Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, exercise or sport.  People engaged in cycling are referred to as "cyclists",  "bicyclists",  or "bikers".  Apart from two-wheeled bicycles, "cycling" also includes the riding of unicycles, tricycles, quadracycles, recumbent and similar human-powered vehicles (HPVs).
Bicycles were introduced in the 19th century and now number approximately one billion worldwide.  They are the principal means of transportation in many parts of the world.
Bicycles provide numerous benefits in comparison with motor vehicles, including the sustained physical exercise involved in cycling, easier parking, increased maneuverability, and access to roads, bike paths and rural trails. Cycling also offers a reduced consumption of fossil fuels, less air or noise pollution, and much reduced traffic congestion. These lead to less financial cost to the user as well as to society at large (negligible damage to roads, less road area required). By fitting bicycle racks on the front of buses, transit agencies can significantly increase the areas they can serve. 
In addition, cycling provides a variety of health benefits. The World Health Organization (WHO) states that cycling can reduce the risk of cancers, heart disease, and diabetes that are prevalent in sedentary lifestyles.  Cycling on stationary bikes have also been used as part of rehabilitation for lower limb injuries, particularly after hip surgery.  Individuals who cycle regularly have also reported mental health improvements, including less perceived stress and better vitality. 
Among the disadvantages of cycling are the requirement of bicycles (excepting tricycles or quadracycles) to be balanced by the rider in order to remain upright, the reduced protection in crashes in comparison to motor vehicles,  often longer travel time (except in densely populated areas), vulnerability to weather conditions, difficulty in transporting passengers, and the fact that a basic level of fitness is required for cycling moderate to long distances.
In many countries, the most commonly used vehicle for road transport is a utility bicycle. These have frames with relaxed geometry, protecting the rider from shocks of the road and easing steering at low speeds. Utility bicycles tend to be equipped with accessories such as mudguards, pannier racks and lights, which extends their usefulness on a daily basis. As the bicycle is so effective as a means of transportation various companies have developed methods of carrying anything from the weekly shop to children on bicycles. Certain countries rely heavily on bicycles and their culture has developed around the bicycle as a primary form of transport. In Europe, Denmark and the Netherlands have the most bicycles per capita and most often use bicycles for everyday transport.  
Road bikes tend to have a more upright shape and a shorter wheelbase, which make the bike more mobile but harder to ride slowly. The design, coupled with low or dropped handlebars, requires the rider to bend forward more, making use of stronger muscles (particularly the gluteus maximus) and reducing air resistance at high speed.
The price of a new bicycle can range from US$50 to more than US$20,000 (the highest priced bike in the world is the custom Madone by Damien Hirst, sold at US$500,000 ),  depending on quality, type and weight (the most exotic road bicycles can weigh as little as 3.2 kg (7 lb) ). However, UCI regulations stipulate a legal race bike cannot weigh less than 6.8 kg (14.99 lbs). Being measured for a bike and taking it for a test ride are recommended before buying.
The drivetrain components of the bike should also be considered. A middle grade dérailleur is sufficient for a beginner, although many utility bikes are equipped with hub gears. If the rider plans a significant amount of hillclimbing, a triple-chainrings crankset gear system may be preferred. Otherwise, the relatively lighter and less expensive double chainring may be better. Much simpler fixed wheel bikes are also available.
Many road bikes, along with mountain bikes, include clipless pedals to which special shoes attach, via a cleat, enabling the rider to pull on the pedals as well as push. Other possible accessories for the bicycle include front and rear lights, bells or horns, child carrying seats, cycling computers with GPS, locks, bar tape, fenders (mud-guards), baggage racks, baggage carriers and pannier bags, water bottles and bottle cages.
For basic maintenance and repairs cyclists can carry a pump (or a CO2 cartridge), a puncture repair kit, a spare inner tube, and tire levers and a set of allen keys. Cycling can be more efficient and comfortable with special shoes, gloves, and shorts. In wet weather, riding can be more tolerable with waterproof clothes, such as cape, jacket, trousers (pants) and overshoes and high-visibility clothing is advisable to reduce the risk from motor vehicle users.
