Biomedical subjects

Håkan Lanshammar

Publications and source records attributed to Håkan Lanshammar.

3 recordsLinked to original sources

PubMed2006

Possibilities of texture based motion analysis.

Analysis of motion patterns in the human locomotion apparatus is important in many clinical areas like orthopaedics, physiotherapy, neurology, and sports medicine. Today marker based human motion analysis (HMA) is completely dominant in the clinical context. Technically, these systems are stable and dependable, and about a dozen variants are commercially available. One drawback of such systems is the time consuming and error prone marker placement. The purpose of the present contribution is to show that it is possible, with existing simple technology and methods, to build systems that do not depend on anatomically placed markers and yet produce an accuracy in knee joint center estimation comparable to marker based systems. It is shown that texture based methods can give estimates of knee joint center of rotation that can be compared to knee joint center estimates from marker based systems. Due to different definitions of the knee joint center position some bias is seen between the two estimates.

Calibration↗

PubMed2005

Using an extended kalman filter for rigid body pose estimation.

Rigid body pose is commonly represented as the rigid body transformation from one (often reference) pose to another This is usually computed for each frame of data without any assumptions or restrictions on the temporal change of the pose. The most common algorithm was proposed by Söderkvist and Wedin (1993, "Determining the Movements of the Skeleton Using Well-configured Markers," J. Biomech., 26, pp. 1473-1477), and implies the assumption that measurement errors are isotropic and homogenous. This paper describes an alternative method based on a state space formulation and the application of an extended Kalman filter (EKF). State space models are formulated, which describe the kinematics of the rigid body. The state vector consists of six generalized coordinates (corresponding to the 6 degrees of freedom), and their first time derivatives. The state space models have linear dynamics, while the measurement function is a non-linear relation between the state vector and the observations (marker positions). An analytical expression for the linearized measurement function is derived. Tracking the rigid body motion using an EKF enables the use of a priori information on the measurement noise and type of motion to tune the filter. The EKF is time variant, which allows for a natural way of handling temporarily missing marker data. State updates are based on all the information available at each time step, even when data from fewer than three markers are available. Comparison with the method of Söderkvist and Wedin on simulated data showed a considerable improvement in accuracy with the proposed EKF method when marker data was temporarily missing. The proposed method offers an improvement in accuracy of rigid body pose estimation by incorporating knowledge of the characteristics of the movement and the measurement errors. Analytical expressions for the linearized system equations are provided, which eliminate the need for approximate discrete differentiation and which facilitate a fast implementation.

Algorithms↗

PubMed2002

Force measurements of postural sway and rapid arm lift in seated children with and without MMC.

OBJECTIVE: The aim was to investigate the horizontal ground reaction forces of seated postural sway and rapid arm lift in children with and without myelomeningocele. BACKGROUND; It is unclear whether children with myelomeningocele have limited control of body posture entirely caused by the impairment in the legs or also by other dysfunction. METHODS: 11 children with myelomeningocele, 10-13 years, and 20 children without physical impairment were investigated. Data were collected by force plate measurements during quiet sitting and during rapid arm lift. The forces were expressed as the corresponding acceleration of the centre of mass. The amplitude and the frequency of the centre of mass acceleration quantified the sway. Movement time, onset and anteroposterior peak acceleration were analysed during arm lift. RESULTS: The children with myelomeningocele had a low sway frequency under both conditions: eyes open and eyes closed. The movement time was longer for these children compared to the controls. The onset of initial anteroposterior centre of mass acceleration preceded the arm lift and was directed forward in both groups. The peak centre of mass acceleration was usually directed backward. CONCLUSIONS: The control of postural sway was different in children with myelomeningocele compared to children without disabilities and this could not be explained by the cele level. The children with myelomeningocele had a slow motor performance of the seated sway and during arm lift. RELEVANCE: Slow motor performance involves functional limitations in the individual child and is important for the therapy program.

Adolescent↗