Plan for today Discuss your assignments detailed on the last slide of the powerpoint for last week on: –Topics/problems in which you are most interested that are related to biomechanics –Results of electronic search for information sources related to a selected topic of interest –Summary of selected biomechanics website Powerpoint presentation on kinematics and their measurement (slides 2-26) Hand out copies of baseball swing warmup study and give reading assignments for next week (February 6)

Kinematics and their measurement Kinematics is the study of bodies in motion without regard to the causes of the motion. Through inverse dynamics, kinematics can provide estimates of net forces or torques Kinematics may be an end in itself, or an intermediate step to estimating kinetic variables –Linear Displacement Velocity Acceleration –Angular Displacement Velocity Acceleration

We need a standard way of describing position in time and space Cartesian coordinate system and frames of reference (see following slides)

Global coordinate system (GCS) & Local coordinate system (LCS)

Degrees of freedom concept The number of independent parameters to define a point (3), or segment (6), in space

Kinematic Analysis Methods & Tools Qualitative vs quantitative –Types and precision of data needed, not the tool available, dictates selection of tool Naked-eye observational procedures –Observational plan necessary to be consistent and reliable View multiple times View from multiple perspectives Focus on parts, then whole, then parts Form a visual-mental image of the performance Use a checklist – borrow or construct your own

Constructing Analysis Checklists Procedures –Study other checklists. Refer to specific sports/skills chapters in textbooks –Read literature to determine what factors are important –List the determinants of skilled and unskilled actions –Arrange checklist in a scale (dichotomous, continuum) Common factors usually included in checklists –Location of COM relative to base of support –Width of base of support –Range and path of movement of various body segments –Sequencing of segment movements –Projection angleof objects released or struck & total body COM –Overall perception of movement’s effectiveness, smoothness, etc.

Analysis of recorded images Qualitative procedures –Contourograms –Point plots –Stick figures Quantitative analysis –Photogrammetry - making accurate measurements from images Photography Videotaping Optoelectric imaging

Analysis using videotaping Initial recording Frame grabbing – converting images to digital form Digitizing – locating segment endpoints in space and time Calculations done by computer –Applying the multiplier or converting coordinates to life size –Smoothing coordinates –Calculating parameters for each segment and body COM »Linear displacement, velocity, acceleration and inverse dynamics »Angular displacement, velocity, acceleration and inverse dynamics –Displaying output (Digital as well as graphic)

Principles of photogrammetry Videotaping and optoelectric imaging systems have replaced film Two types of error must be dealt with: parallax and perspective Must keep image as large as possible, but leave space along the edges Depth of field

Photogrammetry (cont’d) Exposure time – how long it takes to form an image Frame rate –how fast images are formed –For most video cameras frame rate is 30 fps interlaced images per second, or 60 fps if they are split Exposure principles –Exposure time, sensitivity of imaging elements, relative aperture (f-stop), and light intensity. –Focus and depth of field

Photogrammetry (cont’d) Scaling to real size For planar studies –May use a board of known length in plane of motion, perpendicular to optical axis of camera For multi-camera, 3-dimensional studies: –Direct linear transformation Fix cameras with degree convergence angle Place calibration unit containing 6 or more markers in active space and film/tape with each camera Each coordinate is scaled using equations with coefficients derived from the the known location of the calibration points and their location as viewed from each camera

Calibration units for DLT

Do you need surface markers to locate joint centers and segment end points?

Optoelectric imaging – surface markers Optoelectric imaging systems use either active or passive markers

Surface markers for 3-D studies

The VICON System

Derivatives and data smoothing Finite difference, central difference method used to calculate velocity and acceleration Errors are magnified

Angular kinematics

Relationship between linear and angular motion (kinematics) a = r 

Electrogoniometric analysis Can measure either joint (relative) or segment (absolute) angles directly in real time

Electrogoniometry (cont’d)

Accelerometry Can measure vibrations, impacts, and rapidly changing motions Can be as small as a pencil eraser and as light as 1 gram Bi-directional or tri-axial

Example of accelerometry output: Tennis racquet vibrations

Example of accelerometry output: impact accelerations Wayne State Tolerance curve: