How the Human Body Physically Performs

How the Human Body Physically Performs

Exercise physiology shows us how people move and adapt, advancing our knowledge of the possibilities and limits of the human body.
By Christine Yu

On July 26, 2024, the Games of the XXXIII Olympiad will open in Paris, France. Over 10,000 athletes will gather to compete in 329 events across 32 sports—from gymnastics to weightlifting, from equestrian to surfing—pushing the boundaries of human performance. While world-class athletes are gifted genetically, they don’t arrive on the biggest stage in sports by their genes alone. It takes years of dedicated training. And a lot of science.

The field of exercise physiology—and sports science, more broadly—helps researchers understand how bodies adapt to training and move efficiently. It helps people become better athletes, but it also brings us closer to understanding the limits of human performance. 

But studying the science of sports doesn’t just benefit the most active among us. Ultimately, it helps us all.

Why We Study Exercise

Exercise physiology examines how exercise stress affects the body’s physiological systems. “When you put the stress of exercise on the body, all the systems are going to change—heart rate, ventilation, blood flow, core temperature,” says Kimberly Stein, PhD, senior principal scientist at Gatorade Sports Science Institute (GSSI). “Exercise physiologists look at those changes and evaluate what’s safe and what’s not. They also look at adaptations over time, when you train consistently or when you stop exercising.” 

But the field didn’t start with the express purpose of improving athletic performance. For example, when the Harvard Fatigue Laboratory was founded in 1927, its main purpose was to investigate how humans respond to physiological, psychological and sociocultural stress caused by daily activity and industrial work. By understanding what causes the body to tire, researchers could help companies improve worker efficiency. Exercise offered a clear way to study fatigue and “steady state” activity. 

During World War II, the U.S. government contracted the Harvard lab to make recommendations for military operations in extreme heat and cold. This directive drove research to understand how to acclimatize to different environments and what that meant in terms of thermoregulation and hydration. 

“These principles of acclimatization are all the same. You modify them to specific populations,” says Michael Sawka, PhD, adjunct professor of biological sciences at Georgia Institute of Technology. “When athletes got interested in how to acclimatize to the heat, they could take the same basic principles and apply it to sports.” 

Performance Factors

Despite playing collegiate basketball and softball, Stein didn’t necessarily expect her career to involve so much sweat. At GSSI, she oversees the elite athlete services program, which, most of the time, involves sweat testing. 

Regulating body temperature and proper hydration is critical for athletes. “If you don’t sweat appropriately or don’t replenish the fluid and electrolytes lost, you risk heat illness,” Stein says. Dehydration can lead to reduced blood volume, decreased skin blood flow and increased core temperature. It’s a particular concern when exercising in hot, humid environments or when athletes use a lot of equipment, such as in football. 

Hydration also affects physical and cognitive performance. “An approximate 2% loss in body weight is when you start to see performance detriments,” she says.

But there isn’t one hydration strategy that works for everyone. “Everyone has a unique sweat profile in terms of how much you sweat and how much sodium you lose,” she says. So, Stein’s team travels to a team or athlete’s location for sweat testing in their specific environment. 

They weigh athletes before and after practice and note any body weight changes. “It’s old-school, but it’s still the best way to know how much fluid is lost during exercise,” she says. They measure any fluid that leaves the body as urine, as well as anything the athletes eat or drink during practice. Athletes wear a patch on their forearm, which is analyzed for electrolytes. The team then determines each athlete’s individual sweat profile and hydration plan. 

Hydration and thermoregulation are just two factors that influence performance. Muscle, genetics, nutrition, biomechanics, psychology and gender differences all play a role. By studying the mechanisms behind these factors, researchers help build the foundation for better training programs, which benefits not only elite athletes but anyone who wants to be physically active. 

“The goal is to promote an efficient, safe and evidence-based way of training to maximize progress and minimize potential downfalls,” says Christoph Handschin, PhD, professor of pharmacology at the University of Basel in Switzerland.

