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.

Age- and Sex-Related Differences in Motor Performance During Sustained Maximal Voluntary Contraction of the First Dorsal Interosseous

“…The relative contribution of the SPI increases during the sustained contraction but we feel that this would not affect males versus females or young versus older subjects differently. Furthermore, pilot data ( n = 5) showed that the twitches evoked by ulnar stimulation and motor point stimulation resulted in similar estimates of index finger abduction force (see Discussion, Prak et al, 2015 ). The ratio between the maximal forces estimated on basis of the ulnar nerve stimulation and motor point stimulation equaled 0.95 (range 0.86–1.01) suggesting a relatively small contribution of the SPI.…”

“…Self-reported neurologically healthy subjects (age: 19–77, mean 48.84 ± 17.16 years, n = 51; 25 females) were included in the study. Some of these subjects were included as control subjects for earlier experiments ( Steens et al, 2012 ; Prak et al, 2015 ). The University Medical Ethical Committee approved the protocol and the informed consent according to the guidelines of the Declaration of Helsinki (2013), which each subject signed prior to the start of the experiment.…”

“…In contrast to large muscles, the small muscles of the hand are well suited for clinical studies focusing on (chronic) disease-related changes in neuromuscular function ( Sheean et al, 1997 ; Thomas, 1997 ; de Ruiter et al, 2001 ; Thickbroom et al, 2006 ; Steens et al, 2012 ; Murray et al, 2014 ; Prak et al, 2015 ; Santarnecchi et al, 2015 ; Arnold et al, 2017 ). Previously we examined force decline during sustained maximal contractions with the index finger abductor muscle (the first dorsal interosseous, FDI).…”

Properties of the surface electromyogram following traumatic spinal cord injury: a scoping review

“…The pattern of central motor drive of the plantar flexors indicated that individuals with SCI generated greater activation of the plantar flexors during eccentric MVCs compared with isometric or concentric MVCs, likely related to increased efficacy of muscle spindles Ia-α motoneuron transmission during lengthening contractions [ 57 , 58 ]. Finally, the study of voluntary activation of weakened hand intrinsic muscles during sustained contractions indicated that impaired activation due to SCI was more important for explaining weakness compared to muscle atrophy, and greater central fatigue was likely offset by less peripheral fatigue as a result of lower muscle activation [ 59 ].…”

Motor Unit Properties of the First Dorsal Interosseous in Chronic Stroke Subjects: Concentric Needle and Single Fiber EMG Analysis

“…The first dorsal interosseous (FDI) is a commonly used model for in vivo study of neuromuscular function (Galganski et al, 1993; Kalmar and Cafarelli, 2004; Zhou et al, 2012; Prak et al, 2015; Li et al, 2016). It is the only muscle that abducts the index finger and its nerve supply is easily accessible for clinical EMG testing and assessment of motor unit properties (Young and Mayer, 1982).…”