You’re at the gym. The clock on the treadmill reads 20 minutes. You’ve got 10 more minutes to go. Beads of sweat are trickling down your face, and most of the time you like the feeling of sweating it out — it means you’re working hard. But for some reason, today, you’re feeling a lot more tired than you did in the past days. It’s harder to breathe, your muscles feel more achey, and every step feels like a monumental effort. What gives?

When it comes to exercise, it’s easy for us to think only about the physical training we put our bodies through at the gym or on a run. We think only about the amount of weights we’ve added to our barbells or the amount of time we’ve been on the elliptical. In reality, those physical stressors are among thousands of invisible stressors in the environment. They can add up, and they can make it harder for your body to sustain the physical stress you put it through at the gym.

We may forget to think holistically about exercise, but the concept isn’t new. According to ancient traditional Chinese medicine, a person should take into account his or her vital energy — qi (“chi”) — when deciding how much to exercise. According to ancient Indian Ayurvedic wisdom, people should exercise based on their body type. Those with a vata body type, for example, have less endurance and should avoid strenuous exercise. Those with a kapha body type on the other hand, excel at endurance sports. Chinese medicine and Ayurvedic practitioners identify other factors such as diet in energy management, too.

How heavy a burden?

Today, health and fitness experts have begun to get behind concepts like allostatic load and physiological load. In 1993, neuroscientist Bruce McEwen and physiological psychologist Eliot Stellar came up with a concept called allostatic load, which looked at the wear and tear that stress creates on the body. These stressors could come from the environment (through substance abuse, noise or temperature) or from the mind (anxiety or loneliness). In turn, overloading the body with stress could cause your body to go into a heightened or “aroused” state, wreaking havoc on the body’s immune system, impairing cognitive performance, decreasing bone density and muscle tissue and increasing blood pressure. In other words, McEwen and Stellar found that stress can really affect your body’s ability to function properly and take on life’s challenges.

Meanwhile, Paul Chek, founder of the C.H.E.K. Institute, systematized the concept of physiological load, our capacity to handle life’s challenges and surprises. Chek focuses on six categories of stressors—physical, chemical, electromagnetic, psychic, nutritional, and thermal. If you think of your body as a backpack, each stressor adds a load to your backpack. Before long, your body can’t take the load anymore.

For this reason, Chek has all his clients fill out a comprehensive questionnaire to determine what stressors may be impacting their health and then prescribes an exercise regimen accordingly. Someone who is exposed to a lot of chemical and nutritional stress, for example, would be prescribed a lower-impact exercise regimen until the other sources of stress have been dealt with. “A lot of times, doctors and trainers will treat the symptom, like being overweight, without dealing with the root issue,” Chek said.

Since Chek started his institute in 1995, many other trainers have begun to take a holistic approach to exercise as well, not only looking at exercise, but also at the environment the person is exercising in. “The more stress we accumulate, the more ‘physiologically’ our systems are taxed, drained – and our tolerance is decreased. And while sometimes this won’t show up in one or two bouts of exercise, it will inevitably show up soon – usually in the form of illness, injury and/or disease,” said Rodney Corn, co-founder of Personal Training Academy Global.

Here’s a look at the parts of your body that help you carry – or cause you to struggle with – physiological load.


Let’s start at the very top. While much has been written about how exercise helps improve mental health, less has been written about your mood’s impact on your motivation for exercise. This mental component is incredibly important and can determine the physiological load you are able to take on.

There’s the simple fact that when you’ve accumulated a lot of mental stress, it can be difficult to find the motivation to exercise. In 2014, researchers at the Yale Stress Center reviewed years of studies about the correlation between stress and physical exercise. They found that chronically stressed populations, such as caregivers and parents of children with cancer, were much less likely to exercise. They also found that beginning exercisers, or people who did not exercise regularly, were much less likely to exercise when stressed (those who did exercise regularly actually exercised more, in the face of stress).

Corn says that in his training sessions, he asks people to embody anxiety and try to do 20 squats. He’ll then ask them to rate how good the squats felt. Then he’ll ask people to pretend they’ve just won a billion dollars (“not just a million, a billion!” he says) and do 20 squats. Again, they rate how good the squats felt. On average, people rated the anxiety squats around a 3 or 4, and rated the billion-dollar squats around 8 or 9.

