I will say when I train chest, shoulders or triceps I never lock out so as to keep constant tension on the muscle and my triceps are pretty FOOKING big due to this reason I believe. I would only lockout if doing rest pause at the end. I’m big on constant tension.
I will say this...look at a gymnasts triceps. Most have big tricep development. They lock out. Bhanks wants people to Google it...where there is just a bunch of bro science bullshit. The term 'constant tension' is one of those terms I fucking hate. Flex magazine shit. Think about the leg extension....when starting off (one those machines where once you get in, the pad is already pushing your legs back) there is tension on the muscle. When you extend fully, there is tension on the muscle. So, when is there NOT tension on the muscle during the exercise?
Now, if someone wants to ask how much stress is placed on the tendons...be it fully contracted muscle, fully stretched muscle, mid point, etc? I think that is a better question.