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