Whenever people get a Sprinter for the first time, they are amazed at all the differences between Sprinters and domestic vans. One big difference is the side door, the most frequently used door once you're camped somewhere. It's big and heavy and slides instead of opening on hinges. However, there is a trick to minimizing the effort required, and to do this you must understand the dynamics of door operation so that you can use them to your advantage.
My lovely bride and I noticed the difference as soon as we switched from a Chevy to a Sprinter two months ago. She tweaked her knee standing inside facing the door and trying to slide it sideways with her arm and torso muscles. People's muscles just aren't designed to produce high lateral forces. I was facing forward and pulling back, so my knees were fine, but the effort seemed very high. Something was wrong with our techniques; we were out of harmony with the spirit of the Sprinter door.
The first principle of being at one with your Sprinter side door is observation, young master. Look at the tracks the door slides in. The top one is easiest to see, but the bottom one is exactly the same shape. Look at the front end of the track. See that curve inward on the last six inches or so? As the door opens, it doesn't go straight back, it goes OUT and back. Standing inside and pulling the handle back and inward toward the vehicle centerline is applying force at least 45 and more like 60 degrees away from the door's direction of travel. No wonder it's hard. Only when you exert force along the door's destined path are you aligning yourself with the cosmic flow.
I tried a new technique. Stand inside as usual, but closer to the door, and facing straight outward. Pop the latch open with your left hand, but instead of hauling on the handle by the latch, PUSH on the top of the door with your right hand, above the window, out and toward the rear of the vehicle. The path to enlightenment is along the rails the door slides in, not the way you're used to using your muscles. Go with the cosmic flow, and reduce your struggles.
Same thing when you're trying to open the door from the outside – pull OUT and back, don't just try to slide the door backwards. Visualize the tracks inside, and exert force straight along the path the door follows as it opens. Once you get it moving, it will slide all the way back easily.
One more trick I picked up on – you are outside and trying to lock the vehicle, and sometimes the doors won't lock because the side door isn't shut all the way. Rather than opening the door and slamming it shut, check the back edge of the door. If it's popped out a little bit, you can engage the back door latch just by pushing in along the rear door edge. Now the doors will lock – faster, and much less effort.