Traditional processes and services for the delivery of health and care are experiencing a drastic shift to meet people present and future needs. The current proliferation of mobile technologies is helping to pave the path to a new paradigm in which people's health information is timely and ubiquitously available. Concretely, portable and wearable sensors are increasingly utilized to collect data on individuals' biology, psychology and behavior. This valuable information may be used to reduce health risks, optimize health outcomes, understand the determinants of health or even yield new insights into the factors that lead to disease.

Recent surveys show a growing tendency in physicians mobile and digital health adoption. Mainstream applications in the medical domain are principally devoted to learning and informative purposes. Examples of these apps are Medscape [1] and Epocrates [2], which are particularly intended to provide comprehensive and updated information for medical procedures, disease monographs, drug references or practice guidelines. Other applications are mostly useful for primary care practitioners or generalists, such as Calculate by QxMD [3], which provides them with medical calculators and decision support tools that apply to several medical specialties. Electronic reference manuals are also close at hand, as it is the case of Monthly Prescribing Reference [4], an app that incorporates prescribing notes and drug records which facilitate clinical practice and promote the access to the latest advances in treatments [5]. Physicians also increasingly recommend the use of health apps to patients [6]. While most of these apps require users to actively report about their health conditions, e.g., through annotating dietary habits [7] or daily routines [8], new technological trends seek to benefit from the information collected through wearable biomedical devices. Built-in motion sensors readily available in smartphones are used, for example, to detect abnormal conducts such as elderly falls [9] or freezing of gait in Parkinson's patients [10]. Other applications develop on data collected through external wearable health devices capable of inferring sleep disorders [11], detecting cardiovascular illnesses [12], alerting on physical conditions [13] or tracking changes in physiological responses of patients with chronic obstructive pulmonary disease [14].

Despite the rise of mHealth technology, this field is far from mature. Consumers' demand for health apps and sensors clearly outpaces the science needed to understand their benefits, risks and impact on health outcomes. In fact, researchers and developers still need to build and assess the complete spectrum of mHealth technologies, as they create safe, scalable and effective applications. To that end, powerful frameworks and tools that support the development and validation of multidisciplinary mHealth applications are required. There exist various attempts to this respect. For example, [15] provides an open source electrocardiogram signal processing code for quality analysis and atrial fibrillation screening. In [16] the authors present a mobile phone platform to collect users' psychological, physiological and activity information for mental health research. A mobile version of a data processing toolbox originally devised for computer-based architectures and principally used for human behavior modeling is provided in [17]. Distributed signal processing algorithms for the analysis and classification of sensor data are provided as part of a framework for rapid prototyping of body sensor networks in [18]. A mHealth middleware framework integrating multiple interfaces and multiparameter monitoring of physiological measurement is proposed in [19]. Tools to analyze the provenance of mHealth data have also been suggested in [20]. Despite the effort put on the development of health frameworks and tools, past contributions either focus on a specific domain or lack some essential features of health applications.

The main goal of mHealth frameworks is to foster the research and development in health and medical domains as well as to accelerate the market of mobile health technologies and applications. To that end, two supportive objectives are devised: 1) to allow developers to rapidly build applications while integrating a wide spectrum of mobile health devices; 2) to grant simple access, representation and processing of health data collected through heterogeneous resources across several applications.

During the design of a mHealth framework several requirements must be borne in mind, the essential of which are outlined in the following. A certain level of abstraction from heterogeneous resources should be ensured to make hardware and its communication transparent to the developer. For the sake of interoperability, the framework should also define a unified model for multimodal health data. Medical and health applications can operate on a local basis or remotely, thus it should be supported both local and remote storage of health data. Raw medical and physiological data is normally analyzed to extract health knowledge. Accordingly, a mHealth framework should include signal processing, data mining and machine learning techniques tailored to mHealth applications. In this regard, mechanisms to visualize medical and health information in a user-friendly fashion must be also provided for both average users and specialists. Another major requirement refers to the provision of healthcare services such as health delivery, personalized guidelines and intelligent recommendations. Finally, the framework should be modular and extensible to future sensor technologies and application needs.

In the light of the requirements presented in the previous section, a novel framework devised to enable the easy and agile development of mHealth applications leveraging on heterogeneous wearable biomedical devices is proposed. The mHealth Framework implements several functionalities to support resource and communication abstraction, biomedical data acquisition, health knowledge extraction, persistent data storage, adaptive visualization, system management and value-added services. Figure 1 shows the architecture that implements these functionalities and the components of the mHealth Framework.

