Single motor axon conduction velocities of human upper and lower limb motor units. A study with transcranial electrical stimulation

Objectives: To calculate conduction velocities (CV) of single motor axons innervating hand, forearm and leg muscles, weak anodal electrical transcranial stimuli were used and single motor unit potentials were recorded in 17 normal subjects. Methods: The central motor conduction time and neuromuscular transmission delay were subtracted from the latency of unit response to cortical stimulation and single motor axon CV were calculated. Results: In extensor indicis proprius (EIP) units, CV ranged from 30.3 to 76.1 m/s (mean: 51.3±7.1 m/s, 139 units). In first dorsal interosseous (FDI), they ranged from 45.1 to 66.2 m/s (mean: 54.6±2.6 m/s, 88 units). In tibialis anterior (TA), velocities ranged from 27.8 to 55.9 m/s (mean: 41.3±7.5 m/s, 123 units). In FDI units, velocities were compared with those obtained with the F-wave method (range: 50.3–64.5 m/s, mean: 58.1±2.0 m/s). Conclusions: Compared with previously published values, the present method gives better access to slow-conducting units, first recruited by transcranial stimulation and voluntary effort. The spectrum of individual CV was much broader for EIP and TA than for FDI. A linear decline of maximal CV with age was observed, while minimal CV were not affected, suggesting that aging causes a selective loss of the fastest-conducting units.