Six-Day Fast: What Happened to Strength and Muscle Control

Fourteen healthy men went six days without food in a study published on 20 June 2026 in Scientific Reports. Their maximal strength, jump performance and fine motor skills held steady, but they felt more fatigued, and measurements of how the brain and spinal cord drive muscle shifted in ways that only reversed after they started eating again. The study had no control group, so it describes what happened to these men and cannot show that fasting alone caused it.

Read this first

This post reports research. It is not a suggestion to try a six-day fast. A fast of that length carries real medical risks, the men in this study were healthy volunteers taking part in a research protocol, and nothing in the paper's abstract shows that fasting this long is safe or beneficial for anyone else. A multi-day fast is something to discuss with a doctor beforehand and to do only with medical oversight.

This is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting.

What the study did

Researchers at the Lithuanian Sports University in Kaunas wanted to know how physical functioning holds up during a long fast.

Who	14 healthy, physically inactive, non-obese men
What they did	A 6-day fast, followed by 7 days back on their habitual diet
Comparison group	None
What was measured	Mood, fine motor skills, drop jump performance, maximal voluntary contraction of the calf muscles, and several nerve and reflex measures
Registration	Retrospectively registered on ClinicalTrials.gov (NCT05545943)

The muscle tests focused on the ankle plantar flexors, the calf muscles that push your foot down. The measures fall into two kinds.

What the body could do. Fine motor skills (precise hand movements), drop jump performance (stepping off a box and jumping straight back up), and maximal voluntary contraction, which is the hardest a person can push on purpose.

How the nervous system was driving the muscle. These are less familiar:

• H-reflex: a reflex response produced by electrically stimulating a nerve. It reflects how readily the spinal cord passes a signal on to the muscle.
• M-wave: the muscle's direct response to the same stimulation, used as a reference.
• V-wave: a response recorded during a maximal effort, which reflects how much drive is coming down from the brain.
• Central activation ratio: an estimate of how completely the nervous system switches a muscle on during a maximal effort.

What it found

Output was preserved. Fasting had no effect on fine motor skills, drop jump performance or maximal voluntary contraction. On the tests of what these men could actually do, six days without food did not produce a measurable drop.

Fatigue went up. The men reported more perceived fatigue during the fast.

Reflex measures fell from day four. Two ratios, one reflecting spinal reflex excitability (Hmax/Mmax) and one reflecting drive from the brain (Vsup/Mmax), were reduced after both 4 and 6 days of fasting (P < 0.05).

More changes appeared by day six. A decrease in central activation ratio and an increase in H-reflex latency (the reflex took longer to arrive) were observed only after 6 days (P < 0.05).

Refeeding reversed the nervous-system changes. After the return to a habitual diet, central activation ratio and the reflexes went back to pre-fasting values. Mood improved as well: perceived fatigue and depressive symptoms decreased and vigor increased, and this came together with better fine motor performance (P < 0.05).

How to read "strength was preserved"

It would be easy to take the headline as "a six-day fast does not affect your muscles". The abstract supports something narrower.

The men could still produce the same peak force in a brief test. At the same time, they felt worse, and the signals that drive muscle had changed, with more changes at six days than at four. One reading is that the body was compensating, reaching the same output by a different route. Whether that would hold for a longer effort or a longer fast was not tested.

A short maximal push with the calf muscles is also a narrow slice of physical function. Feeling more fatigued is itself a result, and for most people it is the one that would matter.

What the study does not show

• No control group. Every participant fasted. Without a group that ate normally and took the same tests, some changes could come from repeating the tests or from expectations.
• Fourteen people. A study this small can miss real effects. "No effect" on strength means no effect was detected in 14 men, not that none exists.
• Men only, and a specific kind. The participants were healthy, non-obese, physically inactive men. The results say nothing about women, older adults, athletes, or anyone with a medical condition.
• Safety was not the question. The abstract reports no data on blood tests, electrolytes, heart rhythm, body weight or muscle mass, and no adverse events either way. It cannot be read as evidence that a six-day fast is safe.

What this means if you fast

For most people who fast, this study changes very little. It concerns a fast far longer than daily intermittent fasting, and its results do not transfer to shorter fasts in either direction.

What it adds is a caution about how you feel versus how you test. These men kept their peak strength while feeling more fatigued and while their nervous system was measurably working differently. On any longer fast, feeling capable in a short burst is not proof that everything is as usual, and feeling unwell is a reason to stop and eat, not to push through.

If you want background on what the body does as a fast lengthens, see our overview of fasting stages. For the practical side of longer fasts, including when not to continue, see how to structure longer fasts and the hydration and electrolyte checklist.

FAQ

Do you lose strength during a six-day fast? In this study of 14 healthy men, maximal voluntary contraction of the calf muscles did not change over six days. The study was small, had no control group, and did not measure muscle mass or endurance.

Does fasting affect reflexes? The study found reduced reflex amplitude ratios after 4 and 6 days, and a slower H-reflex after 6 days. These returned to pre-fasting values after 7 days of normal eating.

Does this mean a six-day fast is safe? No. The study measured performance and nerve function in 14 healthy men under a research protocol. It was not designed to assess safety, and the abstract reports no safety data.

About QoFast

QoFast is a fasting tracker app for iPhone and Android, for intermittent fasting and multi-day extended fasts. Its extended fasts come with an optional preparation phase and a refeeding phase, and you can log water and mood along the way. QoFast gives general information, not medical advice, and does not replace medical supervision of a long fast. It is free to download with an optional Pro subscription: App Store and Google Play.

Sources

• Solianik R, Eimantas N, Daugė G, Brazaitis M. "The effect of 6-day fasting on physical performance and neuromuscular control in healthy men." Scientific Reports, 20 June 2026. DOI: 10.1038/s41598-026-57312-9, PubMed 42323358. Trial registration: ClinicalTrials.gov NCT05545943.