Rashi explains Yosef was able to carry Ephraim and Menashe on his knees before giving them to Yaakov. How could Yosef carry them both on his knees if they were already old at that time? Is the pshat just that Yosef was really strong?
In order to answer this question, we first should examine the sources behind the question. Which Rashi says "Yosef was able to carry Ephraim and Menashe on his knees before giving them to Yaakov"?
There are two places knees are mentioned that I can tell. The first is Bereishit 48:12:
וַיּוֹצֵ֥א יוֹסֵ֛ף אֹתָ֖ם מֵעִ֣ם בִּרְכָּ֑יו וַיִּשְׁתַּ֥חוּ לְאַפָּ֖יו אָֽרְצָה׃ Joseph then removed them from his knees, and bowed low with his face to the ground.
ויוצא יוסף אתם. לְאַחַר שֶׁנְּשָׁקָם הוֹצִיאָם יוֹסֵף מֵעִם בִּרְכָּיו, כְּדֵי לְיַשְּׁבָם זֶה לַיָּמִין וְזֶה לַשְּׂמֹאל לִסְמֹךְ יָדָיו עֲלֵיהֶם וּלְבָרְכָם: ויוצא יוסף אתם AND JOSEPH BROUGHT THEM OUT — After he (Jacob) had kissed them, Joseph brought them out from between his knees in order to place them one at the right and the other on the left in order that Jacob might thus lay his hands upon them and bless them
That isn't necessarily that he was carrying them on his knees, or that they were sitting on his lap. He could be sitting down, and they could be standing or sitting in front of his spread knees. Or, more figuratively, from in front of him.
Similarly, in 50:23:
וַיַּ֤רְא יוֹסֵף֙ לְאֶפְרַ֔יִם בְּנֵ֖י שִׁלֵּשִׁ֑ים גַּ֗ם בְּנֵ֤י מָכִיר֙ בֶּן־מְנַשֶּׁ֔ה יֻלְּד֖וּ עַל־בִּרְכֵּ֥י יוֹסֵֽף׃ Joseph lived to see children of the third generation of Ephraim; the children of Machir son of Manasseh were likewise born upon Joseph’s knees.
על ברכי יוסף. כְּתַרְגּוּמוֹ גִּדְּלָן בֵּין בִּרְכָּיו: על ברכי יוסף [WERE BORN] ON JOSEPH’S KNEES — The meaning is as the Targum gives it: He brought them up upon his knees.
Our Onkelos, by the way, translates the earlier וַיּוֹצֵא יוֹסֵף אֹתָם, מֵעִם בִּרְכָּיו as וְאַפֵּיק יוֹסֵף יָתְהוֹן, מִן קֳדָמוֹהִי, that he took them from before him. And the later one, יֻלְּדוּ, עַל-בִּרְכֵּי יוֹסֵף, as אִתְיְלִידוּ, וְרַבִּי יוֹסֵף.
If so, no particular great feat of strength is needed.
Welcome to Mi Yodeya, Sam! Thank you for sharing the great question.
Yosef was indeed incredibly strong. The Midrash Bereishis Rabbah (93:7) records the epic battle between Yosef and Yehuda when each displayed their powerful strength. In fact, Yosef kicked a stone pillar and turned it into a pile of pebbles:
מֶה עָשָׂה יוֹסֵף בְּאוֹתָהּ שָׁעָה אוֹתוֹ עַמּוּד שֶׁל אֶבֶן שֶׁהָיָה יוֹשֵׁב עָלָיו בָּעַט בּוֹ וַעֲשָׂאוֹ גַּל שֶׁל צְרוֹרוֹת, מִיָּד תָּמַהּ יְהוּדָה וְאָמַר זֶה גִּבּוֹר מִמֶּנִּי
If Yosef could smash a huge pillar, he could probably carry his two sons.