2D Slow Motion Video Data Motion Analysis Techniques

Question 1

Introduction

Answer 1

• Screening and assessment are important in any medical care model and in physiotherapy, it involves the analysis of movement patterns.
• The analysis of human movement is of great interest for performance, injury prevention, and optimal recovery from injury or trauma.
• Most studies published on human kinematics have used three-dimensional motion analysis systems which are expensive, time-consuming, and generally not widely available in clinical practice.
• With developments in technology, more cost-effective solutions are available for assessing the human body and two-dimensional (2D) imaging has become an alternative to costly 3D systems.
• Two-dimensional video analysis has since become popular in clinical practice as it only requires a digital video camera and digitising software, which are universally available, reasonably cheap, and mostly portable.
• Video analysis involves the “systematic observation and interpretation of video to improve objectivity and reduce bias and subjectivity that are inherent in human observation”.
• Windt et al proposed a clinical decision-making framework consisting of four clinical questions to consider before investing in new technology.
  1. Will the information promised in the new technology be helpful? It has many benefits and indications and is of value when dealing with clients.
  2. Can you trust the information that is provides? Yes, it can be trusted.
  3. Can you integrate, manage and analyse this new information effectively?
  4. Can you implement this new technology in your practice?
• This approach to 2D slow-motion video analysis is based on the frameworks utilised for radiographs/diagnostic imaging with the generic steps:
  1. Record the image
  2. Analyse the image
  3. Store and retrieve the data.

Question 2

Preparation

Answer 2

• The first step in preparing to record involves explaining the purpose / necessity of capturing the image / video to the patient and obtaining their permission.
• “The way you move is contributing to your pain. I would like to analyse the way you move with the help of slow-motion video analysis”. “I see something in the way you move that I would like to show you through the use of an image / video”.
• Planning of the recording session involves many factors, such as:
  1. Staging (while considering privacy)
  2. Background of the image including the lighting
  3. Distance from the camera to the patient
  4. Choosing the camera
  5. Which views to collect
  6. Which movements elicit the pain and/or are the deviant movements
  7. Plan for the end-point of the video (e.g., record the patient moving when in pain)

Question 3

Equipment Considerations and Set Up

Answer 3

1. Chosoe a camera
2. Decide on the orientation of the smartphone / tablet
3. Mount the camera on a stable tripod or alternatively hold it in the hand (stable camera mounting)
4. Maximise the camera-to-subject distance
5. Maximise the size of the image
6. Add a “grid” to the screen
7. Align the optical axis of the camera perpendicular (at an angle of 90degrees) to the plane of motion (be perpendicular to the motion and far away from the subject.
8. Account for parallax error.
   - When performing a video analysis, certain procedures must be carefully followed in order to minimise systematic and random errors.
   - With 2D analysis, the movement being analysed must be confined to a single, pre-defined plane – the plane of motion. Any movement measurements outside of this plane are subject to perspective error / parallax error.
   - Parallax is “a displacement or difference in the apparent position of an object viewed along two different lines of sight” and is caused by viewing an individual at an oblique angle as they walk through the camera field.
   - Camera parallax also occurs when the lens and the viewfinder/display are offset on the camera.
   - This creates the effect that the position or direction of an object appears to differ when viewed from different positions (as seen through the viewfinder versus the lens of the camera).
   - Parallax is most noticeable when objects / people are close to the camera and become relatively insignificant at longer distances. To prevent parallax errors when filming:
9. Only measure angles and linear displacements when the patient is perpendicular to the camera lens (sagittal / side-view movements)
10. The camera can be fixed while the patient moves back and forth turning through 180degrees (anterior-posterior view) – obtain at least 2 views.
11. The patient and the camera can move together – necessitates the use of a gimbal stabilise to keep the camera steady and prevent jerking.
12. Stage a triangle ambulation where the camera is fixed and the patient is turning through 60degree angles. E.g., patient runs/walks from point A to B while we film from the right side. At point B, the patient turns through 180degrees and we film from their left side. At point A, patient turns 60degrees and runs to point B, repeat this a sufficient number of times to obtain a good sampling rate.
13. Obtaining at least 2 views when filming improves accuracy.
14. Place the camera far enough away from the patient being filmed.
15. Ensure optimal lighting of the patient.
16. Select an appropriate frame rate and decide whether to record in regular speed/slow motion. Frames per second is a measure of how motion video is displayed. Each frame is a still image. If a video is captured and played back at 30fps, each second of the video shows 30 distinct still images. The higher the fps, the smoother the motion appears. Standard smartphones and tablets will record at 30fps which is generally sufficient for walker and general patient interactions. High speed activities like running or sporting activities require higher frame rates. Recordings can be made at regular speed and played back in slow motion.
17. Visualise key landmarks on the body – ensure that the body landmarks relevant to the analysis are exposed and can be clearly seen – expose the belt line, poplinteal skin crease at the back of the knee, and for upper extremity expose T1, shoulder blade and elbows.

