Factors that influence muscle torque after spinal cord injury
Bickel, Christopher Scott
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Electromyostimulation (EMS) is often used in rehabilitation settings to evoke contractions in paralyzed muscle. Unfortunately, EMS inherently causes greater fatigue than voluntary activation. Muscles that are unloaded are susceptible to contraction-induced muscle injury. Therefore, the purpose of this study is to investigate factors that can influence torque production, such as variable-frequency train stimulation and contraction-induced muscle injury, in patients with spinal cord injury (SCI). In the first study, able-bodied (AB) and SCI subjects had their Muscle Quadriceps Femoris (m. QF) stimulated with constant-frequency trains (CFT) (six 200-ms square wave pulses separated by 70-ms) or variable-frequency trains (VFT) (identical to CFT except initial interpulse interval separated by 5 ms). After a fatigue protocol that consisted of 180 contractions (50% duty cycle), the isometric peak torque was reduced by 44%, 56%, and 67% in AB, acute SCI, and chronic SCI groups, respectively. The VFTs enhanced the torque-time integral by 18% compared to CFT in AB subjects and VFTs had a much smaller effect in SCI patients. The second study utilized magnetic resonance (MR) images and EMS to evaluate the susceptibility of paralyzed muscle to contraction-induced injury. MR images were taken of the m. QF prior to, immediately post, and three days post EMS, which consisted of 80 isometric contractions. The relative muscle cross-sectional area (CSA) activated and injured was determined by the number of pixels with an elevated T2 signal. EMS resulted in a decline of peak torque by 66% and 37% for SCI and AB subjects, respectively. EMS activated 66% of AB muscle QF of which 2% of the activated muscle was injured. In contrast, the SCI group had 25% of their activated muscle injured due to EMS. These results indicate that VFT stimulation does not appear to augment torque in SCI patients to the same extent as AB subjects. They also suggest that electrically-elicited isometric contractions are sufficient to evoke muscle injury in SCI patients.