Lack of On-Going Adaptations in the Soleus Muscle Activity During Walking in Patients Affected by Large-Fiber Neuropathy

Cutaneous and muscular afferents from the foot and sensory fusion processing: Physiology and pathology in neuropathies

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy.

A computer simulation study on the influences of loss and damage of primary muscle spindle afferents on soleus muscle stretch reflex

Sensory loss is detrimental to sensorimotor control. Several studies have reported that loss and/or damage of primary (Ia) muscle spindle afferents significantly influence the stretch reflex responses of leg and foot muscles. However, a systematic experimental evaluation on how the impairment of Ia muscle spindle afferents affects the stretch reflex is difficult due to technical and ethical issues. In the present study, the aim was to use computer simulations of a multiscale neuromusculoskeletal model to investigate how changes in: i) the number of Ia afferents, ii) the synaptic conductance between Ia sensory fibers and spinal motor neurons (MNs), and iii) the conduction velocities (CVs) of Ia afferents, would influence the stretch reflex of a leg muscle (soleus). Simulation results showed that both anatomical and functional loss of Ia afferents exerted an influence on the amplitude of short-latency stretch reflex response (M1) and the late phase of medium-latency response (M2). Additionally, changes in CVs of Ia afferents mainly influenced the latency of M1 and the amplitude of M2. Our findings provide conceptual evidence that a combination of anatomical and functional loss, as well as changes in CVs of Ia afferents due to demyelination, can explain the stretch reflex responses observed in peripheral neuropathies.

Parallels between astronauts and terrestrial patients - Taking physiotherapy rehabilitation "To infinity and beyond"

Exposure to the microgravity environment induces physiological changes in the cardiovascular, musculoskeletal and sensorimotor systems in healthy astronauts. As space agencies prepare for extended duration missions, it is difficult to predict the extent of the effects that prolonged exposure to microgravity will have on astronauts. Prolonged bed rest is a model used by space agencies to simulate the effects of spaceflight on the human body, and bed rest studies have provided some insights into the effects of immobilisation and inactivity. Whilst microgravity exposure is confined to a relatively small population, on return to Earth, the physiological changes seen in astronauts parallel many changes routinely seen by physiotherapists on Earth in people with low back pain (LBP), muscle wasting diseases, exposure to prolonged bed rest, elite athletes and critically ill patients in intensive care. The medical operations team at the European Space Agency are currently involved in preparing astronauts for spaceflight, advising on exercises whilst astronauts are on the International Space Station, and reconditioning astronauts following their return. There are a number of parallels between this role and contemporary roles performed by physiotherapists working with elite athletes and muscle wasting conditions. This clinical commentary will draw parallels between changes which occur to the neuromuscular system in the absence of gravity and conditions which occur on Earth. Implications for physiotherapy management of astronauts and terrestrial patients will be discussed.

The Neuro-Mechanical Processes That Underlie Goal-Directed Medio-Lateral APA during Gait Initiation

Gait initiation (GI) involves passing from bipedal to unipedal stance. It requires a rapid movement of the center of foot pressure (CoP) towards the future swing foot and of the center of mass (CoM) in the direction of the stance foot prior to the incoming step. This anticipatory postural adjustment (APA) allows disengaging the swing leg from the ground and establishing favorable conditions for stepping. This study aimed to describe the neuro-mechanical process that underlies the goal-directed medio-lateral (ML) APA. We hypothesized that controlled knee flexion of the stance leg contributes to the initial ML displacement of the CoP and to the calibration of the first step. Fourteen subjects initiated gait starting from three different initial stance widths of 15 cm (Small), 30 cm (Medium), and 45 cm (Large). Optoelectronic, force platform and electromyogram (EMG) measurements were performed. During APA, soleus activity diminished bilaterally, while tibialis anterior (TA) activity increased, more so in the stance leg than in the swing leg, and to a larger extent with increasing initial stance width. Knee flexion of the stance leg was observed during APA and correlated with the ML CoP displacement towards the swing leg. ML CoP and CoM displacements during APA increased with increasing stance width. The activity of stance-leg TA was correlated with the degree of knee flexion. Swing-leg tensor fasciae latae (TFL) was also active during APA. Across subjects, when stance-leg tibialis activity was low, TFL activity was large and vice versa. The modulation of the ML CoP position during APA allowed the gravity-driven torque to place the CoM just lateral to the stance foot during step execution. Accordingly, the gravity-driven torque, the ML CoM velocity during step execution, and the step width at foot contact (FC) were lower in the Small and greater in the Large condition. Consequently, the position of the stepping foot at FC remained close to the sagittal plane in all three conditions. Conclusively, coordinated activation of hip abductors and ankle dorsiflexors during APA displaces the CoP towards the swing leg, and sets the contact position for the swing foot.

