Skeletal Muscle Physiology: Fatigue, Fiber Types, and Tension Development

Vocabulary flashcards covering skeletal muscle fatigue mechanisms, muscle fiber types, length-tension relationships, and summation and tetanus.

Muscle Fatigue

A reversible decrease in muscle tension resulting from previous contractile activity that recovers after a period of rest.

Central Fatigue

A psychological feeling of tiredness and a desire to cease activity originating in the central nervous system that precedes physiological cellular fatigue.

Peripheral Fatigue

Muscle fatigue occurring distal to the central nervous system, involving failure of neuromuscular transmission, excitation-contraction coupling, or cross-bridge cycling within the muscle fiber.

$\text{K}^+$ Accumulation in T-Tubules

A peripheral fatigue mechanism where repetitive action potentials cause potassium ions to accumulate in the extracellular lumen of the transverse tubules, altering the muscle fiber membrane potential and action potential conduction threshold.

Accumulation Theories of Fatigue

Proposals that peripheral fatigue is driven by the intracellular buildup of inorganic phosphate ($\text{P}_i$), $\text{ADP}$, and $\text{H}^+$, which interfere with calcium handling and cross-bridge cycling.

Sarcoplasmic Reticulum $\text{Ca}^{2+}$ Impairment

The reduced release, reuptake, and storage of calcium by the sarcoplasmic reticulum during fatigue, caused by inhibited SERCA and RyR function or precipitation of calcium phosphate.

Troponin C Desensitization

A decrease in the calcium sensitivity of troponin C caused by metabolic changes during fatigue, resulting in reduced thin filament activation and diminished cross-bridge cycling.

Myosin ATPase Activity

The enzymatic activity within the myosin head responsible for hydrolyzing ATP, which varies by myosin heavy chain isoform and determines the shortening velocity of a muscle fiber.

Type I Muscle Fibers

Slow-twitch oxidative red muscle fibers that possess slow myosin ATPase activity, a small diameter, high capillary density, abundant myoglobin, numerous mitochondria, and high fatigue resistance suited for posture.

Type IIA Muscle Fibers

Fast-twitch oxidative-glycolytic red muscle fibers featuring fast myosin ATPase activity, intermediate speed of tension development, medium diameter, and high sarcoplasmic reticulum $\text{Ca}^{2+}$-ATPase activity suited for standing and walking.

Type IIX Muscle Fibers

Fast-twitch glycolytic white fibers characterized by fast myosin ATPase activity, large diameter, few mitochondria, low capillary density, and low fatigue resistance suited for quick, forceful movements like jumping.

Myoglobin

An oxygen-binding, iron-rich protein abundant in oxidative muscle fibers that facilitates oxygen diffusion from capillaries and gives red muscle its dark color.

Length-Tension Relationship

The physiological property stating that muscle tension generation is directly proportional to the number of cross-bridges formed, determined by the initial overlap of thick and thin filaments in the sarcomere.

Optimal Resting Length ($l_0$)

The resting sarcomere length ($2.0\text{--}2.3\,\mu\text{m}$) providing maximal overlap between actin and myosin cross-bridges to generate $100\%$ of maximum active tension.

Muscle Twitch

The mechanical response of a single muscle fiber to a single action potential, characterized by a rapid contraction phase and relaxation phase lasting up to $100\,\text{ms}$.

Twitch Summation

The additive increase in mechanical tension observed when consecutive action potentials stimulate a muscle fiber before it has fully relaxed from the previous twitch.

Mechanism of Summation

The process whereby consecutive action potentials release successive pulses of $\text{Ca}^{2+}$ from the SR, keeping troponin complexes activated longer and preventing tropomyosin from covering actin binding sites.

Unfused Tetanus

A sustained, oscillating contractile state produced at intermediate stimulation frequencies (e.g., $25\,\text{Hz}$) where muscle fibers undergo partial relaxation between stimuli.

Fused Tetanus

A smooth, maximal sustained muscle contraction achieved at high stimulation frequencies (e.g., $50\,\text{Hz}$) where the interval between stimuli is too short to allow any relaxation.

