What is GAP in running?

Grade Adjusted Pace, pace adjusted for elevation. It corrects your real pace for the metres you climbed or descended to give the equivalent pace you would have run on flat ground. It is useful for comparing sessions on different routes.

What is cardiac drift and when is it a concern?

It is the rise in heart rate at constant pace over a long session. Up to 5 % is normal in long efforts in the heat; above that threshold it usually indicates dehydration, a short aerobic base or accumulated fatigue.

What do CTL, ATL and TSB mean?

CTL is chronic load (42-day average): your fitness. ATL is acute load (7-day average): recent fatigue. TSB is CTL minus ATL: positive means fresh, negative means you are accumulating fatigue. A persistently very negative TSB signals overload.

How to analyse a running activity

An activity is not analysed by its average pace. Five readings turn a pile of data into decisions for the next session.

Average pace is the most looked-at number and the least informative one: it blends hills, wind, stops and pace changes into a single figure that never actually happened. These five readings do point somewhere.

1. Per-kilometre splits

First, check whether the effort was spread or fell apart. A first kilometre much faster than the average is the classic too-fast start; slower final kilometres are not always a lack of fitness, sometimes they are the bill for those first three minutes. Read splits together with heart rate: if pace holds but heart rate climbs kilometre after kilometre, you are paying for fatigue.

2. GAP: grade adjusted pace

Climbing 40 metres of elevation costs roughly the same as 400 extra metres on the flat. GAP corrects pace for elevation and answers a simple question: what flat pace equals what I just did? Without GAP, a hilly session looks slow when it was not; with GAP, comparisons across days and routes make sense.

3. Time in zones

The zone split is the fastest way to see whether the session did what you intended. An easy run that accumulated 25 minutes in Z3 was not an easy run. Training zones are the reference: without them the split means nothing.

4. Cardiac drift

Compare the first and second half of a long session at constant pace. If pace holds but heart rate rises more than 5 %, there is dehydration, heat or accumulated fatigue. It is the most useful signal for adjusting hydration and for knowing whether your aerobic base is short.

5. Training load

A single session says nothing; the load says whether the whole makes sense. The three usual numbers are CTL (chronic load, your fitness, 42-day average), ATL (acute load, fatigue, 7-day average) and TSB (the difference, telling you whether you are fresh or accumulating). Looking only at the last session is like looking at a photo to decide whether someone is running.

What it looks like in practice

Afterpacer computes and charts all of this together: splits, GAP, zones, drift and load, activity by activity and accumulated per week. The quickest way to see it is to open the demo with a real activity, which needs no account.