The three cases are not equal. Endurance gains, strength is unchanged, and short repeated efforts stay carb territory.
This page follows on from What changes in the body when you cut carbs, which explains the shared mechanism: low insulin, salt and water leaving.
For hard efforts, muscles usually draw on their glycogen. Without carbs, they have to learn to run on fat and ketones, and that takes time: more than a week, with adaptation well advanced by three to four weeks according to Phinney.
Meanwhile, performance often drops, and that is the stretch in which many people judge keto. Once adapted, his small studies show endurance preserved at a moderate pace.
Judging keto during adaptation is judging an engine while its fuel is being changed. The dip of the first weeks says nothing about what comes next.
The engine genuinely changes. Compared with ultra-runners fed on carbs, twenty elite runners adapted for twenty months on average burned 2.3 times more fat at their peak, 1.54 against 0.67 g per minute, and at a higher intensity.
Their muscle glycogen was the same at rest, emptied the same over a three-hour run and refilled the same afterwards. So keto did not drain their backup tank: it mostly taught the body to reach for it later.
That is keto's own advantage in endurance: fuel becomes nearly unlimited, since the fat stores of a lean runner far exceed their 500 g of glycogen. Fewer sugary feeds, fewer stomach troubles, and no wall to hit.
A heavy set of three to five reps lasts a few seconds: it draws on phosphocreatine, which recharges in a minute or two of rest, not on carb stores. Ordinary strength training therefore does fine on keto.
There is an indirect effect too: if fat loss followed, you move less mass for the same force. Everything that carries the body, from pull-ups to stairs, then feels easier.
At race pace, it is another story. In world-class race walkers, a low-carb, high-fat diet worsened their running economy by a few percent and cancelled out the benefit of three weeks of intensified training. At that level, a few percent separate a medal from an anonymous finish. For a Sunday runner, the gap is invisible.
What is felt at every level is something else: short, repeated efforts at high intensity, sprints, intervals, team sports with accelerations, draw on glycogen whatever shape the person is in.
If your sport strings accelerations together or is decided in a sprint, strict keto will cost you something on the day. Nothing stops you training on keto and putting carbs back around races, though.
Adaptation. Three to four weeks of decline, which say nothing about what follows.
Endurance. Twice as much fat burned, a nearly unlimited reserve, no sugary refuelling.
Strength. Untouched: a heavy set runs on phosphocreatine.
Short, repeated efforts. Sprints, intervals, team sports: glycogen remains the best fuel, at every level.
Adaptation and minerals: Phinney, Ketogenic diets and physical performance, Nutrition & Metabolism 2004.
Fat oxidation: Volek et al., Metabolic characteristics of keto-adapted ultra-endurance runners, Metabolism 2016.
Race pace: Burke et al., Low carbohydrate, high fat diet impairs exercise economy, J Physiol 2017.