title: Biomechanical benefits of the Onion-Skin motor unit control scheme.
abstract: Muscle force is modulated by varying the number of active motor units and their firing rates. For the past five decades, the notion that the magnitude of the firing rates is directly related to motor unit size and recruitment threshold has been widely accepted. This construct, here named the After-hyperpolarization scheme evolved from observations in electrically stimulated cat motoneurons and from reported observations in voluntary contractions in humans. It supports the assumption that the firing rates of motor units match their mechanical properties to "optimize" force production, so that the firing rate range corresponds to that required for force-twitch fusion (tetanization) and effective graduation of muscle force. In contrast, we have shown that, at any time and force during isometric voluntary constant-force contractions in humans, the relationship between firing rate and recruitment threshold is inversely related. We refer to this construct as the Onion-Skin scheme because earlier-recruited motor units always have greater firing rates than latter-recruited ones. By applying a novel mathematical model that calculates the force produced by a muscle for the two schemes we found that the Onion-Skin scheme is more energy efficient, provides smoother muscle force at low to moderate force levels, and appears to be more conducive to evolutionary survival than the After-hyperpolarization scheme.
authors.affiliation: NeuroMuscular Research Center, Boston University, Boston, MA 02215, USA; Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, USA; Department of Biomedical Engineering, Boston University, Boston, MA 02215, USA; Department of Neurology, Boston University, Boston, MA 02215, USA; Department of Physical Therapy, Boston University, Boston, MA 02215, USA; Delsys Inc., Natick, MA 01760, USA. Electronic address: cjd@bu.edu.
authors.forename: Carlo J
authors.initials: CJ
authors.lastname: De Luca
authors.affiliation: NeuroMuscular Research Center, Boston University, Boston, MA 02215, USA; Department of Physical Therapy, Boston University, Boston, MA 02215, USA; Delsys Inc., Natick, MA 01760, USA.
authors.forename: Paola
authors.initials: P
authors.lastname: Contessa
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002415:Cats; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D055595:Mechanical Phenomena; D008954:Models, Biological; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology
keywords: AHP; Firing rate; Force; Motor unit; Onion skin