Neural plasticity is the mechanism by which the brain encodes experience and learns new skills, behaviors, and habits in daily life and on the athletic field. Brain cells called neurons form a communication network that serves as the foundation of information processing in the brain. The neural network of the brain holds the capacity to rearrange and strengthen communication efficiency. It is through this process of rearrangement (neural plasticity) that we can experience changes in the way our minds think, feel, and act. This includes everything from changing your backswing in golf or tennis, to developing a new mental routine for shot preparation, or restoring function following biomechanical or nervous system injury. Thus, optimal performance, skill learning, and recovery are achieved when the capacity for neural plasticity is maximized. Research has shown that physical exercise increases the brain’s capacity for plasticity, reflected in part by changes in brain structure and function following exercise training in animal models and humans. Since aging results in gradual neurodegeneration, or loss and dysfunction of brain cells, and decreased neuronal plasticity, aged samples (e.g., 60–80 years) form a platform for studying methods to increase brain plasticity. This entry reviews research on exercise effects on mental performance and the brain and highlights results with aged samples.
Exercise’s Influence on Cognitive Performance
Since the 1960s, studies have shown that physically more active people perform better on tests of cognitive performance compared with physically less active peers. The first studies examined performers’ ability to successfully complete dual tasks by primarily measuring simple response time, the speed to respond quickly and accurately to a flash of light, and discrimination time, the speed to press one key if stimulus A appears and another key if stimulus B appears. Over time, more complex cognitive processes have been examined, such as the ability to switch between tasks, the ability to selectively pay attention and block out distractions, or the ability to inhibit automated responses or habits. Importantly, in these early studies the physically active groups were comprised primarily of competitive athletes. This may present interpretive problems due to the possibility of self-selection, such that athletes may seek and continue sports participation, in part, because of their natural superiority in cognitive processes that benefit sport performance, such as fast response time or the ability to focus amidst distraction. Thus, the best understanding of the effects of physical activity and exercise on mental performance and the brain comes from studying samples that have been well-matched on all characteristics other than physical activity level.
Several reviews have now quantified the effects of physical exercise on mental performance across studies in meta-analyses. Meta-analyses attempt to aggregate results from many studies and group results from similar variables together for the purpose of identifying and comparing replicable effects across studies at either the level of measurement (Does the effect replicate for a specific task?) or construct (Does the effect replicate across tasks that are all theoretically deemed to measure the same construct?). For example, the effect size of exercise training on simple response time could be calculated in different studies that included training and pre and post-tests of simple response time, and then an average could be computed across studies to determine if exercise results in a consistent improvement independent of any one study or laboratory.
Meta-analyses that have examined the question of how exercise training affects different domains of mental performance have demonstrated that exercise has a small to moderate effect on a range of cognitive abilities across the lifespan. In older adults, consistent benefits have been shown in speed of processing, as in simple response time; visuospatial and selective attention, such as the ability to compare line drawings or to selectively attend to stimuli or objects in the environment without distraction; executive function, a set of abilities related to inhibiting unwanted actions, multitasking, or juggling information in one’s mind such as mentally carrying out long-division; and declarative memory, which refers to the ability to remember previous events like the face and name of someone you met at a party last week. Across studies with older adults ages 55 to 80 years, several moderating variables have been identified that may result in greater effects of exercise on mental performance. In regard to exercise type, a combination of strength and aerobic training seems to result in greater effects than either alone. In regard to participant characteristics, women seem to benefit more than men and participants between ages 66 and 70 years may benefit more than younger or older adults. In regard to duration, 30to 45-minute exercise sessions over 6 months have produced a larger benefit than shorter training periods. Since many of these studies included previously sedentary participants, it does not take long for benefits to occur. Yet the question of how long benefits last and what type of exercise is optimal for maintenance of cognitive benefits is open for future research. It is also important to note that moderating variables such as these remain an active area of research in exercise neuroscience.
Exercise’s Influence on Human Brain Structure
Exercise impacts performance in part through enhancement of structural properties of the brain. For example, aging typically results in shrinkage of brain volume in the frontal and temporal association cortices. This can be measured using in vivo brain imaging technology called magnetic resonance imaging (MRI). However, studies have shown that greater physical activity (e.g., distance walked) or moderate aerobic exercise training over 6 months (walking at 60%–70% HR max) among older adults is associated with greater gray matter volume in the frontal and temporal cortices and greater white matter volume in the frontal cortex. Gray matter refers to where neurons expend their energy for information processing and form their connections with communication points called synapses. White matter represents the part of neurons that transmit neuronal activity between different areas of the brain and is composed primarily of myelin, which insulates the transmission “wires” (known as axons) of neurons and increases the speed of neural communication.
