Summary
This protocol describes an objective method to evaluate the movement performance and sensorimotor function of the upper extremity applied to individuals with stroke and healthy controls. A standardized test procedure, kinematic analysis and outcome variables for three-dimensional motion capture of drinking task are provided.
Abstract
Kinematic analysis is a powerful method for objective assessment of upper extremity movements in a three-dimensional (3D) space. Three-dimensional motion capture with an optoelectronic camera system is considered as golden standard for kinematic movement analysis and is increasingly used as outcome measure to evaluate the movement performance and quality after an injury or disease involving upper extremity movements. This article describes a standardized protocol for kinematic analysis of drinking task applied in individuals with upper extremity impairments after stroke. The drinking task incorporates reaching, grasping and lifting a cup from a table to take a drink, placing the cup back, and moving the hand back to the edge of the table. The sitting position is standardized to the individual's body size and the task is performed in a comfortable self-paced speed and compensatory movements are not constrained. The intention is to keep the task natural and close to a real-life situation to improve the ecological validity of the protocol. A 5-camera motion capture system is used to gather 3D coordinate positions from 9 retroreflective markers positioned on anatomical landmarks of the arm, trunk, and face. A simple single marker placement is used to ensure the feasibility of the protocol in clinical settings. Custom-made Matlab software provides automated and fast analyses of movement data. Temporal kinematics of movement time, velocity, peak velocity, time of peak velocity, and smoothness (number of movement units) along with spatial angular kinematics of shoulder and elbow joint as well as trunk movements are calculated. The drinking task is a valid assessment for individuals with moderate and mild upper extremity impairment. The construct, discriminative and concurrent validity along with responsiveness (sensitivity to change) of the kinematic variables obtained from the drinking task have been established.
Introduction
Kinematic analysis describes the movements of the body through space and time, including linear and angular displacements, velocities, and accelerations. The optoelectronic motion capture systems use multiple high-speed cameras that either send out infra-red light signals to capture the reflections from passive markers placed on the body or transmit the movement data from active markers containing infrared emitting diodes. These systems are considered as 'gold standard' for the acquisition of kinematic data1. These systems are valued for their high accuracy and flexibility in measurements of diverse tasks. Kinematic measures have shown to be effective in capturing smaller changes in movement performance and quality that may be undetected with traditional clinical scales2,3. It has been suggested that kinematics should be used for distinction between true recovery (restoration of premorbid movement characteristics) and the use of compensatory (alternative) movement patterns during the accomplishment of a task4,5.
Upper extremity movements can be quantified using end-point kinematics, generally obtained from a hand marker, and angular kinematics from joints and segments (i.e., trunk). End-point kinematics provide information about trajectories, speed, temporal movement strategies, precision, straightness, and smoothness, while angular kinematics characterize movement patterns in terms of temporal and spatial joint and segment angles, angular velocities, and interjoint coordination. End-point kinematics, such as, movement time, speed, and smoothness are effective to capture the deficits and improvements in movement performance after stroke6,7,8 and angular kinematics show whether the movements of joints and body segments are optimal for a specific task. Kinematics from people with impairments are often compared with movement performance in individuals without impairments8,9. End-point and angular kinematics are correlated in a way that a movement performed with effective speed, smoothness, and precision will require good movement control, coordination, and use of effective and optimal movement patterns. For example, a patient with stroke who moves slowly usually also shows decreased smoothness (increased number of movement units), lower maximum velocity, and increased trunk displacement8. On the other hand, improvements in endpoint kinematics, such as movement speed and smoothness might occur independently from the changes of compensatory movement strategies of trunk and arm10. It has been established that kinematic analysis may provide additional and more precise information about how the task is accomplished after an injury or disease, which in turn is essential for individualized effective treatment to reach optimal motor recovery11. Kinematic analysis is increasingly used in clinical studies to describe the movements in people with upper extremity impairments after stroke8,9, to evaluate motor recovery7,12,13 or to determine the effectiveness of therapeutic interventions10,14.
