Sports Science, Physiology, and the Debate Over Women in Ground Combat Units


Every year in professional sports, millions, perhaps billions, of dollars are spent on personnel costs — scouting, recruiting, healthcare and salaries — all in the name of winning a game. The objective is simple: maximizing and bringing to bear the physical abilities of the world’s best athletes. Similarly, in the military, the top of the human performance ziggurat is represented by the combat arms — infantry, armor, artillery, engineers, and at the pinnacle of fitness and physical ability, special operators. Given these similarities, sports medicine — the science of human performance — provides valuable insights into the ingredients in fielding a winning combat team, and the costs of compromising.

The current controversy over opening ground combat to female service members would benefit from the objective analysis of data. In the 1980s, our research group, the Institute of Human Performance, was asked by the Marine Corps to discern the relationship between the Marines’ PFT (physical fitness test) and combat performance. Our efforts and the results of this study are highly relevant to the inflamed debate on women in direct ground combat units. And while opinion inevitably plays a major role in this debate, all sides will benefit by considering our observations principally because they are based on scientific research.

The conclusions based on our research results should influence objective discussion. While this study was conducted nearly 30 years ago, it has relevance today since all the metrics remain unchanged: A minute is still 60 seconds, gravity’s force is still 32 feet per second squared, and consuming one liter of oxygen still burns five calories. While the contentiousness and framing of an issue such as this one change over time, science remains the same.

Research Design

Our group of kinesiologists, exercise scientists, sports medicine physicians, and biomechanics experts scoured the peer-reviewed published research and all the technical documents describing infantry performance as far back as World War I. We then took to the field and embedded as “grunts” in infantry battalions to get a worm’s-eye view of life in the fleet.

Living the life of a rifleman was essential for several reasons. Our credibility was enhanced by sleeping in the same fighting holes, humping through the same knee-deep mud, and bearing all of the customary loss of amenities that make up the working environment of a combat Marine. Doing whatever they were doing brought a certain level of appreciation and established in real time how fitness benefits mobility and load carriage in ways that might not be apparent to observers wearing white lab coats and carrying clipboards.

Over a five-year period, we developed a taxonomy of job tasks for the Military Occupational Specialty (MOS) 0311 (rifleman) in four theaters of combat operations: jungle training at Fort Sherman, Panama’s jungle survival school, High Altitude Winter Training at the Marine Corps’ Mountain Warfare Training Center, desert operations at Twentynine Palms Marine Base, and amphibious operations around the world. All of our observations and measurements resulted in the obvious conclusion that combat tasks at altitude and in the snow had the highest metabolic and muscular fitness demands.

We then created a combat simulation course and randomly selected 200 Marines from two infantry battalions to undergo physiological testing at the Navy’s Health Research Center in San Diego. Later, they spent a day out of their one-month deployment at 9,000 feet performing a series of linked combat tasks.

Our election was to test individuals, thereby avoiding the confounding effects of group interaction. On an individual basis, the weakest link can best be identified, rather than camouflaged by others picking up the slack. All the Marines were tested for marksmanship with their issued M-16A2 at the beginning and the end of the day’s foot movements. The energy costs of the day’s effort were equivalent to those of a half-marathon.

What We Found

In essence, our results demonstrated a clear relationship between marksmanship and fitness. This is an important statistic when you keep in mind that all the Marines were very fit compared to age-matched civilian populations or troops from other services. In an average group of service personnel including non-combat forces, the differences would likely have been staggering.

Physical size is extremely important when carrying loads that approach 100 pounds. The harsh effects of altitude exacerbate the strains on endurance. Small Marines struggled to move towed loads, and their times were considerably slower on long marches employing snowshoes. On road marches, the pace of the group tends to slow to that of the stragglers — hence our attention to the performance characteristics of Marines who were self-pacing. Physical size is a very big part of performance. That’s why the martial arts and other “combat” sports (such as boxing) have weight classes.

A forklift is illustrative of the biomechanics of load carriage and lifting. If I choose to purchase a forklift with a maximum lift of 3,000 pounds, and every lift approaches that maximum, the machine is going to fail in a relatively short time. If I have a functional reserve of 2,000 pounds, the useful lifespan of my forklift is significantly lengthened. A practical, human example is that of firefighters performing on-the-job tasks. Not surprisingly, less-fit firefighters have a higher frequency of lower back injuries. The physiological differences between the sexes are not trivial and are amplified when heavy lifting and marching under load are occupational requirements.

The Marine Corps PFT has general predictive validity but lacks precision when loads are added. Aerobic fitness is an important quality in combat, but running tests provide an advantage to lighter individuals. As loads are added to an individual, the effects to lighter individuals are more deleterious. Simply stated, larger Marines can carry greater loads with less performance degradation.

Our data has great value in cutting through the emotional issues that have obfuscated the question of how combat effectiveness would change with the introduction of women to the combat arms. Our research cannot address risks or benefits related to unit cohesion, equal opportunity, fraternization, conscription, recruitment and retention, harassment, or related issues. But it has much to offer on the issue of combat effectiveness, as well as the related issue of injury potential.

