Also known as: CTL, Chronic Training Load, Fitness score
Training Load is a rolling average of your daily training stress, weighted so the last six weeks count most. One number for how much fitness you have banked. Coaching software usually calls it CTL, for Chronic Training Load.
Training Load answers one question: given everything you have run in the last month and a half, how fit are you today? It is a load-accumulation model, not a measurement of anything in your blood or your lungs. Each session adds a daily training-stress score, and Training Load smooths those scores into a slow trend line, so one brutal day or one day off barely moves it. Climbing over several weeks means you are building fitness. Flat means you have stopped.
Training Load comes from Dr. Eric Banister's fitness-fatigue impulse-response model (1975), which treats every session as an impulse that decays over time into a fitness part and a fatigue part. Coaching software, following Dr. Andrew Coggan's version, usually runs a 42-day exponentially weighted moving average over daily training-stress scores. Day to day that works out as: today's value = yesterday's value + (today's score − yesterday's value) / 42. The box below has the formula in full.
The number scales with however daily stress is scored for your sport and your threshold, so there is no universal good value. A recreational runner on 4 hours a week might sit around 40 to 50. A competitive marathoner on 10+ hours a week can run 90 to 120 and above. The trend matters far more than the level: climbing steadily, or stalled?
Training Load is the fitness half of the model. The same model produces Fatigue, which is short-term load, and Form, which is the balance between the two. On its own, a rising Training Load is a decent proxy for improving aerobic capacity across a block, as long as the rise is gradual. Sports-science guidance generally caps a healthy ramp at roughly 3 to 8 points a week to limit injury and overtraining risk. Read alongside Fatigue and Form, it tells you whether today's tiredness is proportionate to the fitness you bought with it.
Reading Training Load on its own, without Fatigue and Form, is the usual error. A high Training Load with deeply negative Form describes a runner who has built real fitness and is currently too tired to show it, not a runner who is undertrained. The other trap is chasing the number for its own sake. Ramping faster than the 42-day time constant can absorb is a documented route to injury and non-functional overreaching, and it buys no extra fitness.
Doing this yourself means scoring every run against your current threshold pace or power, keeping a 42-day rolling average up to date in a spreadsheet, and redoing the lot whenever your threshold moves.
Miss one threshold update and every score after it drifts. Your Training Load drifts with it, quietly.
Vertical recalculates Training Load, Fatigue and Form from every run that syncs off your watch, re-fits your threshold as your fitness moves, and uses the trend to pick what to schedule next. No spreadsheet.
See my training load, free