Look at the skeleton composition Paragraph 4 supports the thesis in the paragraph Introduction: Almost all sports, whether indoor or outdoor, require stamina and physical fitness

Paragraph 1 (thesis):Perhaps the most demanding sport requiring the most heart and lung action
is marathon running
Body: Although many sports provide very strenuous exercise, they do not utilize all the major
muscles in the way that marathon running does
Paragraph 2: muscles used in running
Paragraph 3: Although a runner may die from a heart attack, most deaths among runners are
caused by incautious running, lack of proper training, or not receiving a thorough physical
examination
Paragraph 4: Another important factor in marathon running is the variety of terrain that adds
even more challenge to the runner's stamina.
Paragraph 5: Most important of all are the statistics of longevity (long life) of marathon runners
Conclusion
by proving that marathon running is very strenuous
by talking about the terrain a runner encounters

by providing that marathon running is very strenuous

Explanation:

author – information on the internet with a listed author is one indication of a credible site. the fact that the author is willing to stand behind the information presented (and in some cases, include his or her contact information) is a good indication that the information is reliable.

date – the date of any research information is important, including information found on the internet. by including a date, the website allows readers to make decisions about whether that information is recent enough for their purposes.

sources – credible websites, like books and scholarly articles, should cite the source of the information presented.

domain – some domains such as .com, .org, and .net can be purchased and used by any individual. however, the domain .edu is reserved for colleges and universities, while .gov denotes a government website. these two are usually credible sources for information (though occasionally a university will assign a .edu address to each of its students for personal use, in which case use caution when citing). be careful with the domain .org, because .org is usually used by non-profit organizations which may have an agenda of persuasion rather than education.

site design – this can be very subjective, but a well-designed site can be an indication of more reliable information. good design make information more easily accessible.

writing style – poor spelling and grammar are an indication that the site may not be credible. in an effort to make the information presented easy to understand, credible sites watch writing style closely.