Unit 3 – Movement Skills and Energy for Physical Activity AREA OF STUDY 2 – HOW DOES THE BODY PRODUCE ENERGY ?

Acute Responses to Exercise

Acute Responses to Exercise

At the onset of exercise the demand for O2 and energy sources by the working muscles increases and the cardiovascular, respiratory and muscular systems respond to meet these demands.

These include:- Cardiovascular System

increase in HR / Stoke Volume/ Cardiac Output/

redistribution of blood flow increased blood pressure increased a-VO2 difference

Respiratory System increased respiratory frequency increased tidal volume increased ventilation

Muscular System increased blood flow depletion of energy substrates increased muscle enzyme activity Increased muscle temperature increased supply and utilisation of O2

increased motor unit and fibre & recruitment

Respiratory SystemACUTE RESPONSES TO EXERCISE

Ventilation

Respiratory system is responsible for the delivery of oxygen to, and the removal of carbon dioxide from working muscles.

At the onset of exercise ventilation is stimulated by messages sent from working muscles to the respiratory centres in the brain – these increase the rate and depth of breathing.

Ventilation = Tidal volume (litres) X RR (breaths per minute)Tidal volume – amount of air expired in one breath

Condition RespiratoryRate

(breaths per minute)

Tidal Volume

(litres)

Ventilation(litres per minute)

Rest 12 0.5 6

Moderate Exercise

30 2.5 75

Maximal Exercise

48 4.0 192

020406080

100

120

140

VE

Litre

s/m

in

Submaximal ExerciseMaximal Exercise

Ventilation Responses During Sub - Maximal and Maximal Exercise

Diffusion

Gas exchange occurs in the lungs at the alveolar- capillary interface and the tissue capillary interface through diffusion.

Diffusion occurs from an area of high pressure to an area of low pressure.

Lungs - 02 high Blood 02 low

- C02 low Blood C02 high

Muscles - 02 low Blood 02 high

- C02 high Blood C02 low

Gas exchange occurs at the alveolar- capillary interface in the lungs.

Cardiovascular SystemACUTE RESPONSES TO EXERCISE

Cardiovascular Responses to Exercise

Cardiac Output (Q) Litres per minute = HR (beats per minute) X SV (millilitres per beat) Stroke volume (SV) - amount of blood

pumped from left ventricle with each beat.

At rest the heart only ejects about 40-50% of blood in the left ventricle.

Stronger ventricular contraction during exercise results in more blood being ejected from the left ventricle and thus a rise in stroke volume.

Heart rate (HR)

Heart rate plays a vital role in increasing cardiac output

Sub-maximal exercise – increases until oxygen demands have been met and levels off.

Maximal – increases linearly until maximum heart rate is achieved.

Heart Rate Values - Resting

Male Average - 72bpm

Female Average - 80bpm

Maximum Heart Rate

220 minus your age

Heart Rates and Exercise

Max – 184Av - 171

In sub max exercise HR

rises then levels off in steady

state.

Cardiac Output – Sub Maximal Exercise

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Cardiac Output

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Stroke Volume

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Heart Rate

Heart Rate

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Recovery

Condition Stroke Volume(mL/beat)

Heart Rate

(bpm)

Cardiac Output(L/min)

Untrained Rest 75 82 6.2

Maximal Exercise

112 200 22.4

Trained Rest 105 58 6.1

Maximal Exercise

126 192 24.2

Changes in SV, HR and Q in Trained and Untrained Individuals at Rest and During Maximal Exercise

Blood Pressure

During exercise increase in cardiac output increases blood pressure. Systolic blood pressure – pressure in arteries following contraction of ventricles as

blood pumped out of heart. Rest – 120 mmHg

Exercise

Aerobic – 130mmHg

Weight training heavy load – 200mmHg

Diastolic blood pressure – the arterial pressure during the interval between heart beats.

Rest – 80 mmHg

Exercise - little if no change / may drop slightly.

Venous return

An increase in cardiac output has to be accompanied by an increase in venous return.

During exercise venous return is increased by:- The muscle pump – contracting muscles result in a pumping action against the

veins forcing the blood towards the heart. Valves in veins prevent backflow.

Respiratory pump – abdominal pressure is increased as diaphragm contacts thus emptying blood in thorax and abdomen towards the heart – they fill during inspiration ready to be emptied again.

Vasoconstriction – reduces capacity of venous system pushing more blood towards the heart.

Blood Volume

Blood volume decreases during exercise (plasma volume can decrease by up to 10% during prolonged exercise).

The magnitude of decrease is dependent on:- exercise intensity

environmental conditions

hydration of the individual

Blood Flow - Rest

Increased blood flow to working skeletal muscles – blood vessels vaso-dilate. Reduced blood flow to less active organs (liver, kidneys, intestine) –blood vessels vaso-constrict.

Rest 5000mlMuscles – 20% (1000mL)

Heart – 4% (200mL)

Skin – 6% (300mL)

Brain – 14% (700mL)Liver – 27% (1350mL)

Kidneys – 22% (1100mL)

Other – 7% (350mL)

Liver – 2% (500mL)Other – 3% (750mL)Heart – 4% (100mL)

Kidneys – 1%(250mL)

Brain – 4%(900mL)

Skin – 2% (600mL)

Muscle – 84% (21000m)

Exercise 25000 mL

Redistribution of Blood Flow - Exercise

Arteriovenous Difference (a-VO2 )

Arteriovenous difference (a-VO2) is the difference between the oxygen content of arterial blood and mixed venous blood.

