This report presents details of how to adopt the acromion marker cluster method of obtaining scapular kinematics when using a passive marker motion capture device. As has been described in the literature, this method provides a robust, non-invasive, three-dimensional, dynamic and valid measurement of scapular kinematics, minimizing skin movement artifact.
The measurement of dynamic scapular kinematics is complex due to the sliding nature of the scapula beneath the skin surface. The aim of the study was to clearly describe the acromion marker cluster (AMC) method of determining scapular kinematics when using a passive marker motion capture system, with consideration for the sources of error which could affect the validity and reliability of measurements. The AMC method involves placing a cluster of markers over the posterior acromion, and through calibration of anatomical landmarks with respect to the marker cluster it is possible to obtain valid measurements of scapular kinematics. The reliability of the method was examined between two days in a group of 15 healthy individuals (aged 19-38 years, eight males) as they performed arm elevation, to 120°, and lowering in the frontal, scapular and sagittal planes. Results showed that between-day reliability was good for upward scapular rotation (Coefficient of Multiple Correlation; CMC = 0.92) and posterior tilt (CMC = 0.70) but fair for internal rotation (CMC = 0.53) during the arm elevation phase. The waveform error was lower for upward rotation (2.7° to 4.4°) and posterior tilt (1.3° to 2.8°), compared to internal rotation (5.4° to 7.3°). The reliability during the lowering phase was comparable to results observed during the elevation phase. If the protocol outlined in this study is adhered to, the AMC provides a reliable measurement of upward rotation and posterior tilt during the elevation and lowering phases of arm movement.
Objective, quantitative measurement of scapular kinematics can provide an assessment of abnormal movement patterns associated with shoulder dysfunction1, such as reduced upward rotation and posterior tilt during arm elevation observed in shoulder impingement2-8. Measurement of scapular kinematics, however, is difficult due to the bone’s deep position and gliding nature beneath the skin surface1. Typical kinematic measurement techniques of attaching reflective markers over anatomical landmarks do not adequately track the scapula as it glides beneath the skin surface9. Various methods have been adopted throughout the literature to overcome these difficulties, including; imaging (X-ray or magnetic resonance)10-14, goniometers15,16, bone pins17-22, manual palpation23,24, and the acromion method3,5,19,25. Each method, however, has its limitations which include: exposure to radiation, projection errors in the case of two-dimensional image based analysis, require repeated subjective interpretation of the location of the scapula, are static in nature or are highly invasive (e.g. bone pins).
A solution to overcome some of these difficulties is to employ the acromion method where an electromagnetic sensor is attached to the flat portion of the acromion25, a flat portion of bone which extends anteriorly at the most lateral part of the scapula leading from the spine of the scapula. The principle idea behind using the acromion method is to reduce skin movement artifact, as the acromion has been shown to have the least amount of skin movement artifact compared to other sites on the scapula26. The acromion method is non-invasive and provides dynamic three-dimensional measurement of scapular kinematics. Validation studies have shown the acromion method to be valid up to 120° during the arm elevation phase when using electromagnetic sensors17,27. When using marker based motion capture devices a series of markers arranged in a cluster, the acromion marker cluster (AMC), is required and has been shown to be valid when using an active-marker motion capture system28 and whilst using a passive-marker motion capture system during arm elevation and arm lowering29.
The use of the AMC with a passive marker motion capture device for measuring scapular kinematics has been used to assess changes in scapular kinematics following an intervention to address shoulder impingement30. The valid use of this method, however, depends on the ability to accurately apply the cluster of markers, the position of which has been shown to affect results31, calibrate anatomical landmarks32 and ensuring arm movements are within a valid range of motion (i.e. below 120° arm elevation)29. It has also been suggested the reapplication of the marker cluster, when using an active marker based motion capture system, was found to be the source of increased error for scapular posterior tilt28. It is, therefore, important to establish the between-day reliability of the acromion method to ensure it provides a stable measure of scapular kinematics. Ensuring that measurements are reliable will enable changes in scapular kinematics, due to an intervention, for example, to be measured and examined. The methods used to measure scapular kinematics have been described elsewhere29,33; the aim of the present study was to provide a step-by-step guide and reference tool for applying these methods using a passive-marker motion capture system, with consideration to the potential sources of error, and to examine the reliability of the measurement method.
NOTE: The use of human participants was approved by the Faculty of Health Sciences Ethics Committee at the University of Southampton. All participants signed consent forms before data collection commenced. For the data presented in this study kinematics were recorded using a passive marker motion capture system consisting of 12 cameras; six 4-megapixel cameras and six 16-megapixel cameras operating at sampling frequency of 120 Hz.
