Distinguish the three muscle types, locate the major skeletal muscles, and explain how slow and fast twitch fibres suit different events

What this dot point is asking

SEAB wants you to distinguish the three types of muscle, locate the major skeletal muscles on the body, and explain how the two main skeletal-muscle fibre types suit different kinds of event. The core idea is that muscle is the engine of movement, and its type and fibre make-up decide what kind of performance it produces.

The answer

The three types of muscle

The body has three muscle types, each with a distinct location and control.

• Skeletal muscle is attached to bones and is under conscious (voluntary) control. It produces all deliberate body movement and works quickly but tires.
• Cardiac muscle is found only in the heart wall and is involuntary. It contracts rhythmically and continuously for life without tiring.
• Smooth muscle lines internal organs and blood vessels and is involuntary. It produces slow, sustained contractions, such as moving food along the gut or narrowing blood vessels.

The major skeletal muscles

For analysis questions you must be able to locate the principal muscles. The main ones are:

• Biceps and triceps at the front and back of the upper arm.
• Deltoid over the shoulder.
• Pectorals across the chest and latissimus dorsi across the back.
• Abdominals at the front of the trunk.
• Quadriceps at the front of the thigh and hamstrings at the back.
• Gluteals of the buttocks and gastrocnemius of the calf.

Muscle fibre types

Skeletal muscle contains a mix of fibre types, and the proportion strongly influences which events a person is suited to.

• Slow twitch (type I) fibres contract slowly and with relatively low force but are highly resistant to fatigue. They have a rich blood supply and use aerobic energy. They dominate in endurance athletes such as marathon runners.
• Fast twitch (type II) fibres contract quickly and with high force but fatigue rapidly. They rely mainly on anaerobic energy. They dominate in power and speed athletes such as sprinters and weightlifters.

Most people have a roughly even mix; elite endurance and power athletes tend to have a higher proportion of the fibre type their sport demands.

Examples in context

Example 1. A weightlifter making a clean and jerk. The lift demands maximal force in a fraction of a second, so it relies on fast twitch fibres in the quadriceps, gluteals and deltoids. The high force and short duration are a textbook fast-twitch demand.

Example 2. A cyclist in a long road stage. Hours of steady pedalling demand fatigue resistance, so slow twitch fibres in the quadriceps and gastrocnemius do most of the work, fed by aerobic energy. This is why endurance cyclists are slow-twitch dominant.

Try this

• Cue. Make a table of the three muscle types with location, control and one role each. (Skeletal: bones, voluntary, movement; cardiac: heart, involuntary, pumping; smooth: organs/vessels, involuntary, sustained contraction.)

• Cue. Name four major skeletal muscles in the leg and one action of each. (Quadriceps: extend the knee; hamstrings: flex the knee; gluteals: extend the hip; gastrocnemius: plantar-flex the ankle.)

• Cue. Explain why a 100 m sprinter and a marathon runner have different dominant fibre types. (The sprint needs maximal short power, favouring fast twitch; the marathon needs sustained fatigue resistance, favouring slow twitch.)