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Pathway
Intermediate
50 min read

Energy and recovery

A six-step reading pathway that follows muscle energy from the second to the night: the instant recharge of ATP, the glycogen stores and their water, then the role of sleep. A closing quiz lets you check what you have taken in.
Muscle energy reads on three time scales: the second, the hour and the night. This pathway crosses them in that order, from the molecule that powers one contraction through to the sleep that shapes the following day.

What this pathway helps you understand

Three questions give the pathway its shape. Why a maximal effort cannot last, even though the cell never runs out of energy. Why a scale can show several hundred grams of difference from one day to the next without any tissue having changed. Why recovery is an active process, most of which is taken up by sleep.
The pathway aims at understanding, not prescription: it sets out what the literature measures, under what conditions, and what it leaves open.

Level, prerequisites and reading time

The pathway is rated intermediate: no background in biochemistry is required, but the energy routes are presented here as running at the same time, which contradicts the successive pathways described elsewhere. Useful, though not compulsory, prerequisite: ATP and phosphocreatine.
The time given is a reading time: around fifty minutes for the six steps, videos included, and not a duration of effort. Progress is saved locally, so you can stop reading and pick up again at the next step.

The steps of the pathway

The six steps, in order

0 of 6 steps completed
  • The immediate fuel
    What the cell spends on every contraction, and the buffer that recharges its tiny reserve.
    8 min read
  • The ATP–phosphocreatine cycle in pictures
    The same mechanics animated, in both directions: spending during effort, recharging during rest.
    8 min read
  • Muscle glycogen and its water
    The carbohydrate store of the muscle, how it is mobilised during effort and the water that goes with it.
    9 min read
  • Why weight varies
    The animation that links the glycogen store, the water held and the number a scale shows.
    8 min read
  • Sleep and recovery
    What a night contains, and what a shortened night takes away first.
    9 min read
  • A night in pictures
    The final step: the night run through cycle by cycle, then two days compared.
    8 min read

What to take away at the end

First takeaway, on the scale of seconds: the muscle stores almost no usable energy, it remakes it continuously through several routes running at the same time.
Certainty level · Established
The energy routes of the muscle run at the same time, and the aerobic contribution becomes dominant earlier than the classic account suggests.
Reviews of metabolic system interaction describe the transfer of a phosphate group from phosphocreatine as the fastest source of ATP, recharged at rest by oxidative metabolism. During continuous maximal exercise, the anaerobic and aerobic contributions even out within a few tens of seconds, depending on the protocol and the participants.
Gastin PB (2001) · Baker JS, McCormick MC, Robergs RA (2010)
Second takeaway, on the scale of hours: glycogen empties and refills along with its water, which moves body mass faster than tissue does.
Certainty level · Probable
Rebuilding muscle glycogen depends on carbohydrate intake and goes together with water being held in the muscle.
Reviews on carbohydrate describe rebuilding as proportional to food intake and to the time available, and express their reference figures in grams per kilogram of body mass. A study in trained men after prolonged exercise in the heat reports roughly three grams of water stored per gram of glycogen: the small sample makes that an order of magnitude, not a constant.
Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011) · Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R (2015)
Third takeaway, on the scale of the night: sleep is the only part of recovery that takes up several hours every day.
Certainty level · Probable
Sleep restriction is associated with poorer sustained performance and alertness, and duration figures are drawn up by age bracket.
A review of sleep and performance reports those impairments from deprivation protocols run over a few nights in young adults. A panel convened by the National Sleep Foundation places the appropriate range at 7 to 9 hours a night for adults aged 18 to 64, stating that it describes a population, not an individual need.
Fullagar HHK, Skorski S, Duffield R, Hammes D, Coutts AJ, Meyer T (2015) · Hirshkowitz M, Whiton K, Albert SM, et al. (2015)
Caution
What this pathway does not do
No step makes a diagnosis or sets an amount or a duration for any given person.
Persistent fatigue, an unexplained change in weight or lasting difficulty sleeping are matters for a healthcare professional who can examine the person concerned.

Check my understanding

How do the energy routes take over from one another during a maximal effort?
Why can body mass vary by several hundred grams in a day?
What do reviews on sleep and performance report?
Choose an answer

Sources for this pathway

  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
  • Baker JS, McCormick MC, Robergs RA (2010). Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
  • Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R (2015). Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. European Journal of Applied Physiology.
  • Fullagar HHK, Skorski S, Duffield R, Hammes D, Coutts AJ, Meyer T (2015). Sleep and athletic performance: the effects of sleep loss on exercise performance, and physiological and cognitive responses to exercise. Sports Medicine.
  • Hirshkowitz M, Whiton K, Albert SM, et al. (2015). National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health.

Read next

  • The ATP–phosphocreatine system — the video
    An animation that follows one ATP molecule through a short, intense effort and then through the rest that follows: it is spent, recharged by phosphocreatine, and backed up by the other pathways. The page carries the same content in text, with its sources and limits.
    With a 3D scene
  • Glycogen and weight swings — the video
    An animation that explains why the number on a scale moves by several hundred grams from one day to the next: muscle glycogen is stored along with water, and that water changes in quantity far faster than tissue does.
    With a 3D scene
  • Sleep and recovery — the video
    An animation that runs through a night of sleep cycle by cycle, then shows what a shortage of sleep changes in physical performance and alertness. The page carries the same content in text, with its sources and the limits of the measurements.
    With a 3D scene
Put it into practice in Shapier

Plan sessions and recovery weeks

Placing rest days and lighter weeks within the week is done in Shapier; Body Lab only explains why that recovery matters.
Plan sessions and recovery weeks
Body Lab explains; Shapier lets you act and track.

Where to go next

Available offline

Offline download is available in the mobile app.

Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • Splitting metabolism into three energy pathways is a teaching convention: the metabolic routes run at the same time, and the durations measured vary with the protocol and the muscle studied.
  • The sleep data come mostly from deprivation induced in a laboratory over a few nights in young adults, which is a poor description of a sleep shortage settled in over time.
  • No step allows the fatigue, weight or sleep of any one person to be interpreted: the pathway explains mechanisms, it does not replace professional advice.
Sources
  • Gastin PB (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine.
  • Baker JS, McCormick MC, Robergs RA (2010). Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism.
  • Burke LM, Hawley JA, Wong SHS, Jeukendrup AE (2011). Carbohydrates for training and competition. Journal of Sports Sciences.
  • Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R (2015). Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. European Journal of Applied Physiology.
  • Fullagar HHK, Skorski S, Duffield R, Hammes D, Coutts AJ, Meyer T (2015). Sleep and athletic performance: the effects of sleep loss on exercise performance, and physiological and cognitive responses to exercise. Sports Medicine.
  • Hirshkowitz M, Whiton K, Albert SM, et al. (2015). National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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