Movement tasks often studied in stroke are pointing and reaching, although the use of functional tasks that incorporate manipulation of real everyday objects is increasing1. Since kinematics of reaching depend on the experimental constraints such as the selection of objects and the goal of the task15, it is essential to assess movements during purposeful and functional tasks in which the real difficulties in individual's daily life will be reflected more closely.
Thus, the aim of this paper is to provide a detailed description of a simple standardized protocol used for kinematic analysis of a purposeful and functional task, drinking task, applied to individuals with upper extremity impairments in acute and chronic stages after stroke. Results from the validation of this protocol for individuals with moderate and mild stroke impairment will be summarized.
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Protocol
All methods described here have been part of the studies approved by the Regional Ethical Review Board in Gothenburg, Sweden (318-04, 225-08).
1. Setting up the Motion Capture System
- Mount 4 cameras on the wall approximately 1.5 - 3 m away from the measurement area at the height of 1.5 - 2.5 m facing the measurement area. Mount one camera on ceiling just above the measurement area (Figure 1). Start the camera system.
- Place the L-shape calibration frame on the table with the short axis in line with the edge of the table and the long axis pointing forward.
NOTE: The coordinate system is defined with X-axes directed forward (anteriorly in the sagittal plane), Y-axis directed laterally (in the frontal plane), and Z-axis directed upward (superiorly, perpendicular to the transversal plane). - Open the 3D tracking and data acquisition software (Track Manager), start calibration by selecting Capture | Calibrate, enter the calibration time of 30 s and click OK.
- Move the wand in all directions throughout the entire measurement area (75 × 75 × 65 cm) above the chair and table to ensure that all 5 cameras capture the wand in as many orientations as possible16,17. After the calibration, the results show on the screen. Accept calibration residuals below 0.5 mm.
- Have the subject, wearing a sleeveless top, sit in a in a height adjustable chair with their back against the chair´s back, the upper arm in neutral adducted position, the palm of the hand resting at the table and the wrist aligned to the edge of the table. Check that the knee, hip, and elbow angles are approximately 90°.
- Place the retroreflective passive markers with double-adhesive tape on the skeletal landmarks18 on the tested hand (third metacarpophalangeal joint), wrist (styloid process of ulna), elbow (lateral epicondyle), right and left shoulder (middle part of acromion), thorax (upper part of sternum), and forehead (notch between eyebrows).
- Place two markers on the cup (upper and lower edge).
2. Procedures for Motion Capture of the Drinking Task
- Place the hard-plastic cup (diameter of 7 cm, height of 9.5 cm) with 100 mL of water 30 cm from the table edge, in the midline of the body. The position of cup on the table is selected deliberately to keep the task performance natural and close to real-life situation.
- Ask the subject to perform the drinking task in a comfortable self-paced speed by i) reaching and grasping the cup, ii) lifting the cup from the table towards mouth, iii) taking a drink (one sip), iv) placing the cup back on the table behind a marked line (30 cm from the table edge), and v) returning to the initial position with the hand on the edge of the table.
- Ensure that subject understands the instructions and can reach the cup comfortably with the less-affected arm without leaning forward.
- Prior each recording, ensure that the start position (initial position) is correct, ask the subject to be ready, start the capture manually and give verbal instruction "you can start now."
- When the subject finishes the task, stop the recording manually.
- Record five trials with short pause between each trial (approximately 30 s), starting with the less-affected arm.
- Check that the data acquisition has been successful (95 - 100% data for each identified marker).
NOTE: Marker data is automatically transferred in real-time to the data acquisition software (Track Manager). A pre-defined Automatic Marker Identification (AIM) model is used for automatic identification of the markers. - When incomplete data are detected, perform extra trials after identifying the problem and adjusting the sitting or marker positions to ensure full visibility of the markers in order to obtain at least 3 successful trials.
NOTE: Possible problems that may occur are that markers may fall off or they are occluded from the cameras viewing angle, which results in incomplete data. However, the camera and marker set-up, as used in this protocol, produces data loss due to gaps only in very rare occasions. In total, the motion capture session takes approximately 10 - 15 min to complete.
