Muscles don't age only because they lose mass: strength declines faster than volume, and neuromuscular junctions are to blame. Neighboring papers in the archive deal with age-related brain maps and MRI imaging of nerve fibers, that is, the nervous system, but not peripheral synapses. This paper fills the gap: it proposes a blood marker of neuromuscular junction degradation that can be measured without a biopsy. That matters because direct methods for assessing synapses in humans are invasive and don't scale for large studies.
This cross-sectional study included 136 men aged 20-92, divided by physical activity level (87 active, 49 inactive). CAF levels were measured in blood serum by ELISA, and the proportion of NCAM-positive fibers was assessed immunohistochemically in biopsies of the vastus lateralis muscle. Functional tests included the 6-minute walk, chair stand, stair climb, 4-meter fast walk, and the Timed Up and Go test. Knee extensor strength and power were assessed on a dynamometer chair and a Nottingham rig, while neurophysiological parameters (central activation, H-reflex, conduction velocity, electromechanical delay) were measured using electrical stimulation and electromyography.
CAF levels rose progressively with age (p<0.05), and were lower in active men, especially after age 70. CAF correlated positively with the proportion of NCAM-positive fibers (r>0, p<0.05), confirming its link to denervation. Higher CAF was associated with worse strength (MVIC), lower-limb power, 6-minute walk distance, walking speed, and functional test results. Among neurophysiological variables, a significant association was found only with electromechanical delay (positive) and, in inactive men, with the H/M ratio (negative). Central activation, conduction velocity, and coactivation did not correlate with CAF.
This work stands out from its neighbors by linking a blood marker to the tissue picture of denervation and functional outcomes in a single cohort. While neighboring papers describe the structure of nervous tissue, this one shows the dynamics of a peripheral synapse through an accessible analyte. This supports CAF as a biomarker of neuromuscular aging, but with a caveat: it reflects only part of the process, not all neurophysiological mechanisms.
The field could move toward longitudinal studies that show whether elevated CAF predicts future functional decline. It would also be promising to study how specific types of training (strength, power, interval) affect CAF dynamics and reinnervation. CAF might be combined with other markers (for example, neurofilaments) to build a fuller picture of neuromuscular health.
The results could influence geriatrics and sports medicine by offering a simple screening indicator of the risk of sarcopenia and falls. They are also important for clinical trials that need a biomarker sensitive to neuromuscular changes.
Conduct longitudinal follow-up of the cohort to assess the prognostic value of CAF. Test whether CAF declines after structured training programs in older adults.
In neuromuscular physiology, it remains unresolved how much synapse degradation, independent of fiber atrophy, determines age-related weakness. This work links a marker of synaptic remodeling to function, but it does not reveal causality and does not include women, whose aging trajectory may differ.
CAF levels in physically active 70-year-old men were comparable to those in inactive 50-year-old men, vividly demonstrating the "rejuvenating" effect of movement on neuromuscular synapses.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.