Urban agriculture is a generic term for a number of ways of the primary food production in urban metropolitan areas and their immediate vicinity for their own needs of the region. Urban agriculture is the practice of cultivating, processing, and distributing food in or around urban areas. Urban agriculture is also the term used for animal husbandry, aquaculture, urban beekeeping, and horticulture. These activities occur in peri-urban areas as well. Peri-urban agriculture may have different characteristics.
In addition to urban forms of horticulture, it also includes animal husbandry in urban areas. The term goes beyond the known forms of urban horticulture (house garden, allotment garden, grave land) and includes also agriculture, Animal husbandry (poultry, domestic rabbits, urban beekeeping or aquaculture / aquaponics), provided they are operated in the urban area and peri-urban zones. The forms of urban agriculture are not tied to any particular legal form (private, communal) or socio-economic intention (self-sufficiency, market production, social exchange).
Urban agriculture can reflect varying levels of economic and social development. It may be a social movement for sustainable communities, where organic growers, “foodies,” and “locavores” form social networks founded on a shared ethos of nature and community holism. These networks can evolve when receiving formal institutional support, becoming integrated into local town planning as a “transition town” movement for sustainable urban development. For others, food security, nutrition, and income generation are key motivations for the practice. In both scenarios, more direct access to fresh vegetables, fruits, and meat products through urban agriculture can improve food security and food safety.
Urban and peri-urban agricultural activities (small livestock, gardens, aquaculture, etc.) have always existed in or near cities for practical reasons of food supply. Since antiquity, cities have created spaces for housing, crafts (then industry) and agriculture. With population growth, fields have gradually disappeared from the center of towns, but smaller plots and very many gardens still occupy a significant place in towns. The short production cycle gives the advantage to this practice. One square meter of garden can provide 20 kg of food per year.
Urban farming is often used synonymously with urban gardening, but there is an essential difference in the scale: while urban horticulture is practiced by subgroups of the general population for the purpose of self-sufficiency, urban agriculture aims to provide products for the general population – also on a commercial basis. In addition, as mentioned at the beginning, urban agriculture explicitly includes, at least in theory, the breeding of (small) livestock in urban areas.
The urban agriculture and, by extension, urban and peri-urban (UPA) is an emerging or re-emerging form of practical agricultural conducted in town. Currently, on a global scale, we are witnessing a growing interest from various actors in society for urban agriculture projects as a vector of ecological transition: sustainable food, social ties and the well-being of populations, projects participatory, environmental education.
Main types
There is no overarching term for agricultural plots in urban areas. Gardens and farms—while not easy to define—are the two main types. According to USDA, a farm is “any place from which $1,000 or more of agricultural products were produced and sold.” In Europe, the term “city farm” is used to include gardens and farms.
Gardens
Many communities make community gardening accessible to the public, providing space for citizens to cultivate plants for food or recreation. A community gardening program that is well-established is Seattle’s P-Patch. The grassroots permaculture movement has been hugely influential in the renaissance of urban agriculture throughout the world. During the 1960s a number of community gardens were established in the United Kingdom, influenced by the community garden movement in the United States. Bristol’s Severn Project was established in 2010 for £2500 and provides 34 tons of produce per year, employing people from disadvantaged backgrounds.
Farms
City farms are agricultural plots in urban areas, that have people working with animals and plants to produce food. They are usually community-run gardens seeking to improve community relationships and offer an awareness of agriculture and farming to people who live in urbanized areas. They are important sources of food security for many communities around the globe. City farms vary in size from small plots in private yards to larger farms that occupy a number of acres. In 1996, a United Nations report estimated there are over 800 million people worldwide who grow food and raise livestock in cities. Although some city farms have paid employees, most rely heavily on volunteer labour, and some are run by volunteers alone. Other city farms operate as partnerships with local authorities.
An early city farm was set up in 1972 in Kentish Town, London. It combines farm animals with gardening space, an addition inspired by children’s farms in the Netherlands. Other city farms followed across London and the United Kingdom. In Australia, several city farms exist in various capital cities. In Melbourne, the Collingwood Children’s Farm was established in 1979 on the Abbotsford Precinct Heritage Farmlands (the APHF), the oldest continually farmed land in Victoria, farmed since 1838.
