Motoneurone afterhyperpolarisation time-course following stroke

• The motoneurone afterhyperpolarisation (AHP) time-course in the paretic tibialis anterior muscle is longer in people with low motor recovery after stroke.
• The prolongation in AHP duration is suggestive of a relationship between motoneurone adaptation and impaired muscle function.
• Two commonly-used techniques to estimate AHP time-course in humans are strongly correlated, suggesting that either technique could be used depending on the purpose of the study.

ObjectiveOur aim was to investigate any changes in the estimated time-course of the afterhyperpolarisation (AHP) in motoneurones innervating the tibialis anterior following stroke, with a secondary objective to compare the results from two different AHP estimation techniques.MethodsMotor units from tibialis anterior on the paretic and non-paretic sides of 15 subjects with chronic stroke were recorded using intramuscular electrodes during voluntary isometric contraction. Participants varied the motor unit firing rate from its lowest rate to approximately 10 Hz. The AHP duration was estimated using the interval death rate (IDR) and transition point methods.ResultsThe AHP decay time-constant was significantly different between sides (paretic: 41.7 ± 8.5 ms, non-paretic: 36.2 ± 6.4 ms). Additionally, the paretic AHP time-constant was significantly longer in participants with low motor recovery (45.9 ± 9.1 ms) than with high motor recovery (39.3 ± 10.0 ms) as measured by CMSA score. The AHP estimates from the two techniques were correlated (r = 0.78).ConclusionsThe AHP time-course prolongation on the paretic side of people with chronic stroke is more pronounced in people with low motor recovery.SignificanceChanges in the motoneurone AHP time course post-stroke were related to muscle function and may play a role in the commonly-observed reduction of motor unit discharge rate during voluntary contractions following stroke.