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ABSTRACT: Short-latency spinally mediated interlimb reflex pathways were recently reported between the left and right soleus muscles in the human lower-limb during sitting. The aim of the current study was to establish if these pathways were observed during a functional motor task such as human gait and modulated by the gait cycle phase and/or electrical stimulation intensity. The second aim was to elucidate on the afferents involved. Two interventions were investigated. First was ipsilateral tibial nerve (iTN) stimulation at motor threshold (MT), 35% of the maximal peak-to-peak M-wave(M-Max) and 85% M-Max (85M-Max) with stimuli applied at 60%, 70%, 80%, 90%, and 100%of the gait cycle of the ipsilateral leg. Second was ipsilateral sural nerve (SuN) and medial plantar nerve (MpN) stimulation at 1, 2, and 3X perceptual threshold at 90% of the gait cycle [corrected]. The root mean squared (RMS) of the contralateral soleus (cSOL) responses were analyzed in a time window, 40-55 ms (or 45-60 ms for subjects >50 y/o) following iTN stimulation. The most consistent responses occurred at 90 and 100% of the gait cycle at higher stimulation intensities of the iTN. Significantly inhibitory responses (P = 0.006) were reported at 60 versus 80% (P = 0.03), 90% (P = 0.006), and 100% (P = 0.002) and 70 versus 90% (P = 0.02) and 100% (P = 0.009) of the gait cycle at 85M-Max. The responses became more inhibitory with increasing stimulation intensities at 80% (P = 0.01), 90% (P = 0.001), and 100% (P = 0.004) of the gait cycle. Stimulation of the MpN and SuN at all stimulation intensities demonstrated no short-latency responses. Therefore, it is unlikely that afferents within these nerves contribute to the response. This is the first study to show short-latency spinally mediated responses in the cSOL following iTN stimulation, during walking. It provides evidence for a new spinal pathway contributing to motor control and demonstrates that the response likely has functional relevance.
Journal of Neurophysiology 02/2011; 105(2):503-11. · 3.32 Impact Factor
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ABSTRACT: Rehabilitation with augmented electrical stimulation can enhance functional recovery after stroke, and cortical plasticity may play a role in this process. The purpose of this study was to compare the effects of three training paradigms on cortical excitability in healthy subjects. Cortical excitability was evaluated by analysing the input-output relationship between transcranial magnetic stimulation intensity and motor evoked potentials (MEPs) from the flexor muscles of the fingers. The study was performed with 25 healthy volunteers who underwent 20-min simulated therapy sessions of: (1) functional electrical stimulation (FES) of the finger flexors and extensors, (2) voluntary movement (VOL) with sensory stimulation, and (3) therapeutic FES (TFES) where the electrical stimulation augmented voluntary activation. TFES training produced a significant increase in MEP magnitude throughout the stimulation range, suggesting an increase in cortical excitability. In contrast, neither the FES nor voluntary movement alone had such an effect. These results suggest that the combination of voluntary effort and FES has greater potential to induce plasticity in the motor cortex and that TFES might be a more effective approach in rehabilitation after stroke than FES or repetitive voluntary training alone.
Experimental Brain Research 08/2008; 191(1):57-66. · 2.39 Impact Factor
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ABSTRACT: The contractile properties of a muscle depend on the activation history of its motor units. At the same time as fatigue seems to impair muscle excitation-contraction coupling, post-tetanic potentiation can augment force production. The effects of post-tetanic potentiation on the mechanical muscle properties of the intact human ankle extensor muscles were investigated by a 4° dorsiflexion of the ankle joint during a sustained contraction. The contraction was elicited by 10 Hz electrical stimulation of the tibial nerve. The changes in the contraction torque and in the intrinsic muscle stiffness of the andle extensors before and after prolonged electrically elicited muscle activation were measured.From the onset of continuous synchronized 10 Hz stimulation to the attainment of maximal torque, the ankle joint torque increased by 47%. At matched background contraction, the prolonged electrically elicited contraction increased the intrinsic muscle stiffness by 49%. The first stretch after prolonged stimulation gave rise to a 17% yield in the background contraction and a 73% yield in the torque increment.The findings imply that with fatigue an increase in the intrinsic stiffness of the pre-stretched muscle might operate as a “safety factor” to compensate for a reduced reflex-induced stiffness, keeping the total muscle resistance at a high level in the active muscle.
Electroencephalography and Clinical Neurophysiology 01/1993;
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ABSTRACT: The influence of muscle (vastus lateralis) length on the muscle fibre conduction velocity (MFCV) and on muscle fatigue was studied in 8 healthy volunteers. In experiment 1, the electromyographic (EMG) responses were evoked by electrical stimulation of the motor point and recorded by a surface electrode array aligned along the muscle fibre direction. The MFCV (determined by cross-correlation) was measured at knee flexions of 5° (full extension), 45°, 90° and 120° with 3 different extension torques. The MFCV declined with increasing muscle length and increased with increasing background torque at knee flexions from 5° to 90°. From 90° to 120° knee flexion of MFCV tended to increase. In experiment 2, the EMG activity at a static fatiguing contraction (80% MVC) was measured at 45° and 90° knee flexion. The EMG was measured until the subject gave up contracting the muscle (endurance). The largest increase in the RMS amplitude and the fastest decreases in the mean power frequency (MPF) and MFCV were found at 90° flexion. The MVC at 45° knee flexion was 35% lower than at 90° and the time until endurance was approximately twice as long for the 45° contraction. The results indicate that muscle length is an important parameter for the propagation velocity of action potentials and for the development of static muscle fatigue.
Electroencephalography and Clinical Neurophysiology 07/1992;
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ABSTRACT: We wished to quantify the dynamic muscle fatigue by electromyographic (EMG) and kinematic parameters during uphill walking (5 km/h, 25%). The muscle coordination between biceps femoris, semitendinosus, vastus lateralis, gastrocnemius medialis, soleus, and tibialis anterior was evaluated by measuring (a) the period from onset of the EMG burst to heel contact, and (b) duration of the EMG burst. The muscle activity of the individual bursts was evaluated by the root mean-square (RMS), the mean power frequency (MPF), and the averaged EMG profile during one stride cycle. The most pronounced differences at the start and end of the endurance test were observed in the semitendinosus and the biceps femoris. The gastrocnemius was recruited substantially closer to heel contact, but no changes were noted in the soleus. The RMS and duration of the EMG burst were changed significantly for the vastus lateralis. The overall pattern of the kinematic profiles (position, and velocity) for the upper and lower leg remained constant, although the muscle activity and coordination changed. During dynamic muscle fatigue, a complex interaction between muscle coordination and muscle performance was noted, but how the central nervous system control this interaction is not known.
Journal of Electromyography and Kinesiology 01/1991; 1(1):1-8. · 1.97 Impact Factor
