RESEARCH ARTICLE Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans Christopher W. MacDonell Æ Tanya D. Ivanova Æ S. Jayne Garland Received: 19 November 2007 / Accepted: 19 April 2008 / Published online: 8 May 2008 Ó Springer-Verlag 2008 Abstract The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneu- rone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units. Keywords Afterhyperpolarization  Motor unit  Human  Discharge rate  Motoneurone Introduction There is a well-established body of knowledge linking the intrinsic properties of the motoneurone with the motor unit type in anaesthetized mammalian preparations (Bakels and Kernell 1993; Burke 1967; Burke et al. 1973, 1982; Cope et al. 1986; Dum and Kennedy 1980; Fleshman et al. 1981; Gardiner 1993; Gardiner and Kernell 1990; Zengel et al. 1985). That is, a motoneurone with a long afterhyperpo- larization (AHP) duration has a low minimal firing rate (Kernell 1965) and a long twitch contraction duration (Bakels and Kernell 1993; Eccles et al. 1958; Gardiner 1993; Gardiner and Kernell 1990; Zengel et al. 1985). Thus, there is a matching of speeds between the twitch characteristics and the motoneurone discharge properties such that motor units with slower twitch contractions will achieve summation of forces at lower (minimal) firing rates (Kernell 1965). In addition, the AHP duration is related to other properties of the motoneurone, for example, it is positively related to input resistance (Zengel et al. 1985) C. W. MacDonell Graduate Program in Neuroscience, University of Western Ontario, London, ON, Canada T. D. Ivanova  S. J. Garland (&) School of Physical Therapy, Elborn College, Room 1588, University of Western Ontario, London, ON N6G 1H1, Canada e-mail: jgarland@uwo.ca S. J. Garland Department of Physiology and Pharmacology, University of Western Ontario, London, ON, Canada 123 Exp Brain Res (2008) 189:23–33 DOI 10.1007/s00221-008-1400-2