Running TSS (rTSS) is calculated using Normalized Graded Pace (NGP), so it is supposed to account for changes in pace when going up or down hill. For some people, this is enough to give them accurate numbers while trail running. In some situations, the NGP isn't enough to correct for the other factors that can affect pace like the surface, wind, short steep pitches up and down that GPS can't detect well, and the rockiness/rootiness/general technical difficulty of trails. In this case, we recommend changing the TSS calculation for trail runs to heart rate based TSS (hrTSS) instead of pace based TSS to see if that more accurately reflects the overall effort and training load of that workout. For this to work correctly you will need to make sure that you have heart rate data in that workout and you have the correct threshold heart rate for running set in your account settings.