Scholars read the oracle bone form two ways: a wooden plough pushed into the soil, or a bent arm with the muscle standing out. Both say the same thing — effort applied against resistance. Either way the character has meant physical force for three thousand years.
努力 (to strive) is the word Chinese parents say most often to children, and 加油 aside, it is the standard encouragement. 力量 is strength, 能力 ability, 压力 pressure. 力 sits inside 男 (male) as a field 田 worked by strength.
A flexed arm, or a plough biting into soil — either way, STRENGTH pushing against something.
力 has only two strokes but appears in over 200 common characters — among the hardest-working components in the language.