When director Robert Zemeckis set out to create a screen version of the best-selling story The Polar Express, he called on the talents of 500 visual-effects specialists at Sony Imageworks to help him create a first-of-its-kind visual marvel that would expand the existing boundaries of film technology. In the $165 million film, super-advanced motion capture technology was used to translate Tom Hanks’ live performance into digital information that allowed him to play the roles of five different characters.
The actor’s smallest expressive movements were incorporated into animated screen characters by strategically placing 194 reflective silver-colored markers or "jewels" at various locations on his face and body. As he acted out each scene, an array of 72 cameras emitted near-infrared light that bounced off the jewels back to the cameras, which captured the visual information needed to represent his movements in three dimensions. Some of the jewels were remarkably small, just 2 or 3 mm in diameter, allowing the team to capture high resolution data and represent important subtleties of human emotion and expression.
First conceived by members of the medical field for the clinical analysis of human movement, motion capture caught the attention of creative minds and spread quickly to the animation and computer game industries. The Vicon System used to create The Polar Express represents a major advancement in motion capture technology, and conquers previous limitations on speed, accuracy and reliability. Besides adding newfound power and possibilities for the film industry, the Vicon System also gives clinicians an unprecedented capacity to quickly and accurately measure and evaluate human movement.
The Baylor Motion and Sports Performance Center was one of the first laboratories in the world to acquire the new Vicon MX System (Oxford Metrics, Ltd., Oxford, UK). The center uses twelve MX40 Motion Capture cameras which utilize a high-speed 4.0 million-pixel sensor and record up to 10,000 frames per second with 10-bit grayscale precision. The cameras locate and track reflective “jewels”, which have been adhered to specific anatomical landmarks, as they move within a calibrated measurement area. The jewels are actually small, spherical markers that are enrobed in a retroreflective tape manufactured by 3M. The tape is coated with the same material used to make the reflective highway signs that glow brightly when struck by a beam from the headlights of an approaching car. Like the highway signs, the tape allows light to be reflected directly back toward its source rather than scattering in all directions. When the cameras, which are ringed with light-emitting diodes, are aimed at the subject, the light pulses bounce back to the camera array. That collective information, when combined and analyzed with a dual-processor PC, precisely identifies the position of each marker sphere in three-dimensional space.
“Physicians who seek helpful information about abnormalities in an adult or pediatric patient’s movement or want to document performance before and after treatment, can refer the patient to the Motion and Sports Performance Center for a clinical study,” notes Fabian Pollo, Ph.D., Director of Research for the Orthopaedic Surgery Department at Baylor University Medical Center at Dallas. “This precise analysis allows clinicians to visualize the patient’s movement from any viewpoint in space, and to quantify the details of their movement. The information obtained can be useful in treatment planning for patients with cerebral palsy, osteoarthritis, sports injury, amputation, spinal cord or brain injury and joint replacement.”
The data obtained with the Vicon MX System lets physicians and researchers analyze common daily activities such as walking and stair climbing in great detail by accurately measuring segment positions, joint rotations, joint forces and subsequent muscle activity.
The assessment includes a kinematic analysis, or study of the geometry of the subject’s gait. From the positional information reflected back to the cameras, the Vicon system creates a three-dimensional skeletal reconstruction or “mesh” to visually model the subject’s movement in real-time. Clinicians can manipulate the skeletal image in three-dimensional space while simultaneously viewing a digital video or a kinematic graph of the movement sequence. The model is used to calculate the joint angle, velocity and acceleration, as well as to analyze segmental motion.
A kinetic assessment of joint forces, torque and power provides information about the loads on a joint and the power generated and absorbed across a joint. Force plates embedded in the floor of the 8-meter-long testing walkway measure the ground reaction forces at the same time the motion capture data is collected.
While the kinematic and kinetic data are being collected, dynamic electromyography (EMG) can be incorporated to record muscle activity and to visualize muscle firing patterns with specific motions. EMG activity is typically recorded with surface electrodes, although fine-wire electrodes can be employed to record the activity of deeper muscles if needed. In the skeletal representation of the patient’s movement, the muscle firing patterns can also be modeled by representing active muscles in bright red, which fade to blue as they become inactive.
This visual EMG data is particularly useful in detecting spasticity or co-contraction of opposing muscles, such as in a patient with cerebral palsy, stroke or brain injury. With this information, physicians can plan the most effective approach for treatment, which may involve orthoses, anti-spasticity medication, or surgical intervention such as the transfer of a portion of muscle to a more functional position to compensate for the biomechanical abnormality.
