10.9: Physical Activity and Fitness
Both maintaining consistent physical activity and having a high level of physical fitness are protective against the chronic diseases which are the primary causes of death. Physical activity refers to the total amount of movement you get throughout the day. Fitness is referring to the conditioning of your body: the capacity of your heart, lungs, and muscles to perform high intensity work. The two are indelibly intertwined of course, since it takes a minimum level of fitness to be physically active, and those who have a high level of fitness tend to do more physical activity. Yet what is interesting is that each is independently protective against cardiovascular disease, diabetes, and some cancers - regardless of body weight, or age, or sex (which are usually also risk factors). In other words, being active and fit is important for health in every body (Foreman, 2020).
The CDC recommends that adults get a minimum of 150 minutes of moderate physical activity or 75 minutes of vigorous physical activity (or some combination of both) per week, including muscle strengthening exercises on at least 2 days of the week. But that’s just the bare minimum. Doubling that amount - 300 minutes of moderate or 150 minutes of vigorous physical activity - can provide substantially greater health benefits. In this way, physical activity and exercise have a dose-response relationship with better health: some is good, and more is better. Children and adolescents are encouraged to do at least 60 minutes per day of moderate to vigorous physical activity, some of which should include muscle and bone strengthening exercises on at least 3 days per week (U.S. Department of Health and Human Services, 2018). There isn’t anything magical that happens between the ages of 17 and 18 that immediately decreases the need for physical activity - shifting from the recommendations for children and teens to those of adults. However, it should be noted that as kids grow they may shift from unstructured play to organized sports and/or exercise. Perhaps the changes in recommendations are more a reflection of societal demands (education, work, etc.) that change how we engage with physical activity (van Sluijs et al., 2021). Of course, there is also an upper limit where frequent, high intensity exercise provides no further benefits for health and longevity - or could even be dangerous. But this excessive amount is typically only the stuff of avid endurance athletes, or a concern when a sedentary person drastically increases their PE all of a sudden (Foreman, 2020).
Most Americans certainly do not get enough physical activity. In 2018, only 54% of adults reported achieving the CDC recommended minimum for minutes of physical activity per week, only 27.6% performed at least 2 days per week of muscle strengthening exercises, and only 24% got both (DNPAO, NCCDPHP, CDC, 2024). According to data from the Behavioral Risk Factor Surveillance System (BRFSS) over the years 2017-2020, over 25% of adults reported getting NO physical activity outside of their work (CDC, 2024d). Adolescence is a time where physical activity tends to decline - we go from energetic, playful children to much more sedentary teens and young adults. This decline in physical activity is sharper for teen girls than boys, likely due in part to the drop off of PE requirements during the last two years of high school combined with gender-skewed participation in organized sports. Nearly 80% of adolescents across the globe do not get enough physical activity, and in high income countries like the U.S., these trends also disproportionately affect teens in lower socioeconomic tiers (van Sluijs et al., 2021).
“Fitness” is traditionally classified as cardiorespiratory fitness (CRF), as measured by a maximal, submaximal, or field test. These tests either directly or indirectly measure the maximum capacity of the human body to use oxygen for physical work, so they essentially measure aerobic (endurance) capacity. Of course there are other measures of fitness including muscular strength, endurance, flexibility, and balance - all of which also have been shown to have protective health benefits. The majority of the research over the last several decades however has been done on CRF and its inverse association with chronic diseases, and particularly risk factors for cardiovascular disease. Poor CRF is considered a stronger risk factor for all-cause mortality than obesity, hypertension, smoking, high cholesterol, or diabetes (Foreman, 2020). And improving CRF and physical activity levels decreases risks for all-cause mortality regardless of weight status. So for folks who have excess body fat, if they also have a moderate to high level of fitness, they are in better health than those with normal weight and low fitness levels (Gaesser & Angadi, 2021).
Getting an accurate picture of the average CRF in Americans is tricky though. While some studies have been able to collect accurate measurements on large groups of people, the less fit are often left out of these measurements because performing a validated CRF treadmill test on them is too risky. Thus, several studies that attempt to take cross-sectional measurements of CRF tend to recruit people who are already physically active or fit, and their subjects tend to be predominantly white, with relatively high incomes and educational attainment. Even with these limitations to the research, it is still apparent that on average, CRF declines with age, and higher CRF is associated with more leisure-time physical activity and lower body mass. CRF also tends to be disproportionately lower in women of color than other demographic groups (Wang et al., 2010).
Physical activity has other benefits besides chronic disease prevention, including supporting mental health. Being active can reduce both acute (immediate) and long-term symptoms of depression and anxiety. Adolescents who are active on at least 5 days per week and those who play sports tend to have a lower risk for depressive symptoms. By contrast, using a computer - a very sedentary behavior - for more than 3 hours per night increases risks for depression (C. H. Wang & Peiper, 2022). Physical activity can support quality, efficient sleep, which also supports mental and emotional health. When it comes to brain gains, most of the evidence for the acute and long-term cognitive benefits of physical activity has been found in children, yet there is growing evidence for the association of CRF with academic achievement in young adulthood (Åberg et al., 2009). Lifelong physical activity is certainly protective against dementia and improving cognition in older adulthood (U.S. Department of Health and Human Services, 2018). In fact, physical activity and exercise combat most of the markers of aging - including muscle loss (sarcopenia), bone loss (osteoporosis), and reductions in physical functioning. There’s likely no other behavior quite as powerful for longevity and quality of life as being physically active (Foreman, 2020).
The other side of the physical activity coin is time spent not moving - or sedentary time. Research from 2019 suggests that American adults may spend an average of 9.5 hours per day sedentary (Matthews et al., 2021). A follow up in 2022 suggested that the COVID-19 pandemic didn’t really change time spent sedentary for most people. Although some folks started working from home and saving time on their commute, that only seemed to translate to more leisure physical activity time for men, not so much for women. Much of that saved commute time was simply spent in sedentary leisure activities. Families with children who had to complete remote schooling tended to decrease their physical activity throughout that time (Matthews et al., 2022). Recent research suggests that long bouts of sedentary time and/or the accumulation of prolonged sedentary hours is leading to chronic diseases and conditions, including CVD, diabetes, some cancers, muscle weakness, low back pain, depression, and more (Foreman, 2020). Even those adults who reach the recommended minimum physical activity may not be able to undo all of the health hazards of sitting for the rest of the day. Per a meta-analysis of sedentary studies and risk of all-cause mortality, it takes about 60-75 minutes of hard exercise to offset the risks of sitting for more than 8 hours each day (Ekelund et al., 2016). In other words, not only do people need to move more for their health, they also need to sit less.