Muscle Fiber Types Explained: Type 1, Type 2a, Type 2x | Muscle Physiology and Training Adaptations

Muscle Fiber Types Explained: Type 1 vs. Type 2a vs. Type 2x

Type 1 -Slow Twitch (slow twitch speed/ slow rate of force development)
Small, red
Recruited first in response to action potential
Slow nerve conduction
HIGHLY fatigue resistant (oxidative) good for endurance – high aerobic enzyme content
Low Myosin ATPase (low contraction speed)
Low power
HIGH capillary & mitochondrial density
Cross country, marathon

Type IIa
Medium, white/red
Larger motor units
Fast contract/relax speed and nerve conduction velocity, intermediate power
More anaerobic enzyme content
Intermediate capillary and mitochondrial density
Low myoglobin

Type IIx
Largest motor units
Largest diameter fiber, white
High recruitment threshold
Fast conduction and contraction
Low fatigue resistance/ endurance
High force and power
Low capillary / mitochondrial density
Large fibers
Sprint, weightlifting

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0:00
0:06 Intro
0:29 Muscle Structure
1:05 Motor Units and Mosaic Distribution
2:15 Size Principle
3:30 Type I Fibers
5:59 Type IIa Fibers
8:52 Type IIx Fibers
10:46 Muscular Adaptations
12:00 Detraining
13:20 Muscle Fiber Hypertrophy
14:20 Myosin Isoform

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