In 1878, Eadweard Muybridge was the first person to perform an experiment that we would nowadays call motion capturing. Using a photo camera, he shot several consecutive pictures of a galloping horse to understand the fundamental biomechanics of a horse’s run. Capturing motion by technological devices with the purpose of analysis is hence a pretty old idea…
Since its early beginnings, motion capturing became a professional industrial branch. In the meanwhile it is used for movies, art, biomechanics, motion analysis, sport training and much more. Various kinds of technologies exist for the different application purposes. Technical standards commonly used in sport clubs are consumer video cameras. They are cheap, readily available and easy to use. However, they do not provide immediate motion feedback and need to be analyzed with the help of an expert (for example your coach). Numerical (and hence comparable) data is usually not obtained. This is unfortunate, since such information might be very useful to enhance the understanding of a motion performance.
Consider for instance the positional, 3-dimensional motion information of selected body points in the following figure…
…shape, body pose and motion is intuitively visible, aren’t they? For biomechanical analyses, one therefore utilizes different motion sensing devices.
Common systems that provide 3-dimensional, kinematic motion properties are: optical marker-based systems, optical markerless systems, mechanical systems, magnetic systems and wearable capture devices as inertial sensors. Every motion capture system is characterized by special system specifications. These influence the individual setup and capture conditions. In the same way, all system properties and requirements vary with regard to the recording environment, the size of the capture volume and the expressiveness of the provided data. These special properties have to be taken into account to select the best method for every application.
Conventional Motion Capture Devices
Camera Based Systems
Optical motion capture systems are widely used in movie and game productions. They base on the tracking of marker positions from multiple camera views and provide very rich data that is easy to interpret. On the other hand, they are very expensive and offer best results in indoor capture conditions. This is because daylight interferes with the tracking of the marker. The size of their capture volume is relatively small and it is relatively difficult and time-consuming to set up the capture system. For these reasons they are less useful for mobile consumer applications and most sport applications.
Markerless systems are based on two-dimensional data of one or more video cameras. They use computer vision algorithms and methods to track motion of objects and humans. It is furthermore possible to obtain depth information from markerless motion capture systems. One popular example here are the Microsoft Kinect camera senors. The main advantage of markerless motion capture systems is that the motion can be captured in a natural capture environment. In other words: subjects are not required to wear special equipment or marker for tracking. The main problem of markerless motion capture is that tracking requires close proximity with the object to be tracked to maintain a sufficient level of accuracy and information content. Objects in large distance to the camera cannot be captured in detail.
Sensors of mechanical motion capture systems are generally attached to the human body with a skeletal-like structure. The performer’s relative motion is then measured over the articulated mechanical parts which move in the same way as the actor. Because the system has an skeletal-like structure, it considerably interferes with the actor’s performance. Therefore, it is not used widely.
Magnetical systems utilize sensors that measure a magnetic field generated by a transmitter. Sensors and source are cabled to an electronic control unit that tracks the range of motion by the relative intensity of the voltage. Markers are not occluded by nonmetallic objects but are very sensitive to magnetic and electrical interference in the environment. Those disturbances affect the magnetic field strongly. The system is furthermore cabled to the electronic control unit and the actor’s mobility restricted, making it less favorable for motions that require freedom of motor activity.
Wearable sensors do not impose any restrictions on the motion with respect to lighting conditions and mobility. The sensors are small and of low weight, and do not need any external cameras, emitters or markers. The most common wearable sensors for sports are inertial measurement units (IMUs) built from accelerometers, magnetic field sensors and gyroscopes. Generally, wireless sensor commands can be sent out using a computer or software device and motions be recorded, saved and viewed accordingly. Restrictions on the capture volume only exist with respect to the maximal distance between sender and receiver of the capture and program commands. Magnetometers can furthermore be sensitive to magnetic and electrical disturbances in the environment.
Motion Capture for Sport Analysis
Most capture systems immediately exclude themselves as prospective data input systems for motion analysis in sports. Although sports generally take place in locally defined environments, their field of activity easily exceeds common capture volumes as they can be covered by camera systems. Optical marker-based and markerless systems herewith need to be neglected. Requiring high mobility, minimal size and weight (to not disturb the athlete), magnetic and mechanical systems also disqualify for use. This leaves wearable sensors as only reasonable choice. But what exactly are they, and how to handle them…? Find out more here…