Make Motocross sessions not affected by speed threshold to get correct TSS
When I use my Garmin watch to track my motocross sessions, trainingpeaks use my speed and thresholds to set TSS.
This makes an incorrect TSS, as my work as such does not give the speed, the motocross bike does. My workload is steering the machine and its power, makin it more like a strength/crossfit session (If you should compare the work).
Motocross is one of the most physically demanding sports you can do, and practically every muscle in your body is activated while doing it.
As an example, last summer I had a VO2max of 56 on mtb, an threshold on MTB of 300 watts/165 HR, and logged and average HR in my 20-30 min. motocross sessions of 190-195 (I am 43 years old)