IB Sports, exercise and health science A.3.3.2 Hydration, Electrolytes, and Energy Availability in Fatigue Notes

Hydration and Fatigue

1. Water makes up approximately 65 to 75% of muscle mass and around 50% of fat mass, and plays a crucial role in thermoregulation, circulation, and muscle function.
2. Maintaining proper hydration levels is essential for optimal muscle performance, as water is involved in:
   1. Transporting nutrients and oxygen to muscles
   2. Removing metabolic waste such as carbon dioxide and lactate
   3. Regulating body temperature during exercise
   4. Maintaining blood volume, which ensures adequate oxygen delivery

Effects of Dehydration on Athletic Performance

Dehydration-Induced Fatigue Cascade

1. Sweat rate rises during exercise, and because sweat is drawn from blood plasma, plasma volume falls.
2. The reduced plasma volume returns less blood to the heart, so cardiac filling pressure falls and each chamber fills with less blood per beat.
3. Reduced filling pressure reduces stroke volume and, with it, cardiac output.
4. To preserve oxygen delivery to working muscle, heart rate rises at constant workload, a compensatory shift known as cardiovascular drift.
5. The same external workload now requires a higher relative cardiovascular cost, so sustainable intensity falls.
6. Reduced plasma volume creates competition for blood flow between muscle and skin, so skin blood flow drops, evaporative heat loss falls, and core temperature rises toward hyperthermia.

Hyperthermia and Performance

1. Risk rises with high ambient temperature, humidity, insufficient air flow, excessive clothing, and lack of shade.
2. Heat moves from the body to the environment by radiation, convection, conduction, and evaporation.
3. In humid air, evaporation is the bottleneck because sweat cannot vaporise against a high water-vapour pressure, so the dominant route for heat loss during heavy exercise shuts down.
4. Working muscle and skin compete for cardiac output, and in heat the heart may fail to supply both, forcing a reduction in intensity.
5. Hyperthermia does not directly impair force production at the muscle in the temperature range reached during normal sport, so reductions in voluntary force come substantially from central nervous system factors.
6. During exercise in heat, brain blood flow falls and electrical activity declines while perceived exertion rises, consistent with a central, brain-temperature-mediated model of heat fatigue.

How Electrolytes Contribute to Muscle Function and Prevent Fatigue

Sodium and Potassium in Membrane Excitation

1. Sodium (Na⁺) sits predominantly in blood plasma and the wider extracellular fluid, while potassium (K⁺) is concentrated inside cells, especially in the nervous system, skeletal muscle, and bone.
2. Water moves across membranes by osmosis, so a change in Na⁺ or K⁺ concentration on either side of a membrane shifts cell volume and fluid distribution almost immediately.
3. At rest the inside of a muscle or nerve cell is slightly negatively charged relative to the outside, and this resting potential depends on the relative concentrations of Na⁺ and K⁺.
4. When the membrane is stimulated, Na⁺ channels open and sodium rushes into the cell, briefly making the interior positive and creating an action potential.
5. K⁺ channels then open and potassium leaves the cell, restoring the negative interior in milliseconds.

Regulation of Fluid Balance

1. Sodium (Na⁺) plays a primary role in maintaining extracellular fluid balance.
2. An imbalance in sodium levels leads to fluid shifts, impairing muscle function and increasing the risk of cramping and fatigue.
3. Potassium (K⁺) is vital for maintaining intracellular fluid balance and regulating muscle contraction.
4. An imbalance can cause muscle weakness or cramping.

Muscle Contraction and Action Potentials

1. Calcium (Ca²⁺) is critical for the contraction process, allowing actin and myosin fibres to interact and generate force.
2. Insufficient calcium reduces muscle contraction efficiency, contributing to fatigue.
3. Magnesium (Mg²⁺) is a cofactor for ATPase activity in myosin and the Na⁺/K⁺ pump, and supports many enzyme reactions in energy production.

Prevention of Fatigue

1. Proper electrolyte levels help maintain the electrical gradient across cell membranes, ensuring proper nerve signalling and muscle contraction.
2. Without these electrolytes, the neuromuscular system becomes less efficient, leading to early fatigue.

Hyponatremia

1. Hyponatremia is an abnormally low plasma sodium concentration, and it is the most relevant electrolyte disturbance in endurance sport.
2. The single largest cause in athletes is excessive intake of hypotonic fluids, especially plain water consumed in large volumes during long events, which dilutes plasma Na⁺ even when total body Na⁺ has not changed.
3. The risk multiplies when heavy fluid intake coincides with high sweat Na⁺ losses, because water is added to plasma at the same time as plasma is losing sodium to sweat.
4. Other contributing factors include high sweat Na⁺ concentration in heat and humidity, hormonal dysfunction affecting renal Na⁺ handling, and certain medications.
5. Mild symptoms include weakness, dizziness, headache, and vomiting, while severe cases progress to fluid accumulation in the brain and lungs, seizures, and death.
6. The mechanism is osmotic: low extracellular Na⁺ pulls water into brain cells, raising intracranial pressure.

