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Tuesday, June 6, 2023

Effects of Muscle Fatigue on 3-Dimensional Scapular Kinematics

Brent Brookbush

Brent Brookbush


Research Review: Effects of Muscle Fatigue on 3-Dimensional Scapular Kinematics

By Nicholas Rolnick SPT, MS, CSCS

Edited by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation:

Tsai NT, McClure PW, Karduna AR. (2003). Effects of muscle fatigue on 3-Dimensional scapular kinematics. Arch Phys Med Rehabil. 84: 1000-1005. ABSTRACT

Muscles of the Scapula
Caption: Muscles of the Scapula

Muscles of the Scapula

Why is this relevant?: Shoulder motion is controlled by passive soft tissue support and active muscular contraction. A change in either passive or active support can alter normal motion leading to pathologies including impingement, rotator cuff tears, and instability secondary to poor scapular positioning. Optimal scapular positioning is believed to be necessary for maintenance of ideal length-tension relationships of the muscles surrounding the shoulder, force production, and assisting with glenohumeral joint stability (1 -2 ). Normal scapular motion consists of progressive upward rotation, external rotation, and posterior tipping with increases in humeral elevation(flexion, scaption and/or abduction) (1 ). Muscle fatigue has been shown to alter scapular kinematics in a shoulder elevation task (3). Infraspinatus and teres minor  are considered the primary external rotators of the glenohumeral joint and have been described as having additional roles including abduction and production of force couples contributing to dynamic stability of the glenohumeral joint (2 ,4). The authors hypothesized that deficiency in these two muscles with a fatiguing protocol would result in sub-optimal scapular positioning and that the level of change would be linearly correlated with the level of muscle fatigue induced.

Study Summary

Study Design Single group, pretest-posttest measurement
Level of Evidence Level 4: Observational studies without controls
Subject Demographics
    • Age: 26 +/- 6 years old
    • Gender: 16 F, 14 M
    • Characteristics:
      • Height - 170 +/- 9 cm
      • Mass - 65 +/- 11 kg
      • Dominant shoulder was tested for each subject

    • Experimental Protocol:
      • Subjects stood with thoracic spine, dominant shoulder and arm exposed
      • Receivers for the program data collection were placed on the thorax at T3, the distal humerus, and over the scapula
      • Axis Designation for kinematic data gathering
        • Scapular - Acromioclavicular joint (superior portion), the inferior angle of the scapula, and the root of the scapular spine (intersection of the medial border and scapular spine)
        • Humeral - Center of the humeral head and the medial and lateral epicondyles
        • Thorax - Sternal notch and the spinous processes of T1 and T7

      • Humeral rotations defined by Euler angle sequence where first rotation is the plane of elevation, the second rotation described the amount of elevation, and the last rotation described the amount of internal/external rotation
      • Scapular rotations defined by Euler angle sequence of internal/external rotation, upward/downward rotation, and anterior/posterior tipping
      • Torque measurement protocol
        • Measured maximal isometric external rotation torque with the arm at the side at 45° of internal rotation and the elbow at 90° of flexion with neutral forearm and wrist position to isolate infraspinatus
        • Dynamometer was used to record force from the dorsal forearm
        • Subjects performed several submaximal contractions followed by 1 maximal isometric contraction of the shoulder external rotators
        • Subjects were instructed to stand still, keep their trunk from leaning forward or backwards, and push as hard as they could and the average 4-second isometric contraction was recorded for each subject

      • Pilot Study Implications
        • Pilot study used to determine the relationship between muscle fatigue and reduction in isometric torque production in the biceps brachii, anterior, middle, and posterior heads of the deltoids, infraspinatus, and upper trapezius muscles
        • 8% reduction in mean power frequency (MPF) was determined to be an indication of muscle fatigue
        • Fatiguing protocol initiated 4 times and shoulder abduction and external rotation strength immediately tested after each fatiguing repetition
          • Only the infraspinatus muscle showed an 8% decrease in MPF with external rotator strength training (that was after the 4th fatiguing set, which resulted in 10% MPF) which was associated with a 25% decrease in torque output.  This was the threshold set for the current study.

      • Fatiguing Protocol
        • Green theraband was attached to a fixed object while the subjects held the other end.
        • Subjects externally rotated their shoulders from 45° of internal rotation to neutral against the resistance of the theraband and then returned to the starting position without retracting their scapula or elevating their arm.
        • Subjects were measured when no more repetitions could be successfully completed; torque was immediately determined.
        • If subjects exhibited 25% or more reduction in torque, they were considered fatigued and they started the experimental protocol; if they exhibited less than a 25% reduction in torque, they underwent another fatiguing cycle.