Quantitative sensory and motor measures detect change overtime and correlate with walking speed in individuals with multiple sclerosis

BACKGROUND

Impairments of sensation, strength, and walking are common in multiple sclerosis (MS). The relationship among these abnormalities and how they change over time remains unclear.

OBJECTIVE

To determine the extent that quantitative lower extremity sensory and motor measures detect abnormalities over time, relate to global disability, and to walking speed in individuals with MS.

METHODS

This prospective, longitudinal analysis evaluated 136 MS subjects. Measures included measures of leg strength, sensation, the Expanded Disability Status Scale(EDSS) and timed 25-foot walk test (T25FW). Mixed effects regression models were used.

RESULTS

Our cohort׳s mean age is 44.3±10.8 years (mean±SD), EDSS score range 0–7.5, 66% were females, and follow-up time was 2.1±1.2 years. Strength significantly changed over time; the RRMS group demonstrated the greatest changes in ADF (3.3 lbs/yr) while the PPMS group showed significant HF changes (−2.1 lbs/yr). Walking speed was affected most by HF, especially in the weakest individuals (HF<20 lbs); T25FW increased by 0.20 s for each 1 lb loss (p=0.001). Likewise T25FW changed by 0.19 s for each 1 lb change in ADF (p<0.01).

CONCLUSION

Quantitative measures detected changes in sensation and strength over time, despite a stable respective functional systems scores of the EDSS. Quantitative measurement tools may improve the sensitivity of disability measures in MS and further investigation of these tools as outcomes in future clinical trials of rehabilitative and neuroreparative interventions is warranted.

Neck proprioception shapes body orientation and perception of motion

This review article deals with some effects of neck muscle proprioception on human balance, gait trajectory, subjective straight-ahead (SSA), and self-motion perception. These effects are easily observed during neck muscle vibration, a strong stimulus for the spindle primary afferent fibers. We first remind the early findings on human balance, gait trajectory, SSA, induced by limb, and neck muscle vibration. Then, more recent findings on self-motion perception of vestibular origin are described. The use of a vestibular asymmetric yaw-rotation stimulus for emphasizing the proprioceptive modulation of motion perception from the neck is mentioned. In addition, an attempt has been made to conjointly discuss the effects of unilateral neck proprioception on motion perception, SSA, and walking trajectory. Neck vibration also induces persistent aftereffects on the SSA and on self-motion perception of vestibular origin. These perceptive effects depend on intensity, duration, side of the conditioning vibratory stimulation, and on muscle status. These effects can be maintained for hours when prolonged high-frequency vibration is superimposed on muscle contraction. Overall, this brief outline emphasizes the contribution of neck muscle inflow to the construction and fine-tuning of perception of body orientation and motion. Furthermore, it indicates that tonic neck-proprioceptive input may induce persistent influences on the subject's mental representation of space. These plastic changes might adapt motion sensitiveness to lasting or permanent head positional or motor changes.

An anterior ankle-foot orthosis improves walking economy in Charcot-Marie-Tooth type 1A patients

BACKGROUND

Ankle-foot orthoses are commonly prescribed in Charcot-Marie-Tooth type 1A disease to improve quality of walking and reduce the risk of falling due to the foot drop.

OBJECTIVES

This study aimed at assessing the effect of an anterior ankle-foot orthosis on walking economy in a group of Charcot-Marie-Tooth type 1A patients.

STUDY DESIGN

Within-group comparisons.

METHODS

7 Charcot-Marie-Tooth type 1A patients (four women and three men; 37 ± 11 years; age range = 22-53 years) were asked to walk on a circuit at their self-selected speeds ('slow', 'comfortable' and 'fast') in two walking conditions: (1) with shoes only and (2) with Taloelast(®) anterior elastic ankle-foot orthoses. Speed of walking and metabolic cost of walking energy cost per unit of distance were assessed at the three self-selected speeds of walking for both walking conditions.

RESULTS

Speed of walking at the three self-selected speeds did not differ between shoes only and anterior elastic ankle-foot orthoses, whereas walking energy cost per unit of distance at comfortable speed was lower in patients using anterior elastic ankle-foot orthoses with respect to shoes only (2.39 ± 0.22 vs 2.70 ± 0.19 J kg(-1) m(-1); P < 0.05).

CONCLUSIONS

In Charcot-Marie-Tooth type 1A patients, the use of anterior elastic ankle-foot orthoses improved walking economy by reducing the energy cost of walking per unit of distance, thus reflecting a lower level of metabolic effort and improved mechanical efficiency in comparison with shoes only.

CLINICAL RELEVANCE

From a practical perspective, Charcot-Marie-Tooth type 1A patients with anterior elastic ankle-foot orthoses can walk for a longer duration with a lower level of physical effort. Improvements in walking economy due to ankle-foot orthoses are likely a consequence of the reduction in steppage gait.

