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?

To read more about ultra-edurance exercise and the affects of pushing too hard on the body, please see “Limits and Effects of Ultra-Endurance Exercise” in the online IDEA Library or in the November-December 2015 print issue of IDEA Fitness Journal. If you cannot access the full article and would like to, please contact the IDEA Inspired Service Team at (800) 999-4332, ext. 7.


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

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.

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.

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.

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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