title: Reduced voluntary drive during sustained but not during brief maximal voluntary contractions in the first dorsal interosseous weakened by spinal cord injury.
abstract: In able-bodied (AB) individuals, voluntary muscle activation progressively declines during sustained contractions. However, few data are available on voluntary muscle activation during sustained contractions in muscles weakened by spinal cord injury (SCI), where greater force declines may limit task performance. SCI-related impairment of muscle activation complicates interpretation of the interpolated twitch technique commonly used to assess muscle activation. We attempted to estimate and correct for the SCI-related-superimposed twitch. Seventeen participants, both AB and with SCI (American Spinal Injury Association Impairment Scale C/D) produced brief and sustained (2-min) maximal voluntary contractions (MVCs) with the first dorsal interosseous. Force and electromyography were recorded together with superimposed (doublet) twitches. MVCs of participants with SCI were weaker than those of AB participants (20.3 N, SD 7.1 vs. 37.9 N, SD 9.5; P < 0.001); MVC-superimposed twitches were larger in participants with SCI (SCI median 10.1%, range 2.0-63.2%; AB median 4.7%, range 0.0-18.4% rest twitch; P = 0.007). No difference was found after correction for the SCI-related-superimposed twitch (median 6.7%, 0.0-17.5% rest twitch, P = 0.402). Thus during brief contractions, the maximal corticofugal output that participants with SCI could exert was similar to that of AB participants. During the sustained contraction, force decline (SCI, 58.0%, SD 15.1; AB, 57.2% SD 13.3) was similar (P = 0.887) because participants with SCI developed less peripheral (P = 0.048) but more central fatigue than AB participants. The largest change occurred at the start of the sustained contraction when the (corrected) superimposed twitches increased more in participants with SCI (SCI, 16.3% rest twitch, SD 20.8; AB, 2.7%, SD 4.7; P = 0.01). The greater reduction in muscle activation after SCI may relate to a reduced capacity to overcome fast fatigue-related excitability changes at the spinal level.
authors.affiliation: Department of Neuroscience, University Medical Center Groningen, University of Groningen, The Netherlands;
authors.forename: Roeland F
authors.initials: RF
authors.lastname: Prak
authors.affiliation: Department of Neuroscience, University Medical Center Groningen, University of Groningen, The Netherlands;
authors.forename: Marwah
authors.initials: M
authors.lastname: Doestzada
authors.affiliation: The Miami Project to Cure Paralysis, Departments of Neurological Surgery, Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, Florida; and.
authors.forename: Christine K
authors.initials: CK
authors.lastname: Thomas
authors.affiliation: Department of Rehabilitation Medicine, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Marga
authors.initials: M
authors.lastname: Tepper
authors.affiliation: Department of Neuroscience, University Medical Center Groningen, University of Groningen, The Netherlands; c.a.t.zijdewind@umcg.nl.
authors.forename: Inge
authors.identifier: http://orcid.org/0000-0003-2146-1212
authors.initials: I
authors.lastname: Zijdewind
mesh_terms: D000328:Adult; D001284:Atrophy; D004328:Drive; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D011597:Psychomotor Performance; D013119:Spinal Cord Injuries / Q000378:metabolism* / Q000503:physiopathology
keywords: doublet force; fatigue; muscle activation; muscle atrophy; twitch interpolation