In 2010, New York City saw the building and opening of the world’s largest privately owned and operated rooftop farm, followed by an even larger location in 2012. Both were a result of municipal programs such as The Green Roof Tax Abatement Program and Green Infrastructure Grant Program.
In Singapore, hydroponic rooftop farms (which also rely on vertical farming) are appearing. The goal behind these is to rejuvenate areas and workforces that have thus far been marginalized. Simultaneously top level pesticide-free produce will be grown and harvested.
Perspectives
Resource and economic
The Urban Agriculture Network has defined urban agriculture as: An industry that produces, processes, and markets food, fuel, and other outputs, largely in response to the daily demand of consumers within a town, city, or metropolis, many types of privately and publicly held land and water bodies were found throughout intra-urban and peri-urban areas. Typically urban agriculture applies intensive production methods, frequently using and reusing natural resources and urban wastes, to yield a diverse array of land-, water-, and air-based fauna and flora contributing to food security, health, ◦livelihood, and environment of the individual, household, and community.
Today, some cities have much vacant land due to urban sprawl and home foreclosures. This land could be used to address food insecurity. One study of Cleveland shows that the city could actually meet up to 100% of its fresh produce need. This would prevent up to $115 million in annual economic leakage. Using the rooftop space of New York City would also be able to provide roughly twice the amount of space necessary to supply New York City with its green vegetable yields. Space could be even better optimized through the usage of hydroponic or indoor factory production of food. Growing gardens within cities would also cut down on the amount of food waste. In order to fund these projects, it would require financial capital in the form of private enterprises or government funding.
Environmental
The Council for Agricultural Science and Technology (CAST) defines urban agriculture to include aspects of environmental health, remediation, and recreation: Urban agriculture is a complex system encompassing a spectrum of interests, from a traditional core of activities associated with the production, processing, marketing, distribution, and consumption, to a multiplicity of other benefits and services that are less widely acknowledged and documented. These include recreation and leisure; economic vitality and business entrepreneurship, individual health and well-being; community health and well being; landscape beautification; and environmental restoration and remediation.
Modern planning and design initiatives are often more responsive to this model of urban agriculture because it fits within the current scope of Sustainable design. The definition allows for a multitude of interpretations across cultures and time. Frequently it is tied to policy decisions to build sustainable cities.
Urban farms also provide unique opportunities for individuals, especially those living in cities, to get actively involved with ecological citizenship. By reconnecting with food production and nature, urban community gardening teaches individuals the skills necessary to participate in a democratic society. Decisions must be made on a group-level basis in order to run the farm. Most effective results are achieved when residents of a community are asked to take on more active roles in the farm.
Food security
Access to nutritious food, both economically and geographically, is another perspective in the effort to locate food and livestock production in cities. The tremendous influx of the world population to urban areas has increased the need for fresh and safe food. The Community Food Security Coalition (CFSC) defines food security as: All persons in a community having access to culturally acceptable, nutritionally adequate food through local, non-emergency sources at all times.
Areas faced with food security issues have limited choices, often relying on highly processed fast food or convenience store foods that are high in calories and low in nutrients, which may lead to elevated rates of diet-related illnesses such as diabetes. These problems have brought about the concept of food justice which Alkon and Norgaard (2009; 289) explain that, “places access to healthy, affordable, culturally appropriate food in the contexts of institutional racism, racial formation, and racialized geographies… Food justice serves as a theoretical and political bridge between scholarship and activism on sustainable agriculture, food insecurity, and environmental justice.”
Some systematic reviews have already explored urban agriculture contribution to food security and other determinants of health outcomes (see)
Impact
Economic
Urban and Peri-urban agriculture (UPA) expands the economic base of the city through production, processing, packaging, and marketing of consumable products. This results in an increase in entrepreneurial activities and the creation of jobs, as well as reducing food costs and improving quality. UPA provides employment, income, and access to food for urban populations, which helps to relieve chronic and emergency food insecurity. Chronic food insecurity refers to less affordable food and growing urban poverty, while emergency food insecurity relates to breakdowns in the chain of food distribution. UPA plays an important role in making food more affordable and in providing emergency supplies of food. Research into market values for produce grown in urban gardens has been attributed to a community garden plot a median yield value of between approximately $200 and $500 (US, adjusted for inflation).
