A favorable change in a physiological variable (for example, reduced blood pressure) is the anticipated and desired outcome to exercise training. Commonly, this is defined in the scientific literature as a change (<0) in the favorable direction. For example, an individual who experiences a reduction in resting blood pressure 10mmHg after three months of aerobic exercise training would be categorized as a responder. An individual who performs regular exercise training, yet undergoes no change in physiological function, is defined as a nonresponder.
Physical inactivity is associated with numerous unhealthy conditions, including obesity, hypertension, Type 2 diabetes, and atherosclerotic cardiovascular disease (CVD), and contributes annually to an estimated 250,000 premature deaths. Conversely, it is well accepted that regular exercise has positive effects on multiple health outcomes related to cardiovascular morbidity and mortality. Moreover, regular exercise is linked with reduced risk of certain forms of cancers, improved psychological health, and an overall improved quality of life. The evidence underpinning the widespread benefits of exercise is so overwhelming that multiple organizations, including the American Council on Exercise, American College of Sports Medicine, and American Medical Association, advocate for exercise to be included as a standard part of disease prevention and treatment in the United States. The consensus from numerous health organizations calls for a minimum of 30 minutes moderate-intensity aerobic exercise on five days each week (or 150 minutes) or vigorous-intensity aerobic exercise for a minimum of 25 minutes on three days each week (or 75 minutes) or an equivalent combination of both.
Since the late twentieth century, it has been established that substantial differences can be noted in individual responsiveness to exercise training. For example, improvement in cardiorespiratory fitness ranges from −5% to +58% following five to six months of standardized aerobic exercise. Moreover, scientific literature highlights a considerable variability in exercise-training-induced responses of common cardiovascular and metabolic risk factors, such as blood pressure and cholesterol levels. The most common explanation for these differences in training responses is genetics. In fact, the HERITAGE Family Study, performed in the 1990s, reported that nearly half (47%) of the variability in cardiorespiratory fitness training was inherited. The wide variability to uniform training has been described using a number of terms, including responders, nonresponders, and adverse responders.
Although it would appear to be intuitive that all previously untrained and sedentary individuals undertaking exercise can expect positive changes to their physiological function and overall health, the scientific literature reports that anywhere from ~20% to 45% of individuals may be nonresponders. As of the mid-2010s, evidence suggested that some individuals actually experience an adverse response when exposed to regular exercise. An adverse response has been defined as an exercise-induced change that worsens cardiometabolic health. Approximately 8%–13% of individuals participating in regular exercise training may be adverse responders.
Exercise programs are designed using the F.I.T.T. Principle. F.I.T.T. is an acronym for the four components of the exercise program: frequency, intensity, time (length), and type of exercise. The overall homeostatic stress of an exercise session is comprised of these components, and in turn provides the stimulus to initiate adaptive physiological responses. Scientific literature has reported that variation in exercise training is one of the most important factors that influences training responsiveness (i.e., responder, nonresponder, or adverse responder).
Exercise intensity is arguably the most critical component of the exercise prescription model. Failure to meet minimal threshold values may result in lack of a training effect, whereas too-high exercise intensity could lead to overtraining and negatively impact adherence to an exercise program. The traditional standard for prescribing exercise intensity is expressed in terms of percentages of heart rate reserve (%HRR) or oxygen uptake reserve (%VO2R). This is the relative percent method. The American College of Sports Medicine (ACSM) recommends an exercise intensity of 40%–59% HRR/VO2R for improving and maintaining cardiorespiratory fitness. Nevertheless, despite a large evidence base supporting the ACSM relative percent recommendation, there is concern that the approach consists of a very large range of acceptable percentages and fails to take into account individual metabolic responses to exercises.
Various lifestyle factors may also influence training responsiveness, including sedentary behavior, nutrition, sleep, and stress. A key preventative step to minimizing training unresponsiveness is to identify those individuals who may be at an increased risk for nonresponse or adverse response to exercise training through appropriate preparticipation screening. For example, during the preparticipation screening process if clients report high amounts of sitting at their desk job throughout the week, this issue can be discussed with a health professional.
Sitting has been coined the new smoking. In fact, excessive sedentary behavior has been linked with numerous chronic diseases, including Type 2 diabetes and cardiovascular disease. Interestingly, this increased risk persists whether individuals exercise regularly or not. Individuals who meet the 150 minutes per week of moderate-intensity exercise recommendation, yet remain sedentary at most other times throughout the week, are referred to as “active couch potatoes.” Research has shown excessive sitting time, independent of exercise, is linked to decreased HDL-cholesterol, increased triglycerides, and elevated blood glucose. Overall, these research findings suggest that poor training response in nonres-ponders and adverse responders may be attributable to sedentary behavior and not a product of the exercise program itself.
Nutrition also affects training responsiveness. Good nutrition is vital to exercise performance. Yet, research done since 2010 has made it increasingly evident that the timing and composition of dietary intake contribute to training responsiveness. For instance, it has been demonstrated that post-exercise consumption of a carbohydrate-protein beverage elicits a reduction in muscle-protein degradation, which contributes to enhanced muscle recovery by creating a positive muscle-protein balance in the skeletal muscle. Similarly, it has been reported that post-exercise carbohydrate consumption significantly increases the rate of muscle glycogen replenishment. Both a positive muscle-protein balance and increased rate of muscle glycogen replenishment will accelerate the recovery process and decrease the time to return to resting homeostasis following exercise training. Consequently, scientific literature suggests that an increased readiness for the next training session is also likely to have a positive influence on training responsiveness.
Increased stress and insufficient sleep are additional factors that may limit overall training responsiveness. For example, scientific literature has reported that higher life-event stress resulted in reduced improvements in various power- and strength-related parameters following a resistance-training program. Prolonged sleep debt has been associated with increased susceptibility to infection and overtraining syndrome. Either of these conditions compromise quality exercise training. Moreover, chronic sleep debt has also been linked to increased circulating levels of cortisol. In turn, it has been suggested that increased cortisol impairs muscle recovery. Overall, elevated stress and inadequate sleep compromise favorable training adaptations and may result in reduced training volume, impaired training recovery, and a predisposition to overtraining.
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American College of Sports Medicine (ACSM), 401 W. Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
American Council on Exercise, 4851 Paramount Dr., San Diego, CA, 92123, (858) 576-6500, (888) 825-3636, ext. 782, Fax: (858) 576-6564, https://www.acefitness.org .
American Heart Association (AHA), 7272 Greenville Ave., Dallas, TX, 75231, (800) 242-8721, http://www.heart.org .
Lance C. Dalleck, PhD