In a nutshell, mHealth data delivered by mobile and biomedical sensors is collected and structured by the Communication Manager. This raw data can be stored in the Storage Manager, further processed by the Data Processing Manager, graphically represented by the Visualization Manager or directly used by the applications built on the mHeath Framework. Moreover, the medical knowledge derived by the Data Processing Manager can also be stored in the Storage Manager, input to advanced functionalities provided by the Service Enablers or used by the mHealth applications. Since the Storage Manager offers persistence, stored data can be offline processed by the Data Processing Manager, graphically represented by the Visualization Manager or accessed by the mHealth applications. Finally, the mHealth Framework offers, by means of the System Manager, functionalities to manage general resources of the mobile or wearable device.

mHealthDroid is the Android implementation of the previously proposed mHealth Framework. It is released open source under the GNU General Public License version 3 and available at [21]. mHealthDroid is devised to operate on the Android operating system version 4.2 ("Jelly Bean"), although it provides backwards compatibility to version 2.3.3 ("Gingerbread"). The managers of the mHealth Framework architecture have been developed using the singleton pattern approach in mHealthDroid.

The mHealthDroid Communication Manager has been implemented to facilitate the incorporation of new Adapters. To do so, it provides a generic Adapter skeleton. The current implementation of mHealthDroid provides the Adapter for Android mobile devices and the Adapter for the Shimmer2 and Shimmer3 wearable devices [22]. The Android Mobile Adapter abstracts the sensors embedded into the mobile device (e.g., GPS, temperature or humidity). Likewise, the Shimmer Adapters provide the means to communicate the wearable device with the mobile device and to map the data to the proprietary format. Shimmer devices provide multiple sensing modalities that span from inertial sensing via accelerometer, gyroscope, magnetometer, and altimeter, to physiological signs measurement such as electrocardiogram or electromyogram, among others.

The Storage Manager incorporates a SQLite database [23] to implement the local persistence functionality. SQLite is a popular database engine on memory constrained systems, like mobile devices, since it runs in minimal stack space and very little heap. The Storage Manager also offers an interface to easily retrieve, based on diverse identifiers (session, device identifier, date, time interval), the data stored in the SQLite database. Database consistency check procedures are implemented by the Storage Manager to ensure integrity in the synchronization between the remote and local storage. The transmission to the remote storage is implemented using a HTTP POST request method, which encloses in the request message's body the JSON [24] representation of the data. mHealthDroid also offers a server side implementation for remote persistence. This implementation builds on a MySQL [25] database and provides PHP scripts that use the MySQLi API [26] to manage the remote database.

The mHealthDroid Data Processing Manager provides an essential set of functionalities typically used in the data processing chain. The Preprocessing module implements two techniques: upsampling to increase the sampling rate and downsampling to reduce the sampling rate. A sliding window approach, widely-used in signal processing problems, is implemented by the Segmentation module. The Feature Extraction module implements some generic statistical features such as mean, variance, standard deviation, zero crossing rate, mean crossing rate, maximum and minimum. The Classification module builds on an open source stripped version [27] of Weka (Waikato Environment for Knowledge Analysis [28]). It provides functionalities to train and validate machine learning models, that can be used for classification purposes. mHealthDroid currently implements Naive Bayes [29], Adaboost [30], Decision Trees [31], Linear Regression [32] and ZeroR [33] classification techniques.

The Visualization Manager builds on the open source library Graphview [34], which has been adapted to fulfill the particular needs of mHealth data representation. The manager allows multiplot visualization, multisignal representation and customization for diverse graph types.

The System Manager offers simple interfaces to access common mobile devices resources (WiFi, 3G, Bluetooth and screen) and builds on the standard Android API [35].

mHealthDroid implements three Service Enablers. The Alerts Enabler provides interfaces to trigger phone calls and text messages. The Notifications Enabler implements text remainders that can be scheduled in a simple way. Moreover, this enabler also provides advanced notifications that can trigger external functionalities or applications. Both Alerts and Notifications Enablers build on the standard Android API. Finally, the Guidelines Enabler provides interfaces to reproduce multimedia content, both locally and remotely stored. The Media Player Android API [36] is used in mHealthDroid to control playback of audio and video files for the local content. For the reproduction of remote multimedia content, the Guidelines Enabler implements a set of functions that build on the YouTube Android Player API [37]. This is particularly practical to access a huge variety of medical and wellbeing content.