Corn says that taking into account a client’s mental state is an important part of his job.

“Sometimes we can create anxiety, and that anxiety will negatively impact not only our mental health, but also our physical performance,” he said. In other words, if you have a load on your mind, it will be a lot more difficult to take on a load at the gym.

Working out while stressed can lead to greater risk for injury, and stress also makes it harder for your body to recover physically. In a small 2012 study, researchers at the University of Texas put 31 undergraduate students through intense exercise and measured the muscle strength right before and after the exercises, and 20 minutes, 40 minutes and 60 minutes after the exercises. Before the exercises, the students filled out a questionnaire about their stress level. The researchers found that students with more stress took a much longer time to recover their maximum muscle strength than students who had less stress. This was true across the board, even when the researchers controlled for fitness levels and experience. Other studies have shown that mental stress can make workouts feel more difficult, which can affect a person’s speed or strength.

Mental health and stress also affect your hormones, which can also decrease your physiological load — a topic we will investigate more deeply later on.

Heart and lungs

The heart gets a lot of attention in exercise classes and articles about exercise. Experts are always talking about increasing your heart rate or getting your heart pumping. There’s a good reason for this — a healthy heart is crucial to your overall well-being, and exercise can help strengthen the heart muscles.

On the other hand, stress on your heart can decrease your physiological-load capacity significantly. While there haven’t been any direct studies looking at energy levels and heart health, studies have shown that a heart under stress can’t perform to its highest capacity. The cardiovascular system is responsible for distributing blood and oxygen to working muscles, delivering nutrients and fuel to active tissues and transporting hormones throughout the body — all jobs that are incredibly important as you exercise. A 2015 study released by the American Heart Association found that high blood pressure and high cholesterol (all indicators of an overloaded heart) increased a person’s risk for sudden cardiac arrest during exercise.

Many of the things that stress the heart also place added stress on the lungs. The respiratory system is critical for oxygenating blood and removing waste from the body, which is even more important during exercise (when the body needs more oxygenated blood and when more waste is produced). The amount of oxygen your body can take in and use is one of the main determining factors for how long your muscles can contract during endurance exercise.

Studies have shown that the best predictor for endurance during exercise is a person’s VO2 max, or the highest rate at which the body can take up and consume oxygen during intense exercise. The cardiovascular system and the respiratory system work together to transport oxygen, and how well those two systems work are the biggest determinants of a person’s VO2 max. Toxins from vehicle exhaust and mold in the home can add stress to your respiratory system, compounding stress on your heart and reducing your VO2 max.


Those two small organs that sit at the base of your lower back, the kidneys: while they are each less than half a pound, they are the body’s workhorse. They also determine how much physiological load your body can take on.

The kidneys are responsible for stimulating the production of red blood cells, which are the cells that carry oxygen to body tissue. The kidneys are also responsible for converting vitamin D into its most active useable form. All of these functions are extremely important when exercising, especially for endurance athletes. Studies, including one by members of Royal Melbourne Hospital and Western Hospital, University of Melbourne and Victoria University, have shown that patients with compromised kidney function are unable to sustain aerobic exercise for the same amount of time as healthy peers of the same age, and develop muscle fatigue more quickly.


Although your hormones aren’t really an organ, they are present throughout your body and are extremely important for physical fitness and recovery. The endocrine system is kind of like the train conductor of your body — it signals when certain metabolic processes should kick into gear to help the body use its energy efficiently. For example, the pituitary gland signals the release of the human growth hormone (HGH) during exercise, which signals the body to increase body, muscle and tissue production.

The adrenal glands release the hormone cortisol, which helps control blood pressure, glucose and acts as an anti-inflammatory agent. Our pancreas produces insulin, which causes our cells to take up glucose from the bloodstream and store it as energy in our muscles. Studies have shown that both estrogen and testosterone helps with muscle recovery. Healthy hormone levels mean that our bodies are functioning smoothly and we have more energy to take on life’s challenges.

Adjusting workouts

Your body can only take on so much stress. Personal trainer Corn likes to use the metaphor of a water bottle: “The amount of water in the bottle represents stress,” he says. “Some people have lots of water in their bottle on some days, but not others; some rarely have much water in their bottle; and still others have almost a full bottle every day. How much water is already in the bottle will directly dictate how much more can be put into the bottle, through exercise or working out.”

