title: Reduced Voluntary Activation During Brief and Sustained Contractions of a Hand Muscle in Secondary-Progressive Multiple Sclerosis Patients.
abstract: BACKGROUND
  Secondary-progressive multiple sclerosis (SPMS) patients have structural cortical damage resulting in increased compensatory cortical activity during (submaximal) performance. However, functional effects of changed cortical output are difficult to measure. The interpolated-twitch technique allows for measurement of voluntary activation (VA) necessary for force production. This study aimed to determine VA, force, and muscle fatigue during brief and sustained contractions in SPMS patients. Because fatigue effects are not confined to the motor system, we additionally examined fatiguing effects on cognitive performance.
  
  
  METHODS
  Twenty-five SPMS and 25 sex-, age-, and education-matched participants performed brief (5 seconds) and sustained (2 minutes) maximal index finger abductions. To evaluate VA, double-pulse twitches were evoked before, during, and after contractions. Additionally, data were compared with data obtained in relapsing-remitting multiple sclerosis (RRMS) patients. Subjects also performed choice-reaction time tasks before and after the sustained contraction.
  
  
  RESULTS
  During brief contractions, VA (85% vs 94%,P= .004) and force (25 N vs 32 N,P= .011) were lower for SPMS patients than controls. During sustained contractions, VA (P= .001) was also lower, resulting in greater force decline (73% vs 63%,P< .001) and reduced peripheral fatigue (19% vs 50%,P< .001). Comparisons with RRMS resulted in lower VA, greater force decline, and greater estimated central fatigue in SPMS. SPMS patients were slower (P< .001) and made more errors (P< .001) than controls, but neither group reduced their performance after the sustained contraction.
  
  
  CONCLUSIONS
  SPMS patients had lower VA than RRMS patients and controls. The importance of voluntary activation for muscle force and fatigability warrants targeted rehabilitation strategies.
authors.affiliation: Department of Neuroscience, University Medical Center Groningen, University of Groningen, Groningen, Netherlands.
authors.forename: Ria
authors.initials: R
authors.lastname: Wolkorte
authors.affiliation: Department of Neurology, University Medical Center Groningen, Groningen, Netherlands.
authors.forename: Dorothea J
authors.initials: DJ
authors.lastname: Heersema
authors.affiliation: Department of Neuroscience, University Medical Center Groningen, University of Groningen, Groningen, Netherlands c.a.t.zijdewind@umcg.nl.
authors.forename: Inge
authors.initials: I
authors.lastname: Zijdewind
mesh_terms: D000328:Adult; D000368:Aged; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006225:Hand / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D020528:Multiple Sclerosis, Chronic Progressive / Q000473:pathology / Q000503:physiopathology*; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D014459:Ulnar Nerve / Q000503:physiopathology
keywords: MS phenotypes; central fatigue; cognitive performance; muscle fatigue; peripheral fatigue; reaction times