      • Experimental Protocol
        • 3 repetitions of maximum arm elevation in the scapular plane (40° anterior to the frontal plane) were performed (trial 1)
        • 3 repetitions after a 5-minute rest interval were performed before onset of fatigue to assess reliability (trial 2)
        • Fatiguing protocol was given to each participant until greater than a 25% reduction in torque was noted, and then 3 repetitions of scapular plane elevation were recorded (trial 3)

  • Inclusion Criteria: Individuals who had no history of shoulder or cervical pain or pathology and who had no shoulder range of motion restrictions
  • Exclusion Criteria: None specified
Outcome Measures
  •  For every repetition, data was collected for every 30° of humeral elevation for all 3 Euler rotations, and the data for each of the 3 repetitions (in trial 3) was averaged and plotted
  • Reliability measurements were conducted by using the data from trials 1 and 2 and the intraclass correlation coefficients (ICCs) and standard error of measurement (SEM) were used to determine intrarater reliability for all 3 scapular rotations
  • Paired t-tests were used to distinguish differences in scapular positioning from trials 2 (pre-fatigue) and 3 (post-fatigue)
  • Relationships between scapular motions and decreases in muscle strength caused by fatiguing exercise were tested with Pearson product-moment correlation coefficients with an α of 0.05 set for all comparisons
  • ICC (3,1) values for 30°,60°,90°,120°, and maximal humeral elevation were calculated and were above 0.9 with SEM from 1.0° - 2.6°
  • Significant effect of fatigue on resting scapular position for all rotations
    • Posterior tipping decreased in the first 90° of humeral elevation, with as much as 4° of relative anterior tipping at the beginning of arm elevation
    • External rotation decreased up to 120° of humeral elevation, with as much as 2.4° of relative internal rotation at the beginning of arm elevation
    • Upward rotation decreased in the first 60° of humeral elevation, with as much as 2.5° of relative downward rotation at the beginning of arm elevation

  • Significant relationship between a decrease in scapular posterior tipping and external rotation torque production due to fatiguing exercise at all humeral elevations except the maximal humeral elevations
  • Correlation coefficients for posterior tipping with the external rotation fatigue was determined to be 0.60 at starting position, 0.59 at 30° of elevation, 0.58 at 60° of elevation, 0.45 at 90° of elevation, and 0.39 at 120° of elevation, suggesting fair-to-good correlations between fatigue and decreases in posterior tipping. No other rotations exhibited significant differences between conditions.
  •  Shoulder external rotation fatigue has an influence on scapular kinematics, producing more relative anterior tipping and internal rotation and less upward rotation in scapular plane humeral elevation when compared to the unfatigued condition in healthy individuals free of shoulder pathologies
Conclusions of the Researchers
  •  Small, but statistically significant alterations in scapular kinematics can be observed after an external rotation fatigue protocol and mostly occurred during the first part of arm elevation in the scapular plane
  • Pearson's correlation coefficients suggest that the larger the imbalance between internal and external rotators of the shoulder girdle under fatigue, the larger the change in relative scapular posterior tipping.

The different rotations measured in the current study. To the left is internal and external rotation, the middle is downward/upward rotation, and the right is anterior/posterior tipping.
Caption: The different rotations measured in the current study. To the left is internal and external rotation, the middle is downward/upward rotation, and the right is anterior/posterior tipping.

The different rotations measured in the current study. To the left is internal and external rotation, the middle is downward/upward rotation, and the right is anterior/posterior tipping.

Review & Commentary:

The importance of the rotator cuff (in particular, infraspinatus and teres minor ) in providing dynamic glenohumeral joint stability has been shown in previous studies (2 ). However, the influence of fatiguing the external rotators and its effect on scapular kinematics had yet to be investigated. The current study utilized a fatiguing protocol and kinematic software to investigate whether fatiguing the external rotators had an influence on resting and dynamic scapular positioning. The results indicate that external rotator fatigue induces suboptimal resting and dynamic scapular positioning especially in the beginning stages of arm elevation. This suboptimal positioning is thought to lead to maladaptive and compensatory strategies to better align the humeral head within the glenoid cavity that may increase the risk of impingement, rotator cuff tears, and/or tendinopathies. Of particular significance was the observation that posterior tipping was the only rotation significantly correlated (0.6 at the beginning of elevation to 0.39 at 120° of elevation) to external rotator fatigue. Previous studies have shown that the moment arm for infraspinatus and teres minor are largest during the initial phase of shoulder elevation, which may explain why the majority of changes were observed during the first 90° of elevation (5).

The current study exhibited a strong methodology by providing a clear protocol with a rationale as to how to best quantify fatigue. First, the authors conducted a pilot study with an additional 15 age-matched subjects to determine the relationship between isometric force production and muscle fatigue. The protocol investigated the muscle fatigue patterns of six shoulder muscles, including the infraspinatus during maximal isometric abduction at 90° of humeral elevation in the scapular plane. Torque productions in abduction and external rotation was obtained for each of the six shoulder muscles after the fatiguing protocol. The authors defined mean power frequency loss of greater than 8% as muscle fatigue. Only the infraspinatus showed decreases in maximal external rotation torque as measured by a mean power frequency of greater than 8%. The fatiguing protocol was implemented into the current study as providing an adequate stimulus to induce fatigue in the desired muscles (the external rotators), allowing the authors to test their hypothesis. Second, the set-up for assessment of kinematic data used similar landmarks to other studies allowing the scapular rotations to be more adequately compared to different conditions (1 ). Finally, the authors conducted a reliability study using the data and determined that their assessments were reproducible (0.9 or greater ICC, or excellent reproducibility) giving their results on scapular kinematics credence. This is especially important since the angles measured are small (< 5°), so errors in measurement need to be smaller than that (in this case, the standard error of measurement was 1-2.6°).

