Spinal Excitation to Enhance Mobility

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, the investigators are initiating a new line of research with the goal of enhancing walking function in older adults 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'). The investigators will use a previously established electrode montage to deliver excitatory tsDCS to the lumbosacral spinal cord during practice of a complex obstacle walking task. The investigators 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. The investigators 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) dosage "A" tsDCS with smooth insoles (active/smooth); 2) dosage "B" tsDCS with smooth insoles (sham/smooth); 3) dosage "A" tsDCS with textured insoles (active/textured); and 4) dosage "B" tsDCS with textured insole (sham/textured). Participants will be blinded to group assignment. While receiving stimulation, participants will engage in walking practice over a standardized obstacle course. Immediately prior to and following the practice, each participant will be assessed while walking over the course. Practice-related gains in performance will be quantified primarily by fastest safe walking speed. Retention of performance gains will also be assessed at a separate later visit. 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 older adults.

Inclusion criteria

• Preferred 10m walking speed < 1.0 m/s
• Intact tactile sensation based on two-point discrimination
• Willingness to be randomized to either intervention and to participate in all aspects of study assessment and intervention

Exclusion criteria

• Diagnosed neurological disorder or injury of the central nervous system, or observation of symptoms consistent with such a condition

• spinal cord injury
• Alzheimer's
• Parkinson's
• stroke, etc.
• Contraindications to non-invasive spinal stimulation including any prior spinal surgical procedure
• Chronic lower back pain
• Obesity, defined as Body Mass Index exceeding 30.

• This is due to the potential influence of body fat on the amplitude of electrical current flow to the spinal cord.
• Use of medications affecting the central nervous system including, but not limited to:

• benzodiazepines
• anti-cholinergic medication and GABAergic medication
• Severe arthritis, such as awaiting joint replacement
• Current cardiovascular, lung or renal disease
• Diabetes
• Terminal illness
• Myocardial infarction or major heart surgery in the previous year
• Cancer treatment in the past year, except for nonmelanoma skin cancers and cancers having an excellent prognosis

• early stage breast or prostate cancer
• Current diagnosis of schizophrenia, other psychotic disorders, or bipolar disorder
• Difficulty communicating with study personnel

• including people who cannot speak English
• Uncontrolled hypertension at rest (systolic > 180 mmHg and/or diastolic > 100 mmHg)
• Bone fracture or joint replacement in the previous six months
• Current participation in physical therapy for lower extremity function or cardiopulmonary rehabilitation
• Current enrollment in any clinical trial
• Planning to relocate out of the area during the study period
• Clinical judgment of investigative team