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. 2009 Nov;23(9):928-38.
doi: 10.1177/1545968309336147. Epub 2009 May 28.

Repeated maximal volitional effort contractions in human spinal cord injury: initial torque increases and reduced fatigue

Affiliations

Repeated maximal volitional effort contractions in human spinal cord injury: initial torque increases and reduced fatigue

T George Hornby et al. Neurorehabil Neural Repair. 2009 Nov.

Abstract

Background: Substantial data indicate greater muscle fatigue in individuals with spinal cord injury (SCI) compared with healthy control subjects when tested by using electrical stimulation protocols. Few studies have investigated the extent of volitional fatigue in motor incomplete SCI.

Methods: Repeated, maximal volitional effort (MVE) isometric contractions of the knee extensors (KE) were performed in 14 subjects with a motor incomplete SCI and in 10 intact subjects. Subjects performed 20 repeated, intermittent MVEs (5 seconds contraction/5 seconds rest) with KE torques and thigh electromyographic (EMG) activity recorded.

Results: Peak KE torques declined to 64% of baseline MVEs with repeated efforts in control subjects. Conversely, subjects with SCI increased peak torques during the first 5 contractions by 15%, with little evidence of fatigue after 20 repeated efforts. Increases in peak KE torques and the rate of torque increase during the first 5 contractions were attributed primarily to increases in quadriceps EMG activity, but not to decreased knee flexor co-activation. The observed initial increases in peak torque were dependent on the subject's volitional activation and were consistent on the same or different days, indicating little contribution of learning or accommodation to the testing conditions. Sustained MVEs did not elicit substantial increases in peak KE torques as compared to repeated intermittent efforts.

Conclusions: These data revealed a marked divergence from expected results of increased fatigability in subjects with SCI, and may be a result of complex interactions between mechanisms underlying spastic motor activity and changes in intrinsic motoneuron properties.

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Figures

Figure 1
Figure 1. Alterations in Knee Extensors Torque and Electromyographic Activity
Note: Typical examples of alterations in knee extensors torque and electromyographic activity from the vastus lateralis, vastus medialis, rectus femoris, and medial hamstring during baseline and repeated maximal volitional efforts (MVEs) in a representative control (A) and spinal cord injury (SCI, B) subject, followed by a single MVE 10 minutes after repeated efforts. Central activation ratios were determined during baseline efforts, every 5th repeated MVE, and at the 10 minutes following repeated efforts.
Figure 2
Figure 2. KE Torques and Quadriceps EMG Activity
Note: Averaged, normalized peak knee extensors (KE) torques (A) and pooled quadriceps electromyographic (EMG) activity (B) during the 1st through 5th, 10th, 15th, and 20th repeated maximal volitional efforts (MVEs) in spinal cord injury (SCI; black) and control (gray) subjects. Asterisks (*) indicate significant increases in torque or EMG during repeated versus baseline MVEs in SCI, or decreases in torque in control subjects. (C) The significant correlation between peak KE torques and pooled EMG for the first 5 repeated MVEs in SCI subjects is also shown (P < .001).
Figure 3
Figure 3. Representative Data
Note: (A) One subject with spinal cord injury (SCI) portraying increasing rate of torque increase (indicated by shorter T20–80 duration; gray bars) during the first 3 repeated maximal volitional efforts (MVEs). (B) The mean alterations in T20–80 and EMG20–80 in subjects with SCI. Differences in T20–80 and EMG20–80 from the 1st to 2nd repeated MVEs were significantly correlated (r = 725; P < .001). EMG, electromyographic data.
Figure 4
Figure 4. Correlation Between CAR Values
Note: A significant negative correlation (P < .001) between central activation ratio (CAR) values determined during baseline maximal volitional efforts and largest relative (percentage) increase in torque during the 1st through 5th repeated effort is demonstrated (A), with a significant curvilinear relationship demonstrated when absolute peak increases in knee extensors (KE) torque are plotted versus CAR (B).
Figure 5
Figure 5. Changes in KE Torques
Note: (A) Repeatability of the observed changes in knee extensors (KE) torques were tested on 7 subjects with spinal cord injury on different days (<5 days between sessions), using an identical testing paradigm of 20 repeated maximal volitional efforts. (B) Assessment of the repeatability of the increase in KE torque during the first 5 repeated efforts was performed on the same day. There were small, nonsignificant differences with repeated testing. MVE, maximum voluntary effort.
Figure 6
Figure 6. Alterations in KE Torque and EMG Activity
Note: (A) Single subject data of the alterations in knee extensors (KE) torque and electromyographic (EMG) activity during a sustained (~30 seconds) maximal volitional effort (MVE). (B) Changes in peak KE torques during repeated (black circles) versus sustained (gray circles) efforts in spinal cord injury. Significant increases in KE torques were observed only with intermittent MVEs, with peak KE torques consistently higher than those achieved during sustained efforts. VL, vastus lateralis; VM, vastus medialis; RF, rectus femoris; MH, medial hamstring.

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