Inclusion Criteria:
Enrollment Criteria for Participant with TBI (TBI+ group) Inclusion criteria
* Age 50-75. People of this age normally do not exhibit substantial balance or cognitive impairments, so observing a clear deficit between groups will provide compelling evidence of a TBI-related effect. This "younger old" age group will also help to avoid other comorbid health conditions of older age that would increase variability (uncertainty) in the data set.
* Prior history of moderate closed-head TBI at least 10 years prior to study enrollment. Single or multiple TBIs are acceptable. All participants who self-report a prior TBI will complete a standardized interview called the Ohio State University TBI Identification Method (OSU TBI-ID) to characterize and confirm the occurrence of a prior moderate TBI. Moderate TBI will be defined as any of the following:
* loss of consciousness from 30 minutes to 24 hours
* and/or alteration of consciousness/mental state for greater than 24 hours
* and/or post-traumatic amnesia for \>1 to \<7 days
* and/or abnormal structural imaging confirmed by medical records
* Affirmative response to the question: "At the present time, do you consider yourself to be fully recovered from the TBI?"
* Score on the Activities Specific Balance Confidence Scale of 90 or higher (out of 100 possible points), which indicates absence of self-reported balance/mobility deficits.
* Score of 19 or higher on the telephone version of the Montreal Cognitive Assessment, which indicates absence of major cognitive impairment.
* living in the community and able to travel to the research site
Exclusion Criteria:
Enrollment Criteria for Participant with TBI (TBI+ group) Exclusion criteria
* Neurological injury or disease other than prior history of moderate TBI
* Current clinically significant post-traumatic stress disorder (PCL score \>30)
* severe arthritis, such as awaiting joint replacement, that would interfere with participation balance/mobility tasks
* Current substance abuse
* Current uncontrolled major depressive episode, history of severe psychiatric illness unrelated to TBI (e.g., bipolar 1 or schizophrenia).
* severe obesity (body mass index \> 35)
* unstable cardiovascular disease (for example, recent angina or uncontrolled high blood pressure)
* lung disease requiring use of supplemental oxygen
* renal disease requiring dