Slow-Twitch and Fast-Twitch Muscle Fibres

© 2015 Thompson Educational Publishing, Inc.

Certain muscles and muscle groups are

more adapted to one energy production

system than another. Exercise

physiologists find it useful to distinguish

different kinds of muscle fibres:

• Slow-twitch muscle fibres are red or

dark in colour, and generate and relax

tension relatively slowly. The trade-off is

that they are

able to maintain a lower level of tension for

long durations.

• Fast-twitch muscle fibres are more pale in

colour, have the ability to tense and relax

quickly, and can generate large amounts of

tension with relatively low endurance levels.

The Role of Myoglobin in Sustaining Energy-Producing Reactions

© 2015 Thompson Educational Publishing, Inc.

The differences in muscle fibre types are due mainly to the

extent to which a muscle relies on oxygen in the production of

energy.

•The protein myoglobin is the oxygen storage unit that

delivers oxygen to working muscles, thereby enabling

energy-producing reactions to be sustained over a

long time period.

•Slow-twitch, red muscle fibres are high in myoglobin

and ideal for endurance activities.

•Fast-twitch fibres (with low myoglobin

concentrations) are more adapted to shorter bursts

of effort.

Myoglobin

Comparison of Fibres

Muscle Fibre Types and Physical Activity

© 2015 Thompson Educational Publishing, Inc.

Slow-twitch muscle fibres

These are ideal for activities

such as long- distance

swimming, cycling, and

running.

Fast-twitch muscle fibres

These can activate at a rate two to

three times faster than slow-twitch

muscles, making them ideal for

the fast, powerful muscle

contractions needed for activities

such as short sprints,

powerlifting, and explosive

jumping.

Type I Fibres

[Slow-Oxidative (SO) Muscle Fibres]

© 2015 Thompson Educational Publishing, Inc.

Kinesiologists distinguish not just two but three different types of

muscle fibre, using a combination of tension-generating features and

the metabolic properties of the fibre.

Type I or slow-oxidative (SO) muscle fibres generate energy slowly,

are more fatigue- resistant, and primarily depend on aerobic

processes.

Type IIA Fibres

[Fast-Oxidative Glycolytic (FOG) Fibres]

© 2015 Thompson Educational Publishing, Inc.

Type IIA or fast-oxidative glycolytic (FOG) muscle fibres are intermediate-type muscle fibres.

• They allow for high-speed energy release as well

as glycolytic capacity.

Type IIB Fibres

[Fast-Glycolytic (FG) Muscle Fibres]

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Type IIB or fast-

glycolytic (FG)

muscle fibres

store lots of

oxygen and

sufficiently high

levels of enzymes

necessary for

quick contraction

without requiring

oxygen.

© 2015 Thompson Educational Publishing, Inc.

Feed Me!

Type 1: They rely on their legs to get

them around. The active muscles, such

as the legs and thighs, are full of blood

vessels. These blood vessels contain

myoglobin (or muscle hemoglobin), which

delivers oxygen to the muscles. The

more myoglobin the muscles contain, the

darker the muscle.

Type 2: white meat is the result of well-

rested muscles. The breast muscles,

which are used for flying, are hardly used

by turkeys. There is no need to have a

rich supply of oxygen delivered to these

muscles.

Determining your fibre type

1 RepMax

number of reps at 80% muscle fiber type

< 7 > 50% fast twitch (FT)

7 to 12 equal proportion of fiber type

> 12 > 50% slow twitch (ST)

MUSCLE BIOPSY

Implications for Training

For elite athletes, differences in muscle fibre types are often very pronounced. Olympic sprinters, for example, may possess as muchas 70-80 percent fast-twitch muscle fibres.Athletes in marathon-style events may possessthat equivalent in slow-twitch fibres.

• But many factors

contribute to an

individual’s athletic

performance.

• Physiology is just one

of them.

© 2015 Thompson Educational Publishing, Inc.

The Puzzle of East Africans’ Dominancein Distance Running

For some time now, athletes from Kenya and Ethiopia have dominated long-distance running.

• These countries

typically account for

medals in the 5000 m,

the 10 000 m, and the42.2 km marathon.

• The achievement of

these runners is one of

the most puzzling and

most studied topics in

sport science.

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Consistently Outstanding Resultsfor East African Men and Women

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On the men’s side, the world records in competitions ranging from the 800 m to the marathon show that 17 of 21 records are held by East Africans.

• On the women’s side, Ethiopian Triunesh Dibaba

won gold at the 2012 Olympics in the 10 000 m,

and two Kenyan women won silver and bronze.

• In the 5000 m event in London, Dibaba’s teammate

Mesaret Defar won gold, with Vivian Cheruiyot

placing second and Dibaba placing third. (Dibaba

had also won the 10 000 m race at the 2008

Olympic Games in Beijing.

Victory for Kenya & Ethiopia at the World Athletics Championships, 2011

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Possible Physiological andCultural Explanations

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• Some researchers theorize that these athletes’ bodies have a

higher capacity to take in and use oxygen—a capacity known as

“VO2 max.”

• Others argue that low overall body fat, long legs, and low resting

heart rates help these athletes run faster, longer, and more

efficiently.

• Children in Kenya and Ethiopia tend to walk and run

everywhere, thus building up a strong background in long-

distance running.

• Budding athletes in these countries often see running as a

means of supporting their families through prize money and

endorsements.