**1. Introduction**

Advances in the field of technology and developments in common transport systems have greatly reduced people's physical activity [1]. In developed countries, the majority of people travel to and from work using motor transport systems, e.g., statistics from 2011 show that, in England and Wales, 85% of the population used motorized transport as their usual commute mode [2]. Nowadays, citizens spend much more time at sedentary activities, such as working in front of the computer. In 2012, data from 66 high and low income countries show that the percentage of adults who spent four or more hours sitting each day was 41.5% [1]. Due to these changes in our lifestyle, the risk of suffering health problems as a result of physical activity is increasingly high [3]. Experts from diverse entities, such as WHO (World Health Organization), recommend an average of 150 min of exercise per week or 30 min daily [4]. Physical activity provides a well-known set of health benefits [5]. Exercise has been proven to reduce the risk of suffering from high blood pressure, stroke and others [4]. It increases cardiorespiratory and muscular fitness, bone health or increased functional health. Moreover, it can help prevent depression [4]. In 2010, the Global Health Observatory (GHO) estimated that the daily physical activity of more than 20% of adults is insufficient [6]. The low exercise, combined with the daily ingestion of fat and calorie rich foods, is leading our society to an obesity epidemic [7].

Nowadays, people who wish to make exercise a part of their daily routine usually go to gyms or sign up for different sports. This commitment implies an economic cost of registration and sports equipment, travel to sports centers, as well as the necessary free time to carry out the activity and a willingness to attend regularly. Performing a team sport (e.g., basketball, football or volleyball) also requires adequate sports facilities, a group of people to carry out the activity and a skill in that sport. For these reasons, many people do not perform physical activity regularly [8,9]. As a result, the daily use of a bicycle in routine trips is an important alternative to the gym or other sports [10].

One of the most widespread ways of fostering active transport among users is by promoting the use of bicycles in the city [11,12]. Biking will not only help people to get fit but will also reduce traffic congestion, environmental contamination, climate change and energetic sustainability [13,14]. The upgrading of infrastructures for cyclists helps provide a positive experience and as a result increases the use of bicycles in the city [15,16]. In recent years, cities have been promoting the use of bicycles through the implementation of bicycle-sharing systems (BSS), in order to allow users to travel short distances by bike [17]. Recent studies have shown that the use of BSS has a positive impact on health [18] and reduces the use of motor vehicles [19,20].

A decisive factor for encouraging the use of bicycles is the development of electric bicycles or e-bikes. An electric bicycle consists of three main elements: Engine, battery and control system. These elements are installed in a conventional pedal assist bicycle, which combines the power exerted by the user with the power supplied by the electric set. In comparison to conventional bicycles, these are power assisted bicycles which can travel greater distances, providing greater mobility and reducing barriers (such as age, physical limitations, steep slopes, and lack of time) [21].

Some people have barriers that make the use of a traditional bike challenging or impossible, however electric bicycles may give them an opportunity to start cycling [22]. Bicycles of this type are essential if an increase in the number of bikers in the city is to be achieved [23]. Electric bicycles can help users get active, especially those who have a sedentary work, can at least do exercise by cycling between home and work [24]. The bicycle industry esteems that the e-bike market will continue to grow. It is estimated that, in the last decade, more than 150 million electric bicycles have been sold worldwide [25]. In 2015, 1.2 million were sold in Europe, and it is estimated that this number will triple in 2022 [25]. The increase of sales is a result of the relatively low cost of these vehicles (generally, less than 1000 euros) and they are increasing in popularity over scooters [26,27]. In addition, the new e-bike BSS systems are increasingly popular among users in comparison to traditional bicycles because they make travel easier and more comfortable when factors such as long travel distance, high temperatures or poor air quality are involved [28]. On the current market, it is possible to find two types of electric bicycles [26]: Throttle electric bikes and Pedal Assist Systems (PAS) e-bikes.

Of these electric bikes, the throttle e-bike [29], offer an acceleration device to the users, similar to the one used on the handlebar of mopeds. With this system, the users can activate and deactivate the assistance of the engine, as well as regulate its intensity. Thus, the user is in total control of the assistance provided by the engine, making the use of bicycles much simpler. The use of pedals and the user's physical implication is optional, given that the user does not have to put any physical effort into activating the accelerator. Generally, these bicycles are used by expert users who wish to have precise control of the level of power supplied by the engine. In many countries, such as Spain, Finland or the UK, there are strict rules that regulate the users of these bicycles. In Spain, the use of these bicycles on public roads is prohibited and, to be able to use them, the user needs to have liability insurance.

Assist level electric bicycles or bicycles with a PAS, function differently. Unlike in throttle electric bicycles where, thanks to the acceleration system, the user does not have to exert any physical pressure on the pedals, in PAS bicycles, it is necessary [29]. These e-bikes incorporate a sensor that registers the pedaling velocity of the user and activate the electric engine when pedaling starts. As to the control of the power supplied by the engine, a set of assist levels is employed which will progressively increase or decrease the power provided. A remote control installed on the bike's handlebar is used to manage assistance levels, it can also be used to interact with any app for smartphones. In this way, the user can freely perform all the operations without removing his hands from the handlebar. The use of these bicycles is also regulated by the law in many European countries. In countries such as Spain and the UK, engine assistance cannot exceed 25 km/h and the electric power of the engine cannot be greater than 250 W [30]. Moreover, the engine has to remain in a resting state when the user stops pedaling. The proposed system is designed for PAS bicycles. Almost all the commercial electric bicycles that we find on the current market are pedal assist.

While electric-bikes lower the levels of physical activity compared with pedaling a regular bike, pedaling a regular bike on flat land for a very short distance is beneficial but not necessarily highly health-inducing. Individuals could go to the gym or engage in sports but that takes time and money. The goal of this work is to increase the levels of physical activity while riding an electric bike, thus allowing the bike to become the gym through an activity that is a routine part of the day. A secondary goal is to achieve an increase in the number of bicyclists, either riding regular bikes or electric-assist bikes. To this end, a personalized pedaling intensity level system for power assisted electric bicycle users has been developed. With this system, users will be able to gradually progress their physical activity, as they travel different routes on their e-bikes. Many studies have shown that riding on a bicycle with constant or incremental velocity in time has significant health benefits [31,32] and contributes to the user's fitness [33].
