TSS algorithm
It is a pity that the TSS calculation algorithm only takes into account duration and heart rate. To value 3 hours of cycling in the same way as 3 hours of running (at the same heart rate) for example does not seem the most accurate: the level of joint and metabolic stress has nothing to do with it.
I suggest making a TSS calculation algorithm that takes more factors into account.
Thank you for your feedback!
The TSS model is based on the metabolic demands of training.
Roughly speaking, this is done by comparing your level of effort to your lactate threshold. This is how we can tell how much “stress” a workout was, and turn that into Fitness/Form/Fatigue that describe your fitness specific to endurance sport.
Generally, heart rate zones for running are higher than for cycling. This means that for TSS to best reflect your efforts, you should make sure your zones are correct for each.
TSS is not meant to account for the mechanical demands of different activities, like running vs cycling vs lifting weights.
This is an interesting problem in sport science, and a very challenging one because there are a lot more variables to consider when trying to quantify the loads that different tissues might experience. The first step is following a good plan from an experienced coach, which will balance stress and recovery. A sound progression will make sure you aren't taking on too much too soon.
Tracking your performance and subjective measures like soreness and the "How did you feel?" question in TrainingPeaks are good ways to monitor your recovery from training and help individualize your plan, since athletes do respond to and recover from training differently.