title: Transposed firing activation of motor units.
abstract: Muscles are composed of groups of muscle fibers, called motor units, each innervated by a single motoneuron originating in the spinal cord. During constant or linearly varying voluntary force contractions, motor units are activated in a hierarchical order, with the earlier-recruited motor units having greater firing rates than the later-recruited ones. We found that this normal pattern of firing activation can be altered during oscillatory contractions where the force oscillates at frequencies ≥2 Hz. During these high-frequency oscillations, the activation of the lower-threshold motor units effectively decreases and that of the higher-threshold motor units effectively increases. This transposition of firing activation provides means to activate higher-threshold motor units preferentially. Our results demonstrate that the hierarchical regulation of motor unit activation can be manipulated to activate specific motoneuron populations preferentially. This finding can be exploited to develop new forms of physical therapies and exercise programs that enhance muscle performance or that target the preferential atrophy of high-threshold motor units as a result of aging or motor disorders such as stroke and amyotrophic lateral sclerosis.
authors.affiliation: NeuroMuscular Research Center, Boston University, Boston, Massachusetts; Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts; Department of Biomedical Engineering, Boston University, Boston, Massachusetts; Department of Neurology, Boston University, Boston, Massachusetts; and Department of Physical Therapy and Athletic Training, Boston University, Boston, Massachusetts cjd@bu.edu.
authors.forename: Carlo J
authors.initials: CJ
authors.lastname: De Luca
authors.affiliation: NeuroMuscular Research Center, Boston University, Boston, Massachusetts; Department of Biomedical Engineering, Boston University, Boston, Massachusetts;
authors.forename: Joshua C
authors.initials: JC
authors.lastname: Kline
authors.affiliation: NeuroMuscular Research Center, Boston University, Boston, Massachusetts; Department of Physical Therapy and Athletic Training, Boston University, Boston, Massachusetts.
authors.forename: Paola
authors.initials: P
authors.lastname: Contessa
mesh_terms: D000200:Action Potentials; D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000502:physiology*
keywords: firing rate; motor units; oscillatory contractions