До седьмого колена: to the seventh generation, way back
Колено is a strange little word. It has a huge range of meanings: knee, elbow, bend, generation, dance figure and stunt. And its conjugation sometimes depends on the meaning, and sometimes on the speaker.
The things you kneel on are колени in the plural, but the pipe, generation, dance and stunt колено are колена in the plural. But you can say встать с колен or коленей (to get up off your knees, with knee in the genitive plural) as you wish. Where are the Russian language police when you need them?
They're probably playing international Scrabble with the English language police, who let you say either kneeled or knelt.
When people are talking about knees, pay attention to whether the person is sitting (сидит) or standing (стоит). Стоять на коленях is to kneel; сидеть у кого-то на коленях is to sit on someone's lap.
And then you need to pay a lot of attention to context. Колено реки (literally the knee of a river) is a bend. Колено трубы (literally the knee of a pipe) is what English speakers call an elbow. Streets with sharp bends are often called some form of коленный (knee-like), like the charming Кривоколенный переулок (Crooked Knee Lane) in Moscow.
When people talk about колено in the context of family professions or revenge, this колено means generation. You're likely to hear the Biblical phrase до седьмого колена (to the seventh generation), which sometimes means seven actual generations and sometimes means way, way back or way, way ahead. У нас считается святым долгом знать своих предков до седьмого колена. (We consider it our sacred duty to know our ancestors back to the seventh generation.) Мы доберёмся до всех его потомков до седьмого колена (We'll get every last one of his family for the next seven generations.)
Колено, or its diminutive form коленце, can also be a dance step or any action that is unexpected, funny, or strange — perhaps because standing up and doing a two-step at a board meeting would be definitely weird. The stunt коленце/колено uses the verb pair выкидывать/выкинуть: Хорошо бы выкинуть такое коленце, выйти замуж за старого друга молодости. (Maybe it would be a good idea to pull off a real stunt — marry my old childhood friend.)
The dance коленце/колено (often plural коленца) usually uses the verb выделывать (to do something to perfection) and sometimes the more expressive verb pair откалывать/отколоть (to chop off). Больше всего нам нравится, когда под русскую песню мы пляшем и выделываем русские коленца. (Most of all we love it when we dance to a Russian song and cut the rug with Russian dance steps.)
If someone has голова как колено, it means he's as bald as a billiard ball. По колено is used when you're up to your knees in water or something worse. But the expression ему море по колено (literally he's up to his knees in the sea) means that he couldn't care less about danger — sea or no sea, he'll forge ahead. Ломать/переломить через колено is to ride roughshod over someone or give a good beating. Мы Австрию переломили через колено (We decimated Austria.) And давать/дать кому-то коленкой под зад (literally to give someone a knee to their backside) means to reject or eject someone.
All of this leaves me слабой в коленях (weak in the knees).