Gradual mechanics-dependent adaptation of medial gastrocnemius activity during human walking

While performing a simple bouncing task, humans modify their preferred movement period and pattern of plantarflexor activity in response to changes in system mechanics. Over time, the preferred movement pattern gradually adapts toward the resonant frequency. The purpose of the present experiments was to determine whether humans undergo a similar process of gradually adapting their stride period and plantarflexor activity after a change in mechanical demand while walking. Participants walked on a treadmill while we measured stride period and plantarflexor activity (medial gastrocnemius and soleus). Plantarflexor activity during stance was divided into a storage phase (30-65% stance) and a return phase (65-100% stance) based on when the Achilles tendon has previously been shown to store and return mechanical energy. Participants walked either on constant inclines (0%, 1%, 5%, 9%) or on a variable incline (0-1%) for which they were unaware of the incline changes. For variable-incline trials, participants walked under both single-task and dual-task conditions in order to vary the cognitive load. Both stride period and plantarflexor activity increased at steeper inclines. During single-task walking, small changes in incline were followed by gradual adaptation of storage-phase medial gastrocnemius activity. However, this adaptation was not present during dual-task walking, indicating some level of cognitive involvement. The observed adaptation may be the result of using afferent feedback in order to optimize the contractile conditions of the plantarflexors during the stance phase. Such adaptation could serve to improve metabolic economy but may be limited in clinical populations with disrupted proprioception.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

Amount and intensity of daily living activities in Charcot-Marie-Tooth 1A patients

BACKGROUND

Charcot-Marie-Tooth 1A (CMT1A) patients show a reduction of spontaneous activities of daily living measured by means of questionnaires or pedometers, which are quite inaccurate compared to recent measurement techniques.

AIM

The study aimed at quantifying daily living activities in CMT1A patients by means of inertial sensors, which give information not only on the amount but also on the intensity of these activities.

MATERIALS AND METHODS

Time and count (amount), and velocity and power (intensity) of 24 h daily living activities were measured in eight patients (20-48 years; Barthel >90; Tinetti >20) and eight healthy individuals, matched for age and gender, by means of a wearable inertial sensor device.

RESULTS

There were no differences between patients and controls in the 24-h distance covered and count of steps. However, count of step climbing and sit to stand were lower in patients than in controls (139.93 ± 141.66 vs. 341.06 ± 164.07 n and 58.23 ± 7.82 vs. 65.81 ± 4.75 n, respectively; P < 0.05) as well as mean daily step-climbing and walking velocities (1.07 ± 0.17 vs. 1.21 ± 0.10 m/sec and 1.16 ± 0.31 vs. 1.87 ± 0.50 m/sec, respectively; P < 0.05). In CMT1A patients there was a positive correlation between strength of the knee extensor muscles and both count of steps climbed (R = 0.80) and sit to stand (R = 0.79).

DISCUSSION AND CONCLUSION

The reduced ability of CMT1A patients to carry out activities at high intensity, which was correlated with strength, suggests that strength training might be a rehabilitation tool for improving the 1 ability to carry out these activities.

Plasticity and modular control of locomotor patterns in neurological disorders with motor deficits

Human locomotor movements exhibit considerable variability and are highly complex in terms of both neural activation and biomechanical output. The building blocks with which the central nervous system constructs these motor patterns can be preserved in patients with various sensory-motor disorders. In particular, several studies highlighted a modular burst-like organization of the muscle activity. Here we review and discuss this issue with a particular emphasis on the various examples of adaptation of locomotor patterns in patients (with large fiber neuropathy, amputees, stroke and spinal cord injury). The results highlight plasticity and different solutions to reorganize muscle patterns in both peripheral and central nervous system lesions. The findings are discussed in a general context of compensatory gait mechanisms, spatiotemporal architecture and modularity of the locomotor program.

Afferent control of walking: are there distinct deficits associated to loss of fibres of different diameter?

OBJECTIVES

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.

The functional role of the triceps surae muscle during human locomotion

Aim

Despite numerous studies addressing the issue, it remains unclear whether the triceps surae muscle group generates forward propulsive force during gait, commonly identified as ‘push-off’. In order to challenge the push-off postulate, one must probe the effect of varying the propulsive force while annulling the effect of the progression velocity. This can be obtained by adding a load to the subject while maintaining the same progression velocity.

Methods

Ten healthy subjects initiated gait in both unloaded and loaded conditions (about 30% of body weight attached at abdominal level), for two walking velocities, spontaneous and fast. Ground reaction force and EMG activity of soleus and gastrocnemius medialis and lateralis muscles of the stance leg were recorded. Centre of mass velocity and position, centre of pressure position, and disequilibrium torque were calculated.

Results

At spontaneous velocity, adding the load increased disequilibrium torque and propulsive force. However, load had no effect on the vertical braking force or amplitude of triceps activity. At fast progression velocity, disequilibrium torque, vertical braking force and triceps EMG increased with respect to spontaneous velocity. Still, adding the load did not further increase braking force or EMG.

