**3. Acute versus chronic exercise participation**

Distinctions need to be made between the *acute* versus *chronic* impact of exercise on a physiological system, in this case, the brain. While it is important to know the short-term impact exercise has on physiological systems from a biological or safe participation standpoint, the establishment of long-term health benefits attributed to exercise exposures must account for the chronic adaptations due to historical (i.e., long-term) participation in an exercise regime. It is well-established that exercise is an acute stressor, thereby resulting in (relatively) immediate elevations in blood flow, heart rate, oxygen uptake, respiration, and increased circulation/uptake of most hormones and many metabolic substrates. However, the question remains, do any of these acute exercise responses, when experienced multiple times throughout the week, over several months to many years (i.e., chronic exposure), impact the brain in such a way as to become neuro-protective and prevent or attenuate neurological degeneration and cognitive decline commonly attributed to unsuccessful brain aging?

#### **3.1 Cross-sectional or outcome study?**

Evaluating the brain at one point in time with a selection of a population is a cross-sectional study. One is able to infer relationships between brain structure/function and a host of variables, ranging from cognitive test scores to health fitness ratings. This is an excellent starting point and is where most of the exercise neurobiology literature is currently focused,

MRI Techniques to Evaluate Exercise Impact on the Aging Human Brain 233

recommended by the American College of Sports Medicine (ACSM, 2009). Precisely identifying each of these components within an exercise neurobiology study makes comparison across studies, replication of results, and advancement of the exercise science of neurobiology much more accurate. Furthermore, it makes providing global recommendations to the public easy. Vaguely described exercise protocols are one of the major pitfalls encountered in the neurobiological literature utilizing exercise as a treatment modality. Often the research outcomes are either un-interpretable or non-generalizable. Manipulation of any one of the five components in the FITT + P paradigm can alter the intervention outcome significantly, and varying more than one component within a study must be done carefully. Ultimately the goal is to determine what type(s) of exercise recommendation(s) will best facilitate brain health maintenance. Reproducibility of the exercise prescription is paramount so that the findings can be applied across various

How "often" one exercises is a critical component of the exercise dose-response. It depends not only upon the health status of the individual, but also the type (or modality) of exercise. To improve cardiovascular health (or aerobic fitness), metabolic and lipid profiles, and body composition, 3 days per week is the recommended minimum number of times one should exercise (ACSM, 2009). However, if a person is at either extreme of the physical fitness continuum, (i.e., extremely deconditioned/inactive versus highly fit/active), then multiple daily sessions of very short duration (i.e. time) or nearly daily sessions of moderately long sessions may be instituted. Hence, there is a distinct relationship between frequency of exercise and duration of exercise. Simply put, the amount of time (in minutes) one should expend in a given exercise session is partially determined by how frequently one is exercising on a daily or weekly basis. Furthermore, the 3-days-per week minimum recommendation only applies to aerobic conditioning. Strength conditioning should be performed 2-3 days per week with the goal to alternate muscle groups being trained, and flexibility training recommendations is a minimum of 2 days weekly. Fitting all three of these exercise components (aerobics, strength, flexibility) into one exercise session can cause an exercise session to require at least one hour of time. Therefore, it is common to break up the exercise program into "aerobic" training days and "strength/flexibility" training days,

How "hard" one exercises has many physiological parameters to consider including heart rate response, perception of effort, and workloads on various types of equipment. All of these factors contribute to the "intensity" of the exercise prescription and are manipulated

If aerobic conditioning is desired, then the recommendation is for one to exercise within a ""stimulus zone". This zone is based upon one's health status and a percentage of one's agepredicted maximum heart rate (220 - age). For the average individual, a "moderate" intensity stimulus zone is recommended. As can be seen in *Table 1*, a moderate heart rate stimulus zone would be 64 – 76% of one's maximal predicted heart rate. So if one is 50 years old, the predicted maximal heart would be 170, and the heart rate training stimulus zone

physical activities and different populations.

**5.1.1 Frequency ("F") of exercise** 

resulting in exercising almost daily.

according to the desired outcomes (Nieman, 2010).

**5.1.2 Intensity ("I") of exercise** 

**5.1.2.1 Heart rate** 

would be 109 to 129 bpm.

likely due to time, facility limitations, and monetary constraints. However, care must be taken when reporting the results from cross-sectional studies. Regardless of the strength of the associations, results should not be reported in such a way as to infer causation. Cross-sectional research has pointed the way towards the need for more controlled, randomized longitudinal outcome studies which can take the significant associations one step further and determine causation of an intervention. While acute outcome studies are able to state how exercise stresses the brain on an immediate basis, only longitudinal outcome studies will be able to recommend more definitive exercise dosing guidelines for maintaining and/or improving brain health over a lifetime. Even then, the recommendations will likely be for specific populations, a specific gender, or specific types of physical activity. It will take several years to arrive at the more global health fitness recommendations now common in the cardiovascular literature. There is plenty of work ahead for innovative exercise-focused neuroscientists.
