Michel Ladouceur

Major role for sensory feedback in soleus EMG activity in the stance phase of walking in man

1 Sensory feedback plays a major role in the regulation of the spinal neural locomotor circuitry in cats. The present study investigated whether sensory feedback also plays an important role during walking in 20 healthy human subjects, by arresting or unloading the ankle extensors 6 deg for 210 ms in the stance phase of gait. 2 During the stance phase of walking, unloading of the ankle extensors significantly (P < 0·05) reduced the soleus activity by 50 % in early and mid‐stance at an average onset latency of 64 ms. 3 The onset and amplitude of the decrease in soleus activity produced by the unloading were unchanged when the common peroneal nerve, which innervates the ankle dorsiflexors, was reversibly blocked by local injection of lidocaine (n= 3). This demonstrated that the effect could not be caused by a peripherally mediated reciprocal inhibition from afferents in the antagonist nerves. 4 The onset and amplitude of the decrease in soleus activity produced by the unloading were also unchanged when ischaemia was induced in the leg by inflating a cuff placed around the thigh. At the same time, the group Ia‐mediated short latency stretch reflex was completely abolished. This demonstrated that group Ia afferents were probably not responsible for the decrease of soleus activity produced by the unloading. 5 The findings demonstrate that afferent feedback from ankle extensors is of significant importance for the activation of these muscles in the stance phase of human walking. Group II and/or group Ib afferents are suggested to constitute an important part of this sensory feedback.

Group II muscle afferents probably contribute to the medium latency soleus stretch reflex during walking in humans

1 The objective of this study was to determine which afferents contribute to the medium latency response of the soleus stretch reflex resulting from an unexpected perturbation during human walking. 2 Fourteen healthy subjects walked on a treadmill at approximately 3.5 km h−1 with the left ankle attached to a portable stretching device. The soleus stretch reflex was elicited by applying small amplitude (∼8 deg) dorsiflexion perturbations 200 ms after heel contact. 3 Short and medium latency responses were observed with latencies of 55 ± 5 and 78 ± 6 ms, respectively. The short latency response was velocity sensitive (P < 0.001), while the medium latency response was not (P= 0.725). 4 Nerve cooling increased the delay of the medium latency component to a greater extent than that of the short latency component (P < 0.005). 5 Ischaemia strongly decreased the short latency component (P= 0.004), whereas the medium latency component was unchanged (P= 0.437). 6 Two hours after the ingestion of tizanidine, an α2‐adrenergic receptor agonist known to selectively depress the transmission in the group II afferent pathway, the medium latency reflex was strongly depressed (P= 0.007), whereas the short latency component was unchanged (P= 0.653). 7 An ankle block with lidocaine hydrochloride was performed to suppress the cutaneous afferents of the foot and ankle. Neither the short (P= 0.453) nor medium (P= 0.310) latency reflexes were changed. 8 Our results support the hypothesis that, during walking the medium latency component of the stretch reflex resulting from an unexpected perturbation is contributed to by group II muscle afferents.

Increase in tibialis anterior motor cortex excitability following repetitive electrical stimulation of the common peroneal nerve

The purpose of this study was to investigate whether repetitive electrical stimulation of the common peroneal nerve (CPN) is associated with changes in the motor response of the tibialis anterior (TA) muscle elicited by focal magnetic stimulation of the motor cortex. Motor evoked potentials (MEP) with a stimulation intensity of 125% of the threshold of the relaxed right TA were obtained before, during, and after repetitive electrical stimulation of the CPN (trains of five pulses of 1xa0ms, at a frequency of 200xa0Hz, repeated every second with a 30-min duration). The MEP of the TA muscle elicited after repetitive electrical stimulation were increased by 104% (range: 18–263%), and the increase was maintained for up to 110xa0min (range: 15–110xa0min) after the end of nerve stimulation. This increase in the MEP of the TA muscle was associated with a decrease in the threshold from the stimulation-response curve. Furthermore, during that period the early component of the TA stretch reflex as well as the latency of the MEP did not significantly change. To further test the origin of the increased MEP, complementary experiments showed that MEP elicited by transcranial electrical stimulation (TES) were also increased, but to a lesser degree (approximately 50%) than MEP elicited by TMS. It can be concluded that short-term nerve repetitive electrical stimulation of the lower extremities in healthy human participants can lead to a long-term increase in the contralateral MEP. As TES is believed to mainly activate the axon and not the soma of the cortical cells, the increased MEP cannot be explained exclusively by changes in the motor cortex cell excitability, but also by changes in subcortical neural structures involved in the excitation of spinal motoneurons. The results of this study allow the speculation that it would be possible to use repetitive electrical stimulation in the rehabilitation of patients with lower limb muscle weakness and spasticity.

