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Physiology

42 topics

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Physiology42 topics

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On this page:Concept Energy System Muscle Action Adaptation Reference

Duration

◆ Concept

25 topics

Active TensionThe tension developed by a muscle's contractile elements — through crossbridge formation and the movement of actin and myosin.

ATP — The Energy MoleculeAdenosine triphosphate is the cell's immediate energy currency; splitting off a phosphate releases energy that crossbridges use to pull actin across myosin.

Bioenergetics — Anaerobic vs. Aerobic Energy ProductionThe umbrella concept for where muscle gets its energy — anaerobic (ATP-PC and glycolytic, no oxygen) versus aerobic (with oxygen).

Three Hypotheses of Muscle SorenessSoreness is explained three ways — torn/connective-tissue damage, metabolic accumulation with an ischemia-pain-spasm cycle (de Vries), and localized spasm of motor units — and they can occur together.

How Exercise Improves Metabolic Health — GLUT4 and Insulin SensitivityA contracting muscle pulls glucose out of the bloodstream through a

Length-Tension RelationshipA muscle develops maximal tension at about 1.2× its resting length, where actin and myosin overlap to form the most crossbridges; tension falls off if it is shorter or longer.

Maximal Muscular Contraction (Overloading the Muscle)Voluntary maximal contractions — like the last rep of a set to failure — are the most effective stimulus for strength, and the basis of "overloading" the muscle.

The Mind-Muscle Connection — Why You Can't Always "Feel" a Muscle Working"Feeling" a muscle work is really a question of which motor units your

Motor Unit / Muscle Fiber RecruitmentForce is set by how many fibers are activated and how fast the motor nerves fire — and slow, controlled lifting recruits more fibers than fast lifting.

Roles of Muscles in Movement (Agonist, Antagonist, Synergist, Fixator)The same muscle plays different roles depending on the task — mover, assister, opposer, neutralizer, or stabilizer — coordinated by reciprocal innervation.

"Muscle Action" vs. "Contraction" — The Terminology ArgumentBecause a muscle can shorten, hold, or lengthen, calling every state a "contraction" is misleading — so the term "muscle action" is proposed instead.

Nature and Function of Muscular Contraction (All-or-None Law, Summation)A muscle shortens by shortening its fibers; each fiber fires completely or not at all, and force is graded by how many fibers fire and how their contractions sum.

Muscle Fiber Arrangements (Penniform vs. Fusiform)Feather-like (penniform) muscles pack more fibers across a bigger cross-section and produce more force; long parallel (fusiform) muscles favor speed but produce less force.

Muscle Fiber Distribution and TrainabilityThe average person is ~52% Type I, 33% Type IIa, 13% Type IIb; the split is genetically fixed, but endurance training can convert IIb fibers toward the oxidative IIa type.

Muscle Fiber Types (Type I, IIa, IIb)Slow Type I fibers resist fatigue but make little force; fast Type IIb make great force but tire quickly; Type IIa are a fast, fatigue-resistant middle ground.

Muscle Soreness — Acute vs. DOMSSoreness comes in two forms — acute pain felt during/right after exercise, and delayed-onset (DOMS) appearing 12–48 hours later.

Muscle ToneMuscle tone is muscle firmness.

Muscular Power (Physiological Definition)The explosive aspect of strength — the product of strength and speed of movement.

Muscular Strength (Physiological Definition)The maximum force a muscle or muscle group can generate.

Recovery and Rebuilding PhysiologyAfter stress, muscle needs time to rebuild to a slightly higher strength level — too little rest causes chronic fatigue and strength loss, too much lets gains slip back.

Sex Differences in Hypertrophy ("Lifting Heavy Makes Women Bulky" Myth)Lifting heavy weights does not make women "bulky" by default — testosterone is the primary driver of rapid, large-scale muscle growth, women have roughly a tenth of men's circulating testosterone, and the visibly muscular physiques people picture take years of dedicated hypertrophy training (often with a caloric surplus, sometimes with exogenous hormones) to build.

