Exercise improves our physical and mental health—that is now beyond debate. The physical benefits are obvious; we know that exercise lowers blood pressure, decreases cholesterol, reduces fat, adds muscle and improves cardiovascular function. But how is it that exercise also reduces stress, anxiety and depression and allows us to maintain focus at work and to think clearly?
You might easily assume that improved physical health drives improved mental health; that a healthy body breeds a healthy mind. But the truth is, we know much more about how exercise affects the body than how it affects the mind. That brings us to the issue of measuring the mind and to the proverbial “mind-body problem”: Is the mind separate from the body? It is a question that has tormented philosophers since René Descartes suggested nearly 400 years ago that mind and body were clearly separate but the brain (the pineal gland to be precise) was the place where mind interacted with body. This Wizard of Oz sort of view suggests that a peek behind the neurobiological curtain will unveil some ethereal forces pulling levers.
Regardless of your view on the issue, accept for the time being that our cognitive, perceptual and emotional faculties—indeed, our sense of self—are all nestled within the 100 billion or so neurons (nerve cells) that make up the brain. So if exercise can have lasting effects on the mind, then exercise must also affect the brain. As it turns out, the brain is incredibly dynamic. It is not hard-wired, as we once believed, and it responds to exercise in much the same way that heart, lungs and muscles do. The brain can change its structure and function by adding new neurons, making new connections between neurons (synapses) and even creating brand-new blood vessels, all in response to different forms of exercise (see Figure 1).
A Powerful Healing Agent for Change
Over the last two decades, neuroscientists have begun to reveal how physical activity—whether it be endurance, strength or skill training—can change the neurochemistry, structure and function of the brain. We are starting to understand how these changes in brain biology affect our cognitive, sensory, motor and emotional behaviors. We are also discovering that the neurobiological imprint of exercise can help treat and possibly even prevent a number of psychiatric disorders (such as depression and anxiety) in addition to neurological disorders (such as stroke, Alzheimer’s disease and Parkinson’s disease).
Exercise Improves Cognitive Function
Although the American College of Sports Medicine (ACSM) recommends at least 30 minutes of moderately intense aerobic exercise 5 days per week (ACSM & AHA 2007), alarmingly it is estimated that 74% of all Americans fail to meet this requirement, and the lack of activity represents a major contributor to rising healthcare costs. Aside from the obvious effects on physical health, there is mounting evidence that a sedentary lifestyle also affects the brain—and in turn lessens mental capacity. Sibley and Etnier (2003) found a clear connection between how much schoolchildren exercised and their cognitive performance: the more aerobic exercise the children engaged in, the better they performed on verbal, perceptual and mathematical tests. The same pattern of results was found in older adults: aerobic training improved cognitive performance (Colcombe & Kramer 2003), and active lifestyles decreased age-related risks for cognitive impairment and dementia (Yaffe et al. 2009). Not surprisingly, these cognitive effects were accompanied by clear changes in brain structure and function.
Exercise Changes Brain Function
The fact that exercise enhances cognition suggests it must have some effects on the brain that outlast the exercise experience itself. That is, exercise must somehow change brain function in a lasting manner. Indeed, research supports this hypothesis; the reduced cognitive capacity in sedentary individuals is also associated with different patterns of brain activity—both at rest and while performing mentally challenging tasks—than those observed in active subjects.
Although a number of brain areas are involved in the complex cognitive tasks that we engage in on a daily basis, the cerebral cortex is a major player. Compared with sedentary people, active individuals show greater baseline levels of cortical activity (Dustman et al. 1990) and more activity in various brain regions when performing cognitive tests (Polich & Lardon 1997). Some cortical areas show increases in activity when we are struggling with a particular task. One such area that is especially sensitive in this regard is the anterior cingulate cortex (ACC). Part of the brain’s limbic system, the ACC has connections with numerous brain areas involved in processing sensory, motor, emotional and cognitive information. This brain area becomes very active during moments of indecision or confusion when we are posed with a problem. After a 6-month walking intervention, people showed decreased ACC activity relative to nonaerobic toning or stretching groups (Colcombe et al. 2004), the implication being that less activity in the ACC contributes to the enhanced cognitive function resulting from exercise.
Exercise-related changes in brain function are not limited to areas of the cortex concerned with cognitive function. Brain areas that are engaged during movement are also affected. One key cortical area is the motor cortex. This strip of tissue contains neurons that send information down to the spinal cord to cause muscle contraction. Individuals engaged in regular exercise show reductions in the amount of activity within the motor cortex when performing simple movements (Voelcker-Rehage, Godde & Staudinger 2010). While this might seem counterintuitive, one interpretation is that the cortex is more efficient at controlling movement and therefore requires less effort to produce movement.
Exercise Changes Brain Structures
Although the structure of the brain is highly complex, it can be broken down into two general components. Gray matter contains all of the neurons and supporting cells, while white matter consists of the axons of these neurons (nerve cell fibers) that carry signals from one area to another. One might compare this to the way in which most large cities are organized, with houses and buildings connected by streets and freeways.
Magnetic resonance imaging (MRI) allows for the measurement of gray and white matter and can reveal something about the way that exercise influences the overall structure of the brain. MRI scans have shown that exercise boosts overall brain volume (Colcombe et al. 2006), increasing both gray matter (Colcombe et al. 2006) and white matter (Gordon et al. 2008). Interestingly, these changes can occur over relatively short periods of time with what might seem like minimal amounts of training. After learning to juggle for only a few weeks, for example, study subjects showed increases in gray matter within regions of the brain concerned with integrating visual and motor information (Draganski et al. 2004).
For more on how exercise affects health and learning, see the sidebars.
Our knowledge of the effects of exercise on the body and mind continues to expand. The development of new technologies is indeed allowing us to “peek behind the cerebral curtain” to see which levers are being pulled—to understand the biology of how exercise improves both body and mind. This knowledge will allow us to harness the brain’s endogenous capacity to adapt to experience and will guide the development of new therapies to treat the damaged or diseased brain as well as to improve our general quality of life. This has become increasingly important as our population ages and the stressors of this fast-paced world mount.
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