3. Data Analysis
- Transfer the recorded data from Track Manager directly into Matlab by clicking File | Export | Directly into Matlab.
- Use the Matlab command at the command prompt: (>> workspace) to see the set of Matlab variables.
NOTE: Key Matlab variables containing data to be used in the instructions and in creating the analysis are:
QTMmeasurements.Frames - the number of frames captured
QTMmeasurements.FrameRate - the number of frames captured per second (240)
QTMmeasurements.Trajectories.Labeled.Count - number of labels (10)
QTMmeasurements.Trajectories.Labeled.Labels - labels as defined in the Track Manager
QTMmeasurements.Trajectories.Labeled.Data - measurement data in a 3D array of 10 x 3 x number of frames, where for each frame and each label the 3 coordinates are recorded - In Matlab, filter the x, y, z values using the (butter) and (filtfilt) instructions with a 6Hz second-order Butterworth filter in in both forward and reverse directions, giving a zero-phase distortion and forth-order filtering.
NOTE: Example
[b, a] = butter(2, 6/240/2); % Cutoff frequency 6Hz and with respect to ½ sampling frequency
xfiltered = filtfilt(b, a, QTMmeasurements.Trajectories.Labeled(1,1,:)); - In Matlab, create a program to use the x, y, z values for each frame sample and each label to calculate the kinematic variables such as tangential velocity of the hand, and joint angles. The kinematic variables are shown in Table 2.
- In Matlab, create a program to break the sequence of samples into 5 logical phases: reaching, forward transport, drinking, back transport, and returning the hand to the initial position (Figure 2). Definitions for start and end of each phase are showed in detail in Table 1.
- In Matlab, use the (plot) instruction to create plots of positions, velocities, joints angles, and angle-angle diagrams.
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Representative Results
The protocol described in this article has been applied to individuals with stroke and healthy controls2,6,8,19,20,21. In total, kinematic data from 111 individuals with stroke and 55 healthy controls have been analyzed in different studies. The upper extremity impairment after stroke was defined as moderate (FMA-UE score 32-57) or mild (FMA-UE score 58-66)8,22,23,24. In healthy controls, no significant differences were found between the dominant and non-dominant arm except for the peak velocity and therefore the non-dominant arm was chosen for comparison 2,8. The majority of data was collected within a large longitudinal cohort study, the Stroke Arm Longitudinal study at University of Gothenburg (SALGOT), which includes a non-selected sample of 122 individuals with stroke and involves the assessments at 3 days post stroke and follow up at 10 days, 4 weeks, 3, 6, and 12 months25.
In summary, our results show that the protocol is feasible in clinical settings since a large number of patients were tested as early as 3 days post-stroke at the acute hospitals stroke unit. Feasibility was also proven by the fact that two experienced physiotherapists managed to calibrate and use the motion capture system on daily basis without any larger technical problems (no support from the system providers was needed during the 3 years of data collection). The data quality was good and the pre-programmed automatic procedures for analyses could generally be applied. Only in few recordings, the phases were not detected correctly, often due to the extra movements in the beginning/end of the movement or when the movement speed was extremely low in patients with more severe impairments. In these cases, the extra trials were often used after a manual inspection of the plotted data. The test protocol demonstrated a good consistency in test-retest in healthy individuals and provided clear and accurate results19.
The movements in every phase of the drinking task and for the entire task are slower (Table 3) in people with stroke, although the relative time spend in each phase is similar to controls21. Similarly, both tangential and angular velocities are lower in people with stroke compared to healthy controls (Table 3). The peak velocity occurred approximately at 38% of the total reaching time in stroke and a 46% in controls, which means that the deceleration phase was prolonged in stroke. This indicates that individuals with stroke need to rely more on the feedback driven movement control during the second half of reaching.
