I'm nearly 57 and have been window cleaning 41 years, wfp for the last ten.
I work with a trolley as it best suits my work. This means moving barrels of water from my house to my vehicle and from vehicle to the trolley.
I've found one sure-fire way to cause myself injury, either to my back or wrist and arms is to move them too quickly off the floor or in and out of the back of my car etc. Therefore, I use a sack truck to move full barrels to my car and never walk anywhere carrying them. I never snatch them up when full, just brace myself and lift them carefully in and out and onto the trolley.
As my trolley is in constant use I've spent some time modifying it to make it as light as I can. It is now only marginally heavier that the PF Nano and is easy to get in and out of my car with only one hand if need be.
The bigger and heavier the pole you use the more inertia it has. Early on I injured my wrists and arms quite a bit due to this. So again, when moving around with pole in hand, I've learned not to make sudden moves or changes in direction of movement. I try to make my movements flow smoothly and keep my equipment as light as possible.
At this point in time I'm finding it enjoyable and reasonably pain free. My only long term issues come from wear n tear due to 30 years of ladder use and one act of poor judgment at work on my part, that resulted in a badly broken ankle 17 years ago.