NATICK, Mass. (April 5, 2018) -- The exercise physiology laboratory located within the U.S. Army Research Institute of Environmental Medicine, or USARIEM, has been bustling with activity this month with researchers collecting bone health data from 30 female research volunteers in an effort to better understand how bones, and hormones that affect bone regrowth, respond when new recruits start their first days of initial military training, or IMT.

Stress fractures and other musculoskeletal injuries not only hurt the Soldier, but, as the number one cause of medical holdovers, they also hurt Army readiness by costing the Army millions of lost or restricted work hours and dollars.

In fact, according to last year's statistics, about 20 percent of female recruits and five percent of male recruits can suffer from some form of stress fracture during IMT due to the novice warrior's inability to withstand unaccustomed, repeated stress to their bodies, such as marching with body armor. Women beginning IMT with poor vitamin D status are especially vulnerable to these injuries.

USARIEM's military performance and nutrition teams made an Army-wide impact in 2017 when it came to preventing injuries and improving readiness. USARIEM researchers conducted separate field studies that led to developing the Occupational Physical Assessment Test, or OPAT, which assesses a recruit's physical performance capabilities to determine if they should be allowed to join the Army, and the Performance Readiness Bar, or PRB, a calcium and vitamin D-fortified snack bar developed to strengthen bones.

Since the Army began administering the OPAT to all recruits in 2017 and since all Army Basic Combat Training schools began distributing the PRB in 2018, Soldiers have been able to begin their careers with a slimmer chance of experiencing career-threatening occupational injuries.

Dr. Erin Gaffney-Stomberg, the principal investigator of the current USARIEM women's bone health laboratory study, and her team realize that in order to create more countermeasures against musculoskeletal injuries, it is also important to understand how the human skeleton responds to militarily-relevant exercise, especially when a freshly recruited Soldier is completing IMT for the first time.

"We know that during IMT, that period of seven to twelve weeks when a civilian first enters the military and undergoes a series of trainings, the risk of stress fracture is higher," said Gaffney-Stomberg, a research physiologist from USARIEM's Military Performance Division. "We also know that bone turnover increases. In other words, bone formation and bone resorption markers or hormones in the blood go up with training. What we do not understand is some of the reasons why these hormones go up and whether or not these changes are beneficial or detrimental to bone."

Gaffney-Stomberg and her team have zeroed in on the parathyroid hormone, or PTH, which is secreted from four parathyroid glands in the neck. According to Gaffney-Stomberg, PTH regulates calcium levels in the blood by increasing the levels when they are too low. The hormone does this through its actions on the kidneys, bones and intestines.

"The parathyroid hormone increases during training for Soldiers and during other types of intense exercise for civilians, and it is known to stimulate the release of calcium from large calcium stores in the bones into the bloodstream, decreasing the formation of new bone," Gaffney-Stomberg said. "In this study, we are researching to understand why PTH goes up with the initiation of exercise."

Both men and women can produce PTH. However, Gaffney-Stomberg noted that targeting women as the study demographic would not only lead to gaining insight into women's bone health, but it would also provide data that Army researchers could use to prevent stress fractures in all Soldiers.

"With women being integrated into combat roles, there is a greater need for research that can help us develop countermeasures against musculoskeletal injuries," Gaffney-Stomberg said. "Once we understand what PTH does in people who are at higher risk of stress fractures, we can use this information to improve existing injury prevention guidance, including developing dietary interventions and proper exercise guidance."

One of the main culprits of stress fractures is recruits completing exercises they are unaccustomed to for longer periods. In order to replicate this common incident during IMT in the lab, the researchers had study volunteers complete a load carriage exercise, which would simulate recruits marching in body armor.

For this study, the research volunteers not only had to be women. They had to be civilians.

"We are trying to understand the initial bone and calcium kinetic response to a militarily relevant exercise," Gaffney-Stomberg said. "We want someone who has not been through military training before, such that their bones are naïve or unaccustomed to that type of exercise."

During the load carriage exercise, each study volunteer wore body armor that was weighted at 30 percent of her body weight. The volunteers walked at different speeds and grades on a treadmill, exercising at about 65 percent of their maximum effort, which the researchers calculated during the beginning of the study through an aerobic fitness test. Study volunteers spent three days in USARIEM's exercise lab out of a six-day period either exercising or resting.

To understand how each volunteer's PTH levels changed during and after the load carriage exercise, the researchers needed a way to track where calcium traveled in the body. The researchers did this by giving the volunteers stable calcium isotopes both orally and through an IV on the first day of the study. Gaffney-Stomberg explained that these isotopes were non-radioactive and served as a safe way to record a person's calcium levels. The researchers recorded these hormone changes by collecting urine and blood samples from volunteers during and after the treadmill exercise.

Since diet can also affect a person's PTH levels, the researchers provided the study volunteers with all of their food, which was supposed to mirror the amounts of calcium each volunteer normally ate at home.

The researchers will continue their data collection for the rest of 2018, and they plan to publish their results by next year. Despite being a small lab, USARIEM has made a significant mark in the Army's effort to prevent injury, and they will continue to do so in the future. This study is one of several USARIEM studies focused on prevention and reduction of musculoskeletal injuries among Soldiers.

"There are multiple factors that can impact a person's risk of developing a stress fracture," Gaffney-Stomberg said. "Some of those factors are not modifiable, such as being female. There are also genetics that can place people at higher risk. Then, there are modifiable factors, such as nutrition status and physical fitness. At this point, there is guidance on what the known risk factors are, and then once a person experiences a bone fracture during training, there are clinical care guidelines for how to treat that person. Typically, that involves taking a calcium supplement, as well as some rehabilitation.

"Unfortunately, about 60 percent of male and female recruits who experience a stress fracture will drop out of the military. If we can reduce the number of people who get injured during IMT, we can reduce the number of people whose military careers have prematurely ended."