Physiology, Health & Exercise
Lesson 15
Body Composition & it’s Measurement
Body Composition &
Measurement
Methods include:
 Densitometry
 Skinfold thickness
 Bioelectrical impedance analysis
 Body Mass Index (BMI)
 Waist/hip ratio
 Mid-upper arm circumference
Limitations of each method
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Body Composition
 Measuring someone’s mass does not tell us
the whole story
 i.e. doesn’t tell us much about their size or
composition of their body
 2 people can have the same mass but one can
have more body fat & the other have less fat
but more muscle
 Level of fat to muscle better indicator of
how healthy someone is
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Body Composition
 Need to determine ratio of fat to lean tissue
(muscle, bone etc)  body composition
 Remember do need some fat
 cell membranes
 Formation of some hormones
 Insulation
 Just not too much fat!!
 It is possible to lose fat from body &
increase or maintain lean tissue  not much
change in mass
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Body Composition
 % of body fat associated with the least
health risk is
 18-25% for females
 13-18% for males
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Body Composition
Why is it important to assess body composition?
1. To assess the health risk associated with too
much or too little fat
2. To monitor weight loss in an obese individual
3. To monitor changes in body composition
associated with some diseases like cancer
4. To monitor the effectiveness of exercise
training programmes in athletes
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Body Composition
How to measure?
1. Densitometry
2. Skinfold thickness
3. Bioelectrical impedance analysis
4. Body Mass Index (BMI)
5. Waist/hip ratio
6. Mid-upper arm circumference
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Densitometry

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Fat is less dense than lean tissue
Fat density is 0.9g cm-3
Fat-free tissue density is 1.1g cm-3
If measure density then can predict the
relative proportions of lean and fat tissue in
the body
Density = mass
volume
 Volume measured by Archimedes Principle!
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Densitometry
 Once density determined, % body fat is
calculated as follows:
% body fat
= 495
density
- 450
 Person A & B both have a mass of 60kg.
Person A displaces 56.9 litres of water and B
displaces 58.3litres.
 Calculate % body fat for both
 Who is overweight?
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Densitometry- answer
 Person A has 19.42% body fat
 Person B has 30.97% body fat
 Person B is overweight
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Densitometry- advantages
 Accurate method
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Densitometry- limitations
 Involves the person being totally submerged
under water
 May be difficult & produce some anxiety
 New method developed using a “Bod Pod”
which uses air displacement.
 Person sits in a small chamber and body
volume calculated by measuring initial volume
of empty chamber minus volume with the
person inside
 Specialised equipment needed
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Densitometry- Bod Pod
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Skinfold thickness
 Most widely used method
 Involves measuring the layer of fat under the
skin (subcutaneous layer) at several specific
sites
 Uses a skinfold caliper
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Skinfold thickness
 4 sites are commonly used:
 Over the biceps at the front of the arm
 Over the triceps at the back of the arm
 Under the shoulder blade at the back
(subscapular)
 Above the hip bone at the side of the body (suprailiac)
 Values added and compared with values in
table to predict body density & then body
fat
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Skinfold thicknessadvantages