Potassium Accumulation During Exercise

1. Repeated muscle contractions cause rapid loss of K⁺ from inside the muscle fibre into the surrounding interstitial space.
2. At high firing rates, the K⁺ leaving the cell with each action potential is not fully recovered before the next contraction, so intracellular K⁺ falls and extracellular K⁺ rises.
3. A small rise in interstitial K⁺ is functionally useful, acting as a biochemical signal that triggers vasodilation in active tissue and increased ventilation to match metabolic demand.
4. Excessive accumulation, however, shifts the membrane voltage in an inhibitory direction and interferes with normal Na⁺ entry, reducing muscle cell excitability.
5. The result is that more motor neuron drive is needed to recruit a given amount of force, and in extreme cases individual fibres become temporarily inexcitable.
6. Rising extracellular K⁺ also stimulates muscle metaboreceptors, contributing to discomfort, raised perceived exertion, and central fatigue.

The Role of ATP, Glycogen, and Fatty Acids in Fueling Muscle Contractions

1. ATP is the immediate source of energy required for muscle contraction.
2. However, ATP stores in muscles are very limited, lasting only a few seconds during intense activity.
3. Therefore, ATP must be rapidly replenished by various metabolic processes.
4. Anaerobic glycolysis produces lactate and H⁺ together; H⁺ (not lactate) drives the pH drop.

How Depletion of Glycogen Stores or Low Availability of Fats Impacts Muscle Endurance and Fatigue

Glycogen Depletion and Fatigue

1. Glycogen is the stored form of glucose found in muscles and liver.
2. During exercise, glycogen is broken down into glucose for ATP production.
3. As glycogen stores are depleted, the body becomes less able to produce ATP, leading to fatigue.

Subcellular Glycogen Pools

1. Glycogen is not stored uniformly across the muscle fibre but is concentrated in three subcellular pools: the subsarcolemmal pool just under the fibre membrane, the intermyofibrillar pool between myofibrils, and the intramyofibrillar pool inside the contractile machinery.
2. Localised depletion of the intramyofibrillar pool impairs ATP supply at the specific sites of excitation-contraction coupling even when whole-muscle ATP appears adequate.
3. The functional consequence is reduced calcium release and slower contraction, which fits the experience of an athlete whose muscle feels weak before any global energy crisis has occurred.

Consequences of Glycogen Depletion

1. Slower ATP production leads to muscle fatigue.
2. Increased reliance on fat metabolism, which is less efficient for high-intensity activity.
3. Higher perceived effort and a reduction in mental clarity due to decreased glucose availability for brain function.

Hypoglycaemia

1. Hypoglycaemia is a low blood glucose concentration, and its consequences for exercise are disproportionately central.
2. The brain has very limited internal glucose stores and depends almost entirely on blood glucose uptake for its energy supply.
3. Below a critical blood glucose concentration, brain glucose uptake falls and central drive to skeletal muscle is reduced, raising perceived exertion and lowering voluntary force.
4. Recent work suggests the main contribution of glycogen depletion and hypoglycaemia to fatigue is central rather than peripheral.
5. Clinical presentation varies sharply: some athletes show nausea, confusion, and dizziness when blood glucose drops, while others show no obvious outward signs at similar concentrations.

Fat Availability and Endurance

1. Fatty acids are the main energy source during long-duration, low-intensity exercise.
2. Fat metabolism requires oxygen, which makes it slower but more efficient for long-term ATP production.
3. When fat stores are insufficient, the body cannot sustain prolonged exercise without a dip in performance.

Suboptimal Availability of Calcium, Sodium, and Potassium

1. Muscle contraction and function rely heavily on the availability of key ions: calcium (Ca²⁺), sodium (Na⁺), and potassium (K⁺).
2. These ions are responsible for generating electrical signals in muscle cells and enabling contraction.
3. When their balance is disrupted, muscle fatigue can occur due to impaired excitation-contraction coupling.

How Imbalances in Calcium, Sodium, and Potassium Contribute to Fatigue

When these ions are not available in optimal concentrations, muscle fatigue occurs due to inefficient signal transmission and weakened contractions.

Effects of Calcium Deficiency (Hypocalcemia)

1. Weak cross-bridge formation reduces force production.
2. Slower muscle relaxation leads to increased stiffness and cramping.
3. Impaired nerve signal transmission, leading to muscle weakness.

Effects of Sodium Deficiency (Hyponatremia)

1. Weaker action potentials due to impaired depolarisation.
2. Reduced muscle contraction efficiency.
3. Symptoms include confusion, dizziness, and muscle weakness.

Effects of Potassium Deficiency (Hypokalemia)

1. Delayed repolarisation causes a slower muscle response.
2. Increased risk of muscle cramps and spasms.
3. Potential for irregular heart rhythms (arrhythmia).

Integration of Hydration, Electrolytes, and Energy Availability

1. Dehydration drives hyperthermia by reducing skin blood flow.
2. Hyperthermia accelerates glycogen use through temperature effects on metabolism.
3. Glycogen depletion and a falling liver glycogen reserve drive hypoglycaemia.
4. Hypoglycaemia amplifies central fatigue.
5. Potassium accumulation in the interstitial space contributes to both peripheral impairment of muscle excitability and central sensory feedback that raises perceived exertion.