Conclusions

Triceps surae is not responsible for the generation of propulsive force but is merely supporting the body during walking and restraining it from falling. By controlling the disequilibrium torque, however, triceps can affect the propulsive force through the exchange of potential into kinetic energy.

Reduced plantar cutaneous sensation modifies gait dynamics, lower-limb kinematics and muscle activity during walking

Peripheral neuropathy is the most common long-term complication in diabetes and is involved in changes in diabetic gait and posture. The regression of nerve function leads to various deficits in the sensory and motor systems, impairing afferent and efferent pathways in the lower extremities. This study aimed to examine how reduced plantar-afferent feedback impacts the gait pattern. Cutaneous sensation in the soles of both feet was experimentally reduced by means of intradermal injections of an anaesthetic solution, without affecting foot proprioception or muscles. Ten subjects performed level walking at a controlled velocity before and after plantar anaesthesia. Muscle activity of five leg-muscles, co-contraction ratios for the knee and ankle joint, ground reaction forces (GRF), spatiotemporal characteristics, joint angles and moments of the hip, knee and ankle were analysed. The intervention significantly lowered plantar sensation, reducing it to the level of sensory neuropathy. Spatiotemporal gait characteristics remained unchanged. The ankle joint was more dorsiflexed which coincided with increased tibialis anterior and decreased gastrocnemius medialis muscle activity during foot flat to mid-stance. In addition, the knee joint was more flexed accompanied by increased biceps femoris activity and higher internal knee-extension moment. With regard to gait dynamics, a delay of the first peak of the vertical GRF was observed. Increased soleus and tibialis anterior muscle activity were found during the end of stance. Short-term loss of plantar sensation affects lower-limb kinematics and gait dynamics, particularly during the first half of stance, and contributes to modified muscle-activation patterns during locomotion.

Effects of reduced plantar cutaneous afferent feedback on locomotor adjustments in dynamic stability during perturbed walking

This study examined the effects of reduced plantar cutaneous afferent feedback on predictive and feedback adaptive locomotor adjustments in dynamic stability during perturbed walking. Twenty-two matched participants divided between an experimental-group and a control-group performed a gait protocol, which included surface alterations to one covered exchangeable gangway-element (hard/soft). In the experimental-group, cutaneous sensation in both foot soles was reduced to the level of sensory peripheral neuropathy by means of intradermal injections of an anaesthetic solution, without affecting foot proprioception or muscles. The gait protocol consisted of baseline trials on a uniformly hard surface and an adaptation phase consisting of nineteen trials incorporating a soft gangway-element, interspersed with three trials using the hard surface-element (2nd, 8th and 19th). Dynamic stability was assessed by quantifying the margin of stability (MS), which was calculated as the difference between the base of support (BS) and the extrapolated centre of mass (CM). The horizontal velocity of the CM and its vertical projection in the anterior-posterior direction and the eigenfrequency of an inverted pendulum determine the extrapolated-CM. Both groups increased the BS at the recovery step in response to the first unexpected perturbation. These feedback corrections were used more extensively in the experimental-group, which led to a higher MS compared to the control-group, i.e. a more stable body-position. In the adaptation phase the MS returned to baseline similarly in both groups. In the trial on the hard surface directly after the first perturbation, both groups increased the MS at touchdown of the disturbed leg compared to baseline trials, indicating rapid predictive adjustments irrespective of plantar cutaneous input. Our findings demonstrate that the locomotor adaptational potential does not decrease due to the loss of plantar sensation.

Quantitative measures detect sensory and motor impairments in multiple sclerosis

BACKGROUND

Sensory and motor dysfunction in multiple sclerosis (MS) is often assessed with rating scales which rely heavily on clinical judgment. Quantitative devices may be more precise than rating scales.

OBJECTIVE

To quantify lower extremity sensorimotor measures in individuals with MS, evaluate the extent to which they can detect functional systems impairments, and determine their relationship to global disability measures.

METHODS

We tested 145 MS subjects and 58 controls. Vibration thresholds were quantified using a Vibratron-II device. Strength was quantified by a hand-held dynamometer. We also recorded Expanded Disability Status Scale (EDSS) and Timed 25-Foot Walk (T25FW). t-tests and Wilcoxon-rank sum were used to compare group data. Spearman correlations were used to assess relationships between each measure. We also used a step-wise linear regression model to determine how much the quantitative measures explain the variance in the respective functional systems scores (FSS).

RESULTS

EDSS scores ranged from 0-7.5, mean disease duration was 10.4 ± 9.6 years, and 66% were female. In relapsing-remitting MS, but not progressive MS, poorer vibration sensation correlated with a worse EDSS score, whereas progressive groups' ankle/hip strength changed significantly with EDSS progression. Interestingly, not only did sensorimotor measures significantly correlate with global disability measures (i.e., EDSS), but they had improved sensitivity, as they detected impairments in up to 32% of MS subjects with normal sensory and pyramidal FSS.

