Just because your wheelbarrow is awesome, that doesn’t mean you shouldn’t use proper body mechanics to avoid injuries. There is no reason for a sore back, hamstrings, or shoulders, at least, not from using your wheelbarrow.
When you use your wheelbarrow, use the power in your legs. When it is time to grab the handles and go, instead of bending over for the handles, do a shallow squat instead. Grab onto the handles and then stand up straight.
I try to always remind myself to keep my shoulders back when I am driving and steering my wheelbarrow around the yard. My general tendency is to let my shoulders roll forward. When you keep your shoulders back, those muscles, along with your neck muscles won’t end up tense and knotted at the end of the day.
One more tip for comfortable wheelbarrow use: Use your gloves!
Blisters on your hands are so painful, but can be easily avoided with a good pair of outdoor gloves.