The push off from the back leg is at its peak fully straightened - the closer you are to that point,the stronger your rear technique will be.
As it happens, Karate taught to always bend the supporting knee, but not only does this put undue pressure on the joints,it also inhibits your transfer of weight.
Muay Thai and TaeKwonDo teach it to straighten, unlike Karate. So they got it right, whether they knew it or not.
Also I’ve done Goju Ryu for almost a decade, so anecdotal. I will also stress that I am aware that Goju Ryu, depending on your organisation and if you’re in the Okinawan or Japanese orginisations, have some differences in technical aspects, how kata are performed and what bunkai are focused on, so I couldnt speak for everyone under the Goju banner.
Odd that your post has now got two things I noticed in your arguements:
One we moved to specifying side kick later
And secondly you seem to think me addressing what you put in your text, regardless of if you stated it originally or not, required you to get defensive about who initally said it, and missing the point that I have simply challenged the arguement. Not an attack on you, so no need to get all shields up
And they are right. You don’t know what a side kick is until you’ve seen TaeKwonDo. The outliers in Karate look more like TaeKwondo, and Karatekas admit this themselves… the supporting leg kick is ideally fully flexed when side kicking, which is why I posted one showing just that (well 99% flexed)…
The roundhouse kick has some benefits having bent legs for lateral movement, especially low kicks. Low kicking fully flexed doesn’t make sense. But the power principle is the same there too. If you go for broke with a roundhouse kick, you ideally extend the joints.
Boxing, you push off the supporting leg when throwing the cross and extend rather than bend the joint … Joe Lewis is correct.