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 techniqu es 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 le arn 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 hei ghts 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 techniqu es (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 Forc e 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