1. Introduction
Over 60 % of the world’s population performs physical activity for less than 30 minutes per week. Well-known consequences of health-damaging everyday habits are obesity [
1,
2], decreasing performance capability/resilience, and the occurrence of chronic secondary diseases [
3]. This development is also reflected in the candidate selection or day-to-day military duties. However, health and fitness are currently the most discussed topics in society when lifestyle is concerned. Many manufacturers try to implement this new idea of all-day measuring of physiological data on the commercial market in order to show the user their activities so that one can deduce how to improve their nutrition and physical activity, including the sleep-wake rhythm – in short, how to improve one entire behavior (consumer-oriented fitness and health offers) in the future. While some systems may stay on standby until needed (e.g., pacemakers that detect arrhythmia), other data should be recorded permanently so that the user or a surveying person in a remote setting is informed online and can change the behavior in real-time [
4]. The digitization of the health sector virtually moves from bottom to top. The consumers set the plan, with the increased health consciousness being the primary reason for this trend. More than 100,000 applications from the health and fitness areas are on the market and can be displayed on a smartphone. In recent years, the so-called wearables (mainly as a bracelet or integrated into a watch) were placed on the market. They can be operated autonomously or connected to a smartphone [
5]. Wearables use simple physiological parameters (movement, heart rate) and convert them into distances covered and calories consumed utilizing specific algorithms. Thus, fitness apps and wearables constitute the interface between leisure time and health offers and are currently the growth engine of the e-health market [
6]. Previously, these opportunities were only available to chronically sick persons and top athletes [
7,
8,
9]. Meanwhile, smartphones are even scheduled for diagnostic purposes, which is critical from a physician's point of view [
10] because it will be possible for amateurs to make examinations, which will not be evidence-based medicine [
11].
In extreme environments where it may be very difficult or even impossible to provide medical assistance in the case of an emergency, telemedical systems which allow remote control of the person(s) are of particular interest. Such a setting may significantly increase health, safety, and performance, not only in the military but also in a civilian setting. Examples are firefighters who must handle extreme heat and the risk of hyperthermia or mountain rescuers who have to work in cold – sometimes extreme cold – environments and face difficult terrain, hypoxia, helicopter noise, and other stressors [
12,
13,
14,
15,
16]. However, to increase health and safety in such situations, real-time monitoring is needed in contrast to the alarm systems mentioned above [
17,
18,
19,
20,
21,
22,
23].
2. History of Telemedicine
Telemonitoring is not a new issue. Dr. Einthoven already demonstrated this in 1906 when he had electrocardiogram (ECG) data transmitted from a hospital to his laboratory using a telephone line [
24]. For astronautics, the first time in history that the medical data transfer principle was applied on a large scale was situation-related. The Russian space agency (RKA, today: Roskosmos) and the US-American space agency (NASA) started with medical telemetry programs at the end of the 1950s in order to check the physiological functions of the astronauts and to transmit the data to Earth [
25]. At that time, television technology was used.
1964 the first interactive T.V. telemedicine project started, permitting medical care using a secure T.V. link between a psychiatric ward in Nebraska and a Norfolk clinic. Due to the enormous expenditure and lack of financial profitability, such projects were terminated at the beginning of the 1970s. Only the emergence of digital communications technology and the strongly criticized medical care of cosmonauts/astronauts led to another increase in research work in the early 1990s. With the development of new mobile communications technologies like infrared, UMTS, and Bluetooth, mobile telemedicine has become a growing market and will presumably remain in the future [
26]. Image digitization and data compression procedures permitted video data transmission via low-bandwidth lines. With the World Wide Web introduced into the Internet by Tim Berners-Lees in 1993, data files stored on different computers could be interconnected [
27]. This enabled second-generation telemedicine to provide medical knowledge in otherwise medically inaccessible regions, i.e., aviation and sea travel and areas like the Arctic or the Antarctic [
28] or Space [
29].
All examinations performed in the laboratory must prove whether the results are valid for daily life circumstances [
30], a problem already emphasized by Nathan Zuntz [
31] at the beginning of the twentieth century. His primary interest was to explore the physiology of animals and humans in stressful situations (physical labor, physical exercise, stays in high altitude places) in laboratory and field studies [
32], [
33] or during high altitude balloon flights [
34], [
35] He developed mobile measuring systems and even included meteorological conditions in his interpretation. His motto was: "The knowledge of the relations between climate and humans […] provides us with the basis for evaluating human performance capability […] (and) also with the foundation of human activity under extreme climatic conditions" [
36]. After his death, this approach faded into obscurity. In physiological studies conducted, there were instead no significant relationships between heart rates measured in the laboratory and the field [
37], whereas other studies yielded different [
38] or only minor associations [
39]. In a multimodally-conceived comparison between laboratory and field, no significant predictability of the reactions could be found, leading to the far-reaching conclusion of relative independence of field data [
40]. So, it can be assumed that data collected in the laboratory are probably valid for some, but not all, daily life situations, i.e., they can hardly be generalized.
