Great question. Scott Semple’s reply of being consistent in how you apply TSS to all your workouts is going to the top priority since TSS and CTL are going to be relevant to you and you alone and are not really very comparable from athlete to athlete. That’s true for mountain athletes not only due to the variable terrain/pace/HR but also to changing sports. Some days you may be running, other days skiing, other days doing a long approach and alpine climb.
All that said and hopefully given the appropriate level of importance we can move on to your question. And the answer is……..drum roll……IT DEPENDS.
I think we can all agree that treadmill running or even steep hiking is easier muscularly than the outdoor equivalent. As for uphill treadmill hiking; I chalk that up to the fact that most treadmills seem wildly optimistic in the elevation they display at the end of your workout. If you simply do the math, in most cases the vertical will be higher than is possible. For example: You hike 1 hour out doors on a 15% (average grade) trail at a brisk pace of 5km/hr. In one hour you will have gained 750m. If I hike 1 hour at 15% on my home treadmill at this pace it will show well over 1000m.
And, we have to take into consideration the downhill, which does not happen in the treadmill or stair machine. It is the down hill that tends to really hit most people harder than the hike up. That is because even though your HR might be low on the down hill you are doing a lot of eccentric work with each foot strike as you, to a greater or less degree, depending on your technical economy, are applying a braking force mainly through your quads. It is this localized muscular soreness that will take you longer to recover from than the uphill work you did which was all concentric.
With all this as background, here is what I do to adjust TSS for the wide variety of workouts my athletes do: First I apply the fudge factor. For a treadmill it will be roughly half of the same elevation gain/loss out doors. Then and most importantly, I see how tired those workouts make them. A normal Zone1-2 run or hike will give about 60TSS/hr. So if the I get a report that they are still feeling this in their legs 2 days later I know that whatever they did had a bigger effect than the 60TSS and boost the TSS for that workout. Some of the vicious ME workouts I have some of my folks do can yield 200-250TSS. This would mean they are still quite stiff and sore 48 hours out.
I know this a long way of telling you that there is no simple answer. If you are shifting to indoor training just make the TSS comparable to the level of fatigue you felt from similar outdoor workouts.