Social
Urban agriculture can have a large impact on the social and emotional well-being of individuals. UA can have an overall positive impact on community health, which directly impacts individuals social and emotional well-being. Urban gardens are often places that facilitate positive social interaction, which also contributes to overall social and emotional well-being. Many gardens facilitate the improvement of social networks within the communities that they are located. For many neighborhoods, gardens provide a “symbolic focus,” which leads to increased neighborhood pride. Urban agriculture increases community participation through diagnostic workshops or different commissions in the area of vegetable gardens. Activities which involve hundreds of people.
When individuals come together around UA, physical activity levels are often increased. This can also raise serotonin levels akin to working out at a gym. There is the added element of walking/biking to the gardens, further increasing physical activity and the benefits of being outdoors.
UPA can be seen as a means of improving the livelihood of people living in and around cities. Taking part in such practices is seen mostly as an informal activity, but in many cities where inadequate, unreliable, and irregular access to food is a recurring problem, urban agriculture has been a positive response to tackling food concerns. Due to the food security that comes with UA, feelings of independence and empowerment often arise. The ability to produce and grow food for oneself has also been reported to improve levels of self-esteem or of self-efficacy. Households and small communities take advantage of vacant land and contribute not only to their household food needs but also the needs of their resident city.
This allows families to generate larger incomes selling to local grocers or to local outdoor markets while supplying their household with the proper nutrition of fresh and nutritional products.
Some community urban farms can be quite efficient and help women find work, who in some cases are marginalized from finding employment in the formal economy. Studies have shown that participation from women have a higher production rate, therefore producing the adequate amount for household consumption while supplying more for market sale.
As most UA activities are conducted on vacant municipal land, there have been raising concerns about the allocation of land and property rights. The IDRC and the FAO have published the Guidelines for Municipal Policymaking on Urban Agriculture, and are working with municipal governments to create successful policy measures that can be incorporated in urban planning.
Over a third of U.S. households, roughly 42 million, participate in food gardening. There has also been an increase of 63% participation in farming by millennials from 2008-2013. US households participating in community gardening has also tripled from 1 to 3 million in that time frame. Urban agriculture provides unique opportunities to bridge diverse communities together. In addition, it provides opportunities for health care providers to interact with their patients. Thus, making each community garden a hub that is reflective of the community.
Energy efficiency
The current industrial agriculture system is accountable for high energy costs for the transportation of foodstuffs. According to a study by Rich Pirog, associate director of the Leopold Center for Sustainable Agriculture at Iowa State University, the average conventional produce item travels 1,500 miles (2,400 km), using, if shipped by tractor-trailer, 1 US gallon (3.8 l; 0.83 imp gal) of fossil fuel per 100 pounds (45 kg). The energy used to transport food is decreased when urban agriculture can provide cities with locally grown food. Pirog found that traditional, non-local, food distribution system used 4 to 17 times more fuel and emitted 5 to 17 times more CO2 than the local and regional transport.
Similarly, in a study by Marc Xuereb and Region of Waterloo Public Health, it was estimated that switching to locally-grown food could save transport-related emissions equivalent to nearly 50,000 metric tons of CO2, or the equivalent of taking 16,191 cars off the road.
Carbon footprint
As mentioned above, the energy-efficient nature of urban agriculture can reduce each city’s carbon footprint by reducing the amount of transport that occurs to deliver goods to the consumer. Such areas can act as carbon sinks offsetting some of the carbon accumulation that is innate to urban areas, where pavement and buildings outnumber plants. Plants absorb atmospheric carbon dioxide (CO2) and release breathable oxygen (O2) through photosynthesis. The process of Carbon Sequestration can be further improved by combining other agriculture techniques to increase removal from the atmosphere and prevent the release of CO2 during harvest time. However, this process relies heavily on the types of plants selected and the methodology of farming. Specifically, choosing plants that do not lose their leaves and remain green all year can increase the farm’s ability to sequester carbon.
Reduction in ozone and particulate matter
The reduction in ozone and other particulate matter can benefit human health. Reducing these particulates and ozone gases could reduce mortality rates in urban areas along with increase the health of those living in cities. A 2011 article found that a rooftop containing 2000 m² of uncut grass has the potential to remove up to 4000 kg of particulate matter and that one square meter of green roof is sufficient to offset the annual particulate matter emissions of a car.