Applying the Data to Today’s Debate

Imagine for a moment that you can build a team from scratch based only on candidates’ physical characteristics. You do not know the sex of the potential team members; you only know how each measures up in those areas that scientifically predict a successful outcome: aerobic fitness (particularly absolute aerobic power unadjusted for body weight), muscular strength (as represented by maximal lifting capacity), and anaerobic power. These are the underlying factors that coaches employ to create a winning sports team. Sport-specific variables can then be added to optimize team performance for a given sport. So, for example, for baseball we could add variables like batting averages, speed to first base, fastball speed, etc. For our military team, marksmanship would weigh very high, as would load carriage and dynamic strength. But the basic fitness dimensions must be present before we add specific skills.

Science is based upon empirical evidence. If we start with a hypothesis that there is no difference between men and women (in Marines, or other groups), we should structure our decision about whether to integrate women into ground combat units in such a way that we maximize the likelihood of making the right decision and minimize the consequences of making the wrong one. The object of combat is to effectively employ violence to defeat the enemy on the battlefield. Accordingly, we want to field a team whose physical characteristics maximize our chances of success.

In statistical analysis, there are two types of decision errors (or wrong decisions) known as Type I and Type II errors. A Type I error is detecting an effect that is not present. A Type II error is failing to detect an effect that is present. If we decide that female Marines perform less well than male Marines when, “in reality,” they are equals, then we have made a wrong decision (Type I error). What are the consequences of this wrong decision? Qualified female Marines would not be assigned to infantry units, and arguably, their promotions or career paths might be affected.

If, however, we decide that female Marines perform equivalently to male Marines, but, “in reality,” they are poorer performers, then we have also made a wrong decision (Type II error). What are the consequences of this wrong decision? In this case we accept people who would reduce combat effectiveness. What would be the results of less qualified Marines being added to a unit? Marines might get killed, and the likelihood of winning on the battlefield is eroded. That’s a pretty severe downside.

Sex Differences, Peak Fitness, and Sustained Combat

Motivated individuals at virtually anytime in their lives can improve their muscular and cardiovascular fitness. In sports, the objective is to achieve peak fitness coincidental with a target event, such as the Olympics. In the military, for many non-combat arms service members, the target event will be graduation from basic combat training. During this period, impressive fitness improvements take place as new service members are challenged to do things that, for many, are outside of their comfort zone.

But basic training is but a first step for those entering combat arms fields, who will progress through job-specific advanced individual training and toward the harsh reality of sustained combat. And it is in this environment of sustained combat that fitness is most severely tested. The practical reality is that no one can stay at his or her peak levels. But in a combat environment, fitness must be kept above a minimum threshold for combat effectiveness. If that threshold is at or near an individual’s peak level, attempts to continuously work at or above his or her daily functional limits will result in injury. This is why study after study shows women (and weaker males) incurring injuries disproportionate to their numbers.

There is also the somewhat more intangible issue of combat leadership to consider, which is also tied to physiology and fitness. There are women, in small numbers that can exceed the physical work capacity of some men — men who are at the tail of the left end of the frequency distribution curve. But because of the reasons discussed above — load-bearing capability, fitness degradation, and the rigors of sustained combat — women are not found at the far right end of the frequency distribution curve, exhibiting the ability to sustain a level of fitness that will allow them to lead from the front, as is so often the practice of infantry officers up to and through the rank of lieutenant colonel. Leadership is paramount in the warrior ethos. It is infectious and critical to esprit d’corps, be it at the squad, platoon or company level.

Focus on the Mission

The worst-case scenario should always be the driving force for training. The whole idea behind training is sweating today to avoid bleeding tomorrow. Our applied research conducted in a harsh environment is still relevant. And, keep in mind, our study only measured a single day’s activity. Performance degradation is bound to happen in a sustained theater of operation. The better news is the more fit at the front end, the longer the degradation slope.

For a number of years, I worked as a fitness consultant for the Washington Redskins. Then-coach George Allen challenged me to lay awake at night, thinking of any scintilla of training, no matter how miniscule, that would give his team an edge over the nefarious Dallas Cowboys. With an intensity rivaling Patton’s, Coach Allen was consumed with winning. Suggesting that we add women to the squad would not have been well received. There is no professional sport where women are present, except in separate leagues.

Our military must focus on winning with similar intensity. If you lose sight of the mission, the war is lost.


Paul O. Davis, Ph.D., FACSM is a fellow of the American College of Sports Medicine and director of First Responder Institute. He is a recognized expert witness, having participated in over 60-employment standards lawsuits and was the Principal Investigator of the multi-year, global environmental physiology study for the Marine Corps. He is the co-author (with Dr. Brian Sharkey) of the seminal textbook Hard Work: Defining Physical Work Performance Requirements. In addition, Dr. Davis is the creator of the Firefighter Combat Challenge, the Military Battle Challenge, and provides color commentary for ESPN, Versus, CBS Sports, and Discovery.