For patients afflicted with osteoarthritis of the knee, the varus moment or varus thrust, which leads to load on the medial compartment of the knee joint, is a critical factor in the destructive cycle that characterizes the disease process. Visualizing and quantifying this damaging force lets physicians devise a strategy to offload the joint and interrupt the cycle of destruction.
For patients who are suspected of having a compromised or injured ACL , motion analysis can detect the abnormal compensating motion that patients commonly exhibit, and allow physicians to plan an effective treatment and prevent eventual joint damage.
Clear, detailed results of the clinical investigation is presented to the referring physician on an interactive CD. The results are presented in a well-organized format that incorporates graphical and tabular results of all relevant data, digital video and an interactive 3-D skeletal model of the patient that can be rotated and viewed from any perspective angle. The results include quantified, high-resolution data collected from the motion sequences that is accurate to less than one millimeter of translation or one degree of rotation.
According to Dr. Pollo, “Unlike older motion analysis systems that often required hours or days to generate useful data, the Vicon MX System allows clinicians to conduct a thorough assessment in a matter of minutes.” While a highly sophisticated laboratory like this one is costly to set up and operate (the Baylor lab includes twelve cameras at a cost of $25,000 each), the Vicon MX system enables a faster, more detailed and accurate analysis of motion than is possible at most centers.
Although the laboratory’s primary function is to evaluate clinical patients, its specialized equipment can also be used to evaluate the biomechanical performance of athletes. Baseball pitching and batting, sprinting, long-distance running, golfing, high jumping, gymnastics, tennis and other sports can all be captured and analyzed in detail, then compared to previous performances. Comparisons can also be made to recorded assessments of elite athletes, helping athletes and their trainers make adjustments to more closely approximate proven movement patterns and optimize performance.
Motion analysis may also help identify issues that can affect the athlete’s susceptibility to injury. The Vicon System helps runners fine-tune detailed aspects of their movement, such as the vertical excursion of their center of mass, or check the position of their foot-ground contact in relation to their body’s center of mass which can reveal mechanical inefficiencies. Biomechanical imbalances or asymmetries that might eventually lead to injury can also be evaluated and addressed with appropriate treatment or training, such as orthotics, physical therapy and stretching regimens, to correct the imbalance.
The analysis of an athlete can be custom tailored to the requirements of the coach and/or trainer. The center’s motion analysis system is portable, so athletes can be evaluated while running on the facility’s indoor oval track, or can be transported to an outdoor location for motion capture in the athlete’s natural environment.
When Baylor researchers presented at a recent Endurance Running Summit sponsored by USA Track & Field in Las Vegas, Nevada, athletes and coaches were impressed not only by the power and accuracy of the Vicon MX system, but by the speed with which results can be obtained. Dr. Pollo notes, “It used to take up to six months to analyze high-speed video data of an endurance runner or athlete. With this system, we can provide precise results in about 15 minutes.”
The Baylor Motion and Sports Performance Center is actively involved in research to enhance the understanding of the relationship between biomechanics and health. Center physician and renowned researcher James W. Brodsky, M.D. is the principal investigator in a clinical trial funded by the Orthopedics Research and Education Foundation which compares the effectiveness of a specialized removable walking boot vs. a total contact cast for the prevention and treatment of pressure-related plantar foot ulcerations in diabetics.
Patients with certain medical conditions such as diabetes can develop neuropathy that results in decreased sensation in their extremities. This sensory deficit places them at risk for developing pressure wounds, which often go unnoticed until they become severe. For these patients, the center uses the Novel Pedar® system (manufactured by Novel Gmbh, Munich, Germany) consisting of a thin, flexible insole equipped with 99 sensors to measure the distribution of pressures under the foot. Plantar pressure assessments can be performed barefoot, between the plantar surface and the shoe, or between the plantar surface and an orthosis. These assessments can help evaluate patients with diabetes, Charcot, or other foot deformities that involve abnormal, potentially destructive pressure distribution on the plantar surface which can lead to serious medical problems.
Other center studies are currently examining long-term outcomes following minimally-invasive hip replacement vs. traditional hip replacement by looking at post-operative biomechanical differences in movement. The results of this study will help physicians determine whether a faster recovery justifies the higher level of technical difficulty and accompanying risk associated with minimally invasive surgery when compared to traditional hip replacement surgery.
The Baylor Motion and Sports Performance Center is located in Baylor Tom Landry Center at 411 North Washington in Dallas. For more information, or to refer a patient or athlete for a clinical assessment, call (214) 820-6300.
