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The CrossFit stimulus—constantly varied high-intensity functional movement coupled with meat and vegetables, nuts and seeds, some fruit, little starch, and no sugar—prepares you for the demands of a healthy, functional, independent life and provides a hedge against chronic disease and incapacity. This stimulus is elegant in the mathematical sense of being marked by simplicity and efficacy. The proven elements of this broad, general, and inclusive fitness, in terms of both movement and nutrition, are what we term our CrossFit Essentials.

In this 2012 review, Tim Noakes argues the traditional, purely biological understanding of fatigue (i.e., that exercise failure occurs when the heart or muscles reach a biochemical limitation) is inconsistent with observed data. Noakes proposes an alternative model — the “central governor” model as opposed to the “brainless” model — rooted in the observations of coaches, athletes, and neuroscientists.

Read MoreFatigue Is a Brain-Derived Emotion That Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis

What is the mechanism behind muscle growth? The exercise-induced muscle damage (EIMD) hypothesis posits intense exercise causes damage to muscle fibers, which then repair and adapt to better tolerate future exposure to stress. Eccentric training is thought to be particularly effective in stimulating hypertrophy (muscle growth). Tyler Hass discusses related studies and evaluates popular theories about promoting muscle growth.

Read MoreMuscle Damage for Size and Strength?

Prof. Timothy Noakes provides evidence in support of Nina Teicholz’s argument that “the story of nutritional science is not, as we would expect, one of sober-minded researchers moving with measured judicious steps.” Teicholz claims it instead falls under “the ‘great man’ theory of history, whereby strong personalities steer events using their personal charisma, intelligence, wisdom, or wits.” According to Noakes, these qualities are precisely what allowed Ancel Keys to convince his colleagues to support his diet-heart and lipid hypotheses — even when the science did not.

Read MoreIt’s the Insulin Resistance, Stupid: Part 10

Applying resistance training to a CrossFit Kids program should be done with the same considerations as with adults: minimize risk and develop adaptations; however, considering the developmental process and length of the available timeframe to see results in this younger age group, perhaps the tension between technique and intensity (speed and load) should lean in favor of technique.

Read the article CrossFit Kids Research Brief: Kids and Resistance Training

In this 2019 trial, Tim Noakes et al. demonstrate that athletes can achieve similar performances on a 5K time trial while following a low-carb or high-carb diet, contrary to the widely assumed understanding that low-carb diets cannot support effective exercise at high intensities. At minimum, this research indicates a low-carbohydrate diet does not negatively affect performance beyond a small impairment seen during an initial two-week transition period.

Read MoreHigh Rates of Fat Oxidation Induced by a Low-Carbohydrate, High-Fat Diet Do Not Impair 5-km Running Performance in Competitive Recreational Athletes

Prof. Tim Noakes separates fact from fiction in his examination of the data from the Framingham Heart Study (FHS). Despite the efforts of some of the study's lead scientists to wrench the data into supporting the association of dietary fat and blood cholesterol concentration with coronary heart disease (CHD), the study instead demonstrated insulin resistance is a significant risk factor for CHD mortality whereas blood cholesterol concentration has little or no practical predictive value, especially after age 50. Noakes also highlights the insights shared by Dr. George Mann, “who was initially one of the scientific leaders of the FHS but later resigned to express his displeasure at the fake science he detected."

Read MoreIt’s the Insulin Resistance, Stupid: Part 9

Individuals on low-carbohydrate diets have lower levels of ingested glucose and smaller liver and muscle glycogen stores. In this 2016 trial, endurance athletes following a low-carbohydrate diet were shown to maintain energy sufficiency during exercise through increased reliance on fat oxidation.

Read MoreGluconeogenesis During Endurance Exercise in Cyclists Habituated to a Long‐Term Low Carbohydrate High‐Fat Diet

This trial indicates metformin suppresses gains in strength and muscle mass associated with resistance training in elderly patients. Its results support previous research suggesting the drug suppresses the benefits of resistance training in multiple populations. In patients who do not require immediate improvements in glycemic control, metformin’s suppression of exercise-induced muscle strength and size gains could increase risk of disability and mortality. These risks may provide an argument for not prescribing metformin to elderly patients. In diabetic and prediabetic patients, it is worth considering non-pharmaceutical treatments that lead to improved glycemic control without reducing the benefits of exercise.

Read the article Metformin Blunts Muscle Hypertrophy in Response to Progressive Resistance Exercise Training in Older Adults

Oxygen, the very thing that gives us life, can also be life-threatening, under certain circumstances increasing the risk for central nervous system oxygen toxicity (CNS-OT) seizure. CNS-OT seizures are a limitation of both hyperbaric oxygen therapy and scuba diving, particularly among Navy SEALS who use closed-circuit rebreathers. Dr. Dominic D’Agostino, an expert in the neuroprotective effects of the ketogenic diet, explains why ketones were investigated as a possible mitigator of CNS-OT seizures. He also summarizes existing research on the effects of exogenous ketone supplements on CNS-OT, describes the potential mechanisms at work, and lists examples of ketones' protective qualities.