1. Participant Preparation
Figure 1: Position of the acromion marker cluster, C7 and T8 anatomical markers. This figure has been modified from Warner, M. B., Chappell, P. H. & Stokes, M. J. Measuring scapular kinematics during arm lowering using the acromion marker cluster. Hum. Mov. Sci. 31, 386-396, doi:http://dx.doi.org/10.1016/j.humov.2011.07.004 (2012).
Figure 2: Marker locations for the sternal notch (IJ), xiphoid process (PX), sternoclavicular (SC), upper arm cluster, ulnar styloid (US), radial styloid (RS).
2. Participant Calibration
NOTE: Locations of the scapula’s anatomical landmarks need to be determined with respect to the acromion marker cluster. Calibration of the landmarks is required for each participant.
Figure 3: Calibration wand used to locate anatomical bony landmark with respect the acromion marker cluster (AMC).
3. Experiment Protocol
4. Post-processing of Kinematic Data
NOTE: The following steps detail the procedure needed to calculate scapular kinematics during the dynamic movement trials. These steps have been described and explored extensively within the literature21,33,34 and the purpose of the following section is to provide a synthesis and step-by-step guide to implementing the modeling steps required to obtain scapular kinematics. The application of these steps is conducted in relevant kinematic modeling software. The software contains commands to enable the creation of local coordinate systems, the conversion of coordinates from a global to local coordinate system, the conversion of coordinates from local to global coordinate systems and the calculation of Euler angle rotations. These steps will allow the scapula, humerus and thorax to be defined as rigid bodies. Subsequently rotation of the scapula with respect the thorax, and the humerus with respect thorax can then be determined.
MUTHX = mid-point between IJ and C7. MLTHX = mid-point between PX and T8. GH = glenohumeral joint center. ELJC = elbow joint center.
Mathematical operators:
^ = cross product of two vectors
|| = absolute value of a vector
Table 1: Local coordinate system for each rigid segment.
5. Data Reduction and Analysis
NOTE: The following data reduction and analysis steps are performed in numerical modeling software (such as MATLAB) that allows manipulation of data matrices. The kinematic data is divided into the elevation and lowering phases of humeral movement, time normalized for each phase of movement, then scapular kinematics are expressed relative to humeral elevation angle.
Fifteen participants who had no known history of shoulder, neck or arm injuries were recruited onto the study (Table 2). To assess intra-rater (between-day) reliability, participants attended two data collection sessions separated by at least 24 hours and a maximum of 7 days. During each data collection session, the same investigator performed the protocol for attaching reflective markers, the acromion marker cluster and anatomical landmark calibrations, as detailed above. The reliability of the kinematic waveform obtained from dynamic trials was assessed using coefficient of multiple correlation (CMC)37. Waveform measurement error was used to assess the amount of error between days (σb)38.
Age (years) | Weight (kg) | Height (m) | Body mass index (kg/m²) | |
Group (n=15) | 24.9 ± 4.4 | 65.8 ± 11.7 | 1.7± 0.1 | 22.6 ± 2.3 |
19 – 38 | 48 – 86 | 1.5 – 1.9 | 18.3 – 36.5 | |
Males (n=8) | 25.1 ± 1.5 | 73.4 ± 9.9 | 1.8 ± 0.06 | 23.2 ± 2.4 |
23 – 27 | 62 – 86 | 1.7 – 1.9 | 19.8 – 26.4 | |
Females (n=7) | 24.6 ± 1.5 | 57 ± 6.3 | 1.6 ± 0.06 | 21.9 ± 2.2 |
23 – 27 | 48 – 68.5 | 154 – 170 | 18.3 – 24.2 |
Table 2. Participant demographics, mean ± standard deviation (SD) and range.
The intra-rater (between-day) reliability produced high CMC (>0.92) for upward rotation and posterior tilt (>0.69) during humeral elevation and lowering in all planes of arm movement. Internal rotation demonstrated lower CMC values (0.44 to 0.76) during all planes of arm elevation and lowering (Table 3). This was also reflected in the waveform measurement error with generally lower error values for upward rotation (σb = 2.7° to 4.4°) and posterior tilt (σb = 1.3° to 2.8°), indicating good reliability, compared to internal rotation (σb = 3.9° to 7.3°) (Table 3). There did not appear to be any bias between days, with similar waveform patterns obtained for upward rotation, posterior tilt and internal rotation during both the elevation and lowering phases (Figure 10).
