Training load does not add up across sports
Three sports, three different tissues, one number pretending they are interchangeable.
If you train in more than one sport, sooner or later something offers to add your loads together. Swim TSS plus bike TSS plus run TSS, one weekly total, a tidy chart. It is a very appealing number and it hides two real problems.
Problem one: the scores are not the same unit
Cycling load is computed from power — an external, absolute measurement. A 250 W effort is 250 W on any bike, any day, at any temperature.
Running load is usually computed from pace with a grade adjustment, or from heart rate. Pace is external but strongly affected by terrain and heat; heart-rate-derived load is internal, drifts with hydration and temperature, and lags the effort.
Swimming load is normally derived from pace or stroke rate, and the dominant variable is technique. A swimmer whose catch improves does the same work at a faster pace, and the load model reads that as a harder session.
Summing them assumes a point of cycling load and a point of swimming load represent the same physiological cost. They do not, and the discrepancy is not a constant you can calibrate away — it varies by athlete and by session type.
Problem two: the tissue does not aggregate
Even if the units agreed, the adaptation and damage would not.
The cardiovascular system is broadly shared — an aerobic hour is an aerobic hour, and fitness built on the bike transfers usefully to running. This is the part where a combined number is almost defensible.
Mechanical loading is not shared at all. Running loads bone, tendon and connective tissue eccentrically, with ground impact. Cycling is concentric, non-impact, joint-friendly. Swimming loads the shoulder and essentially nothing in the legs.
Which means the classic multisport injury is invisible to a summed load. An athlete swaps two run sessions for two bike sessions of equal “load”, total weekly load flat, and their running-specific tissue tolerance decays anyway. Three weeks later they return to normal run volume, at a load the chart says they have been handling all along, and something in the lower leg gives.
The number was flat. The exposure that mattered was not.
What to do instead
Track load per sport, separately, and look at the set.
Keep a cardiovascular aggregate if you find it useful — as one line among several, for answering “have I been doing too much overall”, which it answers reasonably well. But make the per-sport lines the ones you actually watch, because those are the ones that map to a tissue.
The two questions worth asking every week are different from the one the combined chart answers. Is my total aerobic stress sustainable? — the aggregate is fine for that. Is any single tissue getting a step change in exposure? — only the per-sport view can tell you, and that is the question that predicts whether you will be training next month.
The combined number is not useless. It is just answering a narrower question than its tidiness suggests, and treating it as a complete picture is how multisport athletes get hurt during the block where everything looked fine.
Paceva keeps the per-sport lines separate and computes them on the device, from the sessions your watch already recorded.