CONCLUSIONS

Sensory and motor deficits in MS can be quantified using clinically accessible tools and distinguish differences among MS subtypes. We show that quantitative sensorimotor measures are more sensitive than FSS from the EDSS. These tools have the potential to be used as clinical outcome measures in practice and for future MS clinical trials of neurorehabilitative and neuroreparative interventions.

Charcot-Marie-Tooth 1A patients with low level of impairment have a higher energy cost of walking than healthy individuals

The study aimed at quantifying the walking energy cost of a group of Charcot-Marie-Tooth 1A patients (CMT1A), with low severity of walking impairment, in comparison with healthy individuals. Oxygen uptake was measured in 8 patients (age-range 20-48 years; Barthel >90; Tinetti >20) and 8 healthy individuals, matched for age and gender, when walking on a circuit for 5-min at their self-selected speeds ("slow", "comfortable" and "fast"). Both comfortable and fast speeds were lower in patients than in the control group (0.92±0.16 vs 1.16±0.22 and 1.27±0.27 vs 1.61±0.22 m s⁻¹, respectively; P<0.05), whereas walking energy cost per unit of distance was higher in patients than in the control group (P<0.05) at both "comfortable" (2.27±0.35 vs 1.92±0.21 J kg⁻¹m⁻¹) and "fast" speed (3.05±0.35 vs 2.37±0.42 J kg⁻¹m⁻¹). CMT1A patients, therefore, choose to walk slower but with higher metabolic cost compared to healthy individuals, despite no clinically evident walking impairment, which is likely due to altered walking patterns.

Short-term locomotor adaptation to a robotic ankle exoskeleton does not alter soleus Hoffmann reflex amplitude

BACKGROUND

To improve design of robotic lower limb exoskeletons for gait rehabilitation, it is critical to identify neural mechanisms that govern locomotor adaptation to robotic assistance. Previously, we demonstrated soleus muscle recruitment decreased by approximately 35% when walking with a pneumatically-powered ankle exoskeleton providing plantar flexor torque under soleus proportional myoelectric control. Since a substantial portion of soleus activation during walking results from the stretch reflex, increased reflex inhibition is one potential mechanism for reducing soleus recruitment when walking with exoskeleton assistance. This is clinically relevant because many neurologically impaired populations have hyperactive stretch reflexes and training to reduce the reflexes could lead to substantial improvements in their motor ability. The purpose of this study was to quantify soleus Hoffmann (H-) reflex responses during powered versus unpowered walking.

METHODS

We tested soleus H-reflex responses in neurologically intact subjects (n=8) that had trained walking with the soleus controlled robotic ankle exoskeleton. Soleus H-reflex was tested at the mid and late stance while subjects walked with the exoskeleton on the treadmill at 1.25 m/s, first without power (first unpowered), then with power (powered), and finally without power again (second unpowered). We also collected joint kinematics and electromyography.

RESULTS

When the robotic plantar flexor torque was provided, subjects walked with lower soleus electromyographic (EMG) activation (27-48%) and had concomitant reductions in H-reflex amplitude (12-24%) compared to the first unpowered condition. The H-reflex amplitude in proportion to the background soleus EMG during powered walking was not significantly different from the two unpowered conditions.

CONCLUSION

These findings suggest that the nervous system does not inhibit the soleus H-reflex in response to short-term adaption to exoskeleton assistance. Future studies should determine if the findings also apply to long-term adaption to the exoskeleton.

Afferent Contribution to Locomotor Muscle Activity During Unconstrained Overground Human Walking: An Analysis of Triceps Surae Muscle Fascicles

Plantar flexor series elasticity can be used to dissociate muscle–fascicle and muscle–tendon behavior and thus afferent feedback during human walking. We used electromyography (EMG) and high-speed ultrasonography concomitantly to monitor muscle activity and muscle fascicle behavior in 19 healthy volunteers as they walked across a platform. On random trials, the platform was dropped (8 cm, 0.9 g acceleration) or held at a small inclination (up to ±3° in the parasagittal plane) with respect to level ground. Dropping the platform in the mid and late phases of stance produced a depression in the soleus muscle activity with an onset latency of about 50 ms. The reduction in ground reaction force also unloaded the plantar flexor muscles. The soleus muscle fascicles shortened with a minimum delay of 14 ms. Small variations in platform inclination produced significant changes in triceps surae muscle activity; EMG increased when stepping on an inclined surface and decreased when stepping on a declined surface. This sensory modulation of the locomotor output was concomitant with changes in triceps surae muscle fascicle and gastrocnemius tendon length. Assuming that afferent activity correlates to these mechanical changes, our results indicate that within-step sensory feedback from the plantar flexor muscles automatically adjusts muscle activity to compensate for small ground irregularities. The delayed onset of muscle fascicle movement after dropping the platform indicates that at least the initial part of the soleus depression is more likely mediated by a decrease in force feedback than length-sensitive feedback, indicating that force feedback contributes to the locomotor activity in human walking.