Items legally required in some jurisdictions, or voluntarily adopted for safety reasons, include bicycle helmets,  generator or battery operated lights, reflectors, and audible signalling devices such as a bell or horn. Extras include studded tires and a bicycle computer.
Bikes can also be heavily customized, with different seat designs and handle bars, for example.
Many schools and police departments run educational programs to instruct children in bicycle handling skills, especially to introduce them to the rules of the road as they apply to cyclists. In some countries these may be known as bicycle rodeos, or operated as schemes such as Bikeability. Education for adult cyclists is available from organizations such as the League of American Bicyclists.
Beyond simply riding, another skill is riding efficiently and safely in traffic. One popular approach to riding in motor vehicle traffic is vehicular cycling, occupying road space as car does. Alternately, in countries such as Denmark and the Netherlands, where cycling is popular, cyclists are often segregated into bike lanes at the side of, or more often separate from, main highways and roads. Many primary schools participate in the national road test in which children individually complete a circuit on roads near the school while being observed by testers.
Cyclists, pedestrians and motorists make different demands on road design which may lead to conflicts. Some jurisdictions give priority to motorized traffic, for example setting up one-way street systems, free-right turns, high capacity roundabouts, and slip roads. Others share priority with cyclists so as to encourage more cycling by applying varying combinations of traffic calming measures to limit the impact of motorized transport, and by building bike lanes, bike paths and cycle tracks. The provision of cycling infrastructure varies widely between cities and countries, particularly since cycling for transportation almost entirely occurs in public streets. 
In jurisdictions where motor vehicles were given priority, cycling has tended to decline while in jurisdictions where cycling infrastructure was built, cycling rates have remained steady or increased.  Occasionally, extreme measures against cycling may occur. In Shanghai, where bicycles were once the dominant mode of transport, bicycle travel on a few city roads was banned temporarily in December 2003. 
In areas in which cycling is popular and encouraged, cycle-parking facilities using bicycle stands, lockable mini-garages, and patrolled cycle parks are used in order to reduce theft. Local governments promote cycling by permitting bicycles to be carried on public transport or by providing external attachment devices on public transport vehicles. Conversely, an absence of secure cycle-parking is a recurring complaint by cyclists from cities with low modal share of cycling.
Extensive cycling infrastructure may be found in some cities. Such dedicated paths in some cities often have to be shared with in-line skaters, scooters, skateboarders, and pedestrians. Dedicated cycling infrastructure is treated differently in the law of every jurisdiction, including the question of liability of users in a collision. There is also some debate about the safety of the various types of separated facilities.
Bicycles are considered a sustainable mode of transport, especially suited for urban use and relatively shorter distances when used for transport (compared to recreation). Case studies and good practices (from European cities and some worldwide examples) that promote and stimulate this kind of functional cycling in cities can be found at Eltis, Europe's portal for local transport.
A number of cities, including Paris, London and Barcelona, now have successful bike hire schemes designed to help people cycle in the city. Typically these feature utilitarian city bikes which lock into docking stations, released on payment for set time periods. Costs vary from city to city. In London, initial hire access costs £2 per day. The first 30 minutes of each trip is free, with £2 for each additional 30 minutes until the bicycle is returned. 
In the Netherlands, many roads have one or two separate cycleways alongside them, or cycle lanes marked on the road. On roads where adjacent bike paths or cycle tracks exist, the use of these facilities is compulsory, and cycling on the main carriageway is not permitted.  Some 35,000 km of cycle-track has been physically segregated from motor traffic,   equal to a quarter of the country's entire 140,000 km road network.  A quarter of all the trips in the country made on bicycles, one quarter of them to work. Even the prime minister is going to work by bicycle, when weather permits. This saves the life of 6,000 citizens per year, prolong the life expectancy by 6 months, save the country 20 million dollars per year, and prevent 150 grams of CO2 to be emitted per kilometer of cycling, on each bicycle. 
Utility cycling refers both to cycling as a mode of daily commuting transport as well as the use of a bicycle in a commercial activity, mainly to transport goods, mostly accomplished in an urban environment.
