I want you to think back when you first tried learning golf. I don't think there has been any golfer who hasn't entertained the idea that their lead arm needs to be straight in the golf swing. If you think about what your first observation of the golf swing was, it was that you noticed the pro golfer having a straight lead arm.
There is a lot of discussion in popular instruction that says you shouldn't straighten the lead arm... that your arms have to be relaxed when swinging the golf club, but what actually happens in the golf swing? To examine this let's look at other sports. Pick any sport that involves kicking or throwing and you can apply this analogy. let's take for example a tennis player. How are their arms in the beginning of making a serve or volley? It's nice relaxed and ready to go, What about a kicker getting ready to kick? His legs are nice relaxed and ready to go. What about hockey player? Even say a hockey player, I mean you name it, their body is always in a relaxed and predisposed to be making the actual motion,
Now, let's examine what happens during the natural motion of the athlete throwing, kicking, or whatever motion they are making. The arm begins to extend to strike the tennis ball and extends even further after impact, the kicker begins to extend his leg as he approaches the kick and the leg extends further through the kick. The hockey player begins his extension of arms and then the extension increases through the impact area.
So what does this all mean for the golfer and what does it have to do with the lead arm? Well, the exact same thing. When you go for a golf swing, you are in a relaxed state at set up, although your arms are connected more to you sides they are not fully tense and straightened. However as you approach you backswing, your arms begin to start extending toward the top of the swing and even more as you approach the follow through. This will give you that "look" that the lead arm is straightening.
So for all of those who are saying you don't need to keep your arm straight in the golf swing, they are wrong. And to those who say you need to completely keep your arms loose throughout the whole swing, you are also wrong. It's actually a combination of both. Watch the videos of Koepka and DJ. notice how their arms hand and address without and tension and then notice how their arm gradually start tensing up throughout the swing.. Try this!!! You will be amazed!