Biocybernetics and Biomedical Engineering 2012, Volume 32, Number 4, pp. 33–45 * Correspondence to: Jan Celichowski, Department of Neurobiology, University School of Physical Education, 55 Grunwaldzka St., 60-352 Poznań, Poland, e-mail: celichowski@awf.poznan.pl Received 04 April 2012; accepted 24 April 2012 Variability and Plasticity of Motor Unit Properties in Mammalian Skeletal Muscle JAN CELICHOWSKI*, PIOTR KRUTKI Department of Neurobiology, University School of Physical Education, Poznań, Poland Tn the majority of mammalian skeletal muscles, contractile properties of motor units are variable and three main types of these units can be distinguished. The present review summarizes: results of studies of motor unit properties in the medial gastrocnemius muscle and their variability in two species, cats and rats, and studies on differences of motor unit properties in two genders. Moreover, plasticity of motor unit properties in rat medial ga- strocnemius evoked by two kinds of spinal cord injury, total transection and hemisection, is reviewed, and effects of two types of training, treadmill locomotor and whole-body vibration training, are summarized. Finally, changes in the motor unit properties during the aging process are presented. K e y w o r d s: motor unit, plasticity, contractile properties, motor unit action potentials 1. Introduction Motor units, each composed of one motoneuron and a group of muscle fibers inner- vated exclusively by this neuron, are the smallest functional structures of mammalian skeletal muscles. As revealed with a glycogen depletion technique, all muscle fibers within one motor unit are of the same physiological and histochemical type [1–3] and therefore three basic types of muscle fibers: SO, FOG and FG [4] correspond to three basic types of the motor units: S (slow), FR (fast resistant to fatigue) and FF (fast fatigable), respectively [1]. The classification of motor units into three above- mentioned types, first proposed for cat medial gastrocnemius muscle, was confirmed for several other hind limb muscles, first in cats [1, 5, 6] and later in rats [7–9]. Additionally, data for two basic experimental species turned our attention toward interspecies differences in the motor unit contractile properties and the expected