Biomechanics Assignment: Forces and Moments of Forces Analysis

This assignment tests your understanding of two basis concepts need for biomechanics namely forces (and force components) and Moments of forces. Each individual will be given a sequence of data from a subject wearing their usual shoes. They will then analyse the data in detail as described below.

In the assignment section of the ITB myplace site you will find a list of students on the module and an associated file name such as trial01.xls. The data can be found in the item headed forceplate data folder of the assignment section of the myplace site. You can trust this file when you open it in excel. Download your file and open it in Excel.

Your data was collected in our biomechanics laboratory which should look something like this when imported into excel. The sampling frequency was 100Hz the rest should be selfexplanatory. The recording started before the subject hit the forceplate so the force data at the beginning will be all near zero. In this set of data Fx is forward and Fz is vertical.

To complete the assignment you are asked to calculate the position of the centre of pressure during a stance phase and plot the Pedotti diagram for this complete stance phase of the trial at 100Hz. You are then asked to discuss the shape of the plot and the dynamic movement of the centre of pressure together with factors which may affect the results you have obtained.

The first thing to do is to identify the frames when the subject was on the plate. This can best be done by drawing a plot of the vertical force as a line drawing and looking to see at which frame the hit the plate and at which plate they leave the plate. You should remove all the data lines before they hit the plate and all the data lines after they leave the plate. You now have a set of data with which to draw the Pedotti diagram.

The Pedotti or butterfly diagram can be generated once the two force components (Fx and Fz) and the centre of pressure (x) have been tabulated for the complete stance phase being studied. So first you must calculate the location of the centre of pressure along the x-axis. You can do this by calculating the moments generated around the origin of the plate which is 57 mm below the top surface of the plate.�

Accessing your data

To produce the Pedotti diagram the following steps should be taken:-

Step 1: Draw a side view of the force platform with the z-axis running vertically through the centre and the x axis running horizontally through the centre. Using the scale of 1m along the floor represented by 140 mms on your diagram to indicate the overall length of the platform (600mm).

Step 2: For each time instant (i.e. computer sample at 100Hz) locate the point on the x-axis representing the position "x" of the centre of pressure using the same scale as in step 1.

Step 3: Using a second scale of 1000N represented by 200 mm on your diagram calculate the number of millimetres required to represent the vertical component of the GRF "Fz". From the position of the centre of pressure determined in step 2 move this number of mm vertically upwards.

Step 4: now using the same scale as in step 3 calculate the number of millimetres required to represent the horizontal component of the GRF "Fx". Move this number of millimetres (to the left or right depending on the sign) from the position obtained at the end of step 3.

Step 5: Draw the force vector as the resultant of these two force components starting from the location of the COP for that frame,

Repeat steps 2 to 5 for every sample of data (100 Hz) from heel strike to toe off. You should get a graph something like that shown above. You can hand draw the graph if you wish and scan it into your report. If you wish to draw it using excel then you will need to work out how to do this ! It is not straight forward. If you know matlab you can use that which is more straight forward to do. But a hand drawing is fine.

The report will be assessed and the mark obtained will form a contribution to the overall module mark (30%). Each report should provide the following :

1. A brief description of how you would expect the test was conducted.

2. Present one sample calculation of the centre of pressure location and of how the forces were scaled.

3. Present graphs of Fx, Fz and My to a base of time. (so I can check the data you have used for the diagram)

4. Draw the pedotti diagrams. You should use a horizontal scale of 1m along the floor represented by 140 mms on your diagram and a vertical scale of 1000N represented by 200mm on your diagram. This is important so that your drawing may be checked and is similar to others in the class.

5. Discuss the information with particular regard to the style of gait, the usefulness of such information, advantages/disadvantages of test method etc. (This section provides 25% of the total report mark).

6. Present in an appendix the data from the stance phase of gait for the gait cycle so that errors can be checked. This should be in the form of a table where each row represents each frame at 100Hz. The data should be in the following order time, fx, fz, My, COPx

The report must be submitted by 5pm on the Friday the 13th of November 2020 . Penalties for late submission will be applied. You should submit your assignment as an electronic PDF document to the course myplace site. Please be aware that Turnitin software will be used to check for plagiarism and collusion and that the date and time of submission are recorded by the system. The first page of your PDF should be a completed assignment hand in form giving your details and the title of the assignment. You can get this from the myplace site for this module under assignment.

I repeat again please note that drawing the pedotti diagram in Excel is far from straight forward and can be exceedingly time consuming to work out. If you would prefer to hand draw the pedotti diagram on graph paper and then included it in your PDF as a scanned copy then this would be perfectly acceptable as would using matlab. However you should make sure you do not change the scale of the diagrams.

It is your responsibility to check the PDF produced from word is correct and has not corrupted your document. You should do your calculations in excel or matlab and draw you other graphs this way even if you hand draw the pedotti diagram. You should put your report together using word and then save and export it as a PDF before uploading it to the myplace site for the module.

This assignment should not take you long to do if you follow these instructions !