HSS 500 Muscle Phenotype/Fiber Types

Is there a difference in the amount of force produced by the difference muscle fiber types?

NO — regardless of muscle fiber type, if muscles have the same cross-sectional area (CSA), they will have similar force values

Sarcoplasmic reticulum

A specialized membranous network within muscle fibers that stores, releases, & reuptakes calcium ions; plays a central role in the initiation and regulation of muscle contraction & relaxation

• Different muscle fibers/fiber types have differences in the properties of their sarcoplasmic reticulums

Regulatory proteins

Troponin & tropomyosin are regulatory proteins located on the thin (actin) filament

Tropomyosin

Regulatory protein that lies along the actin filament and, at rest, blocks myosin-binding sites on actin

Troponin

Regulatory protein complex composed of three subunits: troponin C (TnC), troponin I (TnI), and troponin T (TnT)

• When calcium is released from the sarcoplasmic reticulum, it binds to troponin C, causing a conformational change that shifts tropomyosin away from actin’s binding sites, thereby allowing cross-bridge formation and force production

Troponin C (TnC)

Subunit of troponin that binds calcium ions

Troponin I (TnI)

Subunit of troponin that inhibits actin-myosin interation

Troponin T

Subunit of troponin that anchors the troponin complex to the tropomyosin

Myosin heavy chain (MHC) isoforms

Form the motor domain of myosin (the “head”) and are considered as the molecular motor of skeletal muscle; used to classify muscle fiber type

• Determine the contractile speed & metabolic characteristics of the muscle fiber; they strongly influence shortening velocity, power output, & fatigue resistance

• The type of myosin found in slow and fast muscle fiber influences the speed related contractile properties of the muscle fiber; they DO NOT seem to influence the CSA or ability to generate force

Myosin light chain (MLC) isoforms

Smaller regulatory & essential proteins associated with the myosin head

Function of essential MLC isoforms

Modulate force production, calcium sensitivity, and cross-bridge kinetics (speed of contraction)

Function of regulatory MLC isoforms

Influence force & rate of force development

Crossbridge cycle

1. Myosin detaches from actin

2. Head of the MHC attaches to actin and released inorganic phosphate (P_i)

3. Head of the MHC changes position leading to the power stroke

4. Adenosine diphosphate (ADP) is released

5. ATP binds to the nucleotide-binding site for its hydrolysis

6. Head of the MHC returns to initial position

**10¹⁷ to 10¹⁸ crossbridges per gram of muscle per second during a contraction

Mitochondria

Membrane-bound organelles within cells that generate ATP through oxidative metabolism, playing a central role in aerobic energy production, metabolic regulation, and cellular adaptation

Myoglobin

Located in the sarcoplasm of muscle fibers, where it binds and stores oxygen and facilitates its diffusion from the sarcolemma to the mitochondria, supporting aerobic metabolism

Myonucleus

A nucleus located within a skeletal muscle fiber that regulates gene transcription and protein synthesis for a defined volume of cytoplasm, supporting muscle fiber growth, maintenance, and adaptation

Classification of muscle fibers (in humans)

The main nomenclature is linked to the MHC isoform composition of the fiber; in humans → low type I, fast type IIA, fast type IIX

• Note that other forms were found in muscles from specific anatomic regions: EO/type IIL (larynx), superfast type IIM (mastication), EO type (eye movements), & β- (equivalent to slow type I in skeletal muscles) and α-cardiac isoforms

Fiber type time to peak tension

• Slow type I: slow

• Fast type IIA: fast

• Fast type IIX: fastest

Fiber type half relaxation time

Half relaxation time: time until force reduced is ½ from peak

• Slow type I: slow

• Fast type IIA: fast

• Fast type IIX: fastest

Fiber type time constant for redevelopment of tension

• Slow type I: slow

• Fast type IIA: fast

• Fast type IIX: fastest

Fiber type maximal unloaded shortening velocity

• Slow type I: slow

• Fast type IIA: fast

• Fast type IIX: fastest

Fiber type maximal isometric tension

• Slow type I: SAME

• Fast type IIA: SAME

• Fast type IIX: SAME

Fiber type resistance to fatigue

• Slow type I: HIGH

• Fast type IIA: MODERATE

• Fast type IIX: VERY LOW

Fiber type cross-sectional area (CSA)