The velocity profiles in people with stroke are segmented and show multiple peaks, which is reflected in the high number of the movement units (NMU). The mean value for the NMU is significantly larger in individuals with stroke compared to controls. Individuals with stroke reach the cup with a more flexed elbow (less elbow extension) and with the shoulder more abducted while drinking compared to the healthy participants, which reflects the compensatory movement pattern in stroke. Even though the glass was positioned within the arm reach, individuals with stroke did lean forward (trunk displacement) approximately 8 cm compared to 3 cm in controls while performing the drinking task. Decreased interjoint coordination between shoulder and elbow joint in reaching was only observed in individuals with higher degree of impairment (moderate stroke) compared to controls. The exact values for kinematics and the magnitude of effect sizes for all groups are shown in Table 3.
The analysis of construct validity of kinematic variables from drinking task showed that movements after stroke can be described with two major factors, the end-point kinematics and kinematics describing angular movement patterns8. Altogether, five measures (movement time, peak velocity, number of movement units, peak angular velocity of elbow joint, and trunk displacement) explained 86% of the variance in kinematic data8. These results are in line with concurrent validity analyses, in which three kinematic variables, movement time (MT), movement smoothness (NMU) and trunk displacement (TD), together explained 67% of the total variance in scores of clinical assessments as assessed with Action Research Arm Test20. The discriminative validity between groups with mild and moderate arm impairment after stroke and controls was good for majority of kinematics, but the largest effect sizes were noted for the smoothness, total movement time, peak angular velocity of elbow (PAVE) and trunk displacement (Table 3)8. Shoulder abduction during drinking is also discriminative between moderate and mild stroke groups. In addition, the same four kinematic variables: MT, NMU, PAVE, and TD demonstrated to be effective in detecting real clinical improvement during the first 3 months after stroke6. Thus, it can be concluded that these four kinematic variables (MT, NMU, PAVE, TD) are reliable, valid and sensitive to changes (responsive) for assessment of upper extremity function and activity after stroke.
Figure 1: The 5-camera motion capture system set-up for drinking task. From each camera, infra-red light flashes reach retro-reflective markers and reproduce the 2D position of the marker in the cameras image sensor with high spatial resolution and accuracy in real-time. The 3D coordinates of the marker are created when two cameras are viewing the same marker from two different angles. Four cameras are mounted on the walls around the testing area facing slightly downward at approximately 2 m distance and one camera is mounted facing down from the ceiling above the measurement area. Please click here to view a larger version of this figure.
Figure 2: Representative velocity profiles for a healthy control (A) and an individual with moderate stroke impairment (B). The phases of the drinking task are shown. Please click here to view a larger version of this figure.
| Phase name | Start | Detected by | End | Detected by |
| Reaching (includes grasping) |
Hand movement begins | The hand marker velocity exceeds 2% of the peak velocity (searched backward from the peak velocity); if this value is higher than 20mm/s the start is tracked backward to a point where the velocity is not less or equal to 20 mm/s | Hand begins to move towards the mouth with the glass | Velocity of the glass exceeds 15 mm/s |
| Forward transport (glass to mouth) |
Hand begins to move towards the mouth with the glass | Velocity of the glass exceeds 15 mm/s | Drinking begins | Distance between the face and glass marker is below 15% of steady state* during drinking |
| Drinking | Drinking begins | Distance between the face and glass marker is below 15% of steady state during drinking | Drinking ends | Distance between the face and glass marker exceeds 15% of steady state during drinking |
| Back transport (glass to table, includes release of grasp) | Hand begins to move to put the glass back to table | Distance between the face and glass marker exceeds 15% of steady state during drinking | Hand releases the glass and begins to move back to initial position | Velocity of the glass below 10 mm/s |
| Returning (hand back to initial position) |
Hand releases the glass and begins to move back to initial position | Velocity of the glass below 10 mm/s | Hand is resting in initial position | Hand marker velocity returned to 2% of the peak velocity |
| *Steady state in the drinking phase indicates an averaged value of the 100 frames around the shortest distance between the face and glass marker | ||||
Table 1: Phase definitions for the start and end of each phase of the drinking task.