The postal services of many countries have long relied on bicycles. The British Royal Mail first started using bicycles in 1880; now bicycle delivery fleets include 37,000 in the UK, 25,700 in Germany, 10,500 in Hungary and 7000 in Sweden. In Australia, Australia Post has also reintroduced bicycle postal deliveries on some routes due to an inability to recruit sufficient licensed riders willing to use their uncomfortable motorbikes. The London Ambulance Service has recently introduced bicycling paramedics, who can often get to the scene of an incident in Central London more quickly than a motorized ambulance. 
Bicycles enjoy substantial use as general delivery vehicles in many countries. In the UK and North America, as their first jobs, generations of teenagers have worked at delivering newspapers by bicycle. London has many delivery companies that use bicycles with trailers. Most cities in the West, and many outside it, support a sizeable and visible industry of cycle couriers who deliver documents and small packages. In India, many of Mumbai's Dabbawalas use bicycles to deliver home cooked lunches to the city's workers. In Bogotá, Colombia the city's largest bakery recently replaced most of its delivery trucks with bicycles. Even the car industry uses bicycles. At the huge Mercedes-Benz factory in Sindelfingen, Germany workers use bicycles, color-coded by department, to move around the factory.[ citation needed]
Bicycles are used for recreation at all ages. Bicycle touring, also known as cyclotourism, involves touring and exploration or sightseeing by bicycle for leisure. Bicycle tourism has been one of the most popular sports for recreational benefit.  A brevet or randonnée is an organized long-distance ride.
One popular Dutch pleasure is the enjoyment of relaxed cycling in the countryside of the Netherlands. The land is very flat and full of public bicycle trails and cycle tracks where cyclists are not bothered by cars and other traffic, which makes it ideal for cycling recreation. Many Dutch people subscribe every year to an event called fietsvierdaagse — four days of organised cycling through the local environment. Paris–Brest–Paris (PBP), which began in 1891, is the oldest bicycling event still run on a regular basis on the open road, covers over 1,200 km (746 mi) and imposes a 90-hour time limit. Similar if smaller institutions exist in many countries.
A study conducted in Taiwan improved the environmental quality for bicyclist tourists which demonstrated greater health benefits in tourists and even in natives. The number of bicyclists in Taiwan increased from 700,000 in 2008 to 5.1 million in 2017. Thus, this resulted in more and safer bicycle routes to be established. When cycling, cyclists take into account the safety on the road, bicycle lanes, smooth roads, diverse scenery, and ride length. Thus, the environment plays a huge role in people's decision factor to utilize bicycle touring more. This study utilized many questionnaires and conducted statistical analysis to come up with the conclusion of cyclists' top 5 factors that they consider before making a decision to bike are: safety, lighting facility, design of lanes, the surrounding landscape, and how clean the environment is. Thus, after improving these 5 factors, they found much more recreational benefits to bicycle tourism. 
Many cycling clubs hold organized rides in which bicyclists of all levels participate. The typical organized ride starts with a large group of riders, called the mass, bunch or even peloton. This will thin out over the course of the ride. Many riders choose to ride together in groups of the same skill level to take advantage of drafting.
Most organized rides, for example cyclosportives (or gran fondos), Challenge Rides or reliability trials, and hill climbs include registration requirements and will provide information either through the mail or online concerning start times and other requirements. Rides usually consist of several different routes, sorted by mileage, and with a certain number of rest stops that usually include refreshments, first aid and maintenance tools. Routes can vary by as much as 100 miles (160 km).
Some organized rides are entirely social events. One example is the monthly San Jose Bike Party which can reach attendance of one to two thousand riders in Summer months.
Mountain biking began in the 1970s, originally as a downhill sport, practised on customized cruiser bicycles around Mount Tamalpais.  Most mountain biking takes place on dirt roads, trails and in purpose-built parks. Downhill mountain biking has just evolved in the recent years and is performed at places such as Whistler Mountain Bike Park. Slopestyle, a form of downhill, is when riders do tricks such as tailwhips, 360s, backflips and front flips. There are several disciplines of mountain biking besides downhill. Cross country, often referred to as XC, all mountain, trail, free ride, and newly popular enduro.
