Extreme sport

Extreme sport (also called action sport and adventure sport) is a media term for certain activities perceived as having a high level of inherent danger.[1][2] These activities often involve speed, height, high level of physical exertion, high specialized gear or spectacular stunts.[1]
At present date, there are no careful studies or statistics of deaths to separate activities with low or normal level of danger from those with high level of danger. It is instead used as a marketing term in promoting events such as the X Games.

Overview

Extreme sports differ from standard sports due to the relatively higher number of inherently uncontrollable variables in extreme sports. Extreme sport athletes compete not only against other athletes, but also against environmental obstacles and challenges. These environmental variables are frequently weather and terrain related, including wind, snow, water and mountains. Because these natural phenomena cannot be controlled, they inevitably affect the outcome of the given sport or event.
In a standard sporting event, athletes compete against each other under controlled circumstances. While it is possible to create a controlled extreme sporting event, there are often variables that cannot be held constant for all athletes. Examples include snow conditions for snowboarders, rock and ice quality for climbers, and wave height for surfers,

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Running 5km time trial is consider extreme sports? Chiong 80-100% of the effort for short distance can consider as high risk sports? Or running at 60-70% for 42k consider extreme sports?

Given the condition "don't stress the heart". Going 5km time trial is definitely higher risk based on the higher effort. Given normal human behaviour, as long as distance is longer, we tend to take it "easier".

What is Extreme sport then? Perceive effort? As long as HR is keep at decent rate, no matter the distance or time, is it safe?

It seems that most people take that whenever the event take a long time to complete, eg 2hr+ is an extreme sport. Playing a competitive badminton or tennis, which might last a couple of hours are not consider extreme, whereas running for the same amount of time is consider. Sigh... Perception!