The oxidative phosphorylation system, also known as aerobic metabolism, is the primary energy system used during low-intensity, prolonged endurance activities. It utilizes oxygen to generate ATP, the body's energy currency, from carbohydrates, fats, and proteins.

Endurance training induces several physiological adaptations, including increased red blood cell count, increased stroke volume, and increased capillary density. These adaptations collectively enhance the body's ability to deliver oxygen to muscles, supporting sustained endurance performance.

Stride frequency, the number of steps taken per minute, is a key biomechanical factor influencing running economy. A higher stride frequency, combined with a shorter stride length, is associated with reduced energy expenditure and improved endurance performance.

Carbohydrates are the primary fuel source for endurance activities. Their main role is to replenish muscle glycogen stores, which are depleted during exercise. Consuming carbohydrates during endurance events helps maintain glycogen levels, ensuring a sustained energy supply for muscles.

Vitamin B1 (Thiamine) is a crucial vitamin involved in the metabolism of carbohydrates. It plays a key role in converting carbohydrates into energy, particularly during high-intensity exercise. Adequate thiamine intake is essential for optimal endurance performance.

Electrolytes, including sodium and potassium, perform several vital functions during endurance activities. They regulate fluid balance, transmit nerve impulses, and maintain muscle contraction. Proper electrolyte balance is crucial for optimal hydration, muscle function, and overall endurance performance.

Mitochondrial density, the number of mitochondria per muscle cell, is a key factor limiting the rate of oxygen consumption during maximal exercise. Mitochondria are the cellular organelles responsible for oxidative phosphorylation, the process by which oxygen is used to generate ATP. A higher mitochondrial density allows for greater oxygen utilization and ATP production, supporting enhanced endurance performance.

The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions that occur in mitochondria. Its primary role in endurance performance is to generate NADH and FADH2, high-energy electron carriers that are used in the electron transport chain to produce ATP.

Glucagon is a hormone produced by the pancreas that plays a crucial role in mobilizing fat stores during prolonged endurance activities. It stimulates the breakdown of glycogen in the liver, releasing glucose into the bloodstream, and promotes the release of fatty acids from adipose tissue, providing an alternative energy source for muscles.

The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane. Its primary function is to generate ATP through oxidative phosphorylation. During this process, high-energy electrons from NADH and FADH2 are transferred through the electron transport chain, releasing energy that is used to pump protons across the mitochondrial membrane. This proton gradient drives the synthesis of ATP.

The 'second wind' phenomenon during endurance activities is primarily attributed to the body's adaptation to substrate utilization. As glycogen stores are depleted, the body shifts from relying primarily on carbohydrates to utilizing fats as the main energy source. This metabolic adaptation allows for a more sustainable energy supply and reduces the perception of fatigue.

The Cori cycle is a metabolic pathway that involves the recycling of lactate produced during glycolysis. Lactate is transported from muscles to the liver, where it is converted back into glucose through a series of enzymatic reactions. This process helps maintain blood glucose levels during prolonged endurance activities, ensuring a continuous supply of energy to muscles.

Increased stride frequency, characterized by taking more steps per minute with a shorter stride length, is associated with improved running economy and reduced energy expenditure during endurance activities. This biomechanical adaptation allows runners to maintain a given pace while expending less energy.

The sympathetic nervous system plays a crucial role during endurance activities by increasing heart rate and blood pressure. This physiological response ensures that oxygen and nutrients are delivered to muscles more efficiently, supporting sustained performance.

'Hitting the wall' during endurance activities is primarily caused by muscle glycogen depletion. When glycogen stores are exhausted, the body can no longer rely on carbohydrates as the main energy source, leading to a sudden drop in performance and fatigue.