Soil decontamination
Vacant urban lots are often victims to illegal dumping of hazardous chemicals and other wastes. They are also liable to accumulate standing water and “grey water”, which can be dangerous to public health, especially left stagnant for long periods. The implementation of urban agriculture in these vacant lots can be a cost-effective method for removing these chemicals. In the process known as Phytoremediation, plants and the associated microorganisms are selected for their chemical ability to degrade, absorb, convert to an inert form, and remove toxins from the soil. Several chemicals can be targeted for removal, including heavy metals (e.g. Mercury and lead), inorganic compounds (e.g. Arsenic and Uranium), and organic compounds (e.g. petroleum and chlorinated compounds like PBC’s).
Phytoremeditation is both an environmentally-friendly, cost-effective and energy-efficient measure to reduce pollution. Phytoremediation only costs about $5–$40 per ton of soil being decontaminated. Implementation of this process also reduces the amount of soil that must be disposed of in a hazardous waste landfill.
Urban agriculture as a method to mediate chemical pollution can be effective in preventing the spread of these chemicals into the surrounding environment. Other methods of remediation often disturb the soil and force the chemicals contained within it into the air or water. Plants can be used as a method to remove chemicals and also to hold the soil and prevent erosion of contaminated soil decreasing the spread of pollutants and the hazard presented by these lots.
One way of identifying soil contamination is through using already well-established plants as bioindicators of soil health. Using well-studied plants is important because there has already been substantial bodies of work to test them in various conditions, so responses can be verified with certainty. Such plants are also valuable because they are genetically identical as crops as opposed to natural variants of the same species. Typically urban soil has had the topsoil stripped away and has led to soil with low aeration, porosity, and drainage. Typical measures of soil health are microbial biomass and activity, enzymes, soil organic matter (SOM), total nitrogen, available nutrients, porosity, aggregate stability, and compaction. A new measurement is active carbon (AC), which is the most usable portion of the total organic carbon (TOC) in the soil. This contributes greatly to the functionality of the soil food web. Using common crops, which are generally well-studied, as bioindicators can be used to effectively test the quality of an urban farming plot before beginning planting.
Noise pollution
Large amounts of noise pollution not only lead to lower property values and high frustration, they can be damaging to human hearing and health. The study “Noise exposure and public health,” found that exposure to continual noise is a public health problem. Examples of the detriment of continual noise on humans to include: “hearing impairment, hypertension and ischemic heart disease, annoyance, sleep disturbance, and decreased school performance.” Since most roofs or vacant lots consist of hard flat surfaces that reflect sound waves instead of absorbing them, adding plants that can absorb these waves has the potential to lead to a vast reduction in noise pollution.
Nutrition and quality of food
Daily intake of a variety of fruits and vegetables is linked to a decreased risk of chronic diseases including diabetes, heart disease, and cancer. Urban agriculture is associated with increased consumption of fruits and vegetables which decreases risk for disease and can be a cost-effective way to provide citizens with quality, fresh produce in urban settings.
Produce from urban gardens can be perceived to be more flavorful and desirable than store bought produce which may also lead to a wider acceptance and higher intake. A Flint, Michigan study found that those participating in community gardens consumed fruits and vegetables 1.4 times more per day and were 3.5 times more likely to consume fruits or vegetables at least 5 times daily (p. 1). Garden-based education can also yield nutritional benefits in children. An Idaho study reported a positive association between school gardens and increased intake of fruit, vegetables, vitamin A, vitamin C and fiber among sixth graders. Harvesting fruits and vegetables initiates the enzymatic process of nutrient degradation which is especially detrimental to water soluble vitamins such as ascorbic acid and thiamin. The process of blanching produce in order to freeze or can reduce nutrient content slightly, but not nearly as much as the amount of time spent in storage. Harvesting produce from one’s own community garden cuts back on storage times significantly.
Urban agriculture also provides quality nutrition for low-income households. Studies show that every $1 invested in a community garden yields $6 worth of vegetables if labor is not considered a factor in investment. Many urban gardens reduce the strain on food banks and other emergency food providers by donating shares of their harvest and providing fresh produce in areas that otherwise might be food deserts. The supplemental nutrition program Women, Infants and Children (WIC) as well as the Supplemental Nutrition Assistance Program (SNAP) have partnered with several urban gardens nationwide to improve the accessibility to produce in exchange for a few hours of volunteer gardening work.
