AQA · A-Level · Physical Education
Course version: 7582 · Version from Sept 2016
Neuromuscular system: Fast glycolytic (type IIx).
Fast glycolytic (type IIx) motor units are built for brief, explosive effort.
Checked against the AQA 7582 specification (opens in a new tab) on .
- Point 4 of 16
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fast glycolytic (type IIx) motor units are built for brief, explosive effort. They contain a high proportion of fast-twitch fibres with very rapid contraction speed and high force output, but low resistance to fatigue because ATP is resynthesised mainly through the anaerobic glycolytic system and phosphocreatine breakdown, with limited oxidative capacity and fewer mitochondria and capillaries. A 100 m sprinter or a shot-putter relies on these units during the acceleration and drive phases. In multiple-choice questions you must distinguish type IIx from type IIa and type I by matching each option to contraction speed, force, fatigue resistance and the dominant energy system, then select the single best description of the type IIx profile.
What earns the marks
- Type IIx motor units produce the fastest contraction speed and the highest force output of the three fibre types.
- ATP is resynthesised predominantly anaerobically, through the phosphocreatine and glycolytic systems.
Common mistakes and how to put them right
Confusing type IIx with type IIa.
Put it right: Type IIa is fast oxidative glycolytic and has greater fatigue resistance, whereas type IIx is fast glycolytic and fatigues quickly.
Claiming type IIx fibres have high aerobic capacity.
Put it right: Their oxidative capacity is low, so they depend on anaerobic ATP resynthesis.
Assuming type IIx fibres are recruited first.
Put it right: Motor units are recruited in order of increasing size, so type I units are generally used before type II units at low forces.
Treating type IIx as the same as type IIb.
Put it right: In this specification the fast glycolytic fibre is named type IIx, so use that term.
Exam tips
- Read each option and check it against all four features: contraction speed, force, fatigue resistance and dominant energy system.
- Eliminate options that describe type I or type IIa characteristics before choosing the best match.
- If two options seem similar, look for the one that correctly links fast contraction with low fatigue resistance and anaerobic ATP resynthesis.
Try the lesson
The start of the Neuromuscular system: Fast glycolytic (type IIx). lesson, made from checked teaching notes for this specification point.
Have a go first
Which energy system provides the majority of the ATP for fast glycolytic (type IIx) muscle fibres during a high-intensity event lasting approximately 45 seconds, such as a 400 m sprint?
Listen
Fast glycolytic (type IIx) motor units are built for brief, explosive effort. They contain a high proportion of fast-twitch fibres with very rapid contraction speed and high force output, but low resistance to fatigue because ATP is resynthesised mainly through the anaerobic glycolytic system and phosphocreatine breakdown, with limited oxidative capacity and fewer mitochondria and capillaries. A 100 m sprinter or a shot-putter relies on these units during the acceleration and drive phases. In multiple-choice questions you must distinguish type IIx from type IIa and type I by matching each option to contraction speed, force, fatigue resistance and the dominant energy system, then select the single best description of the type IIx profile. A strong answer earns marks for these points. Type IIx motor units produce the fastest contraction speed and the highest force output of the three fibre types. ATP is resynthesised predominantly anaerobically, through the phosphocreatine and glycolytic systems. Exam tip: Read each option and check it against all four features: contraction speed, force, fatigue resistance and dominant energy system.
See it
Key words
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Fast glycolytic (type IIx)
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Motor unit recruitment
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See every definition
- Fast glycolytic (type IIx)
- Motor units built for brief, explosive effort, containing a high proportion of fast-twitch fibres with very rapid contraction speed and high force output but low resistance to fatigue.
- Motor unit recruitment
- Motor units are recruited in order of increasing size, so type I units are generally used before type II units at low forces.
Label the diagram
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1Box 1
2Box 2
3Box 3
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