Motor Unit Recruitment

An increase in whole-muscle force development accomplished by activating additional motor units and their associated muscle cells.

Muscle Fatigue

A reversible decrease in muscle tension resulting from previous contractile activity that resolves with rest.

Central Fatigue

A protective sensation of tiredness and a desire to cease physical activity that originates in the central nervous system ($\text{CNS}$) and precedes physiological cellular fatigue.

Peripheral Fatigue

Fatigue resulting from structural or biochemical failure within the motor unit outside the $\text{CNS}$, localized to the neuromuscular junction, excitation-contraction coupling, $Ca^{2+}$ signaling, or the cross-bridge cycle.

T-Tubule Conduction Failure

A mechanism of excitation-contraction coupling failure caused by the accumulation of $K^+$ in the extracellular lumen of the transverse tubules during repetitive action potential firing, which alters the membrane potential and threshold for propagation.

Accumulation Theories of Fatigue

Mechanisms proposing that intracellular buildup of metabolic byproducts—including inorganic phosphate ($P_i$), $ADP$, and $H^+$—directly slows cross-bridge power strokes, reduces sarcoplasmic reticulum $Ca^{2+}$ release and reuptake, and blunts troponin C sensitivity.

Depletion Theories of Fatigue

Mechanisms suggesting fatigue arises from the consumption of key energy substrates, such as phosphocreatine ($PCr$), $ATP$, or glycogen stores, although substrate depletion is not typically the primary limiting factor under normal conditions.

Type I Skeletal Muscle Fibres

Slow-twitch oxidative (red muscle) fibers containing slow myosin ATPase, abundant mitochondria, dense capillaries, and high myoglobin; they have the slowest speed of tension development, long contraction duration, and high fatigue resistance suited for posture.

Type IIA Skeletal Muscle Fibres

Fast-twitch oxidative-glycolytic (red muscle) fibers characterized by fast myosin ATPase, intermediate tension development speed, moderate mitochondria, medium diameter, and high fatigue resistance; used primarily in walking and standing.

Type IIX Skeletal Muscle Fibres

Fast-twitch glycolytic (white muscle) fibers exhibiting fast myosin ATPase activity, large diameter, few mitochondria, and low capillary density; they develop tension fastest, fatigue easily, and are used for jumping and rapid, fine movements.

Myosin ATPase Activity

The enzymatic rate of $ATP$ hydrolysis by different myosin heavy chain isoforms that dictates the maximal shortening velocity of a muscle fiber.

Length-Tension Relationship

The physiological property stating that isometric active tension is directly proportional to the number of cross-bridges formed, which is dictated by the degree of actin and myosin filament overlap prior to contraction.

Optimal Resting Length ($l_0$)

The resting sarcomere length range (between $2.0\text{ and }2.3\,\mu\text{m}$) at which filament overlap maximizes the total number of actin-myosin binding interactions, generating $100\%$ of maximal tension.

Muscle Twitch

The mechanical contractile response of a single muscle fiber to a single action potential, lasting up to approximately $100\,\text{ms}$, while the driving action potential lasts only $1\text{ to }3\,\text{ms}$.

Twitch Summation

The additive increase in mechanical tension observed when consecutive action potentials stimulate a muscle fiber before it has fully relaxed, prolonging elevated intracellular $Ca^{2+}$ and cross-bridge cycling.

Unfused Tetanus

A sustained contractile state produced by intermediate stimulation frequencies (e.g., $25\,\text{Hz}$) where the fiber develops an elevated, oscillatory plateau of tension with partial relaxation between stimuli.

Fused Tetanus

A steady, maximal muscle contraction produced when high-frequency stimulation (e.g., $50\,\text{Hz}$) prevents any relaxation between stimuli, achieving peak tension.

Motor Unit

A single somatic motor neuron and all individual muscle fibers it innervates, serving as the fundamental unit of whole-muscle recruitment and graded force generation.