The mHealthDroid API is organized into a total of six packages, five of which correspond to each of the aforementioned managers, and an additional one comprising utilities and miscellanea functionalities. The Communication Manager package contains a set of seven classes, respectively devoted to define sensor adapters, data structures and communication protocols. The Storage Manager package is consisted of four main classes, including database adapters, storage interfaces and communication for remote persistence. The Data Processing Manager package contains six classes, one for each individual data processing module, and as for the rest of managers, an additional one for the orchestration of all its modules. The Visualization Manager package consists of two main classes that provide the required functionalities for customized data representation and the control of the canvas. Finally, the System Manager package and Service Enablers package sum up to eleven classes to support the functionalities already described above. All these classes total around 500 methods available to the developer for their use. For a thorough description of the API and its methods, the reader is particularly referred to [21].

An exemplary application has been developed to illustrate the potential of mHealthDroid. The main features and components of the mHealth Framework have been used during the implementation of this application. As already envisioned during the conception of this framework, the usage of mHealthDroid proved the rapid application development, which in this case was roughly a couple of days. In fact, most of the application development efforts were mainly devoted to the graphical user interface design, whereas the implementation of the core functionalities were easily built on mHealthDroid. The comprehensive set of predefined functions provided by mHealthDroid allowed the mHealthApp developers to easily interface a wearable sensor through simply three lines of code, create a lightweight database system to store the data in just five code lines, or visualize these data on a graph by calling a few predefined functions. This was possible owing to the level of abstraction and programming simplicity introduced by mHealthDroid, also supported with practical templates and helpful documentation provided together with its implementation [21]. The developed mHealthApp is readily available for download from Google Play at [38]. The main features of this application, health data collection, visualization, storage, guidelines and human behavior inference, are described in the following.

Health data visualization

The information collected by the wearable or mobile sensors can be depicted through the visualization menu. mHealthApp supports diverse representation modes and plot types. An online representation of the signals is provided to visualize the data collected at runtime, e.g., registered motion data (Figure 2 left and bottom-right). This type of representation is particularly useful for specialists to analyze vital sign patterns. Moreover, average values or general statistics of processed data can be also represented, e.g., heart rate averaged by month (Figure 2 top-right). This kind of graphic is specially devised for the average user, although it is also practical for the expert user.

Guidelines for health encouragement and personalized recommendations

mHealth applications are devised to play an important role within the delivery of both traditional and new healthcare services. One of these services corresponds to the empowerment and encouragement of people in their personal health care and wellbeing. mHealthApp comprises an illustrative set of resources to promote education of healthy dietary habits and physical therapy support. The application provides video tutorials and guidelines elaborated by specialists and presented in a categorized and user-friendly fashion (Figure 3). The guidelines may be initiated when the user accesses the corresponding menu or be triggered by personalized notifications. The notifications can be scheduled to a particular date and time by the user.

Inference of human behavior

mHealthApp provides a means to assess users' daily routines by analyzing the motion data collected through the wearable sensors. The application identifies a set of common activities while the user carries them out (Figure 4). To that end, a human activity recognition model is implemented. This model essentially builds on the mHealthDroid Data Processing Manager. The following section provides an extensive description of the methodology used for the development of the behavior inference functionality.

The analysis of human behavior has attracted very much attention in healthcare, assistance and wellness areas during the recent years. The identification or inference of people's conduct, also known as activity recognition, has been proven of particular interest to promote healthier lifestyles [39, 40], prevent unhealthy habits [41, 42], detect anomalous behaviors [43–45] or track conditions [46]. Wearable and mobile technologies are extensively used for the inference of human behavior, which makes activity recognition one of the most prominent domains in mHealth.

This section describes a practical solution to the activity recognition problem, which has been developed as part of mHealthApp and aims at illustrating the potential of mHealthDroid. As it is normal practice in the activity recognition domain, a dataset is collected to train and validate the recognition model. The mHealthApp activity recognition model is defined based on prior solutions proved to perform well in similar problems. In order to determine the statistical performance of the recognition model, an offline evaluation is carried out. This type of performance analysis, which is the most recurrently used in activity recognition, consists of training and validating on multiple complementary subsets of the dataset. Once the model is confirmed to perform satisfactorily, it is implemented in mHealthApp by using the mHealthDroid Data Processing Manager functionalities. To prove the correct functioning of mHealthApp during real-time activity recognition, an online evaluation, which is less frequent in activity recognition studies, is performed.

