The original Chinese symbol for strength li looks like a tool used for ploughing: the upper part is the handle, the lower part is the ploughshare and the vertical stroke is the part to place the foot.
(Do you know how the Chinese character for "man" came about? It's got something to do with the character for strength.)
Ploughing requires a lot of strength, hence li means strength.
Don't you think the modern Chinese character li looks like a picture of a forearm which is flexed to show physical strength?
Common Chinese phrases with li include
li4 liang4 力量 = physical strength
li4 qi4 力气 = effort, strength
li4 bu4 cong2 xin1 力不从心 = even though one's heart wishes (something), one doesn't have the ability (to do it)
li4 zheng1 shang4 you2 力争上游 = to strive for good results
tuan2 jie2 jiu4 shi4 li4 liang4 团结就是力量 = unity is strength
Incidentally, the Chinese symbol for strength combined with the symbol for field tian gives the Chinese character for man - a man working in the fields with a plough. Definitely work that requires strength and work which is done by adult men traditionally.