J.E.J. Duysens

Step characteristics during obstacle avoidance in hemiplegic stroke

Whereas several animal studies have indicated the important role of the motor cortex in the control of voluntary gait modifications, little is known about the effects of cortical lesions on gait adaptability in humans. Obstacle avoidance tasks provide an adequate paradigm to study the adaptability of the stepping pattern under controlled, experimental conditions. In the present study, an exploratory assessment was made of the failure rate, the preferred stepping strategies (step lengthening vs step shortening), and the spatiotemporal stride characteristics (percentage increases in stride length, duration, and velocity of the crossing and postcrossing strides) during obstacle avoidance in 11 hemiplegic stroke patients and seven healthy controls. Patients were less successful in avoiding obstacles than controls (14% failure rate vs 0.5% in controls), independent of whether the affected or the unaffected leg led the obstacle avoidance. The number of failed trials increased systematically when the available response time became shorter. During successful trials, lengthening of the step was generally preferred over shortening. This bias towards step lengthening was more pronounced in stroke patients (step lengthening in 91% of the trials vs 75% in controls), irrespective of the side of obstacle presentation. For both groups, overall strategy preference did not adhere to a principle of minimal foot displacement, since step lengthening was used even if it would be more spatially efficient to shorten the step. No statistically significant group differences were found for the increases in length, duration, and velocity of the crossing and postcrossing strides. However, for a subgroup of more slowly walking patients, large percentage increases were found in crossing stride length, duration, and velocity. Similar results were obtained for the postcrossing stride, indicating that, for this subgroup of patients, restoration of the normal walking cadence was more difficult. Overall, no systematic differences were found between the affected and the unaffected leg in stroke patients with respect to failure rates, stepping strategies, or spatiotemporal measures of obstacle avoidance. The present findings suggest that the ability to adequately modify the stepping pattern in response to imposed spatiotemporal constraints is impaired in persons with stroke, especially when modifications have to be performed under time pressure. In addition, the stepping strategies employed by subjects with stroke are different from those found in controls, possibly to reduce the complexity of the avoidance maneuver and to enhance safety. Finally, unilateral cortical damage results in an impaired ability to avoid obstacles on both sides of the body, suggesting that the reduced ability of stroke patients to negotiate obstacles may be related to problems of a more general coordinative nature.

Older women strongly prefer stride lengthening to shortening in avoiding obstacles.

In the present study the obstacle avoidance strategy during treadmill walking was investigated in ten young (aged 19–32) and ten older females (aged 65–78). Minimisation of displacement of the foot from its original landing position has been proposed to be the main criterion for the selection of alternate foot placement. Each participant performed 60 obstacle avoidance trials. Foot–obstacle configurations were varied in order to obtain both lengthening and shortening avoidance reactions. For each trial it was calculated how much lengthening and how much shortening of the stride was required minimally for successful avoidance. The difference between required lengthening and required shortening was expressed as a percentage of the control stride length and was used as a measure of minimal displacement. The behavior of young females was in agreement with the minimal displacement criterion. The older females, however, exhibited a strong preference for stride lengthening, even in situations in which stride shortening would be highly favorable. The explanation for the long step strategy preference of the older females is discussed in terms of age-related changes in decision-making, differences between young and older persons in the unobstructed gait pattern, and safety considerations.

Cutaneous reflexes evoked during human walking are reduced when self-induced

Reflex responses are often less pronounced when they are self‐induced, but this question has barely been investigated quantitatively. The issue is particularly relevant for locomotion since it has been shown that reflexes elicited during normal gait are important for the regulation of locomotion. The cortex is thought to be involved in the control of reflexes during gait, but it is unclear whether it plays a role in the modulation of these reflexes during the step cycle. During gait, weak electrical stimulation of the sural nerve elicits reflexes in various leg muscles. Are these reflexes different when subjects themselves trigger the stimuli instead of being randomly released by the computer? Cutaneous reflexes were elicited by sural nerve stimulation in 16 phases of the gait cycle in healthy subjects. The stimuli were triggered either by computer or by the subjects themselves. In 6 out of 7 subjects it was observed that the facilitatory responses in leg muscles were smaller and the suppressive responses were more suppressive following self‐generated stimuli. In some muscles such as tibialis anterior (TA) both effects were seen (reduced facilitation at end stance and exaggerated suppression at end swing). In all subjects the modulation of anticipatory influences was muscle specific. In the main group of six subjects, the mean reduction in reflex responses was strongest in the TA (max. 30.7%; mean over 16 phases was 12.5%) and weakest in peroneus longus (PL, max. 10.1%; mean over 16 phases was 2.6%). The observation that facilitation is reduced and suppression enhanced in several muscles is taken as evidence that anticipation of self‐induced reflex responses reduces the excitatory drive to motoneurones, for example through presynaptic inhibition of facilitatory reflex pathways.

Startle responses in Parkinson patients during human gait.