For the reasons mentioned above, it is stated that the advantages of field research, on the one hand, and the questionable external or ecological validity and reliability of test and laboratory findings, on the other hand, have long since been discussed in the literature. But empirical field studies can only sometimes be found [
41]. This is primarily due to the low methodical experience or the need for more suitable equipment. The transferability of examination results from the laboratory to everyday life is considered critical.
For decades, examining humans freely moving during their activities under natural conditions was impossible. Therefore, humans were primarily examined at rest before or after their activities or possibly in their actions, but this needed interruptions. Consequently, active humans’ physiology primarily developed into rest physiology [
42] or the physiology of recovery [
43]. During the transition from rest to activity, the organism's fundamental adaptation processes during the entire training and during the transition from activity to rest could not be recorded under natural conditions [
44]. Moreover, the data’s significance decreases as time passes between stress and measurement. Therefore, a more precise and, thus more significant measurement of physiological framework data must be performed on-site. With all conventional methods, the subjects must be connected to the point of registration through hoses and wires, resulting in a significant number of activities remaining elusive for exact measurement [
45]. Also, measurements where the examiner can't be together with the proband or where the examiner's presence may be perceived as disturbing will have an impact on the data (e.g., the "white coat effect" [
46]).
Moreover, the mental situation impacts the data [
47]. Physical and psychological stress are intertwined; thus, it is not astonishing that permanent stress also results in clinical abnormalities [
48]. This is also true the other way around. These relationships are scientifically edited within the studies' scope on psychophysical connections, show a distinct correlation, and attract attention [
49]. However, laboratory studies are performed in an idealized way, i.e., excluding disturbing influences that exist in the real world [
45]. These studies simulate stress situations, and the influencing environmental factors are systematically standardized or eliminated. Yet, this causes problems when these results are transferred to reality; some examples are listed below:
Examinations of the working and performance capability are performed using conventional, i.e., clinical standards [
50]. The equipment used, consists of medical devices and is thus validated but mostly inaccurate for field equipment [
51].
Functional diagnoses are made based on short-time examinations, which have little to do with the proper working environment regarding the temporal aspect [
40].
Physiological stress indices are generated statically. They constitute the mathematic-functional correlation between a few characteristic physiological parameters [
52], which cannot be generally transferred to the natural, dynamic situation [
53].
Studies are mostly conducted with a few persons with limited statistical power. In addition, "idealized persons" are recruited, i.e., mostly young men aged between 20 and 30 years, sporty, with normal weight, etc. However, the results are then extrapolated and transferred to all other persons. But the transfer can neither be applied to all age groups (age physiology) nor both genders (gender physiology).
In the real human environment, numerous additional influences significantly impact performance capability and resilience [
54]. In this context, the combination of temperature, altitude (oxygen pressure), wind, radiation, precipitation, pressure changes, noise disturbances, etc., must be mentioned [
55]. The complexity of these factors is not taken into account in the laboratory.
In the future, more reliable indices will have to include more parameters on the one hand and take into account intra-individual dynamic factors, e.g., sleep deficit, inadequate food intake, beginning or healing infectious diseases, or influences due to the unique environment (family, friends, etc.) [
56]. “Dynamic” in this context primarily means weighting the individual parameters, which may sometimes impact the person in the respective situation, sometimes more or less intensively [
57].
3. Physiological Monitoring in the Military Area
For decades, the US-American military sector (e.g., U.S. Army Research Institute of Environmental Medicine (USARIEM)) has been working on a technology to support soldiers medically in the theater of operations or during training. The Future Force Warrior (FFW), formerly called Objective Force Warrior (OFW), was part of the Future Combat System (FCS) project of the United States Army [
58]. It enabled infantry members to participate in the Global Information Grid (GIG). It was designed to increase soldiers' survivability, robustness, and networkability on the future battlefield, but it was stopped for political reasons. This program also included the so-called Warfighter Physiological Status Monitor (WPSM) subsystem [
59]. The WPSM had sensors integrated into a suit to monitor each individual soldier's physiological and medical condition (pulse, body temperature, blood pressure, respiration, stress, and posture). If necessary, the WPSM could transfer the corresponding data to the military leaders or the medical personnel when the respective soldier was tired, wounded, or even fatally hit and should calculate a soldier's survival probability during combat [
60]. But this seems to be more an evaluation and an estimation of military combat efficiency, not a caring approach. Work continues on parts of the former program, now called Brigade Combat Team Modernization. The project is subdivided into subareas, including the Early Infantry Brigade Combat Team Capability Package (E-IBCT). The field units will be equipped with a Network Integration Kit (NIK) to per000mit network-centric warfare (NCW). Later, additional elements of the Follow-On Incremental Capabilities are scheduled to supplement the Global Information Grid network; this program was planned to be operational by 2020 [
61]. Due to their orientation towards physical performance optimization, cognitive performance, decision-making, and judgment, deployment health protection, heat and cold stress, high altitude, and pathophysiology, the two latter aspects, in particular, have determined the focal points. During their missions, the US-Americans particularly focus on heat stress and the significant failure rates that go with it. Due to these data on heat sicknesses or heat death, a parameter measurement has been introduced into the U.S. military, where intensive work has been performed (Thermal and Mountain Medicine, USARIEM).
