research is needed to determine if these results are stable in larger populations with injuries. In the meantime, clinicians should consider that multiple assessments of strength and force may be needed to evaluate neurologic function in the individual regions of the trapezius, and these test results should be considered holistically in the context of the patient. Indeed, it is possible that an MVIC for the trapezius involving abduction and resisted extension may provide information on trapezius function as a whole; however, further exploration is needed.