EXSS 323
FORCE MEASUREMENT LAB

This lab was developed by Gerry Smith, Ph.D., formerly of Oregon State University, and is used by permission. Portions in italics are additions/modifications to the original and were made by Gary Christopher, M.S., A.T.C.

·         INTRODUCTION

In this lab the focus of study is vertical ground reaction force (GRF) and subsequent vertical motion of the body.  Reaction forces are equal and opposite to the forces applied by the body to the ground (Newton's 3rd law).  In addition, Newton's 2nd law (F = m a) governs the resulting motion.

A force platform is a device which measures ground reaction force.  It is a three dimensional measuring device, however we will deal only with the vertical component of the ground reaction force.  A free body diagram of the plate and a subject is illustrated below:

For the case in which a person is stationary on the plate, the two forces (FW and R) must be equal in magnitude and opposite in direction.  To propel the center of mass (CM) upward or to brake its downward motion, the person must accelerate the CM upward (positive direction); ma will be positive and the ground reaction force will increase above body weight.  Alternately, in propelling the CM downward or braking its upward motion, the person must accelerate the CM downward (negative direction); ma will be negative and the ground reaction force will decrease below body weight.  When the person is not in contact with the plate, the ground reaction force should equal zero (if the data show a different value, we need to correct for this by subtracting the value from all of the data).

In summary:

    • If acceleration is less than 0, then R is less than FW
    • If acceleration = 0, then R = FW
    • If acceleration is greater than 0, then R is greater than FW

For illustration of the relationship of GRF to the motion it causes, a short movie clip was created.  Note that this is a very large file (about 1.5 Mb) and will require an extended download over a modem.  It will be best used from a direct campus connection.

Play the movie through several times at normal speed to observe the motion and force relationships.  Then advance it one frame at a time and observe where takeoff and landing occur on the GRF graph and how the body motion relates to the ground reaction force.

·         PROCEDURES

Each student will perform the following jumps on the force platform while it monitors the vertical ground reaction force:

1.      With hands on hips from a squat position, perform a maximal vertical jump without counter movement.

2.      With hands on hips from an upright position, perform a maximal vertical jump with counter movement.

During each jump, keep your hands on your hips throughout the jump.  When you land, stand up but do not step off the force plate until directed to do so.  You will receive a force-time file on disk for each jump.  Do not attach the force-time data sheet to your lab after graphing the data.

·         RESULTS

Use the graphing capabilities of a spreadsheet program (e.g. Excel) to graph force versus time for your jumps.  Create a separate graph for each jump.

Manually label the graphs with the following:

1.      Body Weight line

2.      Takeoff (TO)

3.      Landing (L)

4.      Flight Time (FT)

·         GENERAL QUESTIONS:

Answer the following questions in a few paragraphs. Attach your force-time graphs to these written responses and turn in to your lab instructor at the beginning of lab next week.

1.      Discuss the differences in your force-time curves for the vertical jumps with and without counter movement.

2.      For each vertical jump, determine the flight time.  How do flight time and jump height interact?  If we had measured it, which of the two jumps would likely have resulted in the greater jump height?  How did you reach this conclusion?