UMass Amherst KIN 430 - BiomechLabF (6 pages)

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BiomechLabF



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BiomechLabF

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Pages:
6
School:
University of Massachusetts Amherst
Course:
Kin 430 - Biomechanics
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Biomechanics Laboratory F Impulse Momentum in Vertical Jumping Objectives To better understand the impulse momentum relationship Learn to use force platform data to compare two jumping techniques Introduction Determining the height of a jump in the field is often done using the jump and reach method either with a fancy measuring device as seen in the figure above or by placing chalk on the fingertips and making marks on a wall These techniques may be sufficient to monitor progress in an athlete s jumping ability but they don t provide a precise measure of jump height The height of a jump more specifically the vertical displacement of the center of mass may be determined more accurately in the laboratory using a force platform Thus the purpose of this laboratory is to learn how data from a force platform can be used to analyze jump height and to use these techniques to compare jump heights in countermovement jumps CMJ and squat jumps SJ Figure 1 Figure 1 a A stick figure drawing of a countermovement jump with the center of mass shown for movement comparison with b the representation of the squat jump Linthorne 2001 As we know the center of mass of the human body will act as a projectile while airborne Thus if the vertical take off velocity for a jump is known the vertical displacement of the jump i e jump height can be determined using equations of constant acceleration If we know the forces acting on the body in the vertical direction and the velocity at the start of the jump zero if the person is standing still we can use the impulse momentum relationship to determine the take off velocity There is also a simpler way to use force platform data to determine takeoff velocity The total time in the air is equal to the period of time when the vertical ground reaction force GRF is zero i e from takeoff to landing If we assume that the person took off and landed with the same body posture is this a reasonable assumption then the vertical takeoff velocity can be determined from projectile motion considerations In this lab you will compare the estimated heights determined using these two methods for a CMJ and a SJ METHODS Overview A person from your lab group will be asked to stand on a force platform and perform a CMJ and a SJ with hands held on their hips to eliminate performance differences due to arm swing The vertical GRF for each jump will be recorded and analyzed The GRF traces vary between the two jumping techniques Figure 2 The CMJ is characterized by a GRF that drops below body weight BW shown as a dashed horizontal line in figure 2 before rising above body weight while in the SJ the GRF rises above body weight without first decreasing in magnitude For both jumps you will determine the net impulse prior to takeoff and use this value in the impulse momentum relationship to determine the takeoff velocities Separate estimates of the takeoff velocities will be obtained from the time in the air for both jumps Figure 2 Force recordings of a countermovement jump left and a squat jump right Linthorne 2001 Participants A group of 3 4 students will work together at a single workstation Data for jumps using the two different techniques CMJ and SJ will be collected on one volunteer from each group What you need for this lab Bring a USB flash drive aka a thumb drive memory stick removable memory card etc Wear athletic shoes no sandals flip flops or dress shoes in case you are called upon to be the jumper in your group Equipment Each workstation will consist of a desktop computer for data collection and a force plate that interfaces with PASCOairlink The force platform records forces in the vertical direction at a sampling frequency of 100 Hz samples per second Procedures The participant will perform two different jumping techniques on the force platform The first technique will consist of a CMJ followed by a SJ The GRF from each technique will be analyzed using Excel to gain an understanding in the differences between each jumping technique Steps for data collection 1 Choose a jumper a computer operator and 1 2 observers The jumper will first perform a countermovement jump and subsequently will perform a squat jump The computer operator will perform the steps below for each jump and the observer s will make sure the participant is performing the specified jumping technique correctly The force plates are a bit small so the observers need to make sure that the subject landed cleanly on the plate Repeat trials as necessary 1 Open the PASCO Capstone icon on the desktop Select the option that says Two Displays Once a new window has opened click the center icon within the top box and select Graph Once the graph is open choose Select Measurement on the y axis and choose Normal Force N For the x axis select Time 2 Follow the same steps for the bottom box but select Parallel Force N for the y axis measure 3 Before any data are collected zero the force plate With the plate unloaded click the small black round TARE button on the side of the force platform where the cable connects Make sure to set the sampling frequency to 100 Hz at the bottom center of the page 4 Have the participant stand on the force plate Click the red round Record button in the lower left hand corner of the screen A stable line should appear in the Normal Force graph Press the Stop button once a stable measurement is achieved Record the participant s weight in newtons N using the coordinates tool in the task bar above the graphs Weight N 4 Data Collection Before each of the two trials the computer operator will push the red Record button and then call out for the participant to perform the specified jumping technique First the participant will perform the countermovement jump on the force plate stepping on it with both feet A real time force trace will appear as the participant performs the jump Press the red Stop button once the jump is complete 6 The next trial will consist of a squat jump Return to step 4 above and have the jumper perform a squat jump Note that in a true squat jump the GRF should not drop below BW before rising above BW at the beginning of the jump i e there should be no downward dip in the force Be sure to check the force tracing closely and have your subject repeat the jump if necessary 7 After the last jump trial export the data three runs in one file Body weight trial 2 jump conditions to a thumb drive Use the drop down menu Export Export it as a csv file naming it Jump csv Make sure everyone in your


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