Effort in mountain biking
The mountain biking is a very popular sport because it allows appreciating nature (campaign, mountain, etc.) and escaping of road constraints (danger, pollution, etc.)
This sport uses more muscular groups than road cycling but in a particular way.
In road cycling, essentially the thighs (quadriceps) are used and very often in a sit down position.
In mountain biking, the position is more often stand up, pulling more with arms, without counting the bumps, descents, bends which it is necessary to negotiate in particular positions.
The various states of the ground require also from the muscles, tendons, articulations to support more shocks and to work differently (work without movement, quick changes of rhythm, etc.).
Finally, given the bike weight, the size of tires and the nature of the ground, to move a mountain bike require more energy (for the same effort time).
However, you have to keep in mind that road cycling allows concentrating more easily on the effort. In mountain biking, a lot of situations can generate various slow downs.