Jacob Buus Andersen

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.

Transcranial magnetic stimulation and stretch reflexes in the tibialis anterior muscle during human walking

1 Stretch of the ankle dorsiflexors was applied at different times of the walking cycle in 17 human subjects. When the stretch was applied in the swing phase, only small and variable reflex responses were observed in the active tibialis anterior (TA) muscle. Two of the reflex responses that could be distinguished had latencies which were comparable with the early (M1) and late (M3)components of the three reflex responses (M1, M2 and M3) observed during tonic dorsiflexion in sitting subjects. In the stance phase a single very large response was consistently observed in the inactive TA muscle. The peak of this response had the same latency as the peak of M3, but in the majority of subjects the onset latency was shorter than that of M3. 2 The TA reflex response in the stance phase was abolished by ischaemia of the lower leg at the same time as the soleus H‐reflex, suggesting that large muscle afferents were involved in the generation of the response. 3 Motor‐evoked potentials (MEPs) elicited in the TA by transcranial magnetic stimulation (TMS) were strongly facilitated corresponding to the peak of the stretch response in the stance phase and the late reflex response in the swing phase. A similar facilitation was not observed corresponding to the earlier responses in the swing phase and the initial part of the response in stance. 4 Prior stretch did not facilitate MEPs evoked by transcranial electrical stimulation in the swing phase of walking. However, in the stance phase MEPs elicited by strong electrical stimulation were facilitated by prior stretch to the same extent as the MEPs evoked by TMS. 5 The large responses to stretch seen in the stance phase are consistent with the idea that stretch reflexes are mainly involved in securing the stability of the supporting leg during walking. It is suggested that a transcortical reflex pathway may be partly involved in the generation of the TA stretch responses during walking.

Evidence for transcortical reflex pathways in the lower limb of man.

The existence of transcortical reflex pathways in the control of distal arm and hand muscles in man is now widely accepted. Much more controversy exists regarding a possible contribution of such reflexes to the control of leg muscles. It is often assumed that transcortical reflex pathways play no, or only a minor, role in the control of leg muscles. Transcortical reflex pathways according to this view are reserved for the control of the distal upper limb and are seen in close relation to the evolution of the primate hand. Here we review data, which provide evidence that transcortical reflexes do exist for lower limb muscles and may play a significant role in the control of at least some of these muscles. This evidence is based on animal research, recent experiments combining transcranial magnetic stimulation with peripheral electrical and mechanical stimulation in healthy subjects and neurological patients. We propose that afferent activity from muscle and skin may play a role in the regulation of bipedal gait through transcortical pathways.

An actuator system for investigating electrophysiological and biomechanical features around the human ankle joint during gait

A system has been developed which is able to impose a fast ankle rotation during gait. The two-link system consists of a mechanical joint which is strapped to the calf and the foot of the subject. The mechanical joint turns around the ankle joint by means of Bowden wires and it is connected to a motor placed next to a treadmill where the subject is walking. By position feedback from the mechanical joint the motor is regulated in such a may that it follows the movement of the ankle joint without influencing the gait pattern. The system is designed to impose a fast ankle rotation with a displacement of up to 20/spl deg/, e.g., a 8/spl deg/ stretch is performed with a rise time of 32 ms during any time of the gait cycle. The system attached to the subjects leg weights in total 0.9 kg. The system is designed to elicit a stretch reflex in the ankle extensor muscles during walking. The prospect of the system is to investigate electrophysiological and biomechanical features during a naturally evoked stretch reflex of the ankle extensors in both healthy and motor impaired subjects. The system is able to produce a perturbation of the ankle extensors at a walking speed of up to 6 km/h during the total gait cycle.

Impaired stretch reflex and joint torque modulation during spastic gait in multiple sclerosis patients

The modulation of the short latency stretch reflex of the soleus muscle during walking was investigated in seven spastic multiple sclerosis (MS) patients and nine healthy control subjects. Ankle joint stretches were applied by a system which can rotate that ankle joint in any phase of the step cycle during treadmill walking. The torque related to the muscle fibres contracting prior to the stretch and the passive tissues around the ankle joint were measured as the “non-reflex torque”. At the same time the short latency stretch reflex-mediated EMG response was measured. The findings show that the stretch reflex modulation was impaired in spastic patients during walking. The stretch reflex modulation was quantified by a modulation index of on average 50% (range −5 to 100%) in the patients and 93% (78–100%) in the control subjects (P < 0.05). The passive stiffness of the ankle joint was at the same time increased in the patients (P < 0.05). It is proposed that the impaired modulation of the stretch reflex along with increased ankle joint stiffness contribute to the impaired walking ability in spastic MS patients.