Structure of Skeletal MuscleA skeletal muscle is bundles within bundles — fibers (cells) grouped into fascicles, each fiber packed with myofibrils made of actin and myosin arranged into sarcomeres.

Substrate Utilization by Exercise Intensity (the Crossover Concept)As relative exercise intensity rises, the body's dominant fuel shifts from fat toward carbohydrate — a well-established pattern called the "crossover concept" — which is why interval training, by alternating low and high segments, draws on both fuel systems across a single session.

Three Basic Types of Muscle (Smooth, Cardiac, Skeletal)The body has smooth, cardiac, and skeletal muscle; only skeletal muscle is under conscious control and so can be deliberately trained.

Work (Physiological Definition)Work done against resistance equals distance times force.

◆ Energy System

6 topics

Oxygen (Aerobic) Energy SystemThe slow-but-unlimited pathway that uses oxygen to burn carbohydrate, fat (and a little protein) for ATP — the primary fuel for endurance and the engine that restores the anaerobic systems.

Replenishing the ATP-PC Source (Alactacid Half-Life Curve)After intense effort the aerobic system rebuilds depleted ATP-PC on a half-life curve — about 50% back in 20–48 s and essentially full in 3–4 minutes.

ATP-PC (Phosphagen) SystemThe body's immediate, stored anaerobic energy source — ATP and phosphocreatine ready to fire instantly, fueling short, explosive efforts of roughly 0–45 seconds.

Replenishing the Lactic Acid Source (Lactacid Oxygen Debt)Accumulated lactate is mostly metabolized aerobically by many tissues; it clears on a slow half-life (~50% in 25 min, ~95% in 1 h 15 min) — the lactacid portion of oxygen debt.

Lactic Acid (Glycolytic) SystemAn anaerobic pathway that breaks glycogen/glucose down to lactic acid for a net 2 ATP, dominant in all-out efforts of about 1–3 minutes — at the cost of fatiguing acidity.

Phosphocreatine (PC) Breakdown and ATP ResynthesisPC splits into creatine + inorganic phosphate, and that released energy recombines ADP and Pi back into ATP — the coupled reaction at the heart of the phosphagen system.

◆ Muscle Action

4 topics

Concentric (Positive) Muscle ActionThe lifting phase — a dynamic action where the whole muscle shortens, its ends draw closer together, and joint movement occurs.

Eccentric (Negative) Muscle ActionThe lowering phase — a dynamic action where an external force overcomes the muscle and forcibly lengthens it; the strongest action and the chief cause of DOMS.

Isometric (Static) Muscle ActionA static action where the muscle develops force but its ends are prevented from drawing closer together — no change in length, no joint movement.

Positive and Negative Resistance"Positive" is the concentric (shortening) phase and "negative" is the eccentric (lengthening) phase — the lifter's-eye names for the two dynamic actions.

◆ Adaptation

4 topics

Muscle AtrophyThe wasting away or shrinkage of muscle tissue from lack of use or immobilization.

Muscle HyperplasiaThe theorized increase in the *number* of muscle fibers from resistance training — unproven in humans, but if real may account for only 5–10% of size gains.

Muscle HypertrophyGrowth in muscle size from the enlargement of existing fibers — more and larger actin and myosin filaments plus added sarcomeres — driven by long-term resistance training.

Muscle MemoryRetraining after a layoff is widely believed to be faster because muscle fibers permanently keep the extra nuclei gained from prior training — but the best current human evidence says that's not what's actually happening.

◆ Reference

3 topics

Energy Metabolism Summary and ATP YieldsA one-glance outline of the three systems and their ATP yields — stored ATP-PC, net 2 ATP from the lactic acid system, and net 36 ATP from the aerobic system.

General Characteristics of the Energy Systems (Comparison Table)A side-by-side comparison of the ATP-CP, lactic acid, and oxygen systems across oxygen need, speed, fuel, capacity, endurance, and power.

Classification of Exercise and Muscle Action Types (Dynamic vs. Static)Exercise is either dynamic (concentric = shortens, eccentric = lengthens) or static (isometric = no change), mapped to what happens to muscle length.