Both Chek and Corn use questionnaires to determine the amount of accumulated stress their clients already have. These extensive questionnaires look at everything from diet to sleep patterns, and help trainers adjust exercise recommendations accordingly. For clients with high levels of accumulated stress, Corn says he may advocate for a more moderate exercise plan, avoiding high-intensity exercise and decreasing exercise volume. He may advocate for more warm-up time as well. For some clients, Corn says it may be even better to take a rest day.

Chek says he will often recommend the client participate in a modified exercise regimen until those stressors are reduced. “If you continue to push yourself without reducing those stressors, you can seriously endanger your health,” he said.

The environment pulls the trigger

The good news is that you can do a lot to change the environment around you to reduce the amount of stress you place on your body, and increase your physiological-load capacity. For example, clutter, noise, lighting and décor can all affect our mood and stress levels. “The first thing we do with clients is help them de-clutter their space,” said Chicago-based sports psychologist Michele Kerulis. “Clutter can be extremely stressful, and it deters people from exercising.”

You can also take a page out of architects’ notebooks: they’ve known for years that nature helps calm and focus the mind, exposure to sunlight is good for the brain, and soft edges help people feel more comfortable and at ease.

A healthy diet also reduces the amount of stress your body has to manage. Studies have shown that diets high in certain types of fats and sugar can lead to build up of plaque in the arteries, which can narrow the coronary arteries and place additional stress on the organ as it tries to deliver blood to the rest of your body.

You can also try to limit your exposure to chemical toxins, which place a significant burden on your organs. Heavy metals, including cadmium, lead and mercury can damage the kidneys and disrupt hormones. These heavy metals can be found in canned food linings, cigarettes, vehicle emissions, lead paint and certain cosmetics. A study conducted by researchers in England in 2015 found that long-term exposure to small amounts of the herbicide Roundup can cause damage to the kidneys and liver.

Other hormone-disrupting chemicals include BPA (found in plastics and cans), atrazine (found in pesticides), phthalates (found in soft plastics and plastic wrap), PFCs (found in non-stick cookware), organophosphate pesticides and glycol esters (found in household cleaning products). Many of these chemicals have been linked to disruption in estrogen and testosterone levels or disruption of the thyroid, which is responsible for hormone regulation. “The biggest change I tell people they can make is to get rid of all the toxins in their home,” Chek said. “Air scents are the worst, and get rid of the bug spray.”

And your phone and DNA might be triggers too

There is also some evidence that our electronics may be disrupting our hormones as well. A study published in 2014 by researchers in Iran found that excessive use of mobile phones disrupted thyroid function. The World Health Organization advised that mobile phone use be classified under the category “precautionary principle,” in order to keep tabs on the potential negative effects mobile phones could have on human health. In addition, some health practitioners recommend removing all electronics from the bedroom and powering down the wireless router at night.

There’s another reason why changing the environment around you is important: you could be affecting the DNA expression of your children. Researchers involved in the burgeoning field of epigenetics have found that our physical experiences can affect the chemical markers that signal the body to express certain DNA. Whereas in the past, scientists believed that only DNA was passed on from parent to offspring, they now suspect that we pass on those chemical markers as well. This means that even if, say, you have a genetic predisposition to breast cancer, you might carry chemical markers that keep the gene from expressing, and you can pass those chemical markers on to your children.

Scientists have also found that those chemical markers can be affected by your environment and your experiences. A study published in 2012 by researchers from Sweden and Denmark found that when people work out, the chemical markers that switch off genes involved in energy metabolism were removed, which allowed those genes to switch on. The more the person worked out, the more the genes regulating energy metabolism were switched on. Scientists are now investigating whether these exercise induced epigenetic changes can be passed on through generations. The results are mixed so far, but the idea is intriguing.

So the next time you’re at the gym, don’t forget that your body is taking on a lot more than the 10-pound weights you’re lifting … it’s also bearing the burden of living in a modern society. Be kind to your body and make sure that you’re giving it the proper load, in light of the other stressors it has to take on – or even removing some of those stressors. You might even pass on some of that good health to your children and grandchildren.