Japanese people (especially adults) often say “よっこいしょ(yokkoisho)” when they stand up or lift something heavy.
People are able to output their force effectively when they are exhaling or holding their breath.
Therefore, it’s natural thing that we put our muscles while calling out some phrases to reduce burdens on our bodies.
In addition to “よっこいしょ,” we say “どっこいしょ” and “よっこらせ” and “よっこいしょういち,” though those precise etymologies are still unknown.
Note that if you say “よっこいしょういち,” you may be questioned like “what era are you from?” because the phrase is very old Japanese joke derived from “横井 庄一（よこい しょういち）” who were a Japanese soldier about a hundred year ago.
By the way, I learned some countries’ calling out phrases like “よっこいしょ.”
According to the internet, American say “alley-oop,” French and Italian say “hop la,” German say “hau ruck,” and Russian say “Давай(davai).”
