Originally Posted by Rozroz
many thanks for the meantime.
Include in your videos some taken from directly behind the server viewing as best you can along the line of the ball's trajectory. If there is someone to help viewing from behind hold the camera as high as possible. That view shows best the angle of the shoulders and the angle between the arm and wrist at impact.
Take your video in bright outdoor sunlight as many cameras with auto exposure control video will select a faster shutter speed when there is bright light - the fastest shutter speed minimizes motion blur. Many smartphone cameras do this. Repeat the videos until you get some frames very near to impact. If you should be taking video with a newer DSLR, if possible, set the shutter speed manually as high as possible (1/2000 or 1/4000) and also video at 60 fps if available. Many cameras have a high speed video mode such as some Canon Powershot models.