Expressed as millilitres per 100mL of blood –represents the extent to which oxygen is removed from the blood as it passes through the body.

The working muscles have extracted more 02 from the blood to produce energy aerobically.

O2 concentration arterial blood – 20 mL/100 mL

O2 concentration venous blood – 16 mL/100 mL

A-V02 = 4mL/100mL

REST

O2 concentration inarterial blood 20mL/100mL

EXERCISE

O2 concentration invenous blood 4mL/100mL

A-VO2 = 16mL/100mL

During exercise capillaries dilate to:- allow for increases in blood flow.

increase surface area to increase diffusion rates.

Muscular Responses – Increased Blood Flow

Muscular SystemACUTE RESPONSES TO EXERCISE

Motor Unit Recruitment

A motor unit – a motor neuron and all the muscle fibres it stimulates. During exercise:-

- the force developed in muscles increases.- the frequency of messages from the brain increases.

As the intensity needed to apply force increases, so does the number of motor units involved in the recruited.

The body recruits the lower threshold motor units first (slow-twitch – Type I), followed by the higher threshold motor units (fast-twitch Type IIA & IIB).

Energy Substrate Depletion

At the commencement and for the duration of exercise muscles use fuel stores to produce ATP.

Exercise cause a depletion of the following fuel stores:- ATP

Creatine phosphate

Glycogen

Triglycerides

The depletion of these energy stores contributes to fatigue.

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coge

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Time (mins)

Muscle Glycogen Depletion – Sub Max Exercise

Muscle Glycogen

Energy Substrate Depletion

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Gly

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Sprint Bout Number

Glycogen Depletion for Repeated Sprints

Slow Twitch Fibres

Fast Twitch Fibres

Fast twitch fibres are important contributors to force production during high intensity activities.

Lactate

Energy produced via the glycolitic pathway produces lactic acid. Lactic acid quickly disassociates to release hydrogen ions (H+) and form lactate.

During intense exercise muscle and blood lactate rises to high levels. At this point lactate is produced and removed by the body at equal rates.

Lactate Inflection Point (LIP) - reflects the last point where lactate entry into and removal from the blood are balanced. It is identified as the final exercise intensity or oxygen uptake value at which blood lactate concentration is relatively stable.

What happens to lactate? Lactate shuttle - lactate can move freely in

and out of muscle cells and into the blood stream. The heart, brain and slow twitch muscle fibres use lactate as a fuel –oxidation.

Lactate not oxidised can either be converted back to pyruvic acid, which then enters the Krebs cycle to produce energy or can be converted into glucose or glycogen which enters glycolysis to produce energy at a later time.

So lactate should be viewed as a useful form of potential energy that is oxidized during moderate-low intensity exercise, during recovery and at rest.

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4 5.5 7 8.5 10 11.8 13 14.3 16 17 17.8 19 20

Bloo

d La

ctat

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mol

/L)

Running Speed (km/h)

Lactate Levels at Varying Running Intensities

LIP

Blood lactate clearance = lactate production

At exercise intensities above lactate inflection point the rate of lactate clearance is lower than that produced – lactate accumulates in the blood.

Onset of Blood Lactate Accumulation (OBLA)

Body Temperature

As exercise commences heat is a by-product of the breakdown of ATP to energy.

As these reactions become more frequent heat is produced that in turn causes body temperature to rise.

The body controls core temperature through:- stimulating sweat glands in skin to produce

sweat – evaporation of sweat acts as a cooling mechanism.

increasing blood flow to the skin.

Revision Questions

1. Which acute response to a near maximum exercise bout does the graph represent?a. heart rateb. cardiac outputc. stroke volumed. respiratory rate

2. Arteriovenous O2 difference refers to the difference between the oxygen content of arterial blood and mixed venous blood. During exercise O2 concentration in arterial blood is measured at 20mL/100mL and in venous blood the measurement is 4mL/100mL. The arteriovenousdifference at this exercise intensity is:-a. 4mL/100mLb. 5mL/100mLc. 12mL/100mLd. 16mL/100mL

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Rest 2 4 6 8 10 12 14 16 18 20

mL

Time (mins)

Revision Questions

3. The graph indicates the distribution of blood flow at rest and during exercise.

a. Which line represents the percentage of cardiac output flowing to:-

• skeletal muscles

Answer: A

• organs of the body

Answer: B

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Rest 5 10 15 20 30 40 50 60 70 80 90 100Bl

ood

Flow

(% C

ardi

ac O

utpu

t)Exercise Intensity (% max)

Distribution of Blood Flow

Skeletal Muscles

Organs

B

AB

Revision Questions

b. Describe the process in the body which enables redistribution of blood to occur.Answer: redistribution of cardiac output is possible, due to vasoconstriction of arterioles supplying inactive areas of the body and vasodilation of arterioles supplying active muscles.

4. Oxygen uptake involves a number of acute responses which enables oxygen to move from the atmosphere to the skeletal muscle. Outline the role of the following in relation to oxygen uptake:-• Pulmonary ventilationAnswer: minute ventilation (TV x RR) increase to move more air - and thus 02 - in and out of the lungs.• MyoglobinAnswer: Facilitates diffusion of oxygen from blood to the mitochondria. Arterio-venous difference indicates amount of oxygen which has diffused.