Shortly after the introduction of bicycles, competitions developed independently in many parts of the world. Early races involving boneshaker style bicycles were predictably fraught with injuries. Large races became popular during the 1890s "Golden Age of Cycling", with events across Europe, and in the U.S. and Japan as well. At one point, almost every major city in the US had a velodrome or two for track racing events, however since the middle of the 20th century cycling has become a minority sport in the US whilst in Continental Europe it continues to be a major sport, particularly in the United Kingdom, France, Belgium, Italy and Spain. The most famous of all bicycle races is the Tour de France. This began in 1903, and continues to capture the attention of the sporting world.
In 1899, Charles Minthorn Murphy became the first man to ride his bicycle a mile in under a minute (hence his nickname, Mile-a-Minute Murphy), which he did by drafting a locomotive at New York's Long Island.
As the bicycle evolved its various forms, different racing formats developed. Road races may involve both team and individual competition, and are contested in various ways. They range from the one-day road race, criterium, and time trial to multi-stage events like the Tour de France and its sister events which make up cycling's Grand Tours. Recumbent bicycles were banned from bike races in 1934 after Marcel Berthet set a new hour record in his Velodyne streamliner (49.992 km on November 18, 1933). Track bicycles are used for track cycling in Velodromes, while cyclo-cross races are held on outdoor terrain, including pavement, grass, and mud. Cyclocross races feature man-made features such as small barriers which riders either bunny hop over or dismount and walk over. Time trial races, another form of road racing require a rider to ride against the clock. Time trials can be performed as a team or as a single rider. Bikes are changed for time trial races, using aero bars. In the past decade, mountain bike racing has also reached international popularity and is even an Olympic sport.
Professional racing organizations place limitations on the bicycles that can be used in the races that they sanction. For example, the Union Cycliste Internationale, the governing body of international cycle sport (which sanctions races such as the Tour de France), decided in the late 1990s to create additional rules which prohibit racing bicycles weighing less than 6.8 kilograms (14.96 pounds). The UCI rules also effectively ban some bicycle frame innovations (such as the recumbent bicycle) by requiring a double triangle structure. 
The bicycle has been used as a method of reconnaissance as well as transporting soldiers and supplies to combat zones. In this it has taken over many of the functions of horses in warfare. In the Second Boer War, both sides used bicycles for scouting. In World War I, France, Germany, Australia and New Zealand used bicycles to move troops. In its 1937 invasion of China, Japan employed some 50,000 bicycle troops, and similar forces were instrumental in Japan's march or "roll" through Malaya in World War II. Germany used bicycles again in World War II, while the British employed airborne "Cycle-commandos" with folding bikes.
The last country known to maintain a regiment of bicycle troops was Switzerland, which disbanded its last unit in 2003.
Two broad and correlated themes run in bicycle activism: one is about advocating the bicycle as an alternative mode of transport, and the other is about the creation of conditions to permit and/or encourage bicycle use, both for utility and recreational cycling.  Although the first, which emphasizes the potential for energy and resource conservation and health benefits gained from cycling versus automobile use, is relatively undisputed, the second is the subject of much debate.
It is generally agreed that improved local and inter-city rail services and other methods of mass transportation (including greater provision for cycle carriage on such services) create conditions to encourage bicycle use. However, there are different opinions on the role of various types of cycling infrastructure in building bicycle-friendly cities and roads.
Some bicycle activists (including some traffic management advisers) seek the construction of bike paths, cycle tracks and bike lanes for journeys of all lengths and point to their success in promoting safety and encouraging more people to cycle. Some activists, especially those from the vehicular cycling tradition, view the safety, practicality, and intent of such facilities with suspicion. They favor a more holistic approach based on the 4 'E's; education (of everyone involved), encouragement (to apply the education), enforcement (to protect the rights of others), and engineering (to facilitate travel while respecting every person's equal right to do so). Some groups offer training courses to help cyclists integrate themselves with other traffic.
Critical Mass is an event typically held on the last Friday of every month in cities around the world where bicyclists take to the streets en masse. While the ride was founded with the idea of drawing attention to how unfriendly the city was to bicyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city streets.
