: Question about rep ranges and muscle fiber activation. The usual claim is that high reps build slow twitch muscle fibers and low reps build fast ones. However I feel this is not quite right, I believe that to build slow

Vote Up Vote Down   0 Reactions

Login to give Reaction!

You're not increasing the number of Type I Fibers with endurance training.

Essentially your body becomes more efficient in various ways i.e (increasing the number of mitochondria, increasing O2 delivery to muscle, delaying lactic acid onset etc)

Muscle Fiber Types

Skeletal muscle fibers can be classified into three types:

Type I: Slow oxidative fibers
Type IIa: Fast oxidative fibers
Type IIx: Fast glycolytic fibers

Resistance Training: Type IIx --> IIa
Endurance Training: (Cardio or Low Weight High Rep) Adaptions Listed Below

Most muscles contain mixture of fiber types, resulting in a range of
contractile speed and fatigue resistance
All fibers in one motor unit are the same type
Genetics dictate individual’s percentage of each

Training Effects on Muscle Fiber Type

Aerobic (Endurance) Exercise
Such as jogging, swimming, biking leads to increased:

Muscle capillaries
Number of mitochondria
Myoglobin synthesis
Results in greater endurance, strength, and resistance to fatigue
May convert fast glycolytic fibers into fast oxidative fibers

Resistance Exercise (Anaerobic),
Such as weight lifting or isometric exercises, leads to increased

Mitochondria
Myofilaments
Glycogen stores
Connective tissue
Muscle strength and Size (hypertrophy)