W5 - Scaling for Body Size Flashcards
How would you remove the influence that body size has on the peak VO2 measurement?
Divide peak VO2 by body mass
= peak VO2 is now expressed as mL∙kg-1∙min-1
The use of which technique offers a more flexible method for removing the influence of body size?
‘Allometric’ models
If you are provided with a significant positive correlation between body mass and VO2, are you able to make a fair interpretation of peak vO2 between boys + girls or across children of different body sizes?
Scaling procedures are needed to make a valid interpretation of sex differences in VO2 max due to the confounding influence of size.
The strong linear relationship between peak VO2 and body size makes comparisons between people difficult, which child has the highest level of fitness?
What do you need to do first?
‘normalise’ peak VO2 for body mass to allow a fair comparison
What is the point of scaling?
To remove the influence of body size from physiological or performance related measures.
Physiological function or performance can then be interpreted from a qualitative viewpoint as the quantity element has been removed.
What does scaling allow us to do?
Compare vs ref values
Make inter-group comparisons
Longitudinal investigations
Explore relationships
What is scaling concerned with
Selecting a method to adjust for differences in body size between ind or groups.
Only when this is achieved can researchers make appropriate ‘use’ of data on ind or groups.
When is scaling not required
When there is no relationship with body size
What is the most common approach to scale for body size
Ratio standard method
What does the Ratio standard method do?
Expresses 2 variables as a simple ration i.e X/Y
- Y = Physiological variable (i.e peak VO2)
- X = Body size variable (i.e body mass)
Scaled value is then used in subsequent analyses or comparisons + assumed to be size free
What does special circumstance relate to?
Assumed proportionality between Y + X in the ratio standard method.
When is the ratio standard method ONLY valid?
When the special circumstance + actual data intersect on the graph.
What does distortion from the special circumstance introduce?
Statistical error
Below mean = inflated
Above mean = Deflated
How could you check if the ratio standard is valid?
There should be an almost perfect correlation between the variables.
Proportional relationship between the variables (the special circumstance)
No relationship between the scales variables + body size.
What if the ratio standard method is NOT valid?
Ind w/ a lower than average body size are advantaged
Those heavier than average are disadvantaged
- Statistical artefact
Allometric method
What does the ‘b’ value tell us?
Info about the nature of the relationship between the 2 variables.
b>1.0
Y increases at a faster rate than X
b=1.0
Y + X are proportional
– Ratio standard method
b<1.0
X increases faster than Y
How can an allometric relationship between Y + X be derived?
From a log-linear regression
What is the ‘b’ value used for?
To modify the equation used for the ratio standard method
How is ‘b’ used to create a ‘power function ratio’?
Y / X^b
How do you check the log-linear allometric model
Plot the calculates power function ratio against body mass
Y/X^b vs X
What does the log-linear allometric model do?
Produces a scaling factor (b) that when used to create a power function ratio, is uncorrelated with body size.
Allometric methods and running
Allometric methods may better predict running (treadmill) performance in children + adolescents.
What was the strongest predictor of 1-mile performance?
Ratio standard VO2 max.
Surface law
b=0.67
Based on the theoretical relationship between body size + metabolic rate. i.e whether you’re standing, sitting, walking…
What could explain the changing ‘b’ values
Allometric cascade model
What does the allometric cascade model suggest
That the relationship between metabolic rate + body mass is not determined by a ‘1 size fits all’ value.
‘b’ value is determined by the weighting of the ind process involved in energy demand and supply.
The switch from energy demand to energy supply processes from rest to exercise acts to change the b value.
Why may body mass not be the most appropriate variable to scale VO2
Confounded by body composition heterogeneity
Body mass scaling factor increases when body fatness is considered
How can lean mass be measured?
MRI
DXA
‘b’ value for body mass
0.3 to 1.1
Why is there large variance in the ‘b’ value
Sample size
Heterogeneity
Body size + composition
Age, sex + maturity status
Physical activity/training status
Is there a single unifying ‘b’ to use?
No, the value is dependent on metabolic rate + is supported but the allometric cascade model.
