Fiber Type Characteristics

SKELETAL MUSCLE FIBER TYPES & CHARACTERISTICS

Table only represents the three major divisions of fiber type, I, IIa, and IIx. There are several intermediate fiber divisions based on oxidative capacity.

	TYPE I	TYPE IIa	TYPE IIx (*IIb)
NAMES OF FIBERS	slow red	fast red	fast white
"	slow oxidative	fast oxidative-glycolytic "FOG"	fast glycolytic
"	oxidative	glycolytic	glycolytic
"	slow twitch	fast twitch	fast twitch
FIRING ORDER	#1	#2	#3
STIMULATION THRESHOLD	lowest	moderate	highest
FORCE PRODUCTION	lowest	moderate	highest
NERVE SPEED IMPULSE	slowest	moderate	highest
TENSION RATE	lowest	moderate	highest
FATIGABILITY	lowest	moderate	highest
MOTOR UNIT SIZE	smallest	moderate	largest
AEROBIC CAPACITY	highest	moderate	lowest
GLYCOLYTIC CAPACITY	lowest	moderate	highest
ACTIVITIES	endurance	hybrid	explosive
			*IIb - used in earlier literature

HUMAN PERFORMANCE & FIBER TYPE

Fiber type has been shown to play a significant role in elite human performances.
Fiber type specific to the predominate muscles used in an elite performer's activity (i.e., lower, upper body).
- Example: Extreme endurance event performers have been found to have 90 to 95% Type I fibers in calves.
Fiber type appears to be more extreme in elite endurance performers than sprint performers.
- Elite endurance performers, fiber composition in legs typically ranges from 85 to 95% Type I fibers.
- Elite sprint performers, fiber composition in legs typically ranges from 60 to 75% Type II fibers.

Summary
Much of this fiber composition is genetically endowed, but there is increasing evidence that a person's activity pattern may effect in significant ways fiber composition and function.

SKELETAL MUSCLE FIBER TYPES & CHARACTERISTICS

Table only represents the three major divisions of fiber type, I, IIa, and IIx. There are several intermediate fiber divisions based on oxidative capacity.

TYPE I

TYPE IIa

TYPE IIx (*IIb)

HUMAN PERFORMANCE & FIBER TYPE