Urban farming has been shown to increase health outcomes. Gardeners consume twice as much fruit and vegetables than non-gardeners. Levels of physical activity are also positively associated with urban farming. These results are seen indirectly and can be supported by the social involvement in an individual’s community as a member of the community farm. This social involvement helped raise the aesthetic appeal of the neighborhood, boosting the motivation or efficacy of the community as a whole. This increased efficacy was shown to increase neighborhood attachment. Therefore, the positive health outcomes of urban farming can be explained in part by interpersonal and social factors that boost health. Focusing on improving the aesthetics and community relationships and not only on the plant yield, is the best way to maximize the positive effect of urban farms on a neighborhood.
Economy of scale
Using high-density urban farming with vertical farms or stacked greenhouses, many environmental benefits can be achieved on a citywide scale that would be impossible otherwise. These systems do not only provide food, but also produce potable water from waste water, and can recycle organic waste back to energy and nutrients. At the same time, they can reduce food-related transportation to a minimum while providing fresh food for large communities in almost any climate.
Health inequalities and food justice
A 2009 report by the USDA, determined that “Evidence is both abundant and robust enough for us to conclude that Americans living in low-income and minority areas tend to have poor access to healthy food”, and that the “structural inequalities” in these neighborhoods “contribute to inequalities in diet and diet-related outcomes”. These diet-related outcomes, including obesity and diabetes, have become epidemic in low-income urban environments in the United States. Although the definition and methods for determining “food deserts” have varied, studies indicate that, at least in the United States, there are racial disparities in the food environment. Thus using the definition of environment as the place where people live, work, play and pray, food disparities become an issue of environmental justice. This is especially true in American inner-cities where a history of racist practices have contributed to the development of food deserts in the low-income, minority areas of the urban core. The issue of inequality is so integral to the issues of food access and health that the Growing Food & Justice for All Initiative was founded with the mission of “dismantling racism” as an integral part of creating food security.
Not only can urban agriculture provide healthy, fresh food options, but also can contribute to a sense of community, aesthetic improvement, crime reduction, minority empowerment and autonomy, and even preserve culture through the use of farming methods and heirloom seeds preserved from areas of origin.
Environmental justice
Urban agriculture may advance environmental justice and food justice for communities living in food deserts. First, urban agriculture may reduce racial and class disparities in access to healthy food. When urban agriculture leads to locally grown fresh produce sold at affordable prices in food deserts, access to healthy food is not just available for those who live in wealthy areas, thereby leading to greater equity in rich and poor neighborhoods.
Improved access to food through urban agriculture can also help alleviate psychosocial stresses in poor communities. Community members engaged in urban agriculture improve local knowledge about healthy ways to fulfill dietary needs. Urban agriculture can also better the mental health of community members. Buying and selling quality products to local producers and consumers allows community members to support one another, which may reduce stress. Thus, urban agriculture can help improve conditions in poor communities, where residents experience higher levels of stress due to a perceived lack of control over the quality of their lives.
Urban agriculture may improve the livability and built environment in communities that lack supermarkets and other infrastructure due to the presence of high unemployment caused by deindustrialization. Urban farmers who follow sustainable agricultural methods can not only help to build local food system infrastructure, but can also contribute to improving local air, and water and soil quality. When agricultural products are produced locally within the community, they do not need to be transported, which reduces CO2 emission rates and other pollutants that contribute to high rates of asthma in lower socioeconomic areas. Sustainable urban agriculture can also promote worker protection and consumer rights. For example, communities in New York City, Illinois, and Richmond, Virginia have demonstrated improvements to their local environments through urban agricultural practices.
However, urban agriculture can also present urban growers with health risks if the soil used for urban farming is contaminated. Lead contamination is particularly common, with hazardous levels of lead found in soil in many United States cities. High lead levels in soil originate from sources including flaking lead paint which was widely used before being banned in the 1970s, vehicle exhaust, and atmospheric deposition. Without proper education on the risks of urban farming and safe practices, urban consumers of urban agricultural produce may face additional health-related issues.
Implementation
Creating a community-based infrastructure for urban agriculture means establishing local systems to grow and process food and transfer it from farmer to consumer.