Figure 4. A) Local coordinate system of the acromion marker cluster (AMC) as determined by the three markers on the AMC (AMCO, AMCA, AMCM). B) Local coordinate system of the wand using the four markers attached to the wand (M1, M2, M3, and M4). The tip of the wand is subsequently calculated as a point 83 mm from the M1 marker along the X axis of the wand. C) The location of the tip of the wand, which represents the location of the anatomical landmark within the global coordinate system, is determined with respect to the local coordinate system of the AMC. Example kinematic modeling commands are given for each step. This figure has been modified from Warner, M. B., Chappell, P. H. & Stokes, M. J. Measuring scapular kinematics during arm lowering using the acromion marker cluster. Hum. Mov. Sci. 31, 386-396, doi:http://dx.doi.org/10.1016/j.humov.2011.07.004 (2012).
Figure 5. A) The location of the acromion angle landmark with respect to the local coordinate system of the acromion marker cluster. B) The conversion of the acromion angle (AA) landmark from the local to the global coordinate system (black axes).
Figure 6. Local coordinate system of the scapula defined by the locations of the acromion angle (AA), medial spine of the scapula (TS) and the inferior angle (AI) following International Society of Biomechanics Recommendations. Example kinematic modeling commands are provided. This figure has been modified from Warner, M. B., Chappell, P. H. & Stokes, M. J. Measuring scapular kinematics during arm lowering using the acromion marker cluster. Hum. Mov. Sci. 31, 386-396, doi:http://dx.doi.org/10.1016/j.humov.2011.07.004 (2012).
Figure 7. Euler angle rotations of the scapula around each axis, with respect to the thorax, following a rotation sequence of internal rotation (Y), upward rotation (X’) and posterior tilt (Z”). This figure has been modified from Warner, M. B., Chappell, P. H. & Stokes, M. J. Measuring scapular kinematics during arm lowering using the acromion marker cluster. Hum. Mov. Sci. 31, 386-396, doi:http://dx.doi.org/10.1016/j.humov.2011.07.004 (2012).
Figure 8. A) Humeral elevation and lowering with the start and end of each phase denoted by the green dotted lines. B) Humeral angular velocity used to determine the start and end of each phase. The uppermost red dashed line represents the threshold used to determine the start and end of the elevation phase. The lowermost red dashed line represents the threshold used to determine the start and end of the lowering phase. Green dotted lines represent the points at which the angular velocity exceeded the thresholds.
Figure 9. Scapular upward rotation during arm elevation that has been interpolated over 101 data points to normalize with respect to time.
Figure 10. Kinematic waveforms of the scapula for day one (black) and day two (grey). Scapular rotations during sagittal plane arm movement shown are; upward rotation during the elevation (A) and lowering phase (B), posterior tilt during the elevation (C) and lowering phase (D) and internal rotation during the elevation (E) and lowering phase (F). Dashed lines represent ±1 standard deviation.
Scapular rotation | Phase of arm movement | Sagittal plane | Scapular plane | Frontal plane | |||
CMC | Waveform error | CMC | Waveform error | CMC | Waveform error | ||
Internal rotation | Elevation | 0.44 ± 0.3 | 7.3° ± 1.6 | 0.50 ± 0.2 | 6.7° ± 0.8 | 0.44 ± 0.3 | 3.9° ± 1.5 |
Upward rotation | 0.93 ± 0.1 | 3.1° ± 1.6 | 0.94 ± 0.1 | 3.4° ± 1.0 | 0.93 ± 0.1 | 2.7° ± 1.5 | |
Posterior tilt | 0.69 ± 0.2 | 2.3° ± 0.9 | 0.78 ± 0.2 | 1.4° ± 0.5 | 0.82 ± 0.2 | 1.3° ± 0.3 | |
Internal rotation | Lowering | 0.53 ± 0.3 | 7.0° ± 1.4 | 0.45 ± 0.2 | 7.2° ± 1.1 | 0.76 ± 0.2 | 5.4° ± 2.9 |
Upward rotation | 0.94 ± 0.0 | 4.4° ± 1.0 | 0.92 ± 0.1 | 4.3° ± 1.1 | 0.94 ± 0.1 | 3.9° ± 1.7 | |
Posterior tilt | 0.70 ± 0.2 | 2.5° ± 1.4 | 0.77 ± 0.2 | 1.8° ± 0.9 | 0.87 ± 0.1 | 2.8° ± 0.8 |
CMC = Coefficient of multiple correlation.
Table 3. Intra-rater (between-days) reliability of the acromion marker cluster as determined by the coefficient of multiple correlation and waveform error.
