Lecture 4 ExPhys

Fatigue, Muscle Soreness, and Muscle Cramps

Fatigue

  • Definition:
      - Fatigue is a complex phenomenon that varies in meaning among individuals.
      - In Exercise Physiology, it is defined as the general sensation of tiredness accompanied by decrements in muscular performance.
      - An alternative research-based definition quantifies fatigue as the inability to maintain the required power output to continue muscular work at a given intensity.
      - Fatigue is reversible by rest, distinguishing it from muscle weakness or damage.

Causes of Fatigue

  • Perceptions of fatigue vary among people; common responses may include:
      - Lactic acid accumulation
      - Dehydration
      - Low electrolyte levels
      - Overtraining

Complexity of Fatigue

  • Fatigue is a multifaceted phenomenon with causes that range from molecular changes to systemic responses involving the brain or the body.

  • Key causes related to fatigue fall under the following categories:
      - Energy systems
      - Accumulation of metabolic by-products
      - Alterations in neural control of muscle contraction
      - Failure of the muscle fiber’s contractile mechanisms

Types of Fatigue

  • Peripheral Fatigue
      - Involves changes happening at the muscle level such as decreased energy delivery, metabolic by-products accumulation, and failure of the muscle fiber contractile mechanism.

  • Central Fatigue
      - Involves changes in neural control, influenced by factors such as exercise intensity, muscle fiber type, training status, age, diet, and environmental conditions.

Factors Influencing Fatigue

  • Factors that may act in synergy to cause fatigue include:
      - Exercise intensity and glycogen depletion
      - Hydration and electrolyte levels
      - Acclimatization to exercise conditions
      - Nutritional intake

Energy Systems and Fatigue

Phosphocreatine (PCr) Depletion

  • Purpose of PCr:
      - Acts as an energy reservoir to rapidly regenerate ATP during short-term high-intensity efforts.
      - Can only sustain activity for a few seconds.

  • Association with Fatigue:
      - Fatigued muscles show marked PCr depletion.
      - PCr depletes more rapidly than ATP indicating exhaustion is often due to PCr depletion rather than ATP depletion.
      - In prolonged efforts, both ATP and PCr can become depleted.

Glycogen Depletion

  • Muscle glycogen is vital for maintaining ATP concentrations during extended exercise bouts.

  • During exercise lasting more than a few seconds, glycogen becomes the primary fuel.
      - Glycogen stores are limited and deplete quickly.

  • Correlation:
      - Studies suggest a correlation between glycogen depletion and fatigue, especially noted by the phenomenon of “hitting the wall”.

  • Fatigue arises from depletion of glycogen, rather than the rate of its usage.

Muscle Fiber Type and Glycogen Depletion

  • Muscle fibers deplete glycogen in specific patterns influenced by exercise intensity.
      - Type I fibers: Recruited first during light exercise.
      - Type II fibers: Recruited as intensity increases.
      - Fatigue Resistance: Differences in fatigue resistance between fiber types must be acknowledged to understand performance and endurance.

Energy Supply During Prolonged Exercise

  • Muscle glycogen alone cannot supply energy for long-duration activities.

  • Blood glucose, derived from liver glycogen stores, plays a significant role during late stages of endurance exercise.

  • As exercise continues, the liver must ramp up glycogen breakdown to maintain blood glucose supply, but is ultimately limited.

  • Once liver stores are exhausted, muscles rely on their own glycogen reserves, which average approximately 500 g.

Mechanisms of Fatigue Related to Glycogen Depletion

  • Glycogen depletion contributes indirectly to fatigue, potentially through reduced ATP production rates and impaired excitation-contraction coupling due to low glycogen.

Metabolic By-Products and Fatigue

  • Inorganic Phosphate:
      - Accumulates as a by-product of ATP and PCr breakdown and impairs contractile function, impacts calcium release from the sarcoplasmic reticulum (SR), and inhibits ATP breakdown via negative feedback.

  • Lactic Acid:
      - Lactic acid, produced during anaerobic glycolysis, has historically been blamed for fatigue, but recent understanding indicates it is recycled for energy and does not directly cause fatigue.
      - Its associated hydrogen ions lead to muscle acidosis and decreased pH, contributing to sensation and performance reductions.

Neuromuscular Fatigue

Neural Transmission

  • Fatigue at the neuromuscular junction (NMJ) can prevent effective nerve impulse transmission to muscle fibers.

  • Possible effects include:
      - Reduced acetylcholine release
      - Failure of action potentials (AP) reaching the muscle
      - Accumulation of acetylcholine leading to inhibition of muscle relaxation
      - Potassium pooling inhibiting action potential generation

Central Nervous System (CNS) Role

  • Muscle recruitment is partially controlled by the CNS which can limit exertion to prevent injury, following the central governor theory.

Psychobiological Aspects of Fatigue

  • The perception of exertion plays a critical role in endurance performance.
      - Highly motivated athletes may experience varying levels of perceived effort based on individual tolerance and motivation.

  • Techniques that lower the perceived effort can help delay fatigue in endurance events.

Heat and Muscle Temperature

  • Energy expenditure generates heat, increasing core temperature during exercise.

  • Exercising in heat not only poses additional external heat stress but also accelerates carbohydrate utilization, leading to quicker glycogen depletion.

  • High muscle temperature may impair metabolism and function, particularly in hot, humid environments.

Critical Power

  • Definition:
      - Critical Power represents the maximal intensity maintainable without fatigue limiting performance over a prolonged period.

  • It indicates the highest metabolic rate sustained by oxidative metabolism and is related to lactate thresholds.

  • Training Effects:
      - Increased through endurance and high-intensity training; decreases with aging or disease.

Muscle Soreness

Types of Muscle Soreness

  • Acute Muscle Soreness:
      - Experienced during or immediately after exercise, characterized by stiffness or tenderness.
      - Usually resolves within hours, associated with H+ accumulation.

  • Delayed Onset Muscle Soreness (DOMS):
      - Occurs one to two days post-exercise, commonly after eccentric training or a return to training after a break.

Exercise-Associated Muscle Cramps (EAMCs)

  • Defined as involuntary skeletal muscle contractions that can cause significant pain.

  • Commonly reported during or after exercise, lasting up to three minutes.

Theories on EAMCs

  1. Fluid & Electrolyte Theory:
      - Involves sodium, potassium, chloride, magnesium, calcium balance, and the impact of dehydration or overhydration.

  • Strenuous physical effort leads to significant loss of electrolytes and water, affecting muscle function.

  1. Altered Neuromuscular Theory:
      - Factors such as muscle fatigue, energy depletion, intensity, heat, and lack of conditioning contribute to cramps.

Fluid & Electrolyte Changes

  • Excess fluid loss affects plasma volume, which may alter motor neuron mechanics, contributing to cramps.

Altered Neuromuscular Control

  • Relevant mechanisms involve Golgi tendon organs, muscle spindles, and the dynamics of motor neuron activity affecting muscle contraction and fatigue.

Conclusion

  • Understanding fatigue, muscle soreness, and muscle cramps is crucial for improving athletic performance, injury prevention, and recovery strategies. An incomplete understanding of the pathophysiology complicates effective prevention and treatment.