title: Estimating the tendency of motor unit recruitment during steady-hold and rapid contractions using surface EMG and Turns-amplitude analysis.
abstract: OBJECTIVE
  The purpose of this study was to investigate changes in the interference pattern in surface electromyography (EMG), and its relationship with the tendency of motor unit (MU) recruitment during steady-hold and rapid muscle contractions.
  
  
  METHODS
  Fifteen healthy adults (eight females and seven males, 22.6 ± 1.5 years old) performed steady-hold and rapid isometric contractions of the bicep brachii, adductor pollicis, and tibialis anterior muscles at various force levels. Surface EMG recordings were analyzed using Turns-Amplitude Analysis (TAA).
  
  
  RESULTS
  During steady-hold contractions, the number of turns per second (T/s) increased exponentially with force during submaximal contractions, and plateaued after force levels of 66, 70 and 57 % MVC for the tibialis anterior, bicep brachii and adductor pollicis muscles, respectively. These force levels were proximate to the maximal recruitment threshold (MaxRT) reported previously. The slopes of the T/s-force relationships before the MaxRT were significantly greater than the slopes after the MaxRT for all three muscles tested. During rapid contraction, the slopes of the T/s-force relationships were significantly lower than the slopes of the steady-hold contraction at 20-40 % MVC in all three muscles, and for 40-60 % MVC in TA muscles.
  
  
  CONCLUSIONS
  Our results suggested that the changes in the number of turns in surface EMG with respect to muscle force can be used to estimate the force levels at which the majority of the MUs to be recruited, and completion of MU recruitment was observed at lower force levels during rapid muscle contraction.
authors.affiliation: Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No.155, Sec.2, Li-Nong Street, Taipei, 112, Taiwan.
authors.forename: Li-Ling
authors.initials: LL
authors.lastname: Pan
authors.affiliation: Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No.155, Sec.2, Li-Nong Street, Taipei, 112, Taiwan.
authors.forename: Chung-Huang
authors.initials: CH
authors.lastname: Yu
authors.affiliation: Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No.155, Sec.2, Li-Nong Street, Taipei, 112, Taiwan.
authors.forename: Mei-Wun
authors.initials: MW
authors.lastname: Tsai
authors.affiliation: Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No.155, Sec.2, Li-Nong Street, Taipei, 112, Taiwan.
authors.forename: Shun-Hwa
authors.initials: SH
authors.lastname: Wei
authors.affiliation: Department of Physical Therapy and Assistive Technology, National Yang-Ming University, No.155, Sec.2, Li-Nong Street, Taipei, 112, Taiwan. lwchou@ym.edu.tw.
authors.forename: Li-Wei
authors.initials: LW
authors.lastname: Chou
mesh_terms: D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D055815:Young Adult
keywords: Maximal recruitment threshold; Motor unit; Rapid contraction; Steady-hold; Surface electromyography; Turns-Amplitude Analysis