In Europe, this approach has yet to attract much attention. Within the scope of an EDA project, it was attempted from 2010 to develop a new kind of biomonitoring and use it for military purposes, all under the slogan "Soldier Modernization Harmonization – Biosensor Information Demonstrator" [
62]. All in all, it must be noted that this concept did not work, particularly concerning data transfer. Within the Bundeswehr, the "Future Infantryman/In fantrywoman" (German abbreviation: IdZ) project [
63] is the German modernization program for the infantry, which is affected in NATO under the slogan "Future Soldier" and serves the purpose of improving the "personal combat equipment of infantry in the Bundeswehr". The manufacturer's overall system responsibility lies with Rheinmetall Defence Electronics residing in Bremen. The current system status (2021) is IdZ-4, which will be delivered to the Bundeswehr under the designation "Gladius" (implementation phase 2) beginning in 2013. Although many components for the infantryman/infantrywoman have been incorporated into this system or are available, there is no mature physiological monitoring. However, it is obligatory for the employer and the executive personnel responsible to fully comply with their legally imposed overall care responsibility [
64].
In contrast, telemedicine in the Bundeswehr already began before the year 2000 and is still employed successfully [
65]. It is primarily used for transferring X-ray data and on sea-based units and medical facilities during missions abroad. The main purpose of telemedicine is to evaluate symptoms and abnormalities by a medical specialist located in the home country, who can then help initiate the ensuing therapy or make recommendations. As early as 1997, such a system was established in the Bundeswehr and has been extended until now [
66].
4. Occupational Fields of High Responsibility in a Stressful Environment
In aviation, various environmental and aircraft type-related factors impact the human organism, which is not tailored to these conditions from an evolutionary perspective. The said factors must be coped with either behavioral measures or technical solutions. Pilots and flying personnel are significantly affected by stress [
67], especially when the military flight service is concerned [
68]. Considering the job of a pilot acting in extreme flights as the operator of high-performance and agile military aircraft is characterized by high physical stress and mental and neuropsychological strain [
69] within the human-machine system. This can have acute [
70] and chronic [
71] effects, thus also resulting in creeping exhaustion. And the stressors cannot be denied, not only in the real situation but even during training [
72]. Due to the high responsibility in this occupational field, a decline in or loss of operational capability constitutes an increased risk for the persons themselves and others (e.g., passenger transport). In addition to person-specific factors, the respective environmental conditions, which still need to be considered in most of the monitoring performed so far, are essential when evaluating stress situations where the different factors often coincide. There is a high physical [
73], and increasingly psychological, psychophysiological [
74], and cognitive [
75] stress not only in jet aviation but also in modern fixed-wing aircraft used for transport and in helicopters. This trend is additionally enhanced by an ever-increasing use of equipment [
76] (e.g., night vision goggles (NVG)). Unmanned air vehicle (UAV) pilots, who primarily have to cope with mental stress, must not be ignored entirely [
77,
78]. Performing regular medical assessments and maintaining the fitness status through appropriate programs [
79] makes it possible to preserve and ensure flight operations, but more is needed from today's point of view.
Therefore, physiological monitoring would be desirable for various reasons. Literature research is only very fruitful from Russian publications [
80]. In the library of the former Institute of Aviation Medicine (Königsbrück, German abbreviation: ILM), however, reports and treatises from the former East German National People's Army (Nationale Volksarmee, German abbreviation: NVA) could be found [
81]. The paper stated: "The progressive development of aviation technology imposes very high requirements on flying personnel's psychological and physical performance capability” [
82]. Thus, the responsibility of aviation medicine increases as well. In order to meet this responsibility, they must know the stress affecting flying personnel during flight and the response reactions triggered by the human organism. Undoubtedly, these factors can be determined most effectively utilizing corresponding examinations performed during flight combat training”. Beside this data are scarce, focusing on telemetric ECG recordings during flights [
83,
84,
85]. The requirements imposed on the biotelemetric complex, i.e., tele-measuring of biophysical or physiological parameters, during which signal transmission is partly affected using radio or carrier frequency procedures [
86], must include the following characteristics:
No impairment of flight safety and the activities of flying personnel
Small dimensions and low weight
No changes concerning aircraft engineering (except for attachment of the transmitter and the antenna as well as the power supply from the aircraft electrical system for the transmitter)
Low operating and servicing effort
No disturbing influences on airborne equipment and no corresponding disturbances of the biotelemetry device
High stability with regard to flight and environmental factors
Relatively high transmitter power to bridge long distances, with high constancy of the transmission frequency
These aspects are still relevant today and serve as a basis for developing such systems or have already been implemented in the commercial wearables mentioned above. In the concluding remark, the biotelemetry device used is described as valid and operational. The expected assumptions and requirements have been fulfilled so that it has been possible to collect valuable aeromedical data for the purpose of evaluation.
7. Recent Developments: The Mobile Physiological Laboratory
The HealthLab system, which is a core component of the mobPhysioLab
® (KORA Industrial Electronics GmbH, Hambühren, Germany [
127]) and has been under development in Germany for several decades, is a mobile physiological system intended to examine stress situations affecting humans in various working environments. It permits both physiological data collection and the conduct of specific studies on persons in extreme stress situations under the conditions directly prevailing on site.
Contrary to conventional procedures, this system permits continuous, non-invasive physiological data measurements through innovative medical technology. It consists of flexible individual components that connect psychophysiological measuring methods with collecting environmental parameters through information technology. This system's current development status integrates and combines physiological measuring procedures in a flexible, mobile, and relatively small device. This device is based on a multiprocessor system combining a network of satellites with a master. The satellite elements have a respective "intelligence" of their own (i.e., a separate BUS). The satellite elements communicate via a bus with the central processing unit (CPU). In principle, this first seems to be more complex than developing one single measuring unit with four channels and particularly one processor only, but it has several advantages:
The analog and digital measuring techniques are distributed among several small units and can, for example, be placed in a vest.
The connection of unique measuring methods (e.g., core body temperature, heart rate, fundamental vocal frequency, etc.) is possible due to the development of a corresponding satellite, which can be used directly on the system without any further settings.
The central processing unit automatically recognizes the respective satellites and immediately records the data with the satellite's signal-specific parameters.
With the help of the software and hardware provided, users can also develop satellites and programs of their own.
Moreover, the system can monitor the data online and thus already evaluate them.
The described modularity and the versatile applicability regarding adaptation options and the system's mobility have yet to be commercially available in this form. The freely programmable software applications give the user a great variety of modification options, depending on the respective special applications and requirements. Thus, only the individual wearer's appropriate parameters are recorded, making the system extremely flexible and lighter (
Figure 1,
Figure 2,
Figure 3 and
Figure 4).
It has been possible to employ the system in terrestrially extreme environments successfully (i.e., in salt mines at a depth of 1,450 m [occupational medicine], in water applications [therapy], at high altitudes [military] and in the [civilian and military] flying service, even in Space [ESA and NASA, ThermoLab]; IBMP [MARS500, Russia]):
The Double Sensor (Tcore
® [
128,
129,
130,
131]) – A Non-Invasive Device to Continuously Monitor Core Temperature in Humans on Earth and Space [
132]
Metabolic and Physiological Changes in the Human Organism during Overwintering in the Antarctic [
133]
Monitoring of Vital Parameters in a Real-World Environment Using a Flexible Modular Telephysiological System [
134]
Oxygen Supply for Parachutists at Operational Altitudes – Laboratory Studies and their Benefits in an Extreme and a Real-World Environment [
135]
Heat Exposure of Jet Pilots during Air Traffic [
136]
Mars500 Project – Circadian Rhythm and Body Core Temperature in Humans during Long-Term Isolation and Confinement [
137]
The Role of Exercise in Synchronizing the Circadian Timing System during 60 Days of Bed-Rest [
138]
Core Body Temperature Changes during Long-Duration Space Flights [
139]
Currently, this system is advanced by miniaturization, insertion of sensors into a smart textile, and data transfer via radio; with the focus being on the following requirements:
Provision of emergency functions and algorithms [
140]
Development of built-in-test (BIT) functions to ensure operation through monitoring of all necessary functions, including power supply
Algorithms regarding the trend statement of vital parameters during missions
Recording of system parameters
Safety functions for the mission duration (reliability)
Unique algorithms for processing large and complex data quantities with low demand for computing power, yielding a valid diagnostic value
Data safety (especially for military use) and person’s privacy