ILLINOIS STATE UNIVERSITY School of Kinesiology and Recreation KNR 482 FORCE PLATFORM A force platform is an electrical instrument to measure force exerted against the platform Our force platform is interfaced to a computer to display and record force platform output The force output typically presents force on the vertical axis and time on the horizontal axis The graph is known as a forcetime curve The forces of interest to us are those exerted by the force platform back against the body These are called ground reaction forces GRFs GRFs are measured in three orthogonal directions at 90 to each other vertical Fz antero posterior Fy and medial lateral Fx The vertical force is the force exerted up on the body affecting acceleration in the vertical direction The antero posterior force curve provides information on braking and propulsion phases of the body in the anterior posterior direction The medial lateral force affects sidewards motion of the body Four variables quantifying the GRF are of interest Force magnitude force timing rate of force application and impulse Each of these variables is plausibly related to injury and each of these may directly affect the level of skill performance Force magnitude The average force applied and the size of peak forces are both of interest Average force during skill performance is a general indicator of the force to which the body is exposed Peak forces reflect the maximum loads applied during skill performance We report it in N kg Force scaled to mass Force timing is important as this variable reflects coordination of the skill and it also represents how much time the body has to adjust to an imposed load Measured in seconds Rate of force application reflects the interaction of the above factors Rate of force application often referred to as loading rate is measured as the slope of the force time curve Frequently loading rate is measured during the rise of the GRF curve to a peak value Measured in BW s Impulse represents the product of force and it s time of application It is measured as the area under the force time curve Measured in N kg s The impulse causes a change in momentum of a body The impulse momentum relationship is derived from Newton s 2nd Law as follows and F m a 1 a If we substitute Vf Vi t V f V i t into equation 1 for a F m Vf Vi t Multiplying both sides by t yields F t mVf mVi F t impulse the product of force the time of force application mVf mVi reflects the change in momentum of a body Thus an impulse causes the change in momentum of a body impulse momentum relationship This is the You will work in pairs for this project Example GRF data will be collected for each subject You will collect six trials in each of 2 conditions of landing total of 12 good trials Note each of you must identify the two levels of an independent variable ie what constitutes the conditions It can be soft vs stiff landings shoes vs no shoes ankle brace vs no ankle brace or landings from different heights Remember you will collect six trials in each condition so don t make them too difficult Remember you must randomize the conditions and if possible the trials within the conditions Vertical curves will be analyzed and discussed Subjects other on stand in can step must land with one foot completely on the force platform and the the solid floor After landing the subject should come back up to the anatomical position until data collection is complete and they off the force plate Use the following protocol to collect the force platform data 1 Turn the power switches ON at the back of the two black Vicon boxes to the left of the computer 2 Activate the Vicon Nexus program from the desktop 3 Activate live mode by pressing live in the resources pane 4 Make sure the system tab is active and select Force plate only from the drop down menu in the resources pane 5 In the left most dropdown menu of the main screen which reads graph select 3D Perspective Note you should now see yellow shaded cameras as well as a gray shaded square in the middle of a grid representing the forceplate you may need to rotate the screen to get a view of the forceplate this can be accomplished by clicking and holding the left mouse button in the black space while moving the mouse left or right It is a good idea to tap the forceplate with your foot and watch the computer screen for a GRF arrow to make sure the system is working properly 6 Right click on the gray box forceplate and select Zero Level This will tare the forceplate 7 Click the Show Hide data management icon in the upper left hand corner just below the file tab When the data management window appears 2 8 9 10 11 12 13 14 15 16 17 18 19 select the Open Database icon second from left Then double click on 482 Force Plate Lab in the name column Highlight Force Plate Lab by clicking on it and then click the NewSubject icon stick figure inside of yellow circle 4th from left Type in YOUR name and press enter on the keyboard Highlight your name by clicking on it and then press the NewSession icon gray circle with black cutout 5th from left Leave it as Session 1 and press enter Double click on the session name and then close the data management window click on the X in the top right corner In the resources pane make the Subjects tab active by clicking on it Click on the Create a new subject from a template icon middle button just below the subjects tab and select PlugInGait as the template A box will appear asking for the subject name type in your name and press ok Click once on the subject name in the resources pane and a subject parameters properties box will appear Type in the subject s body mass in kg convert from the scale along wall using 1kg 2 2 lbs Also write down this value There are 7 more parameters highlighted red which Vicon needs to accurately calculate kinematics These are not necessary for this lab so you can type in 0 for each of these parameters Right click on your name in the resources pane and select Save Model In the tools pane click on Capture middle icon In the Trial Type drop down menu select 482 FP Verify the session name in the next trial setup is yourname Session 1 If not the subject didn t get selected properly in the data management and must be selected again Redo steps from 11 Above Rename the trial to the format SxCyTz where x is your subject ID y is the condition identifier and z is the trial number ie S3c2t1 You will collect six trials in each condition The trial number will auto increment but you