Week 1: Force Measurement

Question 1

Define ‘Force’

Answer 1

A vector quantity containing both magnitude and direction.

Question 2

How did Nigg (1999, 2007) describe the effects of force?

Answer 2

If a force is able to cause constant deformation of an object it is a static force.
If a force causes acceleration of an object it is a dynamic force.

Question 3

Give five reasons why measuring the force exerted by an athlete may be useful:

Answer 3

1. Provides quantitative feedback and insight into performance.
2. Provides movement patterns for qualitative analysis
3. Highlights injury/ risk factors
4. Identifying foot strike patterns
5. Estimating internal loads

Question 4

How do the mechanisms of measuring force differ in AMTI force plates and Kistler force plates.

Answer 4

AMTI force plates use strain gauge sensors which are deformed and change the conductivity of the plate. The voltage is interpreted relative to the load on the plate.

Kistler force plates use piezoelectric sensors which are quartz crystals that generate charge in response to mechanical strain.
Kistler force plates are a lot more sensitive than AMTI.

Question 5

Describe how a force is detected by a platform and data is collected:

Answer 5

1. An input force is placed on the plate by an object.
2. A signal is detected by the spring in the transducer and converted into an electrical signal
3. The signal is passed through an amplifier
4. Goes through an analogue-digital converter to give an output

Question 6

What assumptions are made by force plate data (based on Newton’s third law)?

Answer 6

Weight is exerted by an object on the force plate and the opposing reaction force is exerted in the opposite direction by the force plate.

The measure of output force is equal to the input force..

Question 7

Define:

1) Fx
2) Fy
3) Fz

Answer 7

Fx = the medial-lateral force
Fy = the anterior- posterior force
Fz = the vertical force

Question 8

What directions are positive and negative moments?

Answer 8

Positive moments occur in the anticlockwise direction.

Negative moments occur in the clockwise direction.

Question 9

Why is high linearity a desirable sensor characteristic in force plates?

Answer 9

Linearity is defined as the difference between input and output- ideally the input force should be the same as the output force (high linearity) but sometimes there is a difference in capturing the input signal. (This can be corrected with calibration).

Question 10

What is hysteresis?

Answer 10

Hysteresis is the amount to which the input-output force relationship is influenced by whether force is increasing or decreasing .

Question 11

What is the shape of a typical vertical GRF for a rear foot striker?

Answer 11

A bimodal peak.

Question 12

What is the equation to calculate the average loading rate?

Answer 12

Peak impact force / time to peak impact force

Question 13

What method is used to calculate the instantaneous loading rate?

Answer 13

The first central difference method

Question 14

When might saturation of an output signal occur?

Answer 14

Where there is insufficient range to capture the input force. Altering the amplifier settings increases the range and reduces the saturation.

Question 15

When would you lose sensitivity while measuring force?

Answer 15

When the range of measurable forces is too great (precisions of measurements)

Question 16

Define Cross-talk

Answer 16

Cross-talk is the interference between recording channels for various force components

Question 17

What is the amplitude ratio?

Answer 17

The difference in magnitude between the peak impact force and the peak output force.

Question 18

What is the lag phase?

Answer 18

The difference in time between the start of the input peak and the start of the output peak.

Question 19

Name four factors what influence GRF:

Answer 19

1. Body Mass
2. Running Style
3. Running Speed
4. Impact interface

Question 20

What is the Centre of Pressure?

Answer 20

A theoretical point where the resultant force is being applied to the plate. It is a displacement measure (X, Y)

Question 21

Give the equations for the centre of pressure conventions for ay and ax

Answer 21

(ay) = Y- component of the COP 
Mx = Fz * Y
Y = Mx / Fz

(ax) = X-component of the COP
- My = Fz * X
X = -My / Fz

Question 22

What is the relationship between running speed and Fz?

Answer 22

As speed increases, vertical force increases

Question 23

Outline the Cavanagh and Lafortune (1980) study findings.

Answer 23

Participants ran over a force plate to obtain typical GRFs for running and examine changes in COP. C and L compared different foot strike patterns and determined changes in velocity of the centre of gravity.
Chalk was placed on the sole of the shoe to determine the relative foot placement in relation to the COP plot.
the five subjects ran at the same speed but had different lengths of ground contact time.
There was a clear difference between foot strike type and graph shape.
The centre of pressure was compared between mid foot and rear foot strikers.
conclusion: it is possible to use COP plots alongside vGRF graphs to confirm foot strike type.

Question 24

What did Hamill et al (2011). Find when collecting foot-strike data for RFS and MFS?

Answer 24

There is also variation in the Fx force. The mid foot striker has greater positive and negative peaks in Fx.
There is also a loss of impact peak in the mid foot strike

Question 25

Outline the key finding from the Nordin et al., 2017 study.

Answer 25

Developed four kinds of foot strikes- there was a smoothing off of impact peak for the forefoot and mid foot strike.

Question 26

What is the relationship between medial-lateral force and medial-lateral movement?

Answer 26

People have queried whether medial-lateral force could be related to medial-lateral movement. But as GRF represents the whole movement of the body and COP is a theoretical point that can move without foot movement. it is not possible to quantify foot position or medial lateral movement with a force plate.