The original Russian term, rabotosposobnost, literally translates into “work ability”. A better translation would be “potential productivity”.
But someone creatively translated it into English as “work capacity”, which instantly changed its meaning. The word “capacity” implies the size of a tank, as in “alactic capacity” or “aerobic capacity”. The Russian term, while including capacity, means a lot more.
Even in the USSR there was vagueness and many conflicting definitions of potential productivity. I will spare you the esoteric discussions and present you with a definition that many Russian experts would agree with—or at least could live with: