Training load is a way to put a number on the total stress your training places on the body, combining how long you trained with how hard. Tracking it turns a vague sense of doing enough into something you can manage - which is what makes progression, recovery, and tapering controllable. It is the single lens that explains why you are improving, plateauing, or breaking down.
What training load actually means
Every session you do imposes a stress on the body. Training load is simply a way to put a number on that stress - to measure how much work a session, a week, or a whole block of training really demanded.
The key idea is that hours alone do not capture it. A two-hour easy hike and a forty-minute interval session are nothing alike in their effect, even though one is three times longer. Training load combines duration and intensity into a single figure, so efforts of different shapes can be compared on one scale. It is often derived from heart rate and time - a training impulse that weights harder minutes more heavily than easy ones.
Why bother measuring it? Because the moment training load is a number, it becomes something you can manage. Progression, recovery, and tapering stop being feel and guesswork and become decisions you can actually make.
Fitness, fatigue, and the gap between them
The most useful way to think about training load is the fitness-fatigue model, first published by Banister and colleagues in 1975 and refined across decades of endurance research (Banister et al., 1975). It is one of the foundations of TTM's algorithm, and the idea is simple.
Every training session leaves two marks on the body at the same time. One is a fitness effect - slow to build, slow to fade, accumulating over weeks. The other is a fatigue effect - fast to rise, fast to fall, clearing within days. What you can actually perform on any given day is the gap between the two.
Train hard and fatigue spikes, hiding the fitness underneath. Recover well and fatigue drains away, leaving that fitness exposed.
This is why a hard block can leave you feeling slow and heavy right when, on paper, you should be at your strongest - the fitness is there, buried under fatigue. And it is why a planned easing so reliably produces a jump in performance rather than a loss of it. The same logic underpins recovery and deloads, covered in the recovery and overtraining guide.
Chronic load, acute load, and form
In practice, training load is usually tracked as two running averages that stand in for fitness and fatigue.
Chronic training load is a long-run average of your training, typically over roughly six weeks. It moves slowly, and because it reflects work absorbed over a long window it is a reasonable proxy for fitness. Building it is the entire point of a training block.
Acute training load is a short-run average, typically over about a week. It moves quickly and reflects recent fatigue - how much you have done lately.
The relationship between the two is what coaches sometimes call form, or training stress balance. When acute load sits well below chronic load, you are carrying real fitness without much fatigue: fresh and sharp. When acute load runs far above chronic load, you are deep in fatigue and not expressing your fitness. Neither extreme is wrong - you need hard, fatigued stretches to build, and fresh ones to perform. The skill is knowing which you want, and when.
Why this matters for mountaineers
For most endurance athletes, form fluctuates across a season of many races. A mountaineer often has one day that counts. The summit window is fixed - by a permit, a guide, a weather forecast, a season - and all the fitness in the world is wasted if you arrive on that day buried in fatigue.
That reframes the whole training block as a training-load problem. The job through the build is to push chronic load - fitness - as high as your body can absorb. The job in the final weeks is to shed acute load - fatigue - without losing the fitness you built. Done right, the gap between them is at its widest on exactly the day you step onto the mountain.
This is what a taper is: not a vague wind-down, but a deliberate, timed reduction in acute load. Cut back too early and chronic load starts to slip. Cut back too late and fatigue is still in your legs at the trailhead. The taper has to be reverse-engineered from the summit date.
The two ways athletes get training load wrong
Almost every training-load mistake is a version of one of these two.
fast
slow
The answer to both is a controlled, progressive ramp with recovery built in: load that rises week on week at a rate the body can convert into fitness, punctuated by regular lighter weeks that let accumulated fatigue clear. Progressive overload and structured recovery are the backbone of block periodisation (Bompa & Haff, 2009), and they are how training load becomes fitness instead of breakdown.
How TTM uses training load
Tracking fitness, fatigue, ramp rate, and form by hand is genuinely hard - it is a moving calculation that changes with every session you do or miss. That calculation is the core of what TTM automates.
A calibrated fitness-fatigue model, recalculated weekly
Train to Mountain runs a calibrated implementation of the Banister fitness-fatigue model behind your plan. It reads what your training actually delivered, separates the slow fitness signal from the fast fatigue signal, and uses the gap between them to decide when to push load and when to ease off. Every Sunday it recalibrates the rest of your plan against the load you genuinely carried - a missed week redistributes, a strong block lets the next one build. And because it knows your summit date, it solves the taper backward from that day so your fitness peaks when it has to. The internal model is proprietary; the science underneath it is well established, and it is set out in full on the science page.
Common questions
What is training load?
Training load is a way to quantify the total stress a session, a week, or a block of training places on the body. It combines how long you trained with how hard, so a short hard interval session and a long easy hike can be compared on the same scale. Tracking training load turns a vague sense of doing enough into a number you can manage - which is what makes progression, recovery, and tapering controllable rather than guesswork.
What is the difference between chronic and acute training load?
Chronic training load is a long-run rolling average of your training, usually over about six weeks. It moves slowly and is a reasonable proxy for fitness. Acute training load is a short-run average, usually over about a week. It moves fast and reflects recent fatigue. The gap between them - sometimes called form or training stress balance - tells you whether you are fresh or buried. High chronic load with low acute load is the fresh, sharp state you want on summit day.
What is the fitness-fatigue model?
The fitness-fatigue model, first published by Banister and colleagues in 1975, describes how every training session leaves two marks at once: a slow-building fitness effect that accumulates over weeks, and a fast-moving fatigue effect that rises and fades within days. What you can actually perform is the gap between the two. It explains why a hard block can leave you feeling worse before a planned easing reveals the fitness underneath.
Why does ramp rate matter in mountaineering training?
Ramp rate is how fast you raise your training load week to week. It matters because fatigue accumulates faster than fitness. Push load up too quickly and fatigue outruns the adaptation, leading to a plateau, injury, or overtraining. Raise it too cautiously and fitness never builds enough to meet the objective. A controlled, progressive ramp with regular recovery weeks is what lets training load become fitness instead of breakdown.
How do I peak for a summit day?
Peaking is a training-load problem. In the final weeks before a climb you reduce acute load through a taper while protecting the chronic load you have built. Fatigue drains away faster than fitness, so the gap between them widens and you arrive fresh and sharp rather than buried. The taper has to be timed backward from the summit date - too early and fitness fades, too late and fatigue lingers. The detail is in our tapering for mountaineering guide.
The takeaway
Training load is the single lens that explains progress. Fitness builds slowly, fatigue rises fast, and performance is the gap between them. Raise load at a rate your body can absorb, build chronic load through the block, then shed acute load into the summit date. Manage that balance and the fitness you worked for is there on the one day it has to be.