biomechanics

Force Data Analysis

Aim

The aim of this session is to introduce you to the methods of performing quantitative kinematic and kinetic analyses of 2D video and force data, using the software Quintic Biomechanics, Bioware, and Excel.

Objectives

To use Excel to calculate average velocity and acceleration for each shoe condition using the times recorded by the timing lights.

To use 2D video data of subjects performing 3 short indoor runs of moderate pace, barefoot and shod, to measure step length, step duration, hip and knee angles for one full running stride.

To use the recorded anteroposterior (F_y), and vertical (F_z) components of the ground reaction force (GRF) acting on a runner during the stance phase of moderate pace running, barefoot and shod, to perform a force-time analysis of the vertical GRF component to calculate key parameters such as impact force peak and load rate.

Instructions for Analysis

Using Bioware and Excel:

Using the instructions (Instructions for accessing BioWare Version 5 (AppV).doc), load the Bioware software. Open the middle barefoot trial for your first subject.

From your barefoot GRF graph, zoom in to the beginning of the trace (or use View > Statistics from the menu bar, or F3, and drag the green ‘start’ and red ‘end’ lines to trim in and out) and note the time when the F_z (vertical) trace commences and enter this data into Table 1 in the Excel spreadsheet ( Heel Contact (t₁, F_z1) in Force Analysis_Blank.xls).

Enter the time and force in N for the remaining key components of the VGRF:

Impact Force Peak (IFP) (t₂, F_z2)
End of Passive Phase (min force after IFP) (t₃, F_z3)
End of Absorption Phase (when F_y=0) (t₄, F_z4)
Toe-off (t₅, F_z5)

Convert the force values from absolute force in N to relative force in terms of body weight (BW) by dividing the absolute force values by the subject’s BW (calculated for you in Table 1). You will need to enter your subject’s mass into the spreadsheet first.

Using the time data in Table 1, calculate the following phases and enter into Table 2 (you will need to enter a formula for this):

Contact time (t₅-t₁)
Time to IFP (t₂-t₁)
Duration of Passive Phase (t₃-t₁)
Duration of Absorption Phase (t₄-t₁)
Duration of Propulsion Phase (t₅-t₄)

The times will be converted to proportions of the stance (contact) phase in the spreadsheet for you by dividing the event/phase time by the total contact time and multiplying by 100 to convert to a percentage.

Load rate in N.s^-1 and BW.s^-1 will then be calculated in the spreadsheet.

Repeat for the shod trial, and for another 9 subjects.

Complete a simple kinesiological analysis of the movement of sprinting, the leg lifting and lowering phases (position and movement of major joints/segments and key muscular activity) (see KA Template on Blackboard).

In your write up of this practical as your assessment your discussion should include the following:

A discussion of possible experimental difficulties and errors and how you attempted to minimise these;
A consideration of the effect of running velocity, wearing footwear, and different foot strike patterns on GRF
An evaluation of how you would use the data you have calculated for providing feedback to your performer for the improvement of their technique / performance.

You need to understand and explain the procedure to calculate force in BW, proportion of stance, and load rate – refer to the lecture notes, formulas in the Excel spreadsheet, and Watkins (2007).

Main Reference

Watkins, J. (2007) An Introduction to Biomechanics of Sport and Exercise. Chapter 2 – Linear Motion, pp48-57. Philadelphia, PA: Churchill Livingstone Elsevier Ltd.