With the advent of wearable technology, there’s a potential goldmine of data from a wide range of people across age, sex, racial and ethnic backgrounds. Not only will these data help researchers understand the spectrum of human health and performance better, they could help make better decisions, too. 

For example, Sawka is part of a team that is using wearable data, artificial intelligence and machine learning to predict heat stroke. Where others could predict heat stroke two to three minutes ahead of time, Sawka says they’re able to predict it 40 to 50 minutes early. “It’s a limited number of cases, but it shows the power of this type of approach,” he says.

Women in Sports

Historically, women have been underrepresented in sports science research. The lack of scientific evidence, coupled with sociocultural beliefs about women’s physical capabilities, has restricted women within the athletic arena. “Our culture limits our physical capacity and keeps us from being our best biological selves,” says Sandra Hunter, PhD, professor of exercise science and director of the Athletic and Human Performance Research Center at Marquette University in Milwaukee. 

In recent years, understanding gender differences has become a major area of focus as researchers attempt to tease apart female physiology and what that might mean in terms of training guidelines. To reach their athletic potential, should women train differently?

Take hydration. Stein notes that there are physiological changes in the luteal phase of the menstrual cycle that might affect thirst, but when drinking liquid freely, whenever and in whatever volume desired, overall fluid balance and fluid retention don’t change. “Most people assume that there are going to be big sex differences. While, technically, there are some differences, women for the most part have similar physiological responses to dehydration,” Stein says. “It doesn’t warrant different hydration recommendations.”

However, there may be other areas where sex differences do make a meaningful difference and may call for different recommendations. Scientists are beginning to uncover these nuances, but more research is needed.

From Olympians to Average People

When you consider that sedentary behavior and lack of physical activity increase disease risk and accelerate aging, it makes sense that researchers want to study muscle and understand how and why exercise confers so many health benefits. “In order to understand the bad, you have to understand the good,” Handschin says. 

Muscle does more than just generate force. “You could argue that it’s not just a tissue, that it’s an organ,” Handschin says. “Repeated bouts of exercise over time leads to remodeling and shifts in cellular aspects like inflammation, metabolism and so forth. We want to understand the molecular mechanisms that control muscle plasticity in health and in disease.” 

Studying exercise allows researchers to understand the whole spectrum of the human body’s physical capacity. Elite athletes represent the upper limits. “We’ve got to know what the 100% physical capacity is because it gives us a boundary. It lets us put that stake in the sand to say this is what we could be,” Hunter says. Researchers can then discover the biological and physiological principles that underpin optimum health and performance, as well as the factors keeping people from being their optimal selves. 

For instance, a major focus of Hunter’s research is on how and when muscles become fatigued. In the lab, she asks participants to perform an exercise, such as a leg extension, lifting a weight equivalent to roughly 20% of their maximum strength. While it might not sound difficult, participants must perform a leg extension once every three seconds for four minutes. Hunter and her team measure the relative decline in muscle power. And with age, muscles fatigue faster. 

When older adults contract their muscles repeatedly, like they might when going up stairs, they tire more quickly. Hunter says it’s not just because they are less strong or powerful. “There are other factors contributing to the greater muscle fatigue that occurs with aging. If we understand the mechanisms and causes, then we can treat it. We can address it,” she says. 

For example, Hunter’s group is performing experiments in which older adults strength-train in conjunction with blood-flow restriction. They’re finding that after eight weeks of three-times-a-week training with the restriction of blood flow, older adults’ muscles don’t fatigue as quickly. This is the type of technique being adopted in physical therapy clinics and by athletic trainers for older adults and athletes to enhance training.

Looking Ahead

Despite the rich literature on the science of sports and exercise, researchers are only scratching the surface. “Many of the things that we do to train are not evidence-based. Maybe that’s the reason why we jump around from training modality to training modality,” Handschin says. “There’s still so much to be learned.”

And as researchers continue to hone our understanding of the science of sports, we can continue to push the boundaries of human performance.

This article was originally published in the July 2024 issue of The Physiologist Magazine. Copyright © 2024 by the American Physiological Society. Send questions or comments to tphysmag@physiology.org.