Ultramarathons and other extreme-
endurance events produce amazing
displays of strength and determination. Because these events push the
limits of athletic performance, they’ve drawn scrutiny from scientists
hoping to learn how much the human body can take.

Long-distance endurance competitions are popular in the United States,
Japan, Europe, South Africa and Korea (Millet & Millet 2012). They’re
typically called ultramarathons, classified as foot races longer
than 26.2 miles (Millet & Millet 2012) or 31 miles or more (Schutz et
al. 2012).

Millet & Millet explain that these races measure either how far runners
can go over multiple days or how fast they can run a specific distance.
Noakes (2006) notes that events of this kind originated in the late
1870s with the emergence of “6-day pedestrian races,” where competitors
walked and jogged as far as they could for 6 consecutive days (usually
starting at 12:01 Monday morning and finishing Saturday at midnight),
often around tiny indoor tracks.

A recent Internet search revealed a long list of ultramarathon
competitions, some claiming to be the toughest, because of terrain and
environmental challenges. Scientists are just starting to understand the
physiological demands and effects of ultra-endurance exercise. Read on
for a summary of the latest research.

How Do Humans Compare With Other Species?

Scientists have noted that humans have more endurance capacity than
every land mammal except sled dogs like the Alaskan husky (Noakes 2006).

Humans have always been walking and running—locomotion research shows we
have been bipeds (standing on two feet) for up to 4.4 million years and
endurance runners for 2 million years (Bramble & Lieberman 2004). We
typically walk at 2.9 miles per hour and run at approximately 5.1–5.6
mph (Bramble & Lieberman 2004). Humans are the only primates capable of
endurance running.

Human legs have unique biomechanics that make running more energy
efficient than walking, because we have a lower-body mass-spring
mechanism in the legs that exchanges kinetic and potential energy
(Brambel & Lieberman 2004). Collagen-rich tendons and ligaments in human
legs release generous amounts of stored energy during the propulsive
phase of running. The body uses this spring mechanism by flexing and
extending more at the knee and ankle, saving about 50% of the metabolic
cost of running (Brambel & Lieberman 2004).

Scientists describe elite endurance athletes as having (genetically) a
much higher than average 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—combined 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 four-footed mammals (Bramble & Lieberman 2004).

How Does Endurance 
Exercise Affect the Brain?

Research exploring whether moderate endurance exercise improves brain
function focuses largely on brain-derived neurotrophic factor, or
BDNF (Seifert et al. 2010). BDNF is a protein that promotes the growth
and maintenance of neurons in the central and peripheral nervous system.

BDNF is active in the hippocampus, cortex and basal forebrain, areas
involved in learning, memory and higher thinking. Seifert et al. note
that daily bouts of endurance training, about 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. The question becomes: Is what’s good for the brain in
moderate doses still good for the rest of the body in ultra-large doses?

Does Ultra-Endurance Exercise Cause Too 
Much Oxidative Stress?

Exercise plays a role in raising oxidative stress levels 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, which attempt to counteract the
harmful effects of ROS.

Oxidative stress has been associated with the development of
atherosclerosis and thus with a higher risk of cardiovascular disease.
The debate over exercise-induced oxidative stress comes down to one
central question: “How much exercise is too much?”

Knez, Coombs and Jenkins (2006), in their review of literature on ultra-

endurance exercise and oxidative damage, observe that ultra-endurance
pursuits raise ROS production and other markers of oxidative stress.
However, they say the research confirms that ultra-endurance exercise is
also associated with elevated antioxidant defenses and potentially
greater CVD protection—noting that higher levels of training may improve
this protective effect. The authors end by stating that research on
ultra-endurance exercise and antioxidant supplementation is inconclusive
for now.

Are There Physical Limits 
to Ultra-Endurance Exercise?

Noakes (2006) confirms that the human body can perform extraordinary
physiological feats when properly nourished and rested. Structurally, it
is mechanically developed to endure great running distances over time.
However, the body’s limits reveal themselves in ultra-endurance exercise

Martin et al. (2010) conducted fatigue research using a 24-hour
treadmill run, contrasting its effects on central (motor neurons and
neurochemicals in the brain) and peripheral (neuromuscular junction
and/or contractile proteins in muscle) mechanisms in the body. Their
study verifies that central fatigue is the principal limiting factor.
The authors hypothesize that a central brain mechanism may exist that
tries to reduce neural drive to working muscles, which could determine
exhaustion levels during ultra-endurance exercise.

