Vertical split-keyboards are often marketed as more ergonomics than traditional keyboards. Is that true? I am especially interested in stress on the epicondyles of the humerus.
I am aware of the study Muss, Timothy M., and Alan Hedge. "Effects of a vertical split-keyboard on posture, comfort and performance." In Proceedings of the Human Factors and Ergonomics Society Annual Meeting, vol. 43, no. 6, pp. 496-500. SAGE Publications, 1999. https://scholar.google.com/scholar?cluster=7634052692663655085&hl=en&as_sdt=0,22 but the sample size was small, the study is almost 20-year-old, and subjects weren't accustomed to vertical keyboards.
Vertical split-keyboards don't have to be position vertically: