Here are some examples of ergonomic "doorknobs" produced by Linear Locksets:
They don't look like ergonomic doorknobs--because they're not knobs at all. Rather than rotate, they latch and unlatch doors via push/pull motions:
And yes, they're lockable:
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"When I was renovating my mother's house, I was faced with a lack of choice of ergonomic DIY hardware, particularly the availability of ergonomic door furniture," writes Australia-based inventor and engineer Thuan Nguyen. "Ergonomic door furniture was a must for my mother as she suffers from chronic Rheumatoid Arthritis and using her fingers and wrist movement was painful when she needed to open a door in her house."
Unable to find anything on the market that met his needs, Nguyen designed the mechanisms and hardware and managed to get Linear Locksets into production. Check out their lineup here.