Exercise is planned physical activity for fitness, strength, or sport. Dose and recovery determine whether it builds or breaks.

Peat's view

Exhaustive exercise can produce dangerously high blood ammonia, similarly to hyperbaric oxygen, high estrogen, or hypothyroidism, and this tends to run alongside excess lactic acid because ammonia stimulates glycolysis.[1] Chronic cardio under low fuel raises stress hormones. Strength and daily walking with enough sugar and protein fit better for many hypothyroid recoveries than marathon volumes. Rest is part of training.

Energy compensation and constrained total expenditure

Doubly-labeled-water measurements across a broad range of physical activity levels found total daily energy expenditure does not keep rising linearly with more activity; instead, expenditure plateaus as activity increases, meaning the body compensates by cutting energy use elsewhere rather than letting total output climb indefinitely.[2] This fits the pattern above: heavy cardio combined with calorie restriction is a double stress-hormone load, and a body already compensating for high output by trimming energy elsewhere has less room left before that compensation starts pulling from thyroid-driven basal metabolism rather than just fat stores.

Practice

Peat drew a practical line between two kinds of muscle contraction. Concentric contraction, the muscle shortening against resistance (walking or cycling uphill, lifting a weight), repairs tissue and improves function without injury. Eccentric contraction, the muscle lengthening under load (walking or running downhill, lowering a weight slowly, deep-stretching under resistance), damages muscle and connective tissue and is the type that causes soreness.

They found that "concentric" contraction, that is, causing the muscle to contract against resistance, improves the muscle's function, without injuring it. (Walking up a mountain causes concentric contractions to dominate in the leg muscles. Walking down the mountain injures the muscles, by stretching them, forcing them to elongate while bearing a load; they call that eccentric contraction.) Old people, who had extensively damaged mitochondrial DNA, were given a program of concentric exercise, and as their muscles adapted to the new activity, their mitochondrial DNA was found to have become normal.

— Ray Peat[3]

Concentric muscle exercise, where you are shortening the muscles under force. Eccentric exercise, like walking downhill, stretches the muscles and makes them sore. Old people with damaged mitochondria can repair them with concentric exercises.

— Ray Peat[4]

Practical takeaway for someone in a recovery/hypothyroid phase: favor walking, stationary cycling, or lifting on flat or uphill ground, and go easy on downhill hiking, running, plyometrics, and heavy eccentric-focused lifting (slow negatives) until Basal temperature and pulse are steady.

Mental imagery and strength

Twelve weeks of daily mental-imagery training, imagining a maximal muscle contraction without any physical movement, increased finger-abduction strength by 35% and elbow-flexion strength by 13.5% in healthy adults, an effect attributed to increased cortical drive to the trained muscles rather than any physical tissue change.[5] This is a real but modest effect relative to actual training load and doesn't substitute for it; it may have practical value during injury recovery or illness when physical training isn't possible.

Muscle-brain signaling and mood

Voluntary running for four weeks raised apelin levels in mouse skeletal muscle, serum, and hippocampus, and reduced depression-like behavior. Muscle-specific deletion of apelin abolished the antidepressant and neurogenic effects of running. Muscle-targeted apelin overexpression reproduced the same benefit without any running at all. Apelin acts through its receptor APJ on hippocampal neurons, activating casein kinase 2 and enhancing NMDA receptor function. This is the pathway through which exercising muscle signals the brain to lift mood.[6]

See also

References

  1. ↑ Ray Peat, "Fatigue, Aging, and Recuperation," raypeat.com.
  2. ↑ Pontzer H, Durazo-Arvizu R, Dugas LR, et al. "Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans," Curr Biol. 2016;26(3):410-417. doi:10.1016/j.cub.2015.12.046. PMID 26832439.
  3. ↑ Adaptive Substances, Creative Regeneration
  4. ↑ Ray Peat, "Ask the Herb Doctor: Serotonin, Endotoxins, Stress", KMUD, 37:33
  5. ↑ Ranganathan VK, Siemionow V, Liu JZ, Sahgal V, Yue GH. "From mental power to muscle power--gaining strength by using the mind," Neuropsychologia. 2004;42(7):944-56. doi:10.1016/j.neuropsychologia.2003.11.018. PMID 14998709.
  6. ↑ Yu J, Cheng T, Guo H, et al. "How muscle talks to brain: apelin protein mediates exercise-induced antidepressant effects," Mol Psychiatry. 2026.