Joint kinetic response during unexpectedly reduced plantar flexor torque provided by a robotic ankle exoskeleton during walking

During human walking, plantar flexor activation in late stance helps to generate a stable and economical gait pattern. Because plantar flexor activation is highly mediated by proprioceptive feedback, the nervous system must modulate reflex pathways to meet the mechanical requirements of gait. The purpose of this study was to quantify ankle joint mechanical output of the plantar flexor stretch reflex response during a novel unexpected gait perturbation. We used a robotic ankle exoskeleton to mechanically amplify the ankle torque output resulting from soleus muscle activation. We recorded lower-body kinematics, ground reaction forces, and electromyography during steady-state walking and during randomly perturbed steps when the exoskeleton assistance was unexpectedly turned off. We also measured soleus Hoffmann- (H-) reflexes at late stance during the two conditions. Subjects reacted to the unexpectedly decreased exoskeleton assistance by greatly increasing soleus muscle activity about 60ms after ankle angle deviated from the control condition (p<0.001). There were large differences in ankle kinematic and electromyography patterns for the perturbed and control steps, but the total ankle moment was almost identical for the two conditions (p=0.13). The ratio of soleus H-reflex amplitude to background electromyography was not significantly different between the two conditions (p=0.4). This is the first study to show that the nervous system chooses reflex responses during human walking such that invariant ankle joint moment patterns are maintained during perturbations. Our findings are particularly useful for the development of neuromusculoskeletal computer simulations of human walking that need to adjust reflex gains appropriately for biomechanical analyses.

Locomotor adaptation and aftereffects in patients with reduced somatosensory input due to peripheral neuropathy

We studied 12 peripheral neuropathy patients (PNP) and 13 age-matched controls with the "broken escalator" paradigm to see how somatosensory loss affects gait adaptation and the release and recovery ("braking") of the forward trunk overshoot observed during this locomotor aftereffect. Trunk displacement, foot contact signals, and leg electromyograms (EMGs) were recorded while subjects walked onto a stationary sled (BEFORE trials), onto the moving sled (MOVING or adaptation trials), and again onto the stationary sled (AFTER trials). PNP were unsteady during the MOVING trials, but this progressively improved, indicating some adaptation. During the after trials, 77% of control subjects displayed a trunk overshoot aftereffect but over half of the PNP (58%) did not. The PNP without a trunk aftereffect adapted to the MOVING trials by increasing distance traveled; subsequently this was expressed as increased distance traveled during the aftereffect rather than as a trunk overshoot. This clear separation in consequent aftereffects was not seen in the normal controls suggesting that, as a result of somatosensory loss, some PNP use distinctive strategies to negotiate the moving sled, in turn resulting in a distinct aftereffects. In addition, PNP displayed earlier than normal anticipatory leg EMG activity during the first after trial. Although proprioceptive inputs are not critical for the emergence or termination of the aftereffect, somatosensory loss induces profound changes in motor adaptation and anticipation. Our study has found individual differences in adaptive motor performance, indicative that PNP adopt different feed-forward gait compensatory strategies in response to peripheral sensory loss.

Fast responses to stepping on an unexpected surface height depend on intact large-diameter nerve fibers: a study on Charcot-Marie-Tooth type 1A disease

The contribution of reflexes from the large myelinated afferents in the control of normal and perturbed gait in humans is a highly debated issue. One way to investigate this topic is by studying normal and perturbed gait in patients lacking large myelinated fibers in the distal limb (Charcot-Marie-Tooth [CMT] type 1A disease). Such patients should have delayed and decreased reflexes if the latter depend on these large myelinated fibers. To elicit the reflexes, both patients and controls had to step on a platform that was either at the same level or lowered by 5 cm. In control subjects, landing on a level surface induced short-latency responses in the biceps femoris and tibialis anterior muscles, whereas such responses were largely absent in the patients. Similarly, stepping down unexpectedly induced a very fast muscle synergy, leading to a brake of the forward propulsion in the controls, which was significantly reduced and delayed (on average 32 ms) in the patients. The observed changes correlated with both sensory and motor deficits. Nevertheless, it is concluded that the results are primarily related to the sensory deficits, since the delayed or absent responses appeared in both upper and lower leg muscles, whereas only the latter showed motor deficits. The data are taken as evidence that large-diameter afferents from the distal leg are essential for fast reflex activations induced by stepping on a level or lowered surface unexpectedly.