The effect of locomotor training combined with functional electrical stimulation in chronic spinal cord injured subjects: walking and reflex studies

With the new developments in traumatology medicine, the majority of spinal cord injuries sustained are clinically incomplete and the proportion is likely to continue to rise. Thus, it is necessary to continue to develop new treatment and rehabilitation strategies and understand the factors that can enhance recovery of walking following spinal cord injury (SCI). One new development is the use of functional electrical stimulation (FES) device to assist locomotion. The objective of this review is to present findings from some recent studies on the effect of long-term locomotor training with FES in subjects with SCI. Promising results are shown in all outcome measures of walking, such as functional mobility, speed, spatio-temporal parameters, and the physiological cost of walking. Furthermore, the change in the walking behavior could be associated with plasticity in the CNS organization, as seen by the modification of the stretch reflex and changes in the corticospinal projection to muscles of the lower leg. In conclusion, recovery of walking is an increasing possibility for a large number of people with SCI. New modalities of treatment have become available for this population but most still need to be evaluated for their efficacy. This review has focused on FES assisted walking as a therapeutic modality in subjects with chronic SCI, but it is envisaged that the care and recovery of SCI in the early phase of recovery could also be improved.

Clinical and Therapeutic Applications of Neuromuscular Stimulation: A Review of Current Use and Speculation into Future Developments

In this paper we present an overview of current research into clinical and therapeutic applications of electrical neuromuscular stimulation (NMS). As this is now such a huge subject we have focused our attention on the therapeutic rather than orthotic uses of stimulation and limited the field almost exclusively to upper limb applications in hemiplegia. The evidence that NMS influences motor re‐learning and how this may be measured is discussed. We have identified the following as the three most important unresolved issues: 1) an understanding of how NMS modifies the interactions within the nervous system, 2) clinical effectiveness of NMS, and 3) inexpensive, simple to insert and reliable controllable implanted systems. We discuss recent research aimed at resolving these issues and based on this we make some suggestions for future research. To resolve these issues we propose: 1) neurophysiologic research into the mechanism through which NMS interacts with the nervous system; 2) large multicenter randomized controlled trials using rigorous methodology that compare different applications of NMs; 3) continued technical development that is closely linked to clinical applications.

The effects of long-term FES-assisted walking on intrinsic and reflex dynamic stiffness in spastic spinal-cord-injured subjects

The effects of long-term functional electrical stimulation (FES)-assisted walking on ankle dynamic stiffness were examined in spinal cord-injured (SCI) subjects with incomplete motor function loss. A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness at different ankle positions while subjects remained relaxed. Intrinsic stiffness dynamics were well modeled by a linear second-order model relating intrinsic torque to joint position. Reflex stiffness dynamics were accurately described by a linear third-order model relating halfwave rectified velocity to reflex torque. We examined four SCI subjects before and after long-term FES-assisted walking (>16 mo). Another SCI subject, who used FES for only five months was examined 12 mo latter to serve as a non-FES, SCI control. Reflex stiffness decreased in FES subjects by an average of 53% following FES-assisted walking, intrinsic stiffness also dropped by 45%. In contrast, both reflex and intrinsic stiffness increased in the non-FES, SCI control. These findings suggest that FES-assisted walking may have therapeutic effects, helping to reduce abnormal joint stiffness.

Review : Walking After Spinal Cord Injury: Control and Recovery

Spinal cord injury is associated with multiple motor problems leading to alterations of walking behavior reflected by a reduced walking speed and changes in the kinematic and electromyographic patterns. This review presents recent developments and concepts emerging from animal and human studies aimed at enhancing recovery of walking following spinal cord injury. Locomotor training, pharmacological interven tions, and their combination have been identified as important approaches in modifying the recovery process following spinal cord injury in both animals and humans. The nervous system still presents great plasticity even several years after spinal cord injury. NEUROSCIENTIST 4:14-24, 1998

Kinematic Adaptations of Spinal Cord-Injured Subjects during Obstructed Walking

Study Design. A case-control study of walking over obstacles. Objective. To characterize and compare the kinematic, anticipatory locomotor adjustments used by people with incomplete spinal cord injuries (SCIs). Methods. The angular and linear kinematics of the lower limb when going over obstacles of low height (0.005 and 0.030 m) were compared between SCI subjects (n = 6) and able-bodied participants (n = 5). Results. The results of this study show that even though SCI participants could adapt their kinematic patterns to go over obstacles, none of the participants used kinematic strategies similar to those of able-bodied participants. Conclusions. This difference could be explained in part by the absence in the SCI participants of increased hip flexion when going over the obstacles. Other confounding factors are discussed.

Parametric modeling of the reflex contribution to dynamic ankle stiffness in normal and spinal cord injured spastic subjects

Dynamic ankle stiffness was estimated at different levels of tonic voluntary contraction in normal and spastic, spinal cord injured (SCI) subjects. A new parallel-cascade identification method was used to identify the reflex contribution to overall stiffness in terms of a static nonlinearity in series with a linear system. Linear dynamics were estimated nonparametrically as impulse response functions (IRFs). Parametric models were fit to these IRFs using non-linear, least-squares methods. A simple, second-order, low-pass model was adequate for some cases, but did not work well with data from normal subjects at high levels of activation or with from spastic subjects. Much better fits were obtained using a third order model.

Soleus Stretch Reflex Inhibition in the Early Swing Phase of Gait Using Deep Peroneal Nerve Stimulation in Spastic Stroke Participants

Objectives. To investigate the feasibility of inhibiting the stretch reflex of the soleus muscle by a conditioning stimulus applied to the deep peroneal nerve in spastic stroke participants during the early swing phase of gait.