| Variable | Specification |
| End-point kinematics | Calculated from the hand marker |
| Movement time, s | Calculated for each phase and as total movement time for the entire task; definitions for start and stop are provided in Table 1 |
| Peak tangential velocity, mm/s | Calculated for reaching phase, combines both arm and trunk movement |
| Time to peak hand velocity, s, % | Absolute and relative values for reaching, characteraizes movement strategy (acceleration and deceleration time) |
| Time to first velocity peak, s, % | Absolute and relative values for reaching, characteraizes the initial movement effort |
| Number of movement units, n | Calculated for reaching, forward transport, back transport, and returning phase. One movement unit is defined as a difference between a local minimum and next maximum velocity value that exceeds the amplitude limit of 20 mm/s, and the time between two subsequent peaks has to be at least 150 ms. The minimum value for drinking task is 4, at least one unit per movement phase. Those peaks reflect repetitive acceleration and deceleration during reaching and correspond to movement smoothness and efficiency. |
| Angular kinematics, degrees | Calculated for shoulder and elbow joint |
| Elbow extension | Minimum angle of elbow flexion detected in the reaching phase, determined by the angle between the vectors joining the elbow and wrist markers and the elbow and shoulder markers |
| Shoulder abduction | Maximum angle in frontal plane detected during reaching and drinking phase, respectively; determined by the angle between the vectors joining the shoulder and elbow markers and the vertical vector from the shoulder marker toward the hip |
| Shoulder flexion | Maximum angle in sagittal plane detected during reaching and drinking, respectively; determined by the angle between the vectors joining the shoulder and elbow markers and the vertical vector from the shoulder marker toward the hip |
| Peak angular velocity of the elbow joint, degrees/s | Peak velocity of the elbow extension detected during the reaching phase |
| Interjoint coordination, r | Temporal cross-correlation of zero time lag between the shoulder flexion and elbow extension during the reaching phase. A Pearson's correlation coefficient closer to 1 indicates stronger correlation and indicates that joint motion of the two joints is tightly coupled. |
| Trunk displacement, mm | Maximum displacement of the thorax marker from the initial position during the entire drinking task |
Table 2: Definitions of kinematic variables used in studies presented in the representative results.
| Kinematic variables, mean (SD) | Healthy | Stroke | Effect size (healthy vs stroke) | Mild stroke |
Effect size (healthy vs mild stroke) |
Moderate stroke | Effect size (mild stroke vs moderate stroke) |
| End-point kinematics | |||||||
| Total movement time, s | 6.49 (0.83) | 11.4 (3.1) | 0.54* | 9.30 (1.68) | 0.46* | 13.3 (2.9) | 0.44* |
| Number Movement Units, (smoothness), n | 2.3 (0.3) | 8.4 (4.2) | 0.54* | 5.4 (2.1) | 0.42* | 11.1 (3.6) | 0.50* |
| Peak velocity in reach, mm/s | 616 (93.8) | 431 (82.7) | 0.54* | 471 (87.7) | 0.37* | 395 (62.0) | 0.22* |
| Peak angular velocity elbow in reach, °/s | 121.8 (25.3) | 64.9 (20.5) | 0.62* | 78.0 (19.3) | 0.57* | 53.3 (13.6) | 0.38* |
| Time to peak velocity in reach, % | 46.0 (6.9) | 38.4 (8.6) | 0.20* | 39.5 (8.7) | 0.15* | 37.5 (8.8) | 0.01 |
| Time to first peak in reach, % | 42.5 (6.9) | 27.1 (12.2) | 0.39* | 33.0 (9.9) | 0.25* | 21.8 (11.9) | 0.22* |
| Angular joint kinematics | |||||||
| Elbow extension in reach-to-grasp, degree | 53.5 (7.8) | 64.1 (11.5) | 0.24* | 60.5 (10.4) | 0.13 | 67.2 (11.9) | 0.09 |
| Shoulder abduction in drinking, degree | 30.1 (10.1) | 47.6 (14.9) | 0.33* | 37.2 (5.3) | 0.07 | 57.1 (14.5) | 0.47* |
| Trunk Displacement, mm | 26.7 (16.8) | 77.2 (48.6) | 0.34* | 50.1 (22.9) | 0.26* | 101.7 (53.4) | 0.30* |
| Interjoint coordination, Pearson r | 0.96 (0.02) | 0.82 (0.35) | 0.08 | 0.95 (0.02) | 0.03 | 0.69 (0.46) | 0.14 |
| * p<0.05; Effect size statistics are calculated as eta squared, η2 | |||||||
Table 3: Kinematic variables for individuals with stroke, for subgroups of moderate and mild upper extremity impairment along with healthy controls. Effect sizes for discrimination between groups above 0.4 (very large effect) are marked bold.