Increases in gray matter from exercise could therefore be from increases in the number of connective branches a neuron forms to communicate with other neurons. In some brain regions like the hippocampus, exercise may actually accelerate normal generation of new neurons (neurogenesis). In contrast, changes in white matter could result from increased myelination production or repair or from increases in the number of axons that branch out from the neuron. Increases in the number and thickness of blood vessels could also contribute to increases in brain volume as measured in humans; blood vessels traverse through gray and white matter and are not well identified on typical brain scans that have been used in most studies to date. However, there is evidence that exercise training increases cerebral blood flow in the hippocampus in humans, which is consistent with animal studies. One reason enhanced blood flow is important is because energy for neuronal processing, and therefore information processing, is transmitted to brain cells through increases in blood flow. Therefore, greater resting cerebral blood flow is thought to predict greater responsiveness to the energy demands of information processing.
In sum, while aging results in gray and white matter volume decline in the frontal and temporal association areas, aerobic exercise has been shown to attenuate this atrophy through mechanisms of neuroplasticity that increase the connective branching of neurons, volume of insulating myelination, density of synaptic connections, and through increased birth and survival of brain cells in the hippocampus. Future research will continue to examine the cellular and molecular mechanisms of changes in human brain volume after exercise training.
Exercise’s Influence on Human Brain Function
Contrasted to brain structure, brain function refers to how well neurons and their support system can coordinate activity to support ongoing thoughts, emotions, perceptions, and behaviors. Using MRI, the effects of exercise on brain function have been studied by either examining how well different parts of the brain respond to demand for information processing, which we call task-evoked functional MRI (fMRI), or by examining how well different regions in the brain activate in teams (functional networks) that we know support coordinated mental performance. Some studies have also used more direct neuronal stimulation methods like transcranial magnetic stimulation (TMS) to study the link between regular exercise and synaptic plasticity.
When examined with task-evoked fMRI, aging studies have examined activation during executive function tasks. Executive function tasks are of interest because they are known to engage the prefrontal cortices, which are areas of the brain that become dysfunctional with increasing age. In turn, studies have found that more aerobically fit older adults have more prefrontal brain activation during executive function performance. For example, one study found that greater aerobic fitness was associated with greater prefrontal activation during the Stroop task, which requires responding to the ink color of a word regardless of what the word says. Because of the automaticity of reading, the Stroop task is cognitively demanding and it requires coordinated brain activity in prefrontal and visual cortex. Importantly, greater fitness was only associated with greater prefrontal activity and not visual cortex activity.
Similarly, a training study found that 6 months of walking training in sedentary older adults resulted in increased prefrontal cortex activity during a task requiring attentional focus and inhibitory control, and that greater prefrontal activity was coupled with greater task performance. These studies support that aerobic exercise benefits mental performance in part through enhancement of prefrontal cortex function. Recent research also supports a beneficial effect of resistance training on brain activation associated with inhibition and memory processes that rely on areas outside the prefrontal cortex, suggesting resistance training may play a complementary role to aerobic training in supporting brain function across the lifespan.
Evidence also exists demonstrating that aerobic exercise is associated with greater coordination of brain activity—in regard to both broad brain networks and to synaptic plasticity in specific neuronal circuits. Brain networks are teams of physically distant regions that work in coordination and provide a system for the brain to carry out highly specific, local processes that feed up to coordinated, complex processes; neural plasticity is the foundation for these functional networks to maintain coordinated teamwork. In one study, older adults with greater aerobic fitness had greater functional coactivation in a brain network known as the default network, whose deterioration has significant implications for cognitive aging, risk for dementia, and a host of developmental psychiatric disorders. Exercise effects were strongest in the lateral and ventromedial prefrontal regions and the temporal cortex, including the hippocampus. Importantly, this research also suggests that greater functional coordination in the default network is associated with some of the cognitive benefits that are linked to aerobic fitness, suggesting this network may be an important component of how exercise improves cognition and decreases risk for dementia in late life. It may also suggest that exercise would be beneficial for developmental disorders related to impaired default network function.
Finally, there is evidence that greater aerobic fitness is associated with greater TMS-induced synaptic plasticity. The basis of learning is the brain’s ability to form new neural connections or to strengthen existing pathways based on experience. One way to study this is to pair stimulation of a hand muscle with electromagnetic stimulation of a corresponding region of motor cortex. The capacity for synaptic plasticity in this circuit can be measured by the increase in reactivity of the hand muscle to activation of the motor cortex following paired training. One study showed that more active adults had greater synaptic plasticity in the specific motor circuit studied. Although this was a cross-sectional study, it presents complementary evidence for the link between aerobic fitness and enhanced synaptic plasticity that may be a generalizable mechanism for the effect of exercise on coordinated brain function and improved learning and performance.
Overall, there is exciting evidence for exercise’s potential to attenuate age-related brain dysfunction, and these results have implications for improving the brain’s capacity to learn and respond adaptively to injury at any age. However, the mechanisms for how this happens are not fully understood and future research should be guided by the need to understand the cellular and molecular basis of these benefits.
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