Muscle fibre types and factors determining muscular strength
Slow-twitch (Type I) fibres resist fatigue and suit endurance; fast-twitch (Type II) fibres give speed and power. Strength depends on muscle size, fibre type and nerve drive.
Types of muscle fibre
| Feature | Type I | Type IIa | Type IIx |
|---|---|---|---|
| Other name | Slow-twitch, slow oxidative | Fast-twitch, fast oxidative-glycolytic | Fast-twitch, fast glycolytic |
| Speed of contraction | Slow | Fast | Fastest |
| Colour and myoglobin | Red, high myoglobin | Red to pink, moderate | White, low |
| Mitochondria and capillaries | Many | Moderate | Few |
| Main energy system | Oxidative | Oxidative and glycolytic | ATP-PC and glycolytic |
| Force and power | Low | High | Highest |
| Fatigue | Very resistant | Fairly resistant | Tires quickly |
| Motor unit size | Small | Large | Largest |
| Typical events | Marathon, long-distance cycling, swimming | 800 m, 1500 m, hockey, football | 100 m sprint, shot put, javelin, weightlifting |
Fibre mix, sport and training
Most untrained people have a roughly even mix of slow and fast fibres in the thigh. Elite distance runners have a high share of Type I fibres; sprinters and jumpers have a high share of Type II. The mix is largely inherited and is measured by a muscle biopsy. Training cannot turn many fast fibres into slow ones, but both endurance and strength training shift Type IIx towards Type IIa, making fast fibres more fatigue-resistant. Fibres are recruited in order of size: Type I for easy work, then IIa, then IIx for maximal efforts.
Factors determining muscular strength
| Factor | Effect on strength |
|---|---|
| Cross-sectional area | A thicker muscle has more actin-myosin units side by side, so more force |
| Fibre type | More Type II fibres give more force and power |
| Nerve (neural) factors | More motor units recruited, faster firing and better timing raise force |
| Muscle length | Strongest near resting length, where actin-myosin overlap is best (length-tension) |
| Joint angle and leverage | Force at the hand or foot changes through the range of movement |
| Speed of movement | Slow concentric moves allow more force; eccentric force is highest (force-velocity) |
| Age | Peaks in the 20s and 30s; falls with age (sarcopenia) unless trained |
| Sex | Men have more muscle mass and testosterone, so greater absolute strength; force per unit of muscle is similar |
| Training | Resistance training increases both nerve drive and muscle size |
| Nutrition, rest and motivation | Enough energy and protein, sleep, a warm-up and motivation all raise force output |
Strength vs power
Muscular endurance is different again: repeating a movement many times without tiring.
A shot-putter with many fast fibres needs protein, enough carbohydrate and creatine stores for repeated power efforts, while a distance runner's slow fibres rely on glycogen and fat. Strength gains need resistance training plus protein within 1.2-2.0 g/kg (ACSM/AND/DC 2016), often towards the upper end while building muscle.
2 marks: “Name the types of skeletal muscle fibre.” Type I (slow oxidative), Type IIa (fast oxidative-glycolytic) and Type IIx (fast glycolytic; called IIb in older books).
5 marks: “Compare slow-twitch and fast-twitch muscle fibres.” Table of speed, colour, myoglobin, mitochondria, energy system, force, fatigue, motor unit size and sport examples.
10 marks: “Explain the types of muscle fibre and the factors determining muscular strength.” Fibre table, fibre mix and sport, effect of training, then ten factors of strength, strength vs power, and a nutrition note.
Slow = Stamina (Type I, red, marathon). Fast = Force (Type II, white, sprint).