Acute caffeine intake increases fasciculation frequency in healthy adults: A quantitative muscle ultrasound study.

OBJECTIVE: To quantify the effect of acute caffeine intake on fasciculation frequency in healthy adults using muscle ultrasound and to explore differences between muscle groups and age categories. METHODS: In this monocentric intraindividual pre-post study, 54 neurologically healthy adults aged 20-50 years underwent ultrasound examination of six muscles: biceps brachii, first dorsal interosseous, abductor pollicis brevis, vastus medialis, gastrocnemius caput laterale, and abductor hallucis. Fasciculations were recorded for 120 s per muscle at baseline and repeated 45 min after administration of a single oral caffeine dose of 2 mg/kg body weight. Changes in fasciculation frequency were analyzed within individuals and compared between participants aged 20-34 and 35-50 years. RESULTS: Acute caffeine intake was associated with an increase in fasciculation frequency across most examined muscles. The largest increases were observed in the gastrocnemius and abductor hallucis, whereas changes in the vastus medialis were minimal. Upper limb muscles showed moderate increases in fasciculation activity. Age-related differences were identified: younger participants demonstrated relatively greater increases in upper limb muscles, while older participants showed more pronounced increases in lower limb muscles. Although interindividual variability was present, the overall pattern suggested enhanced motor unit excitability following caffeine intake. CONCLUSIONS: Acute caffeine intake appears to increase ultrasound-detectable fasciculation frequency in healthy adults. The effect is muscle-dependent and may vary with age. SIGNIFICANCE: Recent caffeine consumption may influence the detection and interpretation of fasciculations during neuromuscular assessment. Consideration of caffeine intake before muscle ultrasound or electromyographic examinations may improve diagnostic accuracy and reduce misinterpretation of benign fasciculation activity. (218 words).