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Unilateral displacement of lower limb evokes bilateral EMG responses in leg and foot muscles in standing humans
No full textDuring upright stance, foot dorsiflexion induced by the movement of a supporting platform elicits a short- (SLR) and a medium-latency response (MLR) in both the soleus and the flexor digitorum brevis muscles; foot plantarflexion elicits a MLR in the tibialis anterior. The SLR is the counterpart of the stretch reflex, but no general agreement exists about the origin of the MLR, though recent results suggest that it is transmitted through group II afferent fibres. Animal studies have shown that group II fibres impinge on interneurones projecting contralaterally as well as ipsilaterally, whereas group I fibres impinge on interneurones which project mainly ipsilaterally. Therefore, we compared the changes in amplitude and latency of the SLRs and MLRs in the right and left limb during postural perturbations induced while subjects maintained both feet on the platform (both-on condition) or while they maintained only one foot on the platform and the other on firm ground (one-on condition). Under the both-on condition, the pattern of EMG responses described above occurred bilaterally. Under the one-on condition, both SLRs and MLRs occurred in the displaced leg. However, whereas the SLRs did not change in amplitude compared with the both-on condition, the MLRs decreased in amplitude to about 50%. MLRs were also present in the non-displaced leg. They were not preceded by any SLR but showed a further decrease in size with respect to the corresponding responses in the perturbed leg. Latency of the MLRs of the perturbed leg increased by about 5 ms passing from the both-on to the one-on condition. In the latter condition, a further increase of 5 ms was observed in the nonperturbed leg with respect to the displaced one. The occurrence of the MLRs but not of the SLRs in the contralateral non-displaced leg is in keeping with the notion that crossed neural pathways fed by spindle group II afferent fibres subserve the MLRs. The changes in latency of the MLRs under the one-on condition compared with both-on give a cue about the synaptic delays along the neural circuit and the time taken by the afferent impulses to cross the spinal cord
Afferent control of walking: are there distinct deficits associated to loss of fibres of different diameter?
No full textOBJECTIVES: To compare the gait pattern in patients affected by different types of neuropathy. METHODS: We recruited healthy subjects (HS, n=38), patients with Charcot-Marie-Tooth disease type 1A (CMT1A) (n=10) and patients with diabetic neuropathy (DNP) (n=12). Neuropathy impairment score and neuropathy score were assessed. Body sway during quiet stance, and spatio-temporal gait parameters were recorded. RESULTS: Most patients had reduced or absent tendon-tap reflexes. Strength of foot dorsiflexor muscles (p<0.05) and conduction velocity (CV) of leg nerves (p<0.0001) were more impaired in CMT1A than DNP, whereas joint-position sense was more affected (p<0.05) in DNP. Body sway while standing was larger in DNP compared to CMT1A and HS (p<0.01 and p<0.0001 respectively). During gait, the distribution of foot sole contact pressure was abnormal in CMT1A (p<0.05) but not in DNP. Velocity and step length were decreased, and foot yaw angle at foot flat increased, in DNP with respect to CMT1A and HS (both variables, p<0.001). Gait velocity and step length were decreased (p<0.005) also in CMT1A, but to a smaller extent than in DNP, so that the difference between patient groups was significant (p<0.0005). Duration of the double support was protracted in DNP compared to CMT1A and HS (p<0.0005). For DNP only, velocity of gait and duration of single support were correlated (p<0.05) both to sway path and lower limb muscle strength. CONCLUSIONS: Changes in both body sway and stance phase of gait were larger in DNP than CMT1A, indicating more impaired static and dynamic control of balance when neuropathy affects the small in addition to the large afferent fibres. Diminished somatosensory input from the smaller fibres rather than muscle weakness or foot deformity plays a critical role in the modulation of the support phase of gait.
SIGNIFICANCE: The analysis of balance and gait in patients with neuropathy can offer a tool for understanding the nature and functional impact of the neuropathy and should be included in their functional evaluation
Standing on a continuously moving platform: is body inertia counteracted or exploited?
No full textWe describe the characteristics of displacement of the head and hip in normal
young subjects standing on a moving platform undergoing continuous sinusoidal
horizontal translation in the antero-posterior direction, at frequencies ranging
from 0.1-1 Hz. The head, hip and malleolus were marked by light-emitting diodes
(LEDs), and the displacement of each LED was quantified by (1) the measure of the
shift during each cycle of translation, (2) the standard deviation (SD) of the
path travelled during the whole trial, (3) the power spectrum (PS) of the signal
and (4) the cross-correlation (CC) between pairs of LED signals. At each
frequency of translation, with eyes open (EO), the displacement of head was
smaller than that of hip, and the displacement of hip was smaller than that of
malleolus. With eyes closed (EC), this order was reversed. The peak value of the
CC functions of the pairs malleolus/head, malleolus/hip and hip/head decreased by
passing from low to high frequency of translation, under both visual conditions,
and decreased more for the pair malleolus/head than malleolus/hip. The lags
between body segment displacements ranged between 30 ms and 150 ms, on average,
the former segment of each pair preceding the latter. The fast Fourier
transformation of hip and head displacement showed a power spectrum peak at the
frequency imposed by the platform translation. The peak was larger with EC than
EO. With EC, another peak appeared at 0.2 Hz, possibly corresponding to the
respiratory frequency. We conclude that, when vision was allowed, subjects
behaved as a non-rigid, noninverted pendulum, and stabilised head in space. When
vision was denied, head oscillated more than the platform, especially at low
translation frequencies. Therefore, the strategy of balance control shifted from
a pendulum to an inverted-pendulum behaviour, passing from active head-and-trunk
control to maximal body compliance to the perturbation
Post-effect on the centre of feet pressure during stance by continuous asymmetric mediolateral translations of a supporting platform-A preliminary study in healthy young adults
No full textVarious diseases are associated with the impaired control of the medio-lateral (ML) position of the centre of feet pressure (CoP), and several manoeuvres have been proposed for enhancing the CoP symmetry. Here, we assessed in healthy standing subjects the feasibility and outcome of a novel protocol entailing a reaction to a continuous asymmetric ML displacement (10 cm) of the support base. The periodic perturbation consisted of a fast half-cycle (0.5 Hz) followed by a slow half-cycle (0.18 Hz). One hundred successive horizontal translation cycles were delivered in sequence. Eyes were open or closed. CoP was recorded before, after, and during the stimulation by a dynamometric platform fixed onto the translating platform. We found that the post-stimulation CoP was displaced towards the direction of the fast half-cycles. The displacement lasted several tens of seconds. Vision did not aect the amplitude or duration of the post-stimulation eect. The magnitude of post-stimulation CoP displacement was related to the perturbation-induced ML motion of CoP recorded during the stimulation. Over the successive perturbation cycles, the time-course of this motion revealed an adaptation phenomenon. Vision moderately reduced the adaptation rate. The findings support the feasibility of the administration of a simple asymmetric balance perturbation protocol in clinical settings to help patients recover the symmetry of the CoP. This protocol needs to be further validated in older populations and in patients
Selective depression of medium-latency leg and foot muscle responses to stretch by an a2 agonist in humans.
No full textThe complex role of spindle afferent input, as evidenced by the study of posture control in normal subjects and patients
No full textAfferent control of body sway in different groups of neuropathic patients: the role of spindle group II fibres
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