Postural Response Latencies Are Related to Balance Control During Standing and Walking in Patients With Multiple Sclerosis

Objective

To understand and examine the relation between postural response latencies obtained during postural perturbations and representative measures of balance during standing (sway variables) and walking (trunk motion).

Design

Cross-sectional.

Setting

University medical center.

Participants

Persons with multiple sclerosis (MS) (n=40) were compared with similar aged control subjects (n=20). There were 20 subjects with MS in the normal walking velocity group and 20 subjects with MS who had slow walking velocity based on a timed 25-foot walk (T25FW) of <5 seconds.

Interventions

None.

Main Outcome Measures

Postural response latency, sway variables, trunk motion variables.

Results

We found that subjects with MS with both slow or normal walking velocities had significantly longer postural response latencies than the healthy control group. Postural response latency was not correlated with the T25FW. Postural response latency was significantly correlated with center of pressure sway variables during quiet standing (root mean square: ρ=.334, P=.04; range: ρ=.385, P=.017; mean velocity: ρ=.337, P=.038; total sway area: ρ=.393, P=.015). Postural response latency was also significantly correlated with motion of the trunk during walking (sagittal plane range of motion: ρ=.316, P=.05; SD of transverse plane range of motion: ρ=−.43, P=.006).

Conclusions

These findings clearly indicate that slow postural responses to external perturbations in patients with MS contribute to disturbances in balance control during both standing and walking.     

Effect of multifocal lens glasses on the stepping patterns of novice wearers

Multifocal lens glasses (MfLs) negatively affect vision, increase falling risk and contribute to gait changes during stepping. Previous studies on the effects of MfLs on gait have focused on experienced wearers. Thus, the initial response of first-time wearers, who may face significant challenges in adapting to these glasses, is not well understood. This study aimed to quantify the effects of MfLs on novice wearers during stepping up and down. Additionally, young adults were compared against a middle-aged adults to determine the validity of convenience sampling in testing novice response to MfLs. Fifteen young adults (18–34 y.o.) and seven middle-aged adults (46–56 y.o.) were recruited to perform stepping trials while wearing progressive MfLs and blank single lens glasses. Participants stepped up and down from a 75 mm and 150 mm step in randomized order. Step placement, minimum toe clearance, lower body kinematics and stepping time were measured during step up. Step placement, minimum heel clearance, vertical forces and stepping time were measured during step down. MfLs significantly increased toe clearance in the lead and trailing legs, hip flexion, knee flexion and stepping time during step up and increased vertical forces and stepping time during step down. Step placement and hip angle explained 17% of the toe clearance variability. Changes during step up suggest a more conservative adaptation while increased forces during step down suggest a reduced level of control. No age group effects were observed, which supports the use of convenience sampling for evaluating the novice response to MfLs.     

Variations of relative anteversion of the femoral neck during walking

Based on the geometric model developed by Netter [11], we determined the different positions of the femoral neck during monopodal support in walking in relation to a fixed frontal plane of reference (relative anteversion). This “relative anteversion” ranges on average from 24° of retroversion at the beginning of support to 15° of anterversion at the end if loading. We then studied the relations possibly existing between relative anteversion and acetabular orientation on the one hand, and the orientation of the resultant of the articular stresses on the other (both being variables during monopodal support in walking). The results showed that relative anteversion is well correlated with variations of position of the acetabulum since, at most, the deviation between the respective axes did not exceed the anatomic deviation due to absolute anteversion of the femoral neck and acetabulum. Lastly, analysis of the relations obtained with the orientation of the resultant of the articular stresses allowed a better comprehension of the functional distribution of forces.     

Angular momentum synergies during walking

We studied the coordination of body segments during treadmill walking. Specifically, we used the uncontrolled manifold hypothesis framework to quantify the segmental angular momenta (SAM) synergies that stabilize (i.e., reduce the across trials variability) the whole body angular momentum (WBAM). Seven male subjects were asked to walk over a treadmill at their comfortable walking speed. A 17-segment model, fitted to the subject’s anthropometry, was used to reconstruct their kinematics and to compute the SAM and WBAM in three dimensions. A principal component analysis was used to represent the 17 SAM by the magnitudes of the first five principal components. An index of synergy (ΔV) was used to quantify the co-variations of these principal components with respect to their effect on the WBAM. Positive values of ΔV were observed in the sagittal plane during the swing phase. They reflected the synergies among the SAM that stabilized (i.e., made reproducible from stride to stride) the WBAM. Negative values of ΔV were observed in both frontal and sagittal plane during the double support phase. They were interpreted as “anti-synergies”, i.e., a particular organization of the SAM used to adjust the WBAM. Based on these results, we demonstrated that the WBAM is a variable whose value is regulated by the CNS during walking activities, and that the nature of the WBAM control changed between swing phase and double support phase. These results can be linked with humanoid gait controls presently employed in robotics.     

New technique for drug application to the spinal cord of walking mice

With the increasing availability of mutant mice that allow the conditional silencing of specific classes of interneurons in the spinal cord by drug application, a method for easily delivering drugs locally on spinal cord segments in adult animals has the potential for providing insights into the functioning of neuronal networks controlling walking. Here we describe a simple technique for this purpose. The drug is applied in high concentrations in a bath created with Vaseline walls around one to three segments of the spinal cord exposed under general anesthetic (isoflurane) combined with a strong, long-lasting analgesic (buprenorphine). After 20 min of drug application the Vaseline and the drug is removed and skin closed. We first document that the surgery and analgesic have no obvious influences on the kinematics of hind leg movements after recovery from the anesthetic, and that the analgesic enhanced locomotor activity. We then describe the influence of applying a glycine-receptor antagonist onto the lumbar segments of the spinal cord to demonstrate that the method is effective in modifying the functioning of neuronal systems in the spinal cord. Combining this method with kinematic and electromyographic recording techniques allows the detailed investigation of the effects of drugs on the walking behavior in adult mice.