MHEALTH dataset

The collected dataset, hereafter MHEALTH dataset, comprises body motion and vital signs recordings, for ten volunteers of diverse profile, while performing 12 physical activities (Table 1). Shimmer2 wearable sensors [22] were used for the recordings. The sensors were respectively placed on the subject's chest, right wrist and left ankle and attached by using elastic straps as illustrated in Figure 5. The use of multiple sensors permits us to measure the motion experienced by diverse body parts, namely, the acceleration, the rate of turn and the magnetic field orientation, thus better capturing the body dynamics. The sensor positioned on the chest also provides 2-lead ECG measurements, which are not used in this work for the development of the recognition model, but rather collected for future work purposes. This information can be used, for example, for basic heart monitoring, checking for various arrhythmias or looking at the effects of exercise on the ECG. The sampling rate used for all sensing modalities is of 50 Hz, which is considered sufficient for capturing human activity. All sessions were recorded using a video camera. The video recording was used to label the data and to check anomalous or unexpected patterns in the signals.

Activity Set
L1: Standing still (1 min)	L7: Frontal elevation of arms (20×)
L2: Sitting and relaxing (1 min)	L8: Knees bending (crouching) (20×)
L3: Lying down (1 min)	L9: Cycling (1 min)
L4: Walking (1 min)	L10: Jogging (1 min)
L5: Climbing/descending stairs (1 min)	L11: Running (1 min)
L6: Waist bends forward (20×)	L12: Jump front & back (20×)

In brackets are the number of repetitions (N×) or the duration of the exercises (min).

This dataset is found to generalize to common activities of daily living, given the diversity of body parts involved in each one (e.g., frontal elevation of arms vs. knees bending), the intensity of the actions (e.g., cycling vs. sitting and relaxing) and their execution speed or dynamicity (e.g., running vs. standing still). The activities were collected in an out-of-lab environment, with no constraints on the way they had to be executed. In any case, the subjects were asked to try their best when executing them. The MHEALTH dataset is readily available for download at [47].

Offline evaluation of the mHealthApp activity recognition model

The offline evaluation of the activity recognition model is performed on the MHEALTH dataset and through a cross-validation process. The cross-validation technique involves partitioning the dataset into complementary subsets, performing the analysis on one subset (training set) and validating the analysis on the other subset (validation or testing set). Although there exist several cross-validation approaches, leave-one-subject-out cross validation and ten-fold cross-validation are the most widely used in activity recognition. In this study the latter validation technique is particularly considered since it leads to a better estimate of the performance of the recognition model [60]. Moreover, the ten-fold cross-validation process is repeated 100 times to ensure statistical robustness and to procure an asymptotic convergence to a correct estimation of the system performance [61].

Diverse metrics may be used to evaluate the performance of the recognition system. The confusion matrix stands out among others, since it summarizes in a single matrix all the information corresponding to the performance evaluation. From this matrix, several other metrics can be simply derived such as the sensitivity (SE), specificity (SP), positive predictive value (PPV), negative predictive value (NPV) and F-score. An extensive review of these and other metrics can be seen in [62].

The results obtained after evaluation are shown in Figure 6 and Table 2. As it can be observed, the developed system provides very promising recognition capabilities for the considered activities. The confusion matrix (Figure 6) is practically diagonal, which represents a performance close to absolute. This is further confirmed for each particular activity, considering the values obtained for each performance metric (Table 2). In fact, daily activities such as standing still, sitting, lying down or walking are unequivocally identified. Subtle misclassifications are seen for other activities that are more dependent on the physical conditions of the subjects, such as running or knees bending. At any rate, from the successful outcomes of this evaluation it can be concluded that the proposed system is a promising activity recognizer.

Activity	SE	SP	PPV	NPV	F-score
L1	1.00	1.00	1.00	1.00	1.00
L2	1.00	1.00	1.00	1.00	1.00
L3	1.00	1.00	1.00	1.00	1.00
L4	1.00	1.00	1.00	1.00	1.00
L5	0.99	1.00	0.99	1.00	0.99
L6	0.97	1.00	0.97	1.00	0.97
L7	1.00	1.00	0.99	1.00	0.99
L8	0.95	1.00	0.97	1.00	0.96
L9	1.00	1.00	0.99	1.00	1.00
L10	0.96	0.99	0.94	1.00	0.95
L11	0.94	1.00	0.96	0.99	0.95
L12	0.99	1.00	0.99	1.00	0.99

Each metric respectively correspond to the sensitivity (SE), specificity (SP), positive predictive value (PPV), negative predictive value (NPV) and F-score.

