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Marker-based Body Motion Analysis
Our motion laboratory has an optical motion capture system (Vicon) synchronized with force plates and wireless electromyography (EMG). Reflective markers are placed on the body following standard protocols such as the Plug-in Gait marker set. High-speed infrared cameras track the markers and reconstruct their 3D trajectories. Combined with ground reaction forces, these trajectories are used in inverse kinematics to compute joint angles and in inverse dynamics to compute joint moments and powers. The same data also provide reference motions for our musculoskeletal simulations.
How we use it
Large gait datasets and individual gait patterns. We built Gait120, an open dataset of human locomotion and EMG. We showed that each person's gait kinematics are unique enough to identify individuals and to detect abnormal gait patterns with machine learning.
Clinical gait analysis. With orthopedic surgeons, we link 3D gait measures such as knee adduction moment, ankle varus moment and ankle power to radiographic findings in knee and ankle osteoarthritis, cerebral palsy and foot deformities.
Estimating loads with fewer sensors. We estimate ground reaction forces from pressure insoles and foot-ground contact models, and predict whole-body motion from a single accelerometer.
Body Motion Analysis with Deformable Bodies
Skin markers are attached to skin and soft tissue, which slide and wobble over the underlying bones during movement. This soft tissue artifact can be as large as the joint motions we want to measure, especially at the thigh. In biomechanics we are usually interested in how the skeleton moves, so we need to separate skin motion from bone motion.
We build deformable human body models that represent how the body surface and soft tissue move over the skeleton. By fitting these models to marker trajectories, we estimate and remove the skin motion and recover the movement of the underlying bones more accurately. The same models can be fitted to ordinary video images. This enables markerless motion capture that estimates 3D body shape and skeletal motion from cameras alone, so movement can be measured outside the laboratory.
Skeletal Motion Analysis with Dual X-ray Fluoroscopy System
Even with deformable body models, skin-based measurement cannot capture the small motions between bones inside a joint, such as how the joint surfaces slide and roll against each other. These motions are often only a few millimeters or degrees, yet they matter for ligament injury, cartilage degeneration and surgical outcomes. To measure them directly, we developed a system that images the skeleton with X-rays from two directions at high speed. From these images we compute the 3D motion of each bone in the joint.
How we use it
Mobile Robotic X-Ray Imaging System for Dynamic Skeletal Imaging : Our high-speed robotic dual X-ray fluoroscopy system follows a target joint, such as the knee or ankle, and captures continuous biplanar X-ray images at 100 Hz while a person walks on a treadmill. CT-based 3D bone models are matched to each pair of images using 3D-to-2D registration. This gives the six-degree-of-freedom motion of every bone in the joint. We also develop the image-analysis methods behind the system: fast generation of simulated X-ray images (digitally reconstructed radiographs), statistical models of bone shape and density, and automated detection of anatomical landmarks.
Natural Skeletal Kinematics of Knee, Ankle, and Foot Joints During Walking : We have measured how healthy joints move during walking. Examples include coupled rotation and translation of the knee, subtalar motion that starts ankle push-off, motion of the midtarsal joint, and relative sliding on the tibiotalar and subtalar joint surfaces. We also study how footwear changes hindfoot motion and, with clinical partners, how the cervical spine rotates.
Abnormal Joint Motions Induced by Musculoskeletal Injuries : We measure how injuries and deformities change joint motion. Examples are anterior-posterior laxity of the knee with ACL insufficiency during the Lachman test, and joint instability in flatfoot during walking.
Effects of Surgical Treatments on Joint Kinematics : We evaluate how well surgery restores natural joint motion. This includes residual knee motion after combined ACL and anterolateral ligament reconstruction, a comparison of single- and double-bundle ACL reconstruction, and hindfoot motion after resection of a tarsal coalition.
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