Spinal excitation to enhance mobility in elderly adults

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It is well known that age-related impairments of the brain and peripheral nerves contribute to a decline in walking function. Age-related impairment of the spinal cord is also a likely contributing factor, as the literature describes a variety of changes in spinal cord structure and function with aging. Specifically, the elderly spinal cord is less excitable, conducts signals more slowly, and is subject to neural noise. Therefore, we are initiating a new line of research with the goal of enhancing walking function in elderly Veterans by intervening on age- related neural impairment of the spinal cord. The objective of the proposed study is to establish the feasibility, preliminary efficacy, and variance of response for using transcutaneous spinal direct current stimulation (tsDCS) and textured shoe insoles to excite spinal locomotor circuits and enhance practice-related performance and retention on an obstacle walking task. Enhanced practice and retention effects will support future efforts to translate this approach into a longer term rehabilitation intervention. Excitatory tsDCS is a non-invasive neuromodulation approach in which a relatively weak electrical current is delivered to the desired region of the spinal cord via electrodes placed on the skin. The electrical current does not cause discharge of action potentials, but rather is designed to bring neurons closer to their discharge threshold by inducing a sub-threshold depolarization of membrane potentials. When combined with a behavioral task, tsDCS has the potential to upregulate neural circuits in a task-specific manner and promote Hebbian neuroplasticity (‘fire together, wire together’). We will use a previously established electrode montage to deliver excitatory tsDCS to the lumbosacral spinal cord during practice of a complex obstacle walking task. We also propose to combine the use of textured shoe insoles with tsDCS. This combinatorial approach may be a potent strategy for simultaneously optimizing spinal responsiveness to input from both descending and ascending excitatory signals to spinal centers of locomotor control. One anticipated benefit of increasing the excitation of spinal locomotor circuits is a reduction in the executive demand of walking, as measured by prefrontal cortical activation. Our research shows that elderly adults rely heavily on compensatory executive control while walking. This is widely considered to be a risk factor for adverse outcomes including falls. We propose a parallel groups study design in which 40 older adults who have walking deficits and who demonstrate a compensatory executive locomotor control strategy will be randomized into one of four groups: 1) active tsDCS with smooth insoles (active/smooth); 2) sham tsDCS with smooth insoles (sham/smooth); 3) active tsDCS with textured insoles (active/textured); and 4) sham tsDCS with textured insole (sham/textured). Participants will be blinded to group assignment. While receiving stimulation, participants will engage in 20 minutes of walking practice over a standardized obstacle course. Immediately prior to and following the practice, each participant will be assessed while walking over the course at their fastest safe pace. Practice- related gains in performance will be quantified by walking speed and other biomechanical metrics. Retention of performance gains will also be assessed at a separate visit 2 days later. tsDCS-induced changes in spinal excitability will be assessed by measuring soleus H-reflex. Executive demand of walking will be assessed as prefrontal cortical activation, measured with functional near infrared spectroscopy (fNIRS). Intervening on age- related impairment of the spinal cord to improve walking function is a promising but untapped area of research. The proposed intervention techniques are low cost and translatable to real-world settings, which enhances the potential long term impact of this work on the well-being of aging Veterans.