I don’t really understand what you mean to be honest! What do you mean by outside the shoulder and above the shoulder? What I would assume you mean is with the arm pointing directly upwards (above the shoulder) or the arm more out to the side (outside the shoulder). Is that what you mean?
I have a qualification in sports therapy so I understand how all the muscle joints work together, but that doesn’t necessarily mean I can properly answer your question I am afraid.
Let’s get this perfectly clear – the body is not intended to play badminton. The impact on the knees and the pressure on the shoulder are far in excess of what we are designed for. However, with the correct strength and conditioning, and with correct technique, it is easy to remain pain free. If you want to play properly, then watching the professionals IS the correct thing to do from my view. Their technique is the most efficient because it makes sense to take advantage of your body’s biomechanics (biomechanics = the most efficient way to produce a result based on the design of the muscles in your body). They HAVE to hit the shuttle in such a way that prevents injury because it is their career at stake at the very least! I would say deviating from the way they hit is more likely to cause injury, as its so much less efficient in many cases. But i am a coach so I am definitely biased
Now to think about where the arm should be: my view is it should be in between vertical and horizontal, but tending more towards horizontal. From my point of view, having the arm point vertically means the shoulder has reached a limit of its range of motion (and pronation is least effective for generating power on smashes). From my view, serious injuries are not likely to come at muscles (muscles are elastic and adaptable!) – the rotator cuff muscles (infraspinatus and teres minor for supination, subscapularis for pronation, supraspinatus for elevating the arm) are used, along with all the other muscles around the shoulder (deltoids, biceps, triceps, rhomboids, pecs, levator scapula, trapezius and probably more). Injuries are much more prone at joints – this could be the ligaments or in some cases the cartilage/bone itself. Overuse of joints could also create calcification which can cause lumps which can cause considerable pain. And injuries are much more likely to happen at joints at the end range of motion, not in the middle – the end of the range of motion is where all the ligaments get stretched, and where the bones are most likely to rub together.
And thats as deep as my thought process goes. Here is the reality: if your body can move in a particular way, then it is normal and healthy and right to move it that way (except ice jockey! which use hip adduction to propel you forwards. Madness). For badminton, the biggest issue I see is incorrect technique, and reaching too vertically on forehands and backhands can cause considerable discomfort – I would consider it incorrect technique. Also lunging and landing from jumps for the lower body. Whether the arm is straight or slightly bent with regards to the elbow. etc etc etc…
I would also note that hitting technique is REALLY about the position of the joints of the shoulder and arm, NOT about whether the contact point is away from the body or not. If you take a “round the head” overhead shot, the shuttle is taken close to the body, typically in line with the shoulder. However, the body has flexed to the side, meaning that if they stood up straight, the arm would be out to the side. There is more variation in the forehand corner depending on how close you get.
But now I am curious: who is telling you to smash in these ways? What makes you think you should trust them? And what does it even matter? I would trust your own experiences: try it all ways, and see if it hurts. Consider all the advice and then do whats best for you. If it hurts, then adjust it so its comfortable. If it doesn’t hurt then carry on. Simples!
p.s. if you are worried about your technique please post a video and I will check for you!