Question 4

Recording the Image / Video

Answer 4

• Rehearing before recording familiarises the patient with the movements to be recorded and allows the clinician to practice the verbal cueing to the patient.
• Following the rehearsal, capture the recording while ensuring that the end-point is achieved. It is important to obtain sufficient sampling to accommodate for the parallax effect.
• Perform a preliminary review and analysis of the video recording with the patient to ensure the video recorded well and there are no videography errors. Assure the patient that this is only a preliminary review and an in-depth analysis will be done after the visit and the results shared at the next visit.
• The initial analysis can be done using the standard video playback function while incorporating the scrubbing function available on most smartphones / tablets.
• Scrubbing entails adjusting the speed of the playback through the use of the dot/button on the slide bar and creates a way to do an initial slow-motion analysis.

Question 5

Video Editing Software Applications

Answer 5

• The video motion analysis can also be done with the use of video editing software which allows for other functions as well. When choosing a video editing software application, desirable functions or features to perform slow-motion analysis are: allow a frame-by-frame analysis and capture still images, split screen function, line drawing function, zoom function in the software, and ability to convert the file format from one operating system to another.
• Kinovea (open-sourse) and Slow Motion Video Function are particularly useful.

Question 6

The Initial Video Analysis

Answer 6

• It is useful to do the initial analysis with the patient in real-time while striving to engage the patient with evoking questions.
• Keep in mind the framework ‘pause-prompt-praise’ when sharing the video with the patient.
• Pause – what do you see?
• Prompt by asking evoking question – do you see ..? What do you think you can change? What ideas do you have to alter the way you walk/run? How important is this?
• Praise – give positive feedback even when weaknesses / problems are identified.

Question 7

Making the Analysis more reliable

Answer 7

• The second question asked by Windt et al is “can you trust the information that is provides?”
• The following factors play a role in enhancing the reliability of the video analysis:
  1. Develop a workflow system that is consistent, thorough and systematic. View and analyse from head-to-toe. View both right and left sides of the body. View the uninvolved side (assuming that it is less deviant). View the involved side.
  2. Define a standard point or time when the movement is observed and measured. When the patient/image is at right angles to the camera. When pain is provoked, for example during the stance phase. When the patient is fatigued enough. When the patient is moving fast enough. When the deviant movement occurs.
  3. Use quality improvement processes – video data allows for repeated observations and analyses of movement without fatiguing the patient or generating additional pain/injury which allows the clinician to determine the reliability of the analysis. Quality improvement processes include self-reflection, peer reviews (video reviews between clinicians) and repeated video observations at different time frames by a solo practitioner.
  4. Be aware of the influence of confirmation biases - our confirmation biases can affect our perception of images. Sharing images / videos with peers can address the possibility of an individual clinician’s confirmation biases.

Question 8

Considerations for Uploading and Transferring Video Data

Answer 8

• Some factors to consider for uploading / transferring the recorded video data include:
• The necessary hardware to get the data from the camera to storage – USB cable / card slot on computer/card USB adapter/Wi-fi.
• Software applications for transferring the video from a smartphone to the patient’s electronic medical record – dedicated clinical imaging applications are emerging – WABA Medical Pics, Secure Image Transfer, Clinical Uploaded, PicSafe.

Question 9

Considerations for Storing and Retrieving Video Data

Answer 9

• After recording the image and doing an initial analysis, it is important to establish a way to store and retrieve the video file as is done with radiographs.
• Factors to consider include:
• Where to store it? If using a personal camera, store the file in a separate folder from personal images. Storage needs privacy, security, encryption and password protection. Rename the video file to the patient’s medical record name and specify the date of recording.