Think about the difference between a short burst of hard effort and work that must continue for an entire 8 hour shift, day after day. A person can briefly work close to their VO2max, but sustaining a high fraction of it for hours causes exhaustion, a rising heart rate that never settles, and falling work output.
Ergonomic studies specifically carried out on Indian workers, accounting for their typical body size and physical conditioning, arrived at a much lower sustainable fraction than the near-maximum values sometimes seen in short-duration lab tests. That accepted sustainable figure is about 35 percent of VO2max, well below the 50, 65 or 80 percent options, which are all too demanding for continuous full shift work.
\[\boxed{35\%\ \text{of VO}_{2max}}\]