The U.S. Army uses wired and wireless systems to monitor real-time performance and safe operating limits of vehicles and aircraft, but no comparable systems exist for soldiers. New wearable technologies could change that.

A chest-worn physiological sensor system such as this one can provide information on a soldier's work- and heat-related strain to team members down-range and leaders at a command post. (U.S. Army)

The limiter to real-time physiological status monitoring (RT-PSM) has not been engineering technology. More than 50 years ago, John Glenn blasted off from Cape Canaveral while his medical team monitored a modest heart rate increase to 110 beats per minute. This ability to remotely monitor physiological signals has improved in terms of size, weight, comfort, cost, and power. Today, for example, recreational athletes commonly monitor their heart rate with commercial body-worn systems.

For at least as long as the space program has been around, the Army has conducted research on wearable monitoring technologies. When then-U.S. Senator John Glenn flew on the Space Shuttle Discovery in 1998, he swallowed a thermometer pill that monitored circadian body temperature rhythms in orbit, using a temperature monitoring system provided by the U.S. Army Research Institute of Environmental Medicine (USARIEM). A chest-worn system created through a successfully completed Army science and technology objective monitored the physiological responses of Austrian daredevil Felix Baumgartner during his 2012 jump from a helium balloon in the stratosphere. Soldier monitoring capabilities have now matured to the point that they can be used to enhance safety and performance during work in hot environments.

Turning physiological data into actionable information needed by soldiers makes a RT-PSM system worth the extra cost, weight, training, and complexity. Raw data is interesting, but not easily interpreted, even by a medic. For example, a high heart rate can variously indicate normal sympathetic activation needed to perform a task, cardiac compensation for hemorrhage or peripheral vasodilation in the heat, or response to an extreme psychological event. Similarly, measurement of core temperature is not, by itself, as useful as it might seem.

The wide range of normal core body temperature among warfighters was not well described until USARIEM field studies using wearable physiological monitors revealed how much core temperatures fell in metabolically challenged Ranger School students, and how high they went during Marine patrolling activities in Iraq and Afghanistan. Laboratory studies would never have revealed these extremes of normal warfighter physiology. Clearly, contextual information and mathematical models that automatically interpret wearable sensor data streams are needed to interpret core temperature and provide actionable, individualized safety and performance information.

Applying the Technology

Soldiers take part in a road march during Ranger assessment at Fort Benning, GA. The “precision medicine” approach based on an individual's documented physiological responses could be particularly useful in high-intensity Ranger training where march and run paces are adjusted for environmental conditions. (U.S. Army)

Applications of RT-PSM technologies include dismounted route-planning decision support tools, performance and safety monitoring in high-risk chemical and biological threat environments requiring full protective gear, and performance and safety training for individuals and small-unit leaders. The use of a RT-PSM to enhance soldier performance and to avoid heat casualties is very different from medical management of casualties after they occur. Medics will bring their own U.S. Food and Drug Administration (FDA)-certified medical devices to diagnose and treat casualties, upload secure medical data to central repositories, and conduct remote telemedicine. In contrast, RT-PSM is not a medical system providing data for medical decisions, but rather a source of useful safety and performance information.

The Army National Guard (ARNG), an early adopter of thermal- or works-train monitoring, is working with the USARIEM and Massachusetts Institute of Technology (MIT) Lincoln Laboratory to define requirements and concepts of operation. The ARNG's Weapons of Mass Destruction Civil Support Teams (WMD-CSTs) train and respond to emergency events in full chemical, biological, radiological, nuclear, and explosive protective gear. Chest-mounted physiological sensors provide work- and heat-strain data to downrange team members and to leaders at a command post. Other applications include the use of wearable sensors to quantify human thermal or work strain during field evaluations of new jungle uniforms performed by the Marine Expeditionary Rifle Squad “Gruntworks,” the human systems integration center at the Marine Corps Jungle Warfare Training Center in Okinawa, Japan.

In another Marine Corps research collaboration with USARIEM, RT-PSM technologies documented the physiological responses of Marines during foot patrols in Iraq and Afghanistan, resulting in knowledge that influenced Marine Corps field doctrine and tactics, techniques, and procedures related to the conduct of patrols.

The current concept is to provide a readout for the individual soldier or squad leader that is comparable to a combination of engine temperature and tachometer, i.e., a thermal work-strain index based on heart rate and core temperature. According to extensive lab and field research by USARIEM, a high index indicates that someone is working close to his or her upper limit of cardiovascular performance and thermal tolerance, and is likely to be stopped by the individual's own physiological limits. This index has also been tested as a simplified, green-yellow-red stoplight risk designation for overall squad and individual squad member status.

Additional research by USARIEM, conducted in collaboration with the Gruntworks and with the Australian military's Defence Science and Technology Group-Melbourne under a project arrangement, demonstrated that heart rate over time can be used to accurately estimate core body temperature, eliminating the need for temperature pills.

Most recently, a task force composed of USARIEM, MIT Lincoln Laboratory, and the Marine Expeditionary Rifle Squad conducted a first test of thermal work-strain monitoring during Marine Corps training using a chest-mounted RT-PSM system that communicated status information to instructors via an operationally acceptable wireless data link. An important lesson learned was that the training cadre, realizing that the trainees were at relatively low thermal work strains and could be further challenged, wanted to use the information to push trainees harder than they would have without the monitoring. This demonstrates the kind of innovation that comes from the end users themselves during iterative field testing, suggesting a “precision medicine” application of RT-PSM to more effectively train each individual according to his or her current level of fitness and acclimatization.

A More Capable Soldier

Information from RT-PSM can also be used in mission planning and route finding. In collaboration with the U.S. Department of Agriculture's Human Nutrition Research Center, the USARIEM tested the use of RT-PSM and novel algorithms to provide continuous pacing guidance to individuals who were tasked with completing a five-mile run within one hour under thermally challenging conditions. They were to pace themselves to complete the mission on time, but not overheat, and to arrive as cool and physically capable as possible. The pacing feedback significantly improved their performance in comparison to a separate trial in which each individual used his or her own pacing strategy.

Robert A. Heinlein captured the imagination of many when he described a wearable physiological status monitor on every soldier in his sci-fi novel “Starship Troopers.” Today, that is as quaint as the old Tom Swift books that imagined a day when spaceships might land on the Moon. Although the current, affordable, wrist-worn technologies have already far surpassed Heinlein's vision, the Army's interest centers on chest-based sensors. Current wrist- and arm-based sensors are power-hungry, largely proprietary, and prone to motion artifacts. They use militarily unacceptable modes of wireless communication, and cannot provide information obtained from the chest, such as respiration rates and body position.

An alternate approach to directly monitoring individual physiological responses is to use rational mathematical models to predict soldier limits. Well-validated USARIEM thermal models can provide mission planning guidance and generalized predictions, but are not intended for real-time use, and will not precisely predict individual responses.

Key targets beyond thermal works-train include assessments of hydration state, readiness and alertness, and musculoskeletal fatigue and strain. These may include unobtrusive sensors to monitor water consumption and loss; goggles that monitor eye responses; communication systems that also measure voice changes, speech content, and breath chemistry; skin sensors that assess stress and alertness; and sensors to monitor extremity temperatures to protect and sustain performance in cold weather. Simple helmet or boot sensors could detect ground impact forces and lower extremity patterns of movement that can provide useful information about impending injury, fatigue, and even behavioral changes.


USARIEM and Lincoln Laboratory have a longer-range plan to combine physiological monitoring with outward-looking detectors to trigger threat alarms that allow soldiers to don protective ensembles. Even military working dogs may benefit from current work to develop thermal work-strain monitors based on collar-worn acoustic sensor systems analyzing panting patterns. Miniature, possibly implantable, body-heat-powered sensors will be even better and are around the corner, leveraged in part by Small Business Innovation Research contracts and by an earlier Army program called Technologies for Metabolic Monitoring. The Army is also leveraging the National Science Foundation's Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies program that is developing the next-generation, ultra-low-power RT-PSM “system on a chip.”

Wearable physiological monitoring technologies are essential tools needed to understand soldier physiology in training and operational field environments. Knowledge gained will lead to new insights into individual and small-unit leader readiness, and help guide changes in field doctrine, materiel development targets and strategies, and trade-space analyses. Physiological models can embody knowledge gained from field and lab studies, and enable predictions for conditions not yet experienced. This capability goes beyond simply duplicating the roles of good leadership and training; it is an important part of what makes real-time monitoring useful.

This article was written by Dr. Reed W. Hoyt and Dr. Karl E. Friedl (COL, USA Retired). For more information, visit the USARIEM at here .