Magnetic fields produced by single motor units in human skeletal muscles

Objective: To develop new non-invasive ways of analyzing human skeletal muscle function, biomagnetic measurement was applied to the vastus lateralis and vastus medialis of 3 healthy adult males using a 64 channel superconducting quantum interference device system. Methods: Discharges from single motor units were detected by simultaneously recorded surface electromyography. Magnetic signals were averaged 64–158 times at zero-crossing timing in the surface electromyographic signal. Results: Six motor units detected in the 3 subjects produced large magnetic fields with peak-to-peak amplitudes of 1–2 pT. Magnetomyographic isofield maps showed current sources arising from motor endplate regions and propagating in directions opposite to fiber ends. The absolute intensity of current moments in muscle fibers within motor units was estimated based on dipole fitting. The estimated moment was 23.9–114.3 nAm for repolarization dipole. Dividing these moment values by the typical dipole moment of 0.286 nAm in a single muscle fiber, the number of muscle fibers in motor unit was estimated to be 84–400. Conclusions: Magnetic recording may provide a new non-invasive way of analyzing and diagnosing human muscle function.