Stamina: how CatLena estimates your glycogen

The body stores carbohydrate as glycogen: most of it in the muscles (about 500 g in an adult) and a smaller part in the liver (about 100 g) [1]. It provides much of the energy for harder workouts. When the store is low, training feels heavier. CatLena’s stamina is an estimate of that store, in grams and as a percentage of how much fits.

Nobody measures glycogen at home: it takes a biopsy or an MRI scan. So CatLena uses a balance model, like a bank statement. There is an account size (how much fits), deposits (the carbohydrate you eat) and withdrawals (what workouts and the body itself burn).

A day in practice

The Stamina card on the Nutrition screen shows your store right now: what you had at the end of yesterday, plus the carbohydrate from meals you have already marked as eaten, minus the workouts you have already marked as done. The resting use of the brain and the rest of the body is taken out for the whole day first thing in the morning, so the number drops when the day starts and rises as you mark meals. Planned meals and workouts that have not happened yet do not count.

Here is a day for a 70 kg man with 15% body fat and a VO₂ max of 52 (capacity of about 634 g):

In the real example below, the card shows 509 g of glycogen (92% of capacity), 212 g of carbohydrate already eaten and 91 g used in the day, counting rest and training.

1. How much fits: capacity

• Lean mass = weight × (1 − body fat %), using the measurements from the Body screen.
• Muscle = a share of lean mass, estimated by sex: 53% in men and 45% in women (50% if sex is not in the profile). An MRI study of 468 adults found muscle to be about 38% of body weight in men and 31% in women [2], which is roughly those shares of lean mass.
• Muscle glycogen = muscle × K, in grams per kilogram of muscle.
• Liver glycogen = 1.4 g per kg of body weight, about 100 g in a 70 kg adult. Direct measurements show it rises after meals and falls in the hours that follow [3].

K rises with fitness. A meta-analysis of 181 studies found about 17 g of glycogen per kg of muscle in men with a VO₂ max near 53, and about 2.5 g more for every 10 points of VO₂ max [4]. CatLena suggests K from your VO₂ max, in steps:

2. What comes in: food

• Carbohydrate minus fiber comes in, because fiber does not turn into glucose [5].
• The brain and nervous system use about 120 g of glucose a day, even at rest. That need is the basis for the minimum carbohydrate recommendation of 130 g a day [5].
• Part of that glucose comes from protein, through gluconeogenesis. CatLena counts 0.35 g of glucose per gram of protein, up to 60 g a day. That sits between the theoretical maximum (0.5 to 0.8 g per gram) [6] and what was measured in people in the first hours after a meal (about 0.17 g per gram) [7].
• The rest of the brain’s use comes out of glycogen, with a minimum of 40 g a day.
• The store does not fill by more than 2.5 g per kg of body weight a day. This is a conservative CatLena limit: in the hours after training, muscle can store much faster than that [8][9].

3. What goes out: workouts

For a run or walk, CatLena takes the workout calories and works out how much came from carbohydrate: glycogen used = calories × carbohydrate fraction ÷ 4 (each gram of carbohydrate has about 4 kcal). The fraction rises with intensity: easy workouts burn more fat, hard workouts burn more carbohydrate. This is the "crossover" concept [10].

Intensity comes from average heart rate as a percentage of your maximum. If you do not enter your maximum, CatLena estimates it as 211 − 0.64 × age, from a study of 3,320 adults who did a maximal test; the typical error is about 11 beats per minute [11].

This curve is a CatLena approximation based on the crossover concept. In trained cyclists measured in the lab, carbohydrate was about 46% of energy at 44% of VO₂ max, 51% at 57% and 76% at 72% [12]. CatLena’s curve is lower at easy intensities, so it tends not to overstate glycogen use on easy days. Heart rate comes from Strava. Without it, for a run CatLena uses your pace compared with your VO₂ max; walks without heart rate, or runs without a VO₂ max, are left out of the use. For easy walks (below 55% of max heart rate), watch calories are reduced by 25%, because they tend to be overestimated.

For strength training, CatLena counts 20 g per session plus 2.2 g per set (40 g when no sets are logged). Biopsy studies show muscle glycogen drops of 12% after one set and 24% after three [13], a drop of about 26% after a heavy leg session [14] and 23% to 44%, depending on fiber type, after a 45-minute session [15]. The per-set formula is CatLena’s own and is in the same range.

4. The daily balance

Today’s balance = yesterday’s balance + net intake − workouts, always between zero and capacity. The calculation starts 120 days before today, at 85% of capacity, and runs day by day up to today.

Each workout also gets a demand badge, from its use relative to capacity: very low (under 5%), low (5% to 12%), moderate (12% to 20%), high (20% to 30%) and very high (over 30%).

CatLena’s own choices

• The K steps by VO₂ max, the share of lean mass that is muscle (53% in men, 45% in women) and liver at 1.4 g per kg follow the studies, but the round numbers are model choices.
• The heart-rate carbohydrate curve, the 20 g + 2.2 g per set for strength, glucose from protein (0.35 g per gram, up to 60 g) and the 2.5 g per kg daily limit do not come from a single study: they are CatLena approximations.
• That is why the absolute value can be off by about 30% to 40%. The change from one day to the next, which depends on the same math, is much more reliable.

How to adjust it

• In Settings › Stamina, you can set K yourself (for example, after a lab assessment) and your real max heart rate from a test.
• You can also choose whether intensity comes from heart rate or pace.
• Leave them empty and CatLena uses the automatic values described above. Without a VO₂ max, K is 15; without a logged body fat %, CatLena uses 18% (men), 26% (women) or 22% (sex not set).