Muscle proprioceptive feedback and spinal networks

This review revolves primarily around segmental feedback systems established by muscle spindle and Golgi tendon organ afferents, as well as spinal recurrent inhibition via Renshaw cells. These networks are considered as to their potential contributions to the following functions: (i) generation of anti-gravity thrust during quiet upright stance and the stance phase of locomotion; (ii) timing of locomotor phases; (iii) linearization and correction for muscle nonlinearities; (iv) compensation for muscle lever-arm variations; (v) stabilization of inherently unstable systems; (vi) compensation for muscle fatigue; (vii) synergy formation; (viii) selection of appropriate responses to perturbations; (ix) correction for intersegmental interaction forces; (x) sensory-motor transformations; (xi) plasticity and motor learning. The scope will at times extend beyond the narrow confines of spinal circuits in order to integrate them into wider contexts and concepts.

Tuning posture to body load: decreases in load produce discrete sensory signals in the legs of freely standing cockroaches

Decreases in load are important cues in the control of posture and walking. We recorded activities of the tibial campaniform sensilla, receptors that monitor forces as strains in the exoskeleton, in the middle legs of freely moving cockroaches. Small magnets were attached to the thorax and body load was changed by applying currents to a coil below the substrate. Body position was monitored by video recording. The tibial sensilla are organized into proximal and distal subgroups that have different response properties and reflex effects: proximal sensilla excite extensor motoneurons while distal receptors inhibit extensor firing. Sudden load decreases elicited bursts from distal sensilla, while increased load excited proximal receptors. The onset of sensory discharges closely approximated the time of peak velocity of body movement in both load decreases and increases. Firing of distal sensilla rapidly adapted to sustained unloading, while proximal sensilla discharged tonically to load increases. Load decreases of small amplitude or at low rates produced only inhibition of proximal activity while decrements of larger size or rate elicited distal firing. These response properties may provide discrete signals that either modulate excitatory extensor drive during small load variations or inhibit support prior to compensatory stepping or initiation of swing.

Stance- and Locomotion-Dependent Processing of Vibration-Induced Proprioceptive Inflow From Multiple Muscles in Humans

We performed a whole-body mapping study of the effect of unilateral muscle vibration, eliciting spindle Ia firing, on the control of standing and walking in humans. During quiet stance, vibration applied to various muscles of the trunk-neck system and of the lower limb elicited a significant tilt in whole body postural orientation. The direction of vibration-induced postural tilt was consistent with a response compensatory for the illusory lengthening of the stimulated muscles. During walking, trunk-neck muscle vibration induced ample deviations of the locomotor trajectory toward the side opposite to the stimulation site. In contrast, no significant modifications of the locomotor trajectory could be detected when vibrating various muscles of the lower as well as upper limb. The absence of correlation between the effects of muscle vibration during walking and standing dismisses the possibility that vibration-induced postural changes can account for the observed deviations of the locomotor trajectory during walking. We conclude that the dissimilar effects of trunk-neck and lower limb muscle vibration during walking and standing reflect a general sensory-motor plan, whereby muscle Ia input is processed according to both the performed task and the body segment from which the sensory inflow arises.

Effects of exercise and creatine on myosin heavy chain isoform composition in patients with Charcot-Marie-Tooth disease

It is not known whether myosin heavy chain (MHC) content changes in response to exercise training or creatine supplementation in subjects with Charcot-Marie-Tooth disease (CMT). Based on previous data, we hypothesized that resistance exercise and creatine would increase the percentage of type I MHC composition in the vastus lateralis muscle and that myosin isoform changes would correlate with improved chair rise-time in CMT subjects. To test this hypothesis, 18 CMT subjects were randomly assigned to either a placebo or creatine group. All subjects performed a 12-week, home-based, moderate-intensity resistance training program. Chair rise-time was measured before and after the training program. Muscle biopsies were obtained from the vastus lateralis before and after the 12-week program. Gel electrophoresis showed a significant decrease (approximately 30%) in MHC type I in CMT subjects given creatine supplementation when compared with placebo. There was a nonsignificant increase in both MHC type IIa (approximately 23%) and MHC type IIx (approximately 7%) in CMT subjects given creatine. Reduced MHC type I content and increased MHC type IIa content correlated with faster chair rise-times (i.e., improved muscle performance). The training-induced change in MHC IIa content was inversely correlated with chair rise-time in CMT subjects given creatine. When the two subject groups were combined, there was a linear, negative relationship between the change in MHC type IIa content and chair rise-time after training and a positive relationship between the training-induced change in MHC type I content and chair rise-time. These data suggest that improved function (chair rise-time) was associated with a lower level of MHC type I and increased MHC type IIa composition. Furthermore, the data are consistent with the hypothesis that creatine supplementation alters MHC composition in CMT patients undergoing resistance training and that MHC changes associated with creatine supplementation can improve muscle function.

