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1. Changes in sensory-evoked synaptic activation of motoneurons after spinal cord injury in man 
NORTON, Jonathan A, BENNETT, David J, KNASH, Michael E, MURRAY, Katie C, and GORASSINI, Monica A
- Brain. 131:1478-1491
Neurology, Neurologie, Sciences biologiques et medicales, Biological and medical sciences, Sciences medicales, Medical sciences, Traumatismes. Maladies dues aux agents physiques, Traumas. Diseases due to physical agents, Traumatismes du système nerveux et du crâne. Maladies dues aux agents physiques, Injuries of the nervous system and the skull. Diseases due to physical agents, Neurologie, Neurology, Pathologie de la moelle épinière, Spinal cord disease, Médula espinal patología, Pathologie du muscle strié, Striated muscle disease, Músculo estriado patología, Pathologie du système nerveux central, Central nervous system disease, Sistema nervosio central patología, Trouble du tonus, Muscle tonus alteration, Trastorno tono muscular, Trouble neurologique, Neurological disorder, Trastorno neurológico, Homme, Human, Hombre, Hypertonie spastique, Spasticity, Hipertonia espástica, Neurone moteur, Motor neuron, Neurona motora, Pathologie du système nerveux, Nervous system diseases, Sistema nervioso patología, Réflexe, Reflex, Reflejo, Traumatisme de la moelle épinière, Spinal cord trauma, Traumatismo de médula espinal, Unité motrice, Motor unit, Unidad motriz, motor unit, reflexes, sensory processing, spasticity, and spinal cord injury
Following spinal cord injury (SCI), prolonged muscle spasms are readily triggered by brief sensory stimuli. Animal and indirect human studies have shown that a substantial portion of the depolarization of motoneurons during a muscle spasm comes from the activation of persistent inward currents (PlCs).The brief (single pulse) sensory stimuli that trigger the PlCs and muscle spasms in chronically spinalized animals evoke excitatory post-synaptic potentials (EPSPs) that are broadened to more than 500 ms, the duration of depolarization required to activate a PIC in the motoneuron. Thus, in humans, we investigated if post-synaptic potentials (PSPs) evoked from brief ( <20 ms) sensory stimulation are changed after SCI and if they are broadened to ≥500 ms to more readily activate motoneuron PlCs and muscle spasms. To estimate both the shape and duration of PSPs in human subjects we used peristimulus frequencygrams (PSFs), which are plots of the instantaneous firing frequency of tonically active single motor units that are time-locked to the occurrence of the sensory stimulus. PSFs in response to cutaneomuscular stimulation of the medial arch or toe of the foot, a sensory stimulus that readily triggers muscle spasms, were compared between non-injured control subjects and in spastic subjects with chronic (>1 year), incomplete SCI. In non-injured controls, a single shock or brief ( <20 ms) train of cutaneomuscular stimulation produced PSFs consisting of a 300 ms increase in firing rate above baseline with an interposed period of reduced firing. Parallel intracellular experiments in motoneurons of adult rats revealed that a 300 ms EPSP with a fast intervening inhibitory PSP (IPSP) reproduced the PSF recorded in non-injured subjects. In contrast, the same brief sensory stimulation in subjects with chronic SCI produced PSFs of comparatively long duration (1200 ms) with no evidence for IPSP activation, as reflected by a lack of reduced firing rates after the onset of the PSF. Thus, unlike non-injured controls, the motoneurons of subjects with chronic SCI are activated by very long periods of pure depolarization from brief sensory activation. It is likely that these second-long EPSPs securely recruit slowly activating PlCs in motoneurons that are known to mediate, in large part, the many seconds-long activation of motoneurons during involuntary muscle spasms.
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2. Volitional muscle strength in the legs predicts changes in walking speed following locomotor training in people with chronic spinal cord injury. 
Yang JF, Norton J, Nevett-Duchcherer J, Roy FD, Gross DP, and Gorassini MA
Physical Therapy[Phys Ther] 2011 Jun; Vol. 91 (6), pp. 931-43. Date of Electronic Publication: 2011 Apr 21.
Adult, Aged, Ankle physiopathology, Electromyography, Female, Hip physiopathology, Humans, Knee physiopathology, Leg physiopathology, Male, Middle Aged, ROC Curve, Young Adult, Exercise Therapy, Muscle Strength, Spinal Cord Injuries physiopathology, Spinal Cord Injuries rehabilitation, and Walking physiology
Background: It is unclear which individuals with incomplete spinal cord injury best respond to body-weight-supported treadmill training.
Objective: The purpose of this study was to determine the factors that predict whether a person with motor incomplete spinal cord injury will respond to body-weight-supported treadmill training.
Design: This was a prognostic study with a one-group pretest-posttest design.
Methods: Demographic, clinical, and electrophysiological measurements taken prior to training were examined to determine which measures best predicted improvements in walking speed in 19 individuals with chronic (>7 months postinjury), motor-incomplete spinal cord injuries (ASIA Impairment Scale categories C and D, levels C1-L1).
Results: Two initial measures correlated significantly with improvements in walking speed: (1) the ability to volitionally contract a muscle, as measured by the lower-extremity manual muscle test (LE MMT) (r=.72), and (2) the peak locomotor electromyographic (EMG) amplitude in the legs (r=.56). None of the demographics (time since injury, age, body mass index) were significantly related to improvements in walking speed, nor was the clinical measure of balance (Berg Balance Scale). Further analysis of LE MMT scores showed 4 key muscle groups were significantly related to improvements in walking speed: knee extensors, knee flexors, ankle plantar flexors, and hip abductors (r=.82). Prediction using the summed MMT scores from those muscles and peak EMG amplitude in a multivariable regression indicated that peak locomotor EMG amplitude did not add significantly to the prediction provided by the LE MMT alone. Change in total LE MMT scores from the beginning to the end of training was not correlated with a change in walking speed over the same period.
Limitations: The sample size was limited, so the results should be considered exploratory.
Conclusions: The results suggest that preserved muscle strength in the legs after incomplete spinal cord injury, as measured by MMT, allows for improvements in walking speed induced by locomotor training.
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