To facilitate food production, cities have established community-based farming projects. Some projects have collectively tended community farms on common land, much like that of the eighteenth-century Boston Common. One such community farm is the Collingwood Children’s Farm in Melbourne, Australia. Other community garden projects use the allotment garden model, in which gardeners care for individual plots in a larger gardening area, often sharing a tool shed and other amenities. Seattle’s P-Patch Gardens use this model, as did the South Central Farm in Los Angeles and the Food Roof Farm in St. Louis. Independent urban gardeners also grow food in individual yards and on roofs. Garden sharing projects seek to pair producers with the land, typically, residential yard space. Roof gardens allow for urban dwellers to maintain green spaces in the city without having to set aside a tract of undeveloped land. Rooftop farms allow otherwise unused industrial roofspace to be used productively, creating work and profit. Projects around the world seek to enable cities to become ‘continuous productive landscapes’ by cultivating vacant urban land and temporary or permanent kitchen gardens.
Food processing on a community level has been accommodated by centralizing resources in community tool sheds and processing facilities for farmers to share. The Garden Resource Program Collaborative based in Detroit has cluster tool banks. Different areas of the city have tool banks where resources like tools, compost, mulch, tomato stakes, seeds, and education can be shared and distributed with the gardeners in that cluster. Detroit’s Garden Resource Program Collaborative also strengthens their gardening community by providing access to their member’s transplants; education on gardening, policy, and food issues; and by building connectivity between gardeners through workgroups, potlucks, tours, field trips, and cluster workdays. In Brazil, “Cities Without Hunger” has generated a public policy for the reconstruction of abandoned areas with food production and has improved the green areas of the community.
Farmers’ markets, such as the farmers’ market in Los Angeles, provide a common land where farmers can sell their product to consumers. Large cities tend to open their farmer’s markets on the weekends and one day in the middle of the week. For example, the farmers’ market of Boulevard Richard-Lenoir in Paris, France, is open on Sundays and Thursdays. However, to create a consumer dependency on urban agriculture and to introduce local food production as a sustainable career for farmers, markets would have to be open regularly. For example, the Los Angeles Farmers’ Market is open seven days a week and has linked several local grocers together to provide different food products. The market’s central location in downtown Los Angeles provides the perfect interaction for a diverse group of sellers to access their consumers.
Benefits
The benefits that UPA brings along to cities that implement this practice are numerous. The transformation of cities from only consumers of food to generators of agricultural products contributes to sustainability, improved health, and poverty alleviation.
UPA assists to close the open-loop system in urban areas characterized by the importation of food from rural zones and the exportation of waste to regions outside the city or town.
Wastewater and organic solid waste can be transformed into resources for growing agriculture products: the former can be used for irrigation, the latter as fertilizer.
Vacant urban areas can be used for agriculture production.
Other natural resources can be conserved. The use of wastewater for irrigation improves water management and increases the availability of fresh water for drinking and household consumption.
UPA can help to preserve bioregional ecologies from being transformed into cropland.
Urban agriculture saves energy (e.g. energy consumed in transporting food from rural to urban areas).
Local production of food also allows savings in transportation costs, storage, and in product loss, what results in food cost reduction.
UPA improves the quality of the urban environment through greening and thus, a reduction in pollution.
Urban agriculture also makes the city a healthier place to live by improving the quality of the environment.
UPA is a very effective tool to fight against hunger and malnutrition since it facilitates the access to food by an impoverished sector of the urban population.
Poverty alleviation: It is known that a large part of the people involved in urban agriculture is the urban poor. In developing countries, the majority of urban agricultural production is for self-consumption, with surpluses being sold in the market. According to the FAO (Food and Agriculture Organization of the United Nations), urban poor consumers spend between 60 and 80 percent of their income on food, making them very vulnerable to higher food prices.
UPA provides food and creates savings in household expenditure on consumables, thus increasing the amount of income allocated to other uses.
UPA surpluses can be sold in local markets, generating more income for the urban poor.
Community centers and gardens educate the community to see agriculture as an integral part of urban life. The Florida House Institute for Sustainable Development in Sarasota, Florida, serves as a public community and education center in which innovators with sustainable, energy-saving ideas can implement and test them. Community centers like Florida House provide urban areas with a central location to learn about urban agriculture and to begin to integrate agriculture with the urban lifestyle.
Urban farms also are a proven effective educational tool to teach kids about healthy eating and meaningful physical activity.