Mobile ankle and knee perturbator

A mobile ankle and knee perturbator has been developed. It consists of a functional joint with an integrated clutch. Four Bowden wires connect the joint to a powerful motor and a double pneumatic cylinder. When needed during any time of the gait cycle, it is possible to impose an ankle rotation by engaging the clutch and rotating the ankle or knee joint with a predefined displacement. The system is designed to investigate electrophysiological and biomechanical features of the human ankle or knee joint during gait.

Sensory feedback to ankle plantar flexors is not exaggerated during gait in spastic hemiplegic children with cerebral palsy

It is still widely believed that exaggerated stretch reflexes and increased muscle tone in ankle plantar flexors contribute to reduced ankle joint movement during gait in children with cerebral palsy (CP). However, no study has directly measured stretch reflex activity during gait in these children. We investigated sensory feedback mechanisms during walking in 20 CP children and 41 control children. Stretch responses in plantar flexor muscles evoked in stance showed an age-related decline in control but not CP children. In swing the responses were abolished in control children, but significant responses were observed in 14 CP children. This was related to reduced activation of dorsiflexors in swing. Removal of sensory feedback in stance produced a drop in soleus activity of a similar size in control and CP children. Soleus activity was observed in swing to the same extent in control and CP children. Removal of sensory feedback in swing caused a larger drop in soleus activity in control children than in CP children. The lack of age-related decline in stretch reflexes and the inability to suppress reflexes in swing is likely related to lack of maturation of corticospinal control in CP children. Since soleus activity was not seen more frequently than in control children in swing and since sensory feedback did not contribute more to their soleus activity, spasticity is unlikely to contribute to foot drop and toe walking. We propose that altered central drive to the ankle muscles and increased passive muscle stiffness are the main causes of foot drop and toe walking.

Interlimb communication to the knee flexors during walking in humans

•  Following unexpected ipsilateral knee extension joint rotations applied during the late stance phase of the gait cycle in humans, a crossed reflex response was observed in the contralateral biceps femoris (cBF) muscle with a mean onset latency of 76 ms. •  Transcranial magnetic and electrical stimulation applied to the primary motor cortex revealed that a transcortical pathway probably contributes to the cBF response. •  We hypothesize that the cBF response signifies a preparation of the contralateral leg for early load bearing, helping the body to maintain dynamic stability during walking. •  This is the first study to show that a transcortical pathway contributes to an interlimb reflex in upper leg muscles. The transcortical nature of the response may allow for more adaptable responses than purely spinally mediated reflexes due to integration with other sensory information.

Input-output properties of the soleus stretch reflex in spastic stroke patients and healthy subjects during walking

The input-output properties of the soleus stretch reflex during walking and sitting were examined in 11 spastic stroke patients and 10 healthy subjects. In the early swing phase, the threshold of the input-output relation was significantly lower in the patient group - on average 108°/s compared to 309°/s in the control group (P=0.02). The slope of the input-output properties was unchanged in patients and in control subjects (P=0.39). In stroke patients we found a minor, but significant, difference in threshold with 108°/s in the early swing phase compared to 74°/s in sitting position at matched TA EMG activity (P=0.01). Furthermore, we found an unchanged slope of 0.25μVs/° during walking and 0.28μVs/° in the sitting position (P=0.21). In contrast, control subjects showed a large difference in the threshold in the early swing phase during walking (309°/s) compared with findings in the sitting position at a matched TA EMG activity (71°/s, P=0.004). The slope was unchanged during walking and when the subject was sitting (P=0.22). There was a significant correlation between clinical spasticity score and stretch reflex threshold in the early swing phase (P=-0.61, P=0.04) and between clinical spasticity score and the slope in the early swing phase (P=0.72, P=0.009). It is concluded that in the early swing phase, the markedly reduced soleus stretch reflex threshold was preventing the stroke patients from making fast dorsiflexion of the foot at the ankle joint and thereby impairing the walking speed.