Last night you trained legs for the first time in weeks, heavy and to the last set. This morning the scale says 83.2 kg (183.4 lb), on Monday it said 82.3 kg (181.4 lb). Almost a kilo more, after a day on which you burned energy.
Your body has not added fat. It is holding water. Glycogen, the carbohydrate store in your muscles, is kept with about three to four parts of water for every part of glycogen (Kreitzman et al. 1992). On top of that, a muscle under strain binds fluid while it repairs itself. Both show up as a blip lasting a few days, not as a new trend.
What happens in your body after a session
Glycogen and its water
Your muscles store carbohydrates as glycogen. Training uses up part of it, and afterwards your body refills the stores. For every part of glycogen, three to four parts of water move into the muscle. The scale weighs that water along, although your fat mass has not changed.
The blip is bigger when the stores were very empty beforehand, for example after a low carb week. They refill quickly after the session and the first proper meal, and the water arrives in one rush. The same mechanism sits behind the jump after a pasta night, described in Why your weight fluctuates every day.
Repair inside the muscle
Unfamiliar or hard effort strains the muscle fibres. Your body repairs them, and fluid collects in the tissue while it does. Sore muscles are the sign of it. The more foreign the effort, the stronger the reaction: your first session after a break or a new programme moves the scale more than the tenth identical session.
Sweat pulls the other way
Straight after training the scale can even show less, because you have lost sweat. Once you drink and eat, that water returns. This number says nothing about fat either, no more downwards than the next morning's number says upwards.
An example with numbers
You weigh yourself every morning. Before the hard session your trend weight is 82.4 kg (181.7 lb). The session is on Tuesday evening, and the week looks like this:
| Day | Scale | Trend weight (approx.) |
|---|---|---|
| Monday | 82.3 kg | 82.4 kg |
| Tuesday | 82.4 kg | 82.4 kg |
| Wednesday | 83.2 kg | 82.5 kg |
| Thursday | 83.0 kg | 82.5 kg |
| Friday | 82.6 kg | 82.5 kg |
| Saturday | 82.3 kg | 82.5 kg |
The daily value jumps by 0.8 kg (1.8 lb) on Wednesday, the trend weight by about 0.1 kg. By Saturday the scale is back at 82.3 kg, and you did nothing except eat and sleep normally. The peak was water, and the arithmetic shows it: one kilo of body weight from fat and tissue takes an energy surplus of about 7,700 kilocalories (Hall 2008). For 0.8 kg that is roughly 6,200 kilocalories above what you burned, on a single day with a workout. Even the biggest dinner does not get you there.
If you compare only daily values, Wednesday shows a gain of 0.8 kg and it is easy to conclude that the training backfired or should be skipped. In the trend, the same week is a straight line. The session took nothing from you on any of those days, and it gave you nothing you now have to get rid of.
How to read the question in a trend curve
No single morning value answers "Did I gain weight from training?". The course does. The trend weight in Trendcurve smooths every entry with the values of the days before it. One high value barely moves the line, a real rise over several weeks does. How the smoothing works is covered in Trend weight instead of the daily number.
Three tools in the app fit this case:
- Ignore a measurement: For a value you consider pure water, you can mark the entry as ignored. It stays visible in the history but no longer feeds the daily value or the trend. This is in Free and Pro. When in doubt, leave it in: the trend evens it out anyway.
- Experiments and markers: Under "Settings & more" → "Experiments and markers" you create a marker of type "Exercise", for example for the start of a new training plan, with a start day, an optional duration and a note. In the chart it appears as a line or, with a duration, as a tinted band. Creating one is possible in Free. The before and after analysis is Pro: it compares the weekly rate of the 28 days before with the duration and needs at least 7 days and 4 weigh-in days per section. The card carries the fixed note "Correlation, not cause".
- Weekday patterns (Pro): If you train on fixed days, the "Patterns" card shows the average gap between your daily value and the trend for each weekday over 13 weeks. If the day after leg day sits above the line again and again, you are looking at your own water pattern.
When it can be more than water
Water explains a blip of a few days. It does not explain a trend that points upward for three weeks. If your trend weight climbs over several weeks although you train, look at your food: people who train more are often hungrier and eat more without noticing. Building muscle also adds weight, but slowly. It never arrives overnight.
What real change looks like and how to tell a plateau from hidden water is covered in How fast should you lose weight?. If you have an injury, a swelling or a gain you cannot explain, ask a doctor. The app is not a medical device.
In practice: weighing around training
The meal after training amplifies the effect. Carbohydrates refill the glycogen stores, and a salty meal after sport, such as pizza or a restaurant dinner on training night, binds additional water. You cannot read the share of each cause from one value, and you do not need to: all three fade within a few days.
Weigh yourself at the same time, best in the morning after getting up, and not straight after a session, so that no sweat loss sits in the number. Log the value on the day after a hard workout too, even if it is higher, and judge only the trend weight at the end of the week. If it sits at last week's level, the blip was water.