Some limitations exist within the current study that should be noted before applying the results to clinical practice. First and foremost, although the authors obtained statistically significant results from their kinematic assessments, the angles were small and there is a chance that the values obtained are not indicative of true scapular kinematic dysfunction and merely an artifact of the testing protocol and recording software. Second, while the fatiguing protocol showed the infraspinatus as the muscle primarily fatigued during external rotation, other muscles not measured such as the supraspinatus or lower trapezius could have been similarly fatigued and influenced the results. Last, the subjects investigated were healthy and free of shoulder pathology. It is not known whether a similar pattern would appear in those exhibiting shoulder pain and pathology.

Why is this study important?

The current study adds to the growing body of literature on factors that influence scapular kinematics. Previous studies have shown that posture, pain, and scapulothoracic fatigue influence the movement of the scapula in dynamic tasks (1 ,3,6). The current study suggests that scapulohumeral muscle fatigue, in particular the external rotators (infraspinatus and teres minor ) will also influence scapular kinematics. Further, kinematic changes occur early and mostly before 90° of elevation in all circumstances with the scapula exhibiting less posterior tipping, external rotation, and upward rotation, predisposing the glenohumeral joint to shoulder pathologies.

How does it affect practice?

Human movement professionals should be aware of where they place external rotator activation in their exercise regimens and pay close attention to the total number of sets they recommend. The current study suggests that fatiguing the external rotators results in scapular changes commonly associated with shoulder and neck pain and faulty posture. (Note: it took 4 sets to induce fatigue, more than the 1-2 recommended sets during a corrective or rehabilitation program) Therefore, if strengthening of the rotator cuff is desired, either use activation drills with lower volume to induce minimal fatigue in the beginning of an exercise session (1 - 2 sets), or perform the exercises at the end of the session.

How does it relate to Brookbush Institute Content?

Whether correcting movement impairments or optimizing motion prior to performance enhancement training, the Brookbush Institute aims to restore and/or maintain optimal length-tension relationships and joint motion. In those exhibiting Upper Body Dysfunction (UBD) , the infraspinatus and teres minor  are commonly long and under-active. Releasing overactive synergists (posterior deltoid ) along with release and lengthening of short/over-active antagonists (pectoralis major and subscapularis ) and mobilization of stiff joints allows activation drills of the under-active targeted muscles (infraspinatus and teres minor ) to be more effective. The Brookbush Institute advises that activation drills for the external rotators be completed prior to other commonly under-active muscles (serratus anterior and lower trapezius ) in UBD . In congruence with the findings of this study, activation exercises are to be performed for 1-2 sets and only 12 - 20 repititions. The videos below demonstrate activation drills for the external rotators and may be implemented before strength training.

Brookbush Institute Videos

Chest Out/Thumbs Out and Unilateral External Rotation:

External Rotation Progression at 90° of Flexion with Scapular Stabilization

Shoulder External Rotator Reactive Activation: Body Blade Reactive Activation

Trapezius and Shoulder External Rotator Reactive Activation:


  1. RESEARCH REVIEW: Thigpen CA, Padua DA, Michener LA, Guskiewicz K, Giuliani C, Keener JD, Stergiou N. (2010). Head and shoulder posture affect scapular mechanics and muscle activity in overhead tasks. Journal of Electromyography and Kinesiology. 20: 701-709.
  2. RESEARCH REVIEW: Reed D, Cathers I, Halaki M, Ginn KA. (2015). Does load influence shoulder muscle recruitment patterns during scapular plane abduction? Journal of Science and Medicine in Sport.
  3. McQuade KJ, Dawson J, Smidt GL. (1998). Scapulothoracic muscle fatigue associated with alterations in scapulohumeral rhythm kinematics during maximum resistive shoulder elevation. JOSPT. 28: 74-80.
  4. Kronberg M, Nemeth G. (1990). Muscle activity and coordination in the normal shoulder: An electromyographic study. Clin Orth Related Res.257: 76-85.
  5. Kuechle DK, Newman SR, Itoi E, et al. (1997). Shoulder muscle moment arms during horizontal flexion and elevation. J Shoulder Elbow Surg. 6: 429-439.
  6. Cools AM, Struyf F, De Mey K, et al. (2013). Rehabilitation of scapular dyskinesis: from the office worker to the elite overhand athlete. BJSM. E1-8.

© 2016 Brent Brookbush

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