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Discussion
The protocol can successfully be used to quantify the movement performance and quality in individuals with moderate and mild upper extremity sensorimotor impairments at all stages after stroke. The feasibility of this protocol has been proved in a clinical setting as early as 3 days post stroke, and showed that the system can be used by trained health professional without specific technical qualifications. Technical expertise is, however, needed to create and develop a program for data analysis. From this aspect, the upper extremity motion capture differs from gait analysis, in which ready-made analysis programs are generally directly provided by the manufacturers. In everyday life, arms and hands can be used in many different tasks involving manipulation and interaction with different object in various sizes, locations, and affordances. This makes each set-up unique. Further, different goals and constraints of the task will also affect the kinematic outcome, since the kinematics are highly task-specific. In the future, more efforts should be made to create a standardized protocol for kinematic analysis of basic tasks, such as, drinking, eating, taking hand to the mouth, and bimanual object manipulation, which would allow a better comparison of the results between different studies.
Based on our early experiences, with a 3-camera capture system, in which the problem with segmentations and gaps was observed, it can be suggested that a 5-camera system that allows different positions for cameras (and one above the measurement area) is optimal for the upper extremity analysis. For a clinically feasible measurement set-up, a simple set-up with limited number of markers and simplified analysis, as described in this protocol can be advocated. When the assessment of movement performance and quality aims to follow patients' recovery, make prediction of future outcomes, select optimal treatment options, or evaluate the effectiveness of treatment and rehabilitation interventions, a simple, easy to use method would be enough. On the other hand, a more comprehensive biomechanical analysis using cluster-based markers would be required for more detailed modeling, particularly when the axial joint rotations and shoulder complex are of interest.
Increased clinical use of kinematic analysis is advocated by many researches in the area of neurology and stroke rehabilitation. Objective and valid methods for evaluation of motor function during natural activities and tasks are of high interest among clinicians and researchers. A recent consensus paper recommends adding kinematic measures in future stroke trials alongside with clinical assessments to distinguish between true recovery and compensation11. Challenges remain though, to determine a core set of kinematic outcomes and tasks for inclusion in trials, and to encourage broader collaboration between investigators to reach consensus11. The current 3D motion capture protocol together with published validation studies of this protocol can be one step on that direction.
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Disclosures
The authors have nothing to disclose.
Acknowledgments
Special thanks to Bo Johnels, Nasser Hosseini, Roy Tranberg and Patrik Almström for help with the initiation of this project. The research data presented in this protocol was gathered at the Sahlgrenska University Hospital.
Materials
| Name | Company | Catalog Number | Comments |
| 5 camera optoelectronic ProReflex Motion capture system (MCU 240 Hz) | Qualisys AB, Gthenburg, Sweden | N/A | Movement analysis system with passive retroreflective markers |
| Markers | Qualisys AB, Gthenburg, Sweden | N/A | Retroleflective passive circular markers, diameter of 12 mm |
| Calibration frame and wand | Qualisys AB, Gthenburg, Sweden | N/A | L-shape calibration frame (defines the origin and orientation of the coordinate system); T-shape wand (300 mm) |
| Qualisys Track Manager | Qualisys AB, Gthenburg, Sweden | N/A | 3D Tracking software |
| Matlab | Mathworks, Inc, Natick, Ca | N/A | Data analysis software |
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