People have ridden two-wheeled self-powered vehicles for transport, recreation and sport since the early 17th century. The materials used in bicycle manufacture have changed significantly and modern bicycles are almost unrecognizable from the wood and iron bikes of old. Despite the technological changes in bicycle manufacture and design, the engine that drives a bike -- your body -- is still the same and riding too far or too fast still results in fatigue. Fatigue is caused by a variety of physiological reactions.
Lactic Acid Buildup
Cycling is predominately an aerobic activity. Fat and carbohydrate is broken down to form an energy yielding substance called adenosine triphosphate, which powers your muscular contractions. If you ride too fast, your body is unable to take in sufficient oxygen for this process to occur. This results in the incomplete breakdown of carbohydrates in an oxygen-free environment. The byproduct of this type of activity is called lactic acid, which is, in simple terms, responsible for the burning sensation you feel when you work very hard. Large amounts of lactic acid will make you slow down or even stop cycling and you will only be able to continue once you have rested and lactic acid levels have reduced.
Shortage of Fuel
Your body uses two main fuels during cycling: fat and carbohydrate. While is it unlikely you will run out of fat during even the longest bike ride, it is possible to run out of carbohydrate. Carbohydrate is stored in your muscles in the form of glycogen. Glycogen is a localized energy store that can only be used by the muscles in which it is locked. If these stores become depleted, your muscles will become fatigued and only recover after adequate rest and nutrition. Runners call this depletion of muscle glycogen stores as hitting the wall whereas cyclists call it bonking. You can minimize glycogen depletion, and therefore fatigue, by consuming adequate carbohydrates, before and during your cycle rides.
Local Muscular Fatigue
Driving your pedals around and around uses your leg muscles -- specifically your quadriceps, hamstrings and gluteus maximus muscles. One of the adaptations you will experience as a result of regular cycling is an increase in local muscular endurance. Increased muscular endurance is the result of an increase in energy producing cells called mitochondria and an increase in size and number of aerobic muscle fibers. The greater your local muscular endurance is, the more resistant your muscles will be to fatigue, which means your muscles can work for longer before tiring.
Poor Cardiovascular Fitness
Your cardiovascular system consists of your heart, lungs, blood and blood vessels, and is responsible for taking in, transporting and utilizing oxygen. If you have poor cardiovascular fitness, your muscles will not receive the essential oxygen they require for prolonged activity and you will have to slow down or even stop cycling. Heart and lung capacity, red blood cell count and capillary density all effect your cycling performance and the better the condition of your cardiovascular system, the less fatigue you will experience. Cardiovascular fitness, and therefore your resistance to fatigue, increases with regular exercise.