Is wearing a SPEAR on your BACK practical, effective or historical?

Is wearing a SPEAR on your BACK practical, effective or historical? Can you draw it quickly or wear one on horseback?
Dawson's awesome video about spears on the back: https://www.youtube.com/watch?v=WbeS1bYB8Io&t

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https://www.kobo.com/au/en/ebook/shadow-of-the-conqueror

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