Cognitive-motor interference in people with multiple sclerosis: a kinematic approach to clarify the effect of cognitive load on walking performance

The simultaneous performance of cognitive tasks during locomotion (or cognitive-motor dual-task) is known to cause performance deficits in either one of, or both tasks. Furthermore, these performance decrements are exacerbated by the presence of motor impairments and cognitive dysfunctions characteristic of numerous neurological diseases, such as multiple sclerosis (MS). In this regard the assessment of walking while performing a cognitive task may represent a relevant outcome measure, because it allows measuring, in a laboratory setting, individual’s ability to cope with walking challenging situations similar to everyday living. The first aim of this thesis is to provide an experimental setup, based on the use of optoelectronic stereophotogrammetry, for obtaining quantitative evaluation of walking biomechanics and motor strategies during dual-task performance in both healthy adults and people with MS (pwMS). Then, this experimental methodology is tested as suitable method not only for detecting, measuring and characterizing disability, but also for testing intervention effectiveness in clinical practice. Specifically, the study is focused on the assessment of spatiotemporal parameters and lower limb kinematics during single- (normal pace walking) and dual-task (walking while performing a discrimination and decision-making). This thesis is composed of four experiments. The first two aimed to measure the effect of cognitive-motor interference on walking biomechanics in terms of spatiotemporal parameters and lower limb joint kinematics. In this regard, a sample of pwMS stratified by disability level (low disability, EDSS 1.0-2.5, n=37; mild to moderate disability, EDSS 3.0-6.0, n=44) and a sample of age- and gender-matched healthy adults (n=41) underwent a 3D kinematic evaluation of single- and dual-task performance using a motion capture system. Differences between conditions and groups were investigated using a two-way repeated ANOVA. The results reported that gait speed and stride length were sensitive motor variables in detecting differences from single- to dual-task condition in both pwMS and unaffected individuals, whereas spatiotemporal parameters closely related to balance control (e.g. step width, double support phase duration) were sensitive to changes only in pwMS with moderate disability. Moreover, those patients showed significant changes in the kinematics of distal joint (shank-foot) and proximal joint (hip), including a reduction in ankle plantarflexion and hip extension peak at the terminal stance phase. These observed changes in more impaired patients are compensatory mechanism to stabilize body posture and allow safe locomotion during complicate dual-task activities. Finally, the other two experiments were designed to provide a clinical application of this methodology, as a tool for quantitatively assessing biomechanics changes after an innovative therapeutic intervention. In this regard, a sample of pwMS (n=34) with mild to moderate disability participated in a bicentric clinical trial. As per protocol, pwMS completed an intervention consisting of either active or sham multiple sessions of transcranial direct current stimulation (tDCS) combined with physical activity, aimed to improve walking performance. Following repeated application of active tDCS, the results obtained from the quantitative gait analysis showed greater improvements in gait velocity, step length and walking endurance. This improvement measured in walking had corresponding effects on walking dual-task performance. In fact, the dual-task cost of gait parameters was significantly reduced after the active tDCS intervention. In conclusion, the quantitative assessment of walking impairments during the execution of functional task in pwMS can support a deep learning of both movement features and motor strategies, which should have implications for the design and validation of clinical intervention aimed at improving functional walking.