People are often told not to lock out a joint.
Usually being warned that it'll damage the joint.
Fingers then get pointed at yoga and some of our "pyjama" martial arts people, specifically here in the West where stretching beyond all sensibility takes precedence over any form of strength work.
Joint issues do feature quite highly in these populations.
But I posit that's more to do with the lack of strength building in favour of stretching.
I think I may have given away the core message of this message in that last sentence.
Maintaining a good balance of strength and mobility around all the joints is the key to freedom of movement.
And part of that freedom is the ability to fully straighten the legs and arms, aka lock out the knees and elbows.
This lockout maybe shouldn't be done at speed, but it should be available.
Failing to reach this point ever will allow some of the bodies smaller muscles get used to a shortened length.
The Popliteus muscle of the knee and the Pronator Teres of the elbow spring to mind.
If you've ever felt pain on the inside of the elbow, that's the Pronator Teres.
The Popliteus lives in around the same space but in the leg.
Part of their roll is to slow down full extension of the joint and then to unlock that joint.
This locking/unlocking is almost always done in conjunction with rotation at the ball and socket above (hip/shoulder)
If you can't lock it, or never go full range, these muscles get used to being short.
Which may lead to an imbalance across that joint, maybe pushing the movement into another joint.
What practical use is there for locking out?
If you watch gymnasts, Kettlebell and Olympic Weightlifters you'll see lovely straight lines of support.
If you're a combat athlete, knowing and having control of this end range may help you prevent injury there.
Don't be afraid to find these locked out positions, ease in and out with a corkscrew like action.
Feel what the body likes and what it doesn't