Metabolism and function · 20 October 2026
Recovery is more than replacing calories
Muscle energy markers and force can recover on different timelines. Adequate fuelling matters, but it is not the only part of recovery.
What has actually recovered?
A person can have eaten, rested and restored one physiological measurement while still being unable to repeat their earlier performance. For EPK, energy per kilogram is a way to organise questions about fuelling. It should not become a claim that every limitation is a shortage of calories.
The useful question is specific: has fuel availability recovered, has the muscle regained force, or does the person simply feel ready? Research can separate these outcomes rather than combine them into one recovery score.
The experiment that separated chemistry from force
Saugen and colleagues studied eight men performing intermittent isometric knee extensions. Magnetic resonance measurements tracked muscle phosphocreatine, inorganic phosphate, ATP and pH, alongside voluntary and electrically stimulated force. Metabolic measures approached control values within five minutes of recovery, while force remained incompletely restored after thirty minutes. ATP concentration remained stable. [1]
The authors proposed impaired excitation–contraction coupling as an explanation. The study did not directly establish a particular calcium-handling defect, and it was not a feeding intervention. The available primary abstract supports the dissociation, but a detailed audit of the full methods and figures remains outstanding.
Concentration, reserve and usable performance
A stable ATP concentration is not the same as no energy being used. Concentration is a snapshot of a maintained pool; turnover concerns the ongoing supply and use of that pool. Likewise, restoring a metabolic marker is not identical to restoring the entire sequence that produces force.
That distinction is a useful extension of the ATP discussion in Controlling Intuitive Appetite, Chapter 35. The book asks readers to consider the processes that make activity possible. These findings make the question more precise: which process is limiting the activity now?
The inference from the Saugen experiment should remain narrow. A persistent force deficit after those measurements have recovered means those measurements alone do not explain readiness. It does not mean nutrition is irrelevant, or that the same mechanism explains mental fatigue after a meeting.
Where carbohydrate availability enters the picture
Ørtenblad and colleagues reported human evidence linking low muscle glycogen after exhausting exercise with impaired calcium release from the sarcoplasmic reticulum, an important part of muscle activation. Their findings support a possible connection between stored carbohydrate and the machinery that produces contraction. [2]
This is a different experimental question from the Saugen study. It should not be used to retroactively diagnose glycogen depletion as the cause of every force deficit in that earlier experiment. Different exercise tasks, measurements and recovery conditions must remain visible.
A study of seven high-level male footballers provides another practical comparison. After a match, despite an optimised diet, muscle force, glycogen, soreness and aspects of calcium handling did not recover together. Calcium uptake recovered sooner than several other outcomes. [3]
Why a wearable cannot settle every recovery question
An estimate of energy expenditure can help describe the demand of a day. It does not directly measure muscle glycogen, calcium release or force. The studies here used laboratory measurements because their questions required them. Translating their findings into an app should not imply that a watch has measured the same processes.
A proposed EPK recovery study could combine intake and activity records with a repeatable functional test. The relevant outcome would be whether the person can reproduce performance under comparable conditions, alongside their experience of fatigue. If the energy estimate improves but performance does not, that disagreement deserves investigation rather than dismissal.
The trial should specify the exercise task, recovery interval and nutrition comparison in advance. A result from repeated isometric knee extensions cannot automatically determine recovery time after endurance racing, resistance training or an ordinary working day.
Fuelling is part of recovery, not a guarantee
Adequate food remains a sensible foundation for testing recovery, but none of these experiments establishes that simply adding calories resolves every impairment. A stronger EPK claim would be that fuelling should be evaluated together with function and training demand.
This changes the practical question from “Have I replaced the calories?” to “Have the conditions for repeating the task been restored?” It leaves room for carbohydrate availability to matter while recognising that the body may still need recovery time after a fuel-related measurement has normalised.
Related EPK articles
Research and guidance
Explore more studies in the research library.
- Saugen E, et al. Dissociation between metabolic and contractile responses during intermittent isometric exercise in man. Exp Physiol. 1997. doi:10.1113/expphysiol.1997.sp004010. Abstract reviewed.
- Ørtenblad N, et al. Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle. J Physiol. 2011;589:711–725.
- Krustrup P, et al. Maximal voluntary contraction force, SR function and glycogen resynthesis during the first 72 h after a high-level competitive soccer game. 2011.
Related working papers
Read the relevant PDFs: EPK–001 to 007 are the revised version 1.2 papers; EPK–008 to 010 are version 1.0 supplements. Earlier PDFs and DOI records remain on the paper archive.