Read MoreKetosis in Extreme Environments: Mitigating Central Nervous System Oxygen Toxicity

In this 2011 trial, resistance exercise is shown to reduce liver fat content even in the absence of weight loss. Resistance exercise improves muscle’s ability to effectively regulate circulating glucose and fat levels, a change that reduces insulin resistance and liver fat buildup. At minimum, this indicates resistance exercise alone may help reverse insulin resistance and fatty liver disease. More importantly, it suggests treatments that induce weight loss, such as other changes to diet and exercise, may drive more rapid and effective metabolic improvements when paired with resistance training.

Read MoreResistance Exercise Reduces Liver Fat and Its Mediators in NAFLD Independent of Weight Loss

This 2012 review summarizes evidence indicating exercise, specifically interval training, maintains or improves skeletal muscle insulin sensitivity and as a result reduces risk of insulin resistance and diabetes. Exercise increases skeletal muscle insulin activity in the obese, while sedentary behavior decreases it. The ability of the muscle to take up and burn fuel (i.e., skeletal muscle oxidative capacity) is a strong predictor of whole-body insulin resistance.

Read MoreExercise and Type 2 Diabetes: New Prescription for an Old Problem

“Exercise training programs may prove to be simple, yet important, methods of enhancing aspects of children’s mental functioning that are essential to cognitive and social development.” Therefore, programming short bouts of relatively high-intensity physical activity can be an efficient way to increase not only fitness but also brain function in a scholastic environment or in a box as part of a CrossFit Kids program.

Read the articleCrossFit Kids Research Brief: Intensity and Cognition

“Strive to blur distinctions between ‘cardio’ and strength training. Nature has no regard for this distinction.” But how does CrossFit blur this distinction given that strength/power training and cardiovascular training are at different ends of the power spectrum? More simply put, when you do “Grace” or “Fran” or “Angie” or even “Linda,” what kind of training are you doing? Is it power, strength, or cardio? Can they be combined? To what extent do they overlap? Obviously, they in fact do, and this is one of CrossFit’s huge contributions to fitness, but it flies in the face of much of the accepted knowledge in exercise science. How does it work? What are the mechanisms? These are complex questions and the answers depend on many factors.

Read the article Human Power Output and CrossFit Metcon Workouts

Increasing time spent doing high-impact physical activity as a youth is a simple and direct way to improve skeletal health. One of the programming directives offered at the CrossFit Kids Certificate Course is including impact-loading exercises on a daily basis. This simple addition results in meaningful and significant benefits not only in terms of the improved fitness it generates through these plyometric exercises but also with respect to increased skeletal health in the long term.

Read the articleCrossFit Kids Research Brief: Bone Density

Metabolic training refers to conditioning exercises intended to increase the storage and delivery of energy for any activity. Ultimately, the CrossFit position on metabolic conditioning, or “cardio,” is summed in two points: Anaerobic training can match endurance training for aerobic benefits. Metabolic training with varying and mixed exercise modalities avoids specificity of adaptation allowing for additional first wave-cardiovascular/respiratory adaptations and increased functional strength.

Read the article Metabolic Conditioning

This 2016 trial found that 12 weeks of high-intensity interval training (HIIT) significantly improved cardiometabolic function and liver fat content in diabetic subjects. HIIT subjects saw significant increases in a variety of markers of cardiovascular health, including healthful regeneration of cardiac muscle tissue and improved cardiac contractile capabilities, as well as reversal of certain forms of cardiovascular degeneration associated with Type 2 diabetes. The same subjects also showed a 39% decrease in mean liver fat content, with four of the 11 subjects in the HIIT group seeing a reduction from clinically significant liver fat levels to “normal” liver fat.

Read MoreHigh intensity intermittent exercise improves cardiac structure and function and reduces liver fat in patients with type 2 diabetes

“Given the strong evidence for a direct role of physical activity in the prevention of insulin resistance, and the fact that exercise training increases mitochondrial biogenesis and improves glucose tolerance and insulin action in individuals with insulin resistance and type 2 diabetes, the question of why such a potent modulator of these conditions is not more commonly prescribed is perplexing and should be of utmost concern to medical health care professionals worldwide. To continue to attack the growing health burden by investing almost exclusively in strategies that target secondary and tertiary treatment of chronic disease states (i.e. pharmaceutical interventions) is extremely short sighted: primary defence mechanisms (i.e. exercise/diet and lifestyle interventions) will decrease disease prevalence by preventing these conditions in the first place!”

Read MoreExercise as a Therapeutic Intervention for the Prevention and Treatment of Insulin Resistance

In the final installment of his hyponatremia series, Prof. Tim Noakes describes how three industry-funded studies inadvertently produced data that would prove inconvenient for Gatorade and the Gatorade Sports Science Institute. Together, the studies showed that sodium balance has no impact on the extent to which blood sodium concentration falls during exercise and that abnormalities in the regulation of body water content explain the development of EAH and EAHE. In short, they demonstrated how sports drink marketing had contributed to the EAHE epidemic in the U.S. after 1981. Noakes explains how the studies’ authors generated conclusions that would be more palatable for the industry. He also describes how various scientific journals responded when he began pointing out the studies’ errors.

Read MoreThe Hyponatremia of Exercise, Part 12