Running jolts the skeletal system, producing a bodily shock wave up to
three to four times a person’s weight at faster speeds (Bramble &
Lieberman 2004). Fortunately, the larger surface areas of major
lower-extremity joint surfaces and increased lower-body bone mass help
to dissipate these impact forces.

However, lower-body overuse injuries remain common in runners, and
research is still trying to understand why some runners are more
susceptible than others. Schutz et al. (2012) report a common problem
for endurance runners is a lower-leg pain syndrome, often called shin
splints. Fallon (1996) confirmed an injury specific to ultra-endurance
runners, calling it “ultramarathoner’s ankle.” Schutz et al. explain
that this chronic pain is a manifestation of overuse pathologies of the
muscles, fasciae, tendons and bone tissues of the lower leg.

What Are the Clinical 
Concerns With Ultra-Endurance Exercise?

In a position paper on immune function and exercise, a panel of world
experts underscore that extreme amounts and/or intensities of exercise
may have negative implications for the immune system (Walsh et al.
2011). This type of training can make people vulnerable to increased
risk of infection, illness and clinical sepsis, a life-threatening
response to infection that can lead to tissue damage and organ failure.

O’Keefe and colleagues (2012) note that regular exercise is a
cornerstone for producing optimal cardiovascular exercise. However, the
researchers caution that chronic training and participation in
ultra-endurance events like marathons, ultramarathons, Ironman® distance
triathlons and very long distance bicycle races may overload the heart’s
atria (upper chambers) and right ventricle, acutely impairing the right
ventricular ejection.

Months to years of such training may eventually make a person more prone
to unfavorable heart arrhythmias (irregular resting heart rates).
Importantly, however, the researchers highlight that lifelong vigorous
exercise (up to 1 hour daily) is associated with low mortality rates and
higher functional capacity for activities of daily living.

Summary Thoughts

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 (see
Figure 1). However, it does appear that we need to educate our clients
that even though safe upper-dose boundaries have not been fully
determined, humans do have endurance limits, and going beyond these
limits can be bad for our health.


Bramble, D.M., & Lieberman, D.E. 2004. Endurance running and the evolution of Homo. Nature, 432, 345-52.

Fallon, K.E. 1996. Musculoskeletal injuries in the ultramarathon: The 1990 Westfield Sydney to Melbourne Run. British Journal of Sports Medicine, 30 (4), 319-23.

Knez, W.L., Coombes, J.S., & Jenkins, D.G. 2006. Ultra-endurance exercise and oxidative damage: Implications for cardiovascular health. Sports Medicine, 36 (5), 429-41.

Martin, V., et al. 2010. Central and peripheral contributions to neuromuscular fatigue induced by a 24-h treadmill run. Journal of Applied Physiology, 108 (5), 1224-33.

Millet, G.P., & Millet, G.Y. 2012. Ultramarathon is an outstanding model for the study of adaptive responses to extreme load and stress. BMC Medicine, 10, 77.

Noakes, T.D. 2006. The limits of endurance exercise. Basic Research in Cardiology, 101 (5), 408-17.

O’Keefe, J.H., et al. 2012. Potential adverse cardiovascular effects from excessive endurance exercise. Mayo Clinic Proceedings, 87 (6), 587-95.

Schutz, U.H.W., et al. 2012. The TransEurope FootRace Project: Longitudinal data acquisition in a cluster randomized mobile MRI observational cohort study on 44 endurance runners at a 64-stage 4,486 km transcontinental ultramarathon. BMC Medicine, 10, 78.

Seifert, T. et al. 2010. Endurance training enhances BDNF release from the human brain. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 298 (2), R372-77.

Walsh, N.P., et al. 2011. Position statement. Part one: Immune function and exercise. Exercise Immunology Review, 17, 6-63.

Tony Nuñez, PhD

Tony Nuñez, PhD, is an assistant professor of human performance and sport at the Metropolitan State University of Denver. He teaches exercise science and specializes in resistance training and metabolic conditioning. During his time in the private sector, he worked as a personal trainer, strength and conditioning coach, business owner and personal training supervisor. Tony has presented his research at national and international conferences around the U.S. and has published in peer-reviewed and professional journals, including IDEA Fitness Journal.

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