Recumbent stepping has similar but simpler neural control compared to walking

The purpose of this study was to compare muscle activation patterns and kinematics during recumbent stepping and walking to determine if recumbent stepping has a similar motor pattern as walking. We measured joint kinematics and electromyography in ten neurologically intact humans walking on a treadmill at 0 and 50% body weight support (BWS), and recumbent stepping using a commercially available exercise machine. Cross correlation of upper and lower limb electromyography patterns between conditions revealed high correlations for most muscles. A principal component analysis revealed that the first factor accounted for more muscle activation signal content during recumbent stepping (81%) than during walking (70%). This indicates that the motor pattern during walking is more complex than during stepping. Cross correlation analysis found a high correlation between factors for recumbent stepping and walking (R = 0.54), though not as high as the correlation between factors for walking at 0% BWS and walking at 50% BWS (R = 0.68). There were substantial differences in joint kinematics between walking and recumbent stepping, most notably in hip, elbow, and shoulder motions. These results suggest that although the two tasks have different kinematic patterns, recumbent stepping relies on similar neural networks as walking. Individuals with neurological impairments may be able to improve walking ability from recumbent stepping practice given similarities in neural control between the two tasks.

The effects of sensory loss and walking speed on the orbital dynamic stability of human walking

Peripheral sensory feedback is believed to contribute significantly to maintaining walking stability. Patients with diabetic peripheral neuropathy have a greatly increased risk of falling. Previously, we demonstrated that slower walking speeds in neuropathic patients lead to improved local dynamic stability. However, all subjects exhibited significant local instability during walking, even though no subject fell or stumbled during testing. The present study was conducted to determine if and how significant changes in peripheral sensation and walking speed affect orbital stability during walking. Trunk and lower extremity kinematics were examined from two prior experiments that compared patients with significant neuropathy to healthy controls and walking at multiple different speeds in young healthy subjects. Maximum Floquet multipliers were computed for each time series to quantify the orbital stability of these movements. All subjects exhibited orbitally stable walking kinematics, even though these same kinematics were previously shown to be locally unstable. Differences in orbital stability between neuropathic and control subjects were small and, with the exception of knee joint movements (p=0.001), not statistically significant (0.380p0.946). Differences in knee orbital stability were not mediated by differences in walking speed. This was supported by our finding that although orbital stability improved slightly with slower walking speeds, the correlations between walking speed and orbital stability were generally weak (r(2)16.7%). Thus, neuropathic patients do not gain improved orbital stability as a result of slowing down and do not experience any loss of orbital stability because of their sensory deficits.

Afferent-mediated modulation of the soleus muscle activity during the stance phase of human walking

The aim of this study was to investigate the contribution of proprioceptive feedback to the amplitude modulation of the soleus muscle activity during human walking. We have previously shown that slow-velocity, small-amplitude ankle dorsiflexion enhancements and reductions applied during the stance phase of the step cycle generate, respectively, increments and decrements on the ongoing soleus activity. We have also shown that the increments in soleus activity are at least partially mediated by feedback from group Ia fibres. In the present study, we further investigated the afferent-mediated contribution from muscle group II afferents, cutaneous and proprioceptive afferents from the foot, and load-sensitive afferents to the soleus EMG. Slow-velocity, small-amplitude ankle trajectory modifications were combined with the pharmaceutical depression of group II polysynaptic pathways with tizanidine hydrochloride, anaesthetic blocking of sensory information from the foot with injections of lidocaine hydrochloride, and modulation of load feedback by increasing and decreasing the body load. The depression of the group II afferents significantly reduced the soleus response to the ankle trajectory modifications. Blocking sensory feedback from the foot did not have an effect on the soleus muscle activity. Changes in body load affected the ongoing soleus activity level; however, it did not affect the amplitude of the soleus EMG responses to the ankle trajectory modifications. These results suggest that the feedback from group II afferents, and possibly from load-sensitive afferents, contribute to the amplitude modulation of the soleus muscle activity during the stance phase of the step cycle. However, feedback from cutaneous afferents and instrinsic proprioceptive afferents from the foot does not seem to contribute to this muscle activation.

The posture-related interaction between Ia-afferent and descending input on the spinal reflex excitability in humans

The separate and combined depressive effects induced by vibration and standing on the soleus H-reflex have been studied by administering Achilles' tendon vibration in prone position and during stance. Without vibration, H-reflex amplitude was larger under prone than standing condition. Vibration reduced the reflex both in prone position and even more during stance. When vibration was superimposed to inclined stance (greater EMG background), the reflex was reduced of the same absolute amount as when it was superimposed to normal stance. When vibration was superimposed on stance with minimal or no background EMG, the reflex disappeared. These results confirm that both upright posture and vibration have a strong depressive effect on the H-reflex. They also show that muscle activity during stance is enough for overcoming the reflex depression. These findings provide information about the origin of the disfacilitatory effects on the monosynaptic reflex pathway, contribute to the understanding of the posture-related mechanisms responsible for the modulation of the spinal reflex excitability, and allow arguing in favour of a minor but adaptable role for the short latency stretch reflex in the control of quiet unperturbed stance.