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Limits and Effects of Ultra-Endurance Exercise
Tony Nunez, M.S. and Len Kravitz, Ph.D.

Long-distance endurance competitions have become very popular in the United States, Japan, Europe, South Africa and Korea (Millet and Millet, 2012). Many of these events are often referred to as ultramarathons, which are often classified as foot races greater than 26.2 miles (Millet and Millet) or 31 miles or longer (Schutz et al., 2012). Millet and Millet explain that some of these races may be completed over multiple days, with the champions covering the most distance in a set time period, or they are races to cover a specific distance in the shortest time. Noakes (2006) states these organized events originated in the late 1870s, with the arrival of the Six-Day Pedestrian races, where competitors walked and jogged as far as they could for 6 straight days (usually starting at 12:01am Monday morning and finishing Saturday at midnight), often around tiny indoor tracks. A recent ultramarathon Internet search led to a listing of countless ultra-marathon competitions, some proclaiming to be the toughest (due to terrain and environmental challenges). Although depictions of amazing displays of physical and mental determination, the physiological demands and effects associated with ultra-endurance exercise are just starting to be understood by scientists, and reviewed in this column.

What are the Capabilities of Humans to be Endurance Runners?
Research regarding human locomotion has determined that humans have been bipeds (standing/walking on two feet) for as long as 4.4 million years ago (Bramble and Lieberman 2004). The typical walking speed for a human is 2.9 miles per hour and most women and men habitually switch to running at approximately 5.1-5.6 mph (Bramble and Lieberman). Interestingly, humans are the only primates (mammals that include monkeys, apes, and humans) capable of doing endurance running (Bramble and Lieberman). Bramble and Lieberman elucidate that running is more energetically efficient than walking due to a lower body mass-spring mechanism in the legs that exchanges kinetic and potential energy very uniquely. The collagen-rich tendons and ligaments in the legs release a generous amount of stored energy during the propulsive phase of running. The body utilizes this spring mechanism by flexing and extending more at the knee and ankle in running which is estimated to save approximately 50% of the metabolic cost of running (Brambel and Lieberman). Continuing, the scientists describe elite endurance athletes as having (genetically) a much greater percentage of slow contracting, oxidative and fatigue-resistant muscle fibers in the primary muscles of the leg. In all, this extensive energy efficient system of springs in the legs, along with lower extremity limb length relative to body size, slow twitch muscle fiber composition, and expansive aerobic metabolism capabilities enables humans to run longer distances at higher speeds than most quadruped mammalians (Bramble and Lieberman). The distances that humans are able to run fall just short to that of the Siberian huskies and Alaskan malamute dogs (Noakes 2006).

What are the Effects of Endurance Exercise on the Brain?
Research regarding the improvement in brain function with moderate endurance exercise is well documented, focusing largely on brain-derived neurotrophic factor (BDNF) (Seifert et al. 2010). BDNF is a protein found in the central and peripheral nervous system that promotes the growth and maintenance of neurons. In the brain BDNF is active in the hippocampus, cortex, and basal forebrain, which are areas involved in learning, memory, and higher thinking. Seifert et al. summarize that daily bouts of endurance training, approximately 60 minutes per session, at an average intensity of 70% of maximal heart rate enhance resting levels of BDNF in the brain, suggesting that endurance training promotes brain health.

Does Ultra-Endurance Exercise Lead to Too Much Oxidative Stress?
It has been established that exercise plays a role increasing oxidative stress in the body. Oxidative stress is defined as an imbalance between free radicals, also called reactive oxygen species (ROS) produced in the mitochondria of contracting muscles, and antioxidants (substances which attempt to counteract the harmful effects of ROS). Oxidative stress has been shown to be associated with the development of atherosclerosis, and thus an increased risk to cardiovascular disease. The debate in regards to exercise-induced oxidative stress has been focused on one central question: “How much exercise is too much exercise?” Knez, Coomes and Jenkins, in their review of literature on ultra-endurance exercise and oxidative damage summarize that ultra-endurance increases ROS production and other markers of oxidative stress. However, the authors sum up that the research confirms that ultra-endurance exercise is also associated with elevated antioxidant defenses, and potentially greater cardiovascular disease protection. They note that higher levels of training status may positively influence the magnitude of adaptation of the protection. The authors conclude that research with ultra-endurance exercise and antioxidant supplementation is inconclusive at this time.

What are the Physical Limitations to Ultra-Endurance Exercise?
Naokes (2006) describes the human body as being capable of extraordinary physiological feats when properly nourished and rested. Structurally, the human body is mechanically developed to endure great running distances over time. Yet, certain limitations present themselves in these ultra-endurance exercise events.
In their fatigue research on the central (i.e., motor neurons and neurochemicals in the brain) and peripheral (neuromuscular junction and/or contractile proteins in muscle) mechanisms contributing to a 24-hour treadmill run, Martin et al. (2010) confirm that central fatigue is the principle limiting factor. The authors hypothesize that this result may reflect the existence of a central brain mechanism aimed at reducing neural drive to the working muscles to limit the level of exhaustion during ultra-endurance exercise.

Running jolts the skeletal system producing a bodily shock wave up to 3 to 4 times a person's body weight at faster speeds (Bramble and Lieberman, 2004). Fortunately, the larger surface areas of major lower extremity joint surfaces and increased lower body bone mass helps to dissipate these impact forces. However, lower body overuse injuries in runners can be a common occurrence, and understanding which runners are more susceptible than others is still being researched today. Schutz, et al. 2012 report a common problem for endurance runners is a lower leg pain syndrome, often called 'shin splints'. Fallon confirmed this injury to be specific to ultra-endurance runners, calling it “ultramarahon ankle.” Schutz, et al explain that this chronic pain is a manifestation of overuse pathologies of the muscles, fascias, tendons and bone tissues of the lower leg.

What are the Clinical Concerns with Ultra-Endurance Exercise?
In their position paper on immune function and exercise, a panel of world experts underscore that extreme amounts and/or intensties of exercise may have negative implications for the immune system (Walsh et al. 2011). This type of training can make persons vulnerable to increased risk of infection, illness, and clinical sepsis (life-threatening response to infection which can lead to tissue damage and organ failure).

O'Keege and colleagues (2012) note that regular exercise is a cornerstone for producing optimal cardiovascular exercise. However, the researchers caution that chronic training and competing in ultra-endurance events such as marathons, ultramarathons, ironman distance triathlons, and very long distance bicycle races, may lead to overload of the heart atria (upper chambers) and right ventricle, acutely impairing the right ventricular ejection. Months to years of this training may eventually make a person more prone to unfavorable heart arrhythmias (irregular resting heart rates). Importantly, the researchers highlight that lifelong vigorous exercise (up to one hour daily) is associated with low mortality rates and higher functional capacity for activities of daily living.

Summary Thoughts
The excitement and participation in ultramarathons and extreme endurance events is surging. The physiological research validates that with progressive training we are remarkably capable of completing these endurance challenges. However, it does appear that we need to educate our clients that (yet to be fully determined) safe upper-dose limits do exist for all of us with ultra-endurance conditioning, and going beyond these limits may make one susceptible to adverse health consequences.

@bio:Tony Nuñez, M.S., is an exercise science doctoral student at the University of New Mexico, Albuquerque. His research interests include metabolic conditioning and disease prevention, sports performance, and exercise nutrition for general population. He enjoys surfing, golfing, team sports and resistance training.

@bio:Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and American Council on Exercise Fitness Educator of the Year.

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