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Non-invasive
Relatively cheap
Portable
Quick
Accurate once the skill has been mastered
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Skinfold thickness- limitations
 Errors associated with measurer skill
 Does not take into account unusual fat
distribution
 Difficult in the fat and very obese
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Bioelectrical impedance
analysis (BIA)
 Relies on fact that fat is an insulator of
electricity whereas fat-free tissue (approx
73% salty water) is a good conductor
 Insulators have a higher resistance or
impedance to the flow of a small electric
current
 Greater resistance  greater % body fat
 Electrodes placed on the back of the right
hand and right foot while subject is lying
down
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Bioelectrical impedance
analysis (BIA)
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BIA- advantages
 Requires little or no technical skill by the
operator
 Portable
 Quick- takes less than a minute to perform
 Only requires removal of a sock!
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BIA- limitations
 Any disturbance in hydration level (e.g.
dehydration or oedema) will affect the
accuracy of the results
 Changes in skin temperature can also affect
conduction of the electrical current
 Tends to overestimate body fat in very lean,
muscular people and underestimate in obese
people- so not as accurate
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Body Mass Index (BMI)
 Most commonly used index of over or
underweight
BMI =
body mass
height2
 Units are kg/m2
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Body Mass Index (BMI)
Classification
BMI (kg/m2)
Underweight
<18.5
Associated
health risks
Low
Normal
18.5- 24.9
average
Overweight
Moderate
Obese class I
Obese class II
Obese class III
>25.0
25.0-29.9
30.0-34.9
35.0-39.9
>40
Increased
Moderately
Severely
Very severely
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Body Mass Index (BMI)caution!
 Classification can result in an individual being
classified as overweight or obese when have
a relatively low % body fat, but large
muscular bulk
 E.g. body builders, weight lifters & other
athletes with large muscle bulk
 Body-builder with mass 130kg & height 1.90m
would have BMI of 36.01kg/m2 & be classed as
obese class II
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Body Mass Index (BMI)
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Body Mass Index (BMI)questions
 Try the following questions:
1. Person A is 150cm tall & has a body mass of
65kg. What is her BMI?
2. What classification would she have?
3. How much weight would she have to lose for
her BMI to be within the normal range?
4. Person B is 170cm tall and weighed 69kg. He
recently lost 15kg on a diet. Explain how his
BMI has changed & what could happen if he
continues to lose weight.
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Body Mass Index (BMI)answers
1. Person A has a BMI of 28.9kg/m2
2. She is moderately overweight.
3. She would have to lose 9kg
65 - (24.9 x 1.5 x 1.5)
4. At start B has BMI of 23.9kg/m2  normal
After weight loss BMI is 18.7kg/m2  close
to underweight which could have a negative
impact on his health
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BMI - advantages
 Easy to calculate
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BMI- limitations
 Can lead to wrong classification as does not
differentiate between high % fat and high %
muscle
 Important to differentiate between large
muscle bulk and excess fat
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Waist/Hip ratio
 The distribution of fat in the body rather
than total quantity is more important
 People classed as apples (android) or pears
(gynoid)
 Apples  extra abdominal fat (“pot-belly”)have a higher risk of CHD, diabetes etc
 Pears  extra fat around the hips & thighs
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Waist/Hip ratio
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Waist/Hip ratio
 Use a measuring tape to check the waist and
hip measurements.
- Measure your hip circumference at it's
widest part.
- Measure your Waist Circumference at the
belly button or just above it.
 Use the same units for both!
 Repeat 3 times and take an average
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Waist/Hip ratio
 For women 0.8  pear shape & > 0.8  apple
 For men 1.0  pear shape & > 1.0  apple
 At risk values are > 1.0 for men & > 0.8 for
women
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Waist/Hip ratio
 It is possible to have a high BMI and a normal
waist measurement if you are a fit, lean,
muscular man. This is why your waist to hip
ratio is a better guide to your risk of heart
disease.
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Waist/Hip ratio - advantages
 Better estimate than BMI for some
 Easy to calculate
 Requires little equipment
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Waist/Hip ratio - limitations
 Takes no account of body composition
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Mid-upper arm circumference
 Is the circumference of the left upper arm,
measured at the mid-point between the tip
of the shoulder and the tip of the elbow
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Mid-upper arm circumference
- advantages

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Easy to calculate
Requires little equipment
Easy to perform
Quick to carry out
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Mid-upper arm circumference
- limitations
 Better for estimating under-nutrition
 Lack of data upon which to decide useful cutoff points
 changing patterns of skeletal muscle and
subcutaneous fat as people age
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Your mission…..
Work out your body composition using the
following methods:
1. Skinfold thickness
2. Body mass index
3. Waist/hip ratio
4. Mid-upper arm circumference
 Record 1 & 2 on your personal profile sheet
 Make a comment on your observations
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