Cyclists form associations, both for specific interests (trails development, road maintenance, bike maintenance, urban design, racing clubs, touring clubs, etc.) and for more global goals ( energy conservation, pollution reduction, promotion of fitness). Some bicycle clubs and national associations became prominent advocates for improvements to roads and highways. In the United States, the League of American Wheelmen lobbied for the improvement of roads in the last part of the 19th century, founding and leading the national Good Roads Movement. Their model for political organization, as well as the paved roads for which they argued, facilitated the growth of the automobile.
As a sport, cycling is governed internationally by the Union Cycliste Internationale in Switzerland, USA Cycling (merged with the United States Cycling Federation in 1995) in the United States, (for upright bicycles) and by the International Human Powered Vehicle Association (for other HPVs, or human-powered vehicles). Cycling for transport and touring is promoted on a European level by the European Cyclists' Federation, with associated members from Great Britain, Japan and elsewhere. Regular conferences on cycling as transport are held under the auspices of Velo City; global conferences are coordinated by Velo Mondial. 
The health benefits of cycling outweigh the risks, when cycling is compared to a sedentary lifestyle. A Dutch study found that cycling can extend lifespans by up to 14 months, but the risks equated to a reduced lifespan of 40 days or less.  Mortality rate reduction was found to be directly correlated to the average time spent cycling, totaling to approximately 6500 deaths prevented by cycling.  Cycling in the Netherlands is often safer than in other parts of the world, so the risk-benefit ratio will be different in other regions.  Overall, benefits of cycling or walking have been shown to exceed risks by ratios of 9:1 to 96:1 when compared with no exercise at all, including a wide variety of physical and mental outcomes.   
The physical exercise gained from cycling is generally linked with increased health and well-being. According to the World Health Organization (WHO), physical inactivity is second only to tobacco smoking as a health risk in developed countries,  and is associated with 20-30% increased risk of various cancers, heart disease, and diabetes  and tens of billions of dollars of healthcare costs.  The WHO's 2009 report  suggests that increasing physical activity is a public health "best buy", and that cycling is a "highly suitable activity" for this purpose. The charity Sustrans reports that investment in cycling provision can give a 20:1 return from health and other benefits.  It has been estimated that, on average, approximately 20 life-years are gained from the health benefits of road bicycling for every life-year lost through injury. 
Bicycles are often used by people seeking to improve their fitness and cardiovascular health. Recent studies on the use of cycling for commutes have shown that it reduces the risk of cardiovascular outcomes by 11%, with slightly more risk reduction in women than in men.   In addition, cycling is especially helpful for those with arthritis of the lower limbs who are unable to pursue sports that cause impact to the knees and other joints. Since cycling can be used for the practical purpose of transportation, there can be less need for self-discipline to exercise.
Cycling while seated is a relatively non- weight bearing exercise that, like swimming, does little to promote bone density.  Cycling up and out of the saddle, on the other hand, does a better job by transferring more of the rider's body weight to the legs. However, excessive cycling while standing can cause knee damage  It used to be thought that cycling while standing was less energy efficient, but recent research has proven this not to be true. Other than air resistance, there is no wasted energy from cycling while standing, if it is done correctly. 
Cycling on a stationary cycle is frequently advocated as a suitable exercise for rehabilitation, particularly for lower limb injury, owing to the low impact which it has on the joints. In particular, cycling is commonly used within knee rehabilitation programs, to strengthen the quadriceps muscles with minimal stress on the knee ligaments.  Further stress of the knee can be relieved by changing seat heights and pedal position to improve the rehabilitation. Cycling is also used for rehabilitation after hip surgery to manage soft-tissue healing, control swelling and pain, and allow a larger range of motion to the nearby muscles earlier during recovery.  As a result, many institutions have established a rehabilitation protocol that involves stationary cycling as part of the recovery process. One such protocol offered by Mayo Clinic recommends 2–4 weeks of cycling on an upright stationary bike following hip arthroscopy, starting from 5 minutes per session and slowly increasing to 30 minutes per session.  The goal of these sessions are to reduce joint inflammation and maintain the widest range of motion possible with limited pain.
