It requires muscle tension to lock out your elbows or your knees and this is not a natural, relaxed position to maintain. Doing so directs stress towards the weaker parts of the joints. No matter how strong your muscles, such a position can expose these weaker areas to damage and injury. To take full advantage of our natural bone support and to make best use of our larger muscle groups, our knees and elbows should be in a natural bend short of being locked.
I use a swing draw and maintain a natural arm position throughout the swing. Some teach to push out towards the target to slightly lock the elbow and then relax to obtain this natural slight bend. Others teach to obtain and maintain a locked position for target archery, but I'm of the opinion that this increases the chance of injury when shooting flat hand with a heavy longbow. Not sure if using a push-pull technique to reach a natural arm position is as risky as locking out, but using a swing draw is far less taxing than the push-pull, which is why I use it for heavy bows.