Falls frequently occur in patients with Parkinson’s disease (Bloem et al. 2001). One potential source for such falls during walking might be caused by the reaction to loud noises. In normal subjects startle reactions are well integrated in the locomotor activity (Nieuwenhuijzen et al. 2000), but whether this is also achieved in Parkinson patients is unknown. Therefore, in the present study, the startle response during walking was studied in eight patients with Parkinson’s disease and in eight healthy subjects. To examine how startle reactions are incorporated in an ongoing gait pattern of these patients, unexpected auditory stimuli were presented in six phases of the step cycle during walking on a treadmill. For both legs electromyographic activity was recorded from biceps femoris and tibialis anterior. In addition, we measured the stance and swing phases of both legs, along with the knee angles of both legs and the left ankle angle. In all subjects and all muscles, responses were detected. The pattern of the responses, latency, duration, and phase-dependent modulation was similar in both groups. However, the mean response amplitude was larger in patients due to a smaller habituation rate. No correlation was found between the degree of habituation and disease severity. Moreover, a decreased habituation was already observed in mildly affected patients, indicating that habituation of the startle response is a sensitive measure of Parkinson’s disease. The results complement the earlier findings of reduced habituation of blink responses in Parkinson’s disease. With respect to behavioral changes in healthy subjects we observed that startle stimuli induced a shortening of the step cycle and a decrease in range of motion. In the patient group, less shortening of the subsequent step cycle and no decrease in range of motion of the knee and ankle was seen. It is argued that the observed changes might contribute to the high incidence of falls in patients with Parkinson’s disease.

Children with spastic hemiplegia are equally able as controls in maintaining a precise percentage of maximum force without visually monitoring their performance

In this study the hypothesis was tested that children with spastic hemiplegia rely more on externally guided visual feedback when trying to keep force constant with their affected hand (AH) as compared to their non-affected hand (NAH) and as compared to controls. An isometric force task in which a cursor had to be moved to a visually specified target that disappeared half way the task, was performed by 19 children with cerebral palsy (CP), spastic hemiplegia, aged between 5 and 16 years and an aged matched control group. It was found that the absolute deterioration of performance after withdrawal of target visualization did differ between AH, NAH and controls. The absolute error was smaller and the variability was larger in the hemiplegic hand. However, the normalized force error and co-efficient of variation increased similarly between groups. Furthermore, power spectrum density analysis of the force signal showed that both hands in both groups had a similar loss in the energy in the 2-3 Hz range when target visualization was removed. These results suggest that CP children are equally able to produce stable force without visually monitoring their performance than children without CP, provided they are allowed to operate within their own force range.

Lopen op je ruggenmerg

Lopen doen we vaak automatisch, reflexmatig, ‘op ons ruggenmerg’. Bij proefdieren is aangetoond dat automatisch lopen te maken heeft met schakelingen in het ruggenmerg (centrale patroongenerator) en met toevoer naar die schakelingen van de juiste sensorische informatie (bijvoorbeeld over de beenbelasting tijdens lopen). Men denkt dat bij de mens het automatisch lopen op dezelfde principes berust. Bij mensen met een onvolledige dwarslaesie is het daarom mogelijk deze schakelingen te oefenen met de geschikte sensorische informatie zodat er een verbetering in loopvaardigheid kan optreden (loopbandtraining).In de neurorevalidatie draait herstel vaak primair om herstel van motoriek, in het bijzonder loopmotoriek. Een afwijkend looppatroon is voor veel artsen een aanduiding voor een bepaalde aandoening. Voor patiënten is lopen erg belangrijk. Voor hen is vaak het belangrijkste moment in de revalidatie de overstap van een rolstoel naar zelfstandig kunnen lopen. ‘Kun je alweer lopen?’ is een van de eerste vragen die veel patiënten te horen krijgen. In dit artikel staat lopen en herstel van lopen dan ook centraal. Er zal telkens worden aangeven wat we verworven hebben aan nieuwe basale kennis en hoe die kennis vertaald kan worden naar de praktijk van de preventie en patiëntenzorg.

Veroudering van het bewegingsapparaat: over struikelen, vallen en uitglijden

Struikelen, uitglijden en vallen zijn gedragingen die maar al te vaak voorkomen bij patiënten in de neurorevalidatie. De laatste jaren zijn er heel wat studies opgezet met het doel beter te begrijpen wat er precies gebeurt bij dit soort kleine ongevallen met dikwijls ernstige gevolgen.Globaal kan gezegd worden dat er voor de neurorevalidatie sprake is van een heel nieuw werkterrein. Vroeger stelde men zich tevreden met het bestuderen van functionaliteit in termen van onverstoord lopen of staan (zie ook Duysens & Smits-Engelsman, 2002). Maar tegenwoordig is het duidelijk dat dit te beperkt is, omdat patiënten in het dagelijks leven worden geconfronteerd met veel complexere situaties. Met de huidige nieuwe richting in het bewegingsonderzoek wordt veel dichter aangesloten bij die complexe situaties. Als vallen wordt veroorzaakt door struikelen en uitglijden dan is het nodig om te begrijpen hoe patiënten struikelen en uitglijden. We moeten een studie ontwerpen in het laboratorium om te onderzoeken hoe patiënten reageren tijdens die handelingen en waarom die reacties eerder tot een val leiden dan bij gezonde proefpersonen.