Factors Affecting Performance and Muscle Fatigue

Vocabulary flashcards covering the physiological mechanisms of fatigue, factors influencing athletic performance across different event durations, and muscle recruitment concepts from the lecture.

Muscle Fatigue

The decline in force or power output during repeated muscle contractions, where power is calculated as force multiplied by shortening velocity.

Sites of Fatigue Model

A physiological framework mapping candidate locations of fatigue from central motor recruitment in the brain down through peripheral excitation-contraction coupling to actin-myosin cross-bridge interaction.

Central Fatigue

Fatigue driven by the central nervous system, characterized by reductions in motor unit recruitment and motor unit firing frequency.

Brachioradialis Motor Unit Firing Rate Reduction

During sustained isometric contractions at $25\text{--}30\text{\%}$ MVC, motor unit firing rates decrease by $20\%$ at the midpoint of the contraction and by $30\%$ at task failure (Calder et al., 2008).

Peripheral Fatigue: Neural Factors

Fatigue mechanisms occurring downstream of the central nervous system, including inability of the sarcolemma Na+/K+ pump to maintain action potential frequency or an action potential block in the T-tubules.

Peripheral Fatigue: Mechanical Factors

Impairments in cross-bridge cycling and tension development, often caused by high $\text{H}^+$ concentration inhibiting $\text{Ca}^{2+}$ release from the SR, impairing $\text{Ca}^{2+}$ binding to troponin, or slowing $\text{Ca}^{2+}$ re-uptake.

Peripheral Fatigue: Energetics of Contraction

Reductions in maximal force caused by high rates of ATP hydrolysis accumulating metabolites such as $\text{ADP}$, $\text{P}_i$, and $\text{H}^+$, where $\text{P}_i$ inhibits cross-bridge binding to actin (Nelson & Fitts, 2014).

Free Radicals

Highly reactive molecules containing unpaired electrons in their outer orbital produced during exercise; they damage proteins (myosin and troponin), limit strong cross-bridge binding, and disrupt potassium homeostasis (Reid, 2001).

Cellular Redox State vs. Isometric Force Curve

An inverted U-shaped relationship showing that optimal levels of reactive oxygen species maximize force output, whereas excessively reduced or oxidized states impair muscle performance.

Order of Muscle Fiber Type Recruitment

The progressive recruitment of muscle fiber types with increasing exercise intensity: Type I fibers up to $\sim 40\%\,\dot{V}\text{O}_2\text{max}$, Type IIa fibers at $\sim 40\text{--}75\%\,\dot{V}\text{O}_2\text{max}$, and Type IIx fibers above $75\%\,\dot{V}\text{O}_2\text{max}$.

Ultra Short-Term Performances

Athletic events lasting less than $10\text{ seconds}$, limited primarily by Type II muscle fiber recruitment, motivation, skill, and phosphocreatine (PC) / anaerobic glycolysis energy systems.

Short-Term Performances

Athletic events lasting $10\text{--}180\text{ seconds}$, shifting from $70\%$ anaerobic at $10\text{ s}$ to $60\%$ aerobic at $180\text{ s}$, where high $\text{H}^+$ accumulation limits performance and ingestion of buffers like bicarbonate ($\text{HCO}_3^-$) can improve performance.

Bicarbonate Buffering System

The chemical equilibrium $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+$ catalyzed by carbonic anhydrase to buffer excess hydrogen ions generated during high-intensity exercise.

Moderate-Length Performances

Aerobic events lasting $3\text{--}20\text{ minutes}$, requiring energy expenditure near maximal oxygen uptake ($\dot{V}\text{O}_2\text{max}$) with aerobic ATP contribution rising from $60\%$ at $3\text{ min}$ to $90\%$ at $20\text{ min}$.

Intermediate-Length Performances

Aerobic events lasting $21\text{--}60\text{ minutes}$, typically run at less than $90\%\,\dot{V}\text{O}_2\text{max}$, where running economy, lactate threshold, and percentage of Type I muscle fibers are key determinants.

Sub 2-Hour Marathon Requirement

The physiological requirement for an elite distance runner to sustain a $\dot{V}\text{O}_2$ of $67\,\text{ml}\cdot\text{kg}^{-1}\cdot\text{min}^{-1}$ over the marathon distance (Jones et al., 2021).

Long-Term Performances

Aerobic events lasting $1\text{--}4\text{ hours}$, where liver and muscle glycogen stores decline, making carbohydrate intake during exercise and hydration critical for performance maintenance.

Hyponatremia

A condition of abnormally low blood sodium levels affecting approximately $4\%$ of ultra-endurance athletes, caused by electrolyte loss from dehydration/vomiting or severe sodium dilution from excessive fluid intake.