Relationship between the rise-time of single-fibre action potentials and radial distance in human muscle fibres.

OBJECTIVE: This study aims to check the theoretical predictions of the small changes in the rise-time of human single muscle fibre action potentials (SFAPs) experimentally when the recording electrode is in the proximity of the fibre.

METHODS: Using a single-fibre electrode, 93 sets of consecutive SFAPs were recorded under needle movement from the m. tibialis anterior of four normal subjects. In each SFAP set, the excitation source was assumed to remain unchanged and thus only the fibre-electrode distance varied between consecutive discharges. The changes of SFAP rise-time with peak-to-peak amplitude (V(pp)) within each SFAP set were used to study the relationship between rise-time and radial distance.

RESULTS: SFAP rise-time was relatively unchanged when V(pp) was greater than 1.0-1.5 mV. For V(pp) smaller than 1.0-1.5 mV, the sensitivity of rise-time to changes in radial distance increased considerably.

CONCLUSIONS: Although the SFAP rise-time is practically not influenced by changes in radial distance in the proximity of muscle fibres, future work is necessary to confirm the hypothesis of using the rise-time for estimating changes in the intracellular action potential (IAP) depolarisation phase.

SIGNIFICANCE: The studies are useful in diagnostics and following the results of treatment in patients suffering from muscular dystrophy or myopathy, they are known to be accompanied by severe changes in the duration of IAP.