The choice of methodology for determining scapular kinematics is crucial, and consideration of the validity, reliability and its appropriateness for the research study should be given. Various methods have been adopted throughout the literature but each method has its limitations. The acromion marker cluster overcomes a number of these limitations, such as projection errors from 2D imaging or requiring repeated interpretation of the location of the scapula by providing non-invasive dynamic kinematic measurement of the scapula. However, the AMC method is still susceptible to skin movement artifact, particularly at higher arm elevation angles and brings into question the validity of the method at these higher arm positions. A previous study that assessed the validity of the method outlined in the present study, has shown that at arm elevation above 120 degrees the measurement error becomes too large and the method is no longer valid29. However, the study also demonstrated that when the arm returns to a position below 120 degrees following arm high arm elevation the acromion marker cluster method remains valid29. It is possible to reduce the errors at higher arm elevation angles by performing the calibration of the anatomical landmarks with the arm elevated32. However, this increases the error at lower arm elevation angles. Therefore, it is important to consider the aims of the study for which scapular kinematics are being determined and decide the optimal arm elevation position with which to calibrate the anatomical landmarks.
In order for any measurement technique to be considered a viable tool it is important to establish its reliability. The data presented in the present paper have shown that the acromion marker cluster can be classified as having excellent to good between-day reliability for scapular upward rotation and posterior tilt respectively. These finding were observed when examining the entire kinematic waveform during the elevation and lowering phases, demonstrating that the acromion marker cluster is a reliable method of measurement during both phases of arm movement. In a previous studies, the repositioning of the acromion marker cluster had been shown to adversely affect reliability27,28, particularly the reliability of scapular posterior tilt when comparing different investigators.28 The results from the present study, however, demonstrate that posterior tilt was a reliable measurement between days. Differences in methodology between the study of van Andel (2008) and the present study which include the type of motion capture system (active marker vs. passive marker), and the design and attachment site of the acromion marker cluster may account for the differences observed. In addition, it is known that the positioning of the acromion marker cluster onto different areas of the acromion affects the accuracy of the measurement31. Although the present study demonstrated good between day reliability, care must be taken when attaching the acromion marker cluster to the participant to ensure valid and reliable results are obtained.
Although good and excellent reliability was observed for upward rotation and posterior tilt, internal rotation of the scapula demonstrated poor to fair reliability when examining the entire kinematic waveform. This is in agreement with previous studies that have also found lower CMC results for internal rotation (0.82) and greater error (4.3°) when compared to upward rotation and posterior tilt (CMC = 0.94 and 0.85, error = 3.3° and 3.4° respectively)39,40. Internal rotation is, therefore, the least reliable of the scapular rotations. The reason why internal rotation has poorer reliability may be due to the lower range of motion (~5°) observed compared to other scapular rotations. The reported errors in the kinematic waveforms range from 3.9° to 7.3° meaning that the errors are in some cases larger than the motion taking place. In addition, within participant variability is inherently large3,18,41. The poor reliability may, therefore, not be as a result of the measurement technique, but rather the inherent individual variability coupled with a small range of motion. Caution should be taken when examining repeated measurements of internal scapular rotations.
The aim of measuring scapular kinematics is to quantify scapular dyskinesis, which is often observed clinically in patients with shoulder impingement1, and subsequently assess the changes in scapular kinematics following treatment interventions to reduce the effects of shoulder impingement30. The technique described in the present study has been used to demonstrate alterations in scapular kinematics in a group of individuals with shoulder impingement following a motor control retraining exercise30 and has been shown to be valid29 and reliable.
The authors have nothing to disclose.
This work lies within the multidisciplinary Southampton Musculoskeletal Research Unit (Southampton University Hospitals Trust/University of Southampton) and the Arthritis Research UK Centre for Sport, Exercise and Osteoarthritis. The authors wish to thank their funding sources; Arthritis Research UK for funding of laboratory equipment (Grant No: 18512) and Vicon Motion System, Oxford UK for providing funding for a PhD studentship (M.Warner). The authors also wish to thank the participants, and Kate Scott and Lindsay Pringle for their help with participant recruitment.
Name of Material/ Equipment | Company | Catalog Number | Comments/Description |
Passive marker capture system | Vicon Motion Systems | N/A | |
Nexus | Vicon Motion Systems | N/A | Data capture software |
Bodybuilder | Vicon Motion Systems | N/A | Modeling software |
14 mm retro reflective markers | Vicon Motion Systems | VACC-V162B | |
6.5mm retro reflective markers | Vicon Motion Systems | VACC-V166 | |
Calibration wand | Vicon Motion Systems | N/A | |
Plastic base | N/A | N/A | Constructed 'in-house' |
Matlab | Mathworks | N/A | Numerical modelling software |