Six Days of Fasting Didn’t Kill Athletic Performance, But It Did Rewire the Nervous System

Six days without food sounds like the kind of deprivation that should wreck athletic performance. Strength gone, coordination shaky, reflexes dulled. But a new study published in Scientific Reports found something more surprising: maximal muscle force, drop jump performance, and fine motor skills all held steady through a complete six-day fast in healthy men, even as the nervous system made measurable compensatory adjustments beneath the surface. The catch is that this is early, preliminary research with a small sample, and what the body preserved during fasting came with hidden neural costs that only resolved once normal eating resumed.

Fasting Preserved Maximal Voluntary Contraction and Jump Output

Fourteen non-obese, healthy, physically inactive men completed the six-day fasting protocol and a subsequent seven-day return to habitual diet. Researchers measured neuromuscular properties of the ankle plantar flexors throughout, tracking maximal voluntary contraction (MVC, the peak force a muscle can produce when a person tries as hard as possible), drop jump performance, and fine motor skill tasks. On all three counts, fasting had no statistically significant effect. The men could still generate the same peak muscle torque on day six as they could at baseline, and their jump mechanics were unchanged.

Subjective perceived fatigue, however, rose during the fast. The gap between how tired participants felt and how their muscles actually performed is one of the more intriguing findings: the machinery kept running even as the operator reported feeling progressively worse.

Neural Signaling at Spinal and Supraspinal Levels Changed Measurably

Under the preserved surface performance, the nervous system was doing something different. The H-max/M-max amplitude ratio, a standard index of spinal motoneuron excitability derived from the H-reflex and M-wave, fell significantly after both four and six days of fasting (P < 0.05). So did the V-sup/M-max ratio, which reflects the descending drive from supraspinal motor centers during voluntary contraction, also after four and six days. In plain terms: the spinal cord became less excitable, and the brain was sending weaker voluntary drive signals to the muscles, yet force output did not drop.

The central activation ratio (CAR), a measure of how fully the central nervous system activates a muscle during MVC, was also tracked. Taken together, these metrics suggest the body found compensatory pathways to maintain gross motor output even as its primary neural control routes downregulated. Whether that compensation has limits under more extreme or prolonged conditions is precisely the kind of open question this preliminary study cannot answer.

How the Study Was Designed and What It Can Measure

The researchers used a within-subjects design: the same fourteen men were measured before, during, and after fasting, giving each participant a personal baseline for comparison. Neuromuscular assessment focused on the ankle plantar flexors and used a three-reflex approach, tracking the V-wave (voluntary), H-wave (spinal reflex), and M-wave (direct motor nerve response) to disentangle changes at different levels of the motor pathway. This methodological layering is what makes the study more than a simple before-and-after: it can localize where in the motor hierarchy the adaptations occurred.

The trial was retrospectively registered on ClinicalTrials.gov (NCT05545943). The sample size of fourteen is small by clinical standards, and the participants were physically inactive at baseline, which limits how readily findings translate to trained athletes or to clinical populations who might fast for medical reasons. No explicit limitations section appeared in the abstract, but the modest sample and the single-population focus are worth keeping in mind when weighing the results.

Mood, Fatigue, and Fine Motor Skills After Refeeding

Once participants returned to their habitual diet, the picture shifted noticeably. Perceived fatigue and depressive symptoms decreased, vigor increased, and fine motor performance actually improved beyond pre-fast baseline (P < 0.05). The neural reflex ratios that had declined during fasting returned toward baseline as well. The mood improvements and the fine motor gains appeared together, which raises an unresolved question about directionality: did better mood enable sharper motor control, or did neural recovery drive mood improvements, or did both simply track the same underlying metabolic restoration?

Researchers studying how subjective states shape motor output have noted the difficulty of separating psychological from physiological contributors in small human trials, and that problem applies here. The refeeding data are suggestive, but a study of this size cannot definitively attribute the post-fast motor gains to any single mechanism.

Open Questions About Fasting, Neural Adaptation, and Who This Applies To

The finding that gross physical performance survives a six-day fast in sedentary men is genuinely preliminary but notable. It challenges a straightforward assumption that energy restriction maps directly onto functional decline. The neural data complicate that story further: the body does adapt at the reflex level, it just adapts in ways that do not immediately manifest as lost strength or coordination in this population.

What remains unknown is how those neural adaptations would behave under heavier physical demands, over longer fasting periods, or in trained individuals whose baseline neural drive is already high. Research in related fields has consistently pushed toward larger, more representative samples before drawing practice-relevant conclusions, and this study sits firmly in the hypothesis-generating phase. It identifies a dissociation between neural control signals and functional output during prolonged fasting that warrants follow-up with larger cohorts, active populations, and perhaps sport-specific performance tasks.

For now, the takeaway is modest and specific: in fourteen healthy sedentary men, six days of fasting altered how the nervous system managed muscle activation without translating those changes into measurable losses in strength, jumping, or fine motor skill. The nervous system, it appears, has more flexibility than the hunger pangs suggest.

Evidence rating

INVESTIGATIONAL — 55/100

Caveats:

• Very small sample (n=14 men only), limiting generalizability.
• No control group; this is an uncontrolled before-after design.
• Retrospectively registered trial, which introduces risk of outcome reporting bias.
• Results are specific to healthy, physically inactive, non-obese men and may not apply to other populations.
• Not yet independently replicated.

Uncontrolled before-after trial in 14 healthy men with no comparison group. Demonstrates associations between 6-day fasting and neuromuscular adaptations, but cannot establish causation or rule out confounding. Retrospective registration and single-study design with no independent replication limit confidence in the findings.

References

1. Rima Solianik, Nerijus Eimantas, Goda Daugė, Marius Brazaitis (2026). The effect of 6-day fasting on physical performance and neuromuscular control in healthy men. Scientific Reports. https://doi.org/10.1038/s41598-026-57312-9
2. Sandra Carroll, Gayathri Embuldeniya, Julia Abelson, Michael McGillion, Alexandre Berkesse, Jeff Healey (2017). Questioning patient engagement: research scientists&rsquo; perceptions of the challenges of patient engagement in a cardiovascular research network. Patient Preference and Adherence. https://doi.org/10.2147/ppa.s135457
3. The past, present, and future of consumer research – PMC – NIH. NIH. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7313075/