Do you have to reach all the way to the ground in order for your dog to lie down? While this isn't the end of the world, it's actually quite easy to teach a dog to lie down with you standing up straight. Your back will definitely thank you!
This method being used here is taught by Jean Donaldson, found of The Academy for Dog Trainers. The overall idea is to break it down into 3 steps. If your dog is doing the first step 4 out of 5 or 5 out of 5 times, you're ready to increase the difficulty and move to the next step. This is explained more in the video. The end goal is for your dog to lie down for a hand signal while you're standing up straight.
Want to get this on a verbal cue? Teach this sequence and once your dog can do it for the small hand signal, add the verbal cue in just before. You can read more about that here.