How do I track my strength training at the gym on my Apple watch

I can't take time to stop and open the app after every exercise and change what I'm doing specifically, that's simply unreasonable. And half the exercises I do, on standard gym equipment, are not listed as individual exercises. So how can I start my Apple watch tracking my workout without having to go change it non-stop through out my one hour strength trainings at the gym? Or is this not possible? If it isn't possible, then why have a watch to help with fitness?