Online evaluation of the mHealthApp activity recognition model

The online validation allows us to corroborate the potential of the mHealthApp activity recognition model at runtime. The recognition model implemented in mHealthApp builds on the complete MHEALTH dataset. The validation is performed on a different set of subjects to the one considered for the model training. A total of five volunteers were asked to perform the complete activity set (Table 1). Each activity was carried out during 30 seconds as it was considered to be enough given the characteristics of the exercises. The activities were performed in various outdoor scenarios for the users' convenience and to reduce their awareness as much as possible. During the executions both user's activity and smartphone's screen (Figure 4) were recorded on video for the evaluation of the system performance. This was considered to be a more tractable approach than using commentary sheets for the activities annotation. The performance is evaluated by comparing both actual and detected activities based on the observation of the video recordings. Actual and predicted activities are aligned taking into account the 1-second delay associated to the data segmentation process. Transitions among the activities of interest are left out of the study since a null-class rejection schema has not been explicitly implemented [63].

The activities detected by mHealthApp, for each user, are depicted in Figure 7. In broad strokes, it can be said that the system shows good recognition capabilities. Only a few outliers or misclassifications are observed. For example, during the identification of "sitting and relaxing", the model sometimes interprets that the users are bending their waist forward or elevating their arms. This is explained by some abrupt movements observed during the execution of this activity for some of the participants. Similarly, some errors are found for the detection of "knees bending or crouching", which is confused here again with "waist bend forwards". This is a consequence of some difficulties encountered by part of the users while performing this exercise, which translated into a moderate sway back and forth. Finally, a few misclassifications are observed among "walking", "jogging" and "running", which are basically originated from the varying cadence with which these activities were executed by the subjects. All these conclusions are further supported by the confusion matrix (Figure 8) and the individual performance metrics for each activity (Table 3). In summary, the developed mHealthApp activity recognition model is not only shown to operate in an experimental basis but also proved to work well during the normal use of the system.

Activity	SE	SP	PPV	NPV	F-score
L1	1.00	1.00	1.00	1.00	1.00
L2	0.93	1.00	1.00	0.99	0.97
L3	1.00	1.00	1.00	1.00	1.00
L4	1.00	1.00	1.00	1.00	1.00
L5	1.00	0.99	0.88	1.00	0.94
L6	1.00	0.99	0.91	1.00	0.95
L7	1.00	0.99	0.94	1.00	0.97
L8	0.90	1.00	1.00	0.99	0.95
L9	1.00	1.00	1.00	1.00	1.00
L10	0.87	1.00	1.00	0.99	0.93
L11	1.00	1.00	1.00	1.00	1.00
L12	1.00	1.00	1.00	1.00	1.00

Each metric respectively correspond to the specificity (SP), sensitivity (SE), positive predictive value (PPV), negative predictive value (NPV) and F-score.

A novel mHealth framework intended to facilitate the development of mobile health applications in a simple and agile fashion has been presented in this paper. The framework has been designed taking into account crucial requirements of mHealth technologies and applications. This work has also introduced mHealthDroid, an open source implementation of the proposed mHealth Framework that operates on the Android OS. This implementation aims at bringing together mobile devices and heterogeneous multimodal sensors including both research and commercial systems. mHealthDroid supports basic and advanced features of mHealth applications such as resource and communication abstraction, biomedical data acquisition, health knowledge extraction, persistent data storage, adaptive visualization, system management and value-added services such as intelligent alerts, recommendations and guidelines.

An exemplary app has also been provided along with this work to showcase the potential of mHealthDroid. mHealthApp implements mechanisms for collection and visualization of health data, persistent remote storage and personalized health and wellbeing guidelines. This app also provides an advanced means to detect and track human behavior, a functionality that has been further validated in this work through extensive experimentation. The agility and simplicity gained by using the mHealthDroid API is proved through the reduced time required for developing an app of these characteristics.

Ultimately, mHealthDroid aims at bringing together developers, healthcare professionals, academics and health enthusiasts to exchange ideas and cooperate in the definition of valuable tools for a healthier world. Accordingly, the authors encourage the community to contribute to this innovative platform by supporting the use of the latest sensors, incorporating new behavioral algorithms or simply making use of it for the development of novel mobile health applications.

Written informed consent was obtained from the participants for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.