• Slow type I: SAME

• Fast type IIA: SAME

• Fast type IIX: SAME

Fiber type myofibrillar volume density

• Slow type I: SAME

• Fast type IIA: SAME

• Fast type IIX: SAME

Fiber type sarcoplasmic reticulum volume density

Type II fibers: have more sarcoplasmic reticulum density → superior release/reuptake of Ca+ → faster contraction

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: LARGEST

Fiber type mitochondrial volume density

Type I fibers: more mitochondrial density → more efficient ATP production → fatigue resistance

• Slow type I: LARGEST

• Fast type IIA: intermediate

• Fast type IIX: smallest

Fiber type triglyceride concentration

• Slow type I: highest

• Fast type IIA: intermediate

• Fast type IIX: lowest

Fiber type glycogen concentration

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: highest

Fiber type ATP concentration

• Slow type I: SAME

• Fast type IIA: SAME

• Fast type IIX: SAME

Fiber type creatine phosphate (CP) concentration

• Slow type I: lowest

• Fast type IIA: higher

• Fast type IIX: higher

Fiber type concentration of glycolytic enzymes (phosphofructokinase [PFK], triosephosphate dehydrogenase)

Type II fibers: equipped to undergo chemical reactions involved in anaerobic metabolism

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: highest

Fiber type concentration of glycogenolysis eynzme (phosphorylase)

Type II fibers: equipped to undergo chemical reactions involved in anaerobic metabolism

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: highest

Fiber type concentration of lactate metabolism enzyme (lactate dehydrogenase [LDH])

Type II fibers: equipped to undergo chemical reactions involved in anaerobic metabolism

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: highest

Fiber type concentration of Krebs cycle enzymes (succinate dehydrogenase [SDH], citrate synthase [CS])

Type I fibers: equipped to undergo chemical reactions involved in aerobic metabolism

• Slow type I: highest

• Fast type IIA: intermediate

• Fast type IIX: lowest

Fiber type capillary density

• Slow type I: highest

• Fast type IIA: intermediate

• Fast type IIX: lowest

Fiber type Krogh cylinder volume

Krogh cylinder volume: the volume of tissue supplied by a single capillary, defined geometrically as a cylinder whose radius extends halfway to the neighboring capillaries

• Slow type I: lowest

• Fast type IIA: intermediate

• Fast type IIX: highest

**Type I fibers: more capillaries & O₂ → lower Krogh cylinder volume

Fiber type myoglobin concentration

• Slow type I: highest

• Fast type IIA: intermediate

• Fast type IIX: lowest

Variation in muscle fiber composition

Muscles in the human body contain a mixture of muscle fibers (some muscles are predominantly composed of fast muscle fibers & some muscles are predominantly composed of slow muscle fibers)

• Genetics, blood levels of hormones, and exercise habits influence the distribution of muscle fibers across the different types

Muscle fiber type plasticity

Muscle fiber composition can adapt to changes in physiological states

• It was initially proposed that muscle fibers follow a transition scheme that can be described as: I ↔ IIA ↔ IIX ↔ IIB

• However, research has shown that MHC isoforms of human muscle fibers can transition via large changes (i.e. directly from I to IIx)

Polymorphism

The initial belief that muscle fibers express only one type of MHC isoform has been dismissed; there is an extensive proportion of so-called hybrid of polymorphic muscle fibers in human and non-human species.

• Muscle fibers can express anywhere from 1 to 3 MHC isoforms in humans

• Within the same muscle fibers, the properties of the machinery can vary between sarcomeres

Myonuclear plasticity and polymorphism

A typical skeletal muscle fiber contains from 100 to 200 myonuclei per mm of length; the gene expression programs vary along the length of the muscle fiber as evidenced by variations in the MHC mRNA isoform expression between myonuclei within the same muscle fiber

• Ultimately, the concept of fiber type might oversimplify the physiological reality