I like the idea of applying some type of HR check to MTB rides that affects the tabulated strain score for low/high/peak.
There is likely some characteristic of MTB rides that would allow those activities to be analyzed differently. Or at least those portions of MTB rides that involve lots of technical maneuvers including descents.
For example, an obvious rise in HR when cadence is near zero compared to coasting on the road which would normally mean the opposite.
Or high strain intermittent power bumps indicating a tough technical section where the power is inconsistent but HR is steadily rising.
IOW some recognizable pattern that would allow XO server to identify offroad activities and apply a scaling factor to the strain score.