As a response to the increased global sedentary lifestyles and consequent overweight and obesity, one response that has been adopted by many organizations concerned with health and environment is the promotion of Active travel, which seeks to promote walking and cycling as safe and attractive alternatives to motorized transport. Given that many journeys are for relatively short distances, there is considerable scope to replace car use with walking or cycling, though in many settings this may require some infrastructure modification, particularly to attract the less experienced and confident.
An Italian study assessed the impact of cycling for commute on major non-communicable diseases and public healthcare costs. Using a health economic assessment model, the study found a lower incidence of type 2 diabetes, acute myocardial infarction, and stroke in individuals that cycled compared to those that did not actively commute. This model estimated that public healthcare costs would reduce by 5% over a 10-year period. 
The effects of cycling on overall mental health has often been studied. A European study surveying participants from seven cities about self-perceived health based on primary modes of transportation reported favorable results in the bicycle use population.  The bicycle use group reported predominantly good self-perceived health, less perceived stress, better mental health, better vitality, and less loneliness. The study attributed these results to possible economic benefits and senses of both independence and identity as a member of a cyclist community. An English study recruiting non-cyclist older adults aged 50 to 83 to participate as either conventional pedal bike cyclists, electrically assisted e-bike cyclists, or a non-cyclist control group in outdoor trails measured cognitive function through executive function, spatial reasoning, and memory tests and well-being through questionnaires.  The study did not find significant differences in spatial reasoning or memory tests. It did however find that both cyclists groups had improved executive function and well-being, both with greater improvement in the e-bike group. This suggested that non-physical factors of cycling such as independence, engagement with the outdoor environment, and mobility play a greater role in improving mental health.
A 15-month randomized controlled trial in the U.S. examined the impact of self-paced cycling on cognitive function in institutionalized older adults without cognitive impairment.  Researchers used three cognitive assessments: Mini-Mental State Examination (MMSE), Fuld object memory evaluation, and symbol digit modality test. The study found that long-term cycling for at least 15 minutes per day in older adults without cognitive impairment had a protective effect on cognition and attention.
Cycling suffers from a perception that it is unsafe.   This perception is not always backed by hard numbers, because of under reporting of accidents and lack of bicycle use data (amount of cycling, kilometers cycled) which make it hard to assess the risk and monitor changes in risks.  In the UK, fatality rates per mile or kilometre are slightly less than those for walking.  In the US, bicycling fatality rates are less than 2/3 of those walking the same distance.   However, in the UK for example the fatality and serious injury rates per hour of travel are just over double for cycling than those for walking.  Thus if a person is, for example, about to undertake a ten kilometre journey to a given destination it may on average be safer to undertake this journey by bicycle than on foot. However, if a person is intending, for example, to undertake an hour's exercise it may be more dangerous to take that exercise by cycling rather than by walking.
Despite the risk factors associated with bicycling, cyclists have a lower overall mortality rate when compared to other groups. A Danish study in 2000 found that even after adjustment for other risk factors, including leisure time physical activity, those who did not cycle to work experienced a 39% higher mortality rate than those who did. 
Injuries (to cyclists, from cycling) can be divided into two types:
- Physical trauma (extrinsic)
- Overuse (intrinsic).
Acute physical trauma includes injuries to the head and extremities resulting from falls and collisions. Most cycle deaths result from a collision with a car or heavy goods vehicle, both motorist and cyclist having been found responsible for collisions.    A third of collisions between motorists and cyclists are caused by car dooring.  However, around 16% of serious cyclist injuries reported to police in the UK in 2014 did not involve any other person or vehicle. 
Of a study of 518 cyclists, a large majority reported at least one overuse injury, with over one third requiring medical treatment. The most common injury sites were the neck (48.8%) and the knees (41.7%), as well as the groin/buttocks (36.1%), hands (31.1%), and back (30.3%). Women were more likely to suffer from neck and shoulder pain than men. 
Many cyclists suffer from overuse injuries to the knees, affecting cyclists at all levels. These are caused by many factors: 
- Incorrect bicycle fit or adjustment, particularly the saddle.
- Incorrect adjustment of clipless pedals.
- Too many hills, or too many miles, too early in the training season.
- Poor training preparation for long touring rides.
- Selecting too high a gear. A lower gear for uphill climb protects the knees, even though muscles may be well able to handle a higher gear.
Overuse injuries, including chronic nerve damage at weight bearing locations, can occur as a result of repeatedly riding a bicycle for extended periods of time. Damage to the ulnar nerve in the palm, carpal tunnel in the wrist, the genitourinary tract  or bicycle seat neuropathy  may result from overuse. Recumbent bicycles are designed on different ergonomic principles and eliminate pressure from the saddle and handlebars, due to the relaxed riding position.
Note that overuse is a relative term, and capacity varies greatly between individuals. Someone starting out in cycling must be careful to increase length and frequency of cycling sessions slowly, starting for example at an hour or two per day, or a hundred miles or kilometers per week. Bilateral muscular pain is a normal by-product of the training process, whereas unilateral pain may reveal "exercise-induced arterial endofibrosis".  Joint pain and numbness are also early signs of overuse injury.
A Spanish study of top triathletes found those who cover more than 186 miles (300 km) a week on their bikes have less than 4% normal looking sperm, where normal adult males would be expected to have from 15% to 20%.  
Much work has been done to investigate optimal bicycle saddle shape, size and position, and negative effects of extended use of less than optimal seats or configurations.
Excessive saddle height can cause posterior knee pain, while setting the saddle too low can cause pain in the anterior of the knee. An incorrectly fitted saddle may eventually lead to muscle imbalance. A 25 to 35 degree knee angle is recommended to avoid an overuse injury. 
Cycling has been linked to sexual impotence due to pressure on the perineum from the seat, but fitting a proper sized seat prevents this effect.     In extreme cases, pudendal nerve entrapment can be a source of intractable perineal pain.  Some cyclists with induced pudendal nerve pressure neuropathy gained relief from improvements in saddle position and riding techniques. 
The National Institute for Occupational Safety and Health (NIOSH) has investigated the potential health effects of prolonged bicycling in police bicycle patrol units, including the possibility that some bicycle saddles exert excessive pressure on the urogenital area of cyclists, restricting blood flow to the genitals.  Their study found that using bicycle seats without protruding noses reduced pressure on the groin by at least 65% and significantly reduced the number of cases of urogenital paresthesia. A follow-up found that 90% of bicycle officers who tried the no-nose seat were using it six months later. NIOSH recommends that riders use a no-nose bicycle seat for workplace bicycling.  
One concern is that riding in traffic may expose the cyclist to higher levels of air pollution, especially if he or she travels on or along busy roads. Some authors have claimed this to be untrue, showing that the pollutant and irritant count within cars is consistently higher,  presumably because of limited circulation of air within the car and due to the air intake being directly in the stream of other traffic. Other authors have found small or inconsistent differences in concentrations but claim that exposure of cyclists is higher due to increased minute ventilation  and is associated with minor biological changes.  A 2010 study estimated that the gained life expectancy from the health benefits of cycling (approximately 3–14 months gained) greatly exceeded the lost life expectancy from air pollution (approximately 0.8–40 days lost).  However, a systematic review comparing the effects of air pollution exposure on the health of cyclists was conducted, but the authors concluded that the differing methodologies and measuring parameters of each study made it difficult to compare results and suggested a more holistic approach was needed to accomplish this.  The significance of the associated health effect, if any, is unclear but probably much smaller than the health impacts associated with accidents and the health benefits derived from additional physical activity.
Oxford English Dictionary (Second ed.). Oxford University Press. 1989.
cycling: The action or activity of riding a bicycle etc.
- Oxford English Dictionary (Second ed.). Oxford University Press. 1988.
cyclist: One who rides a cycle or practises cycling.
Oxford English Dictionary (Second ed.). Oxford University Press. 1989.
bicyclist: One who rides a bicycle.
"Oxford English Dictionary". www.oed.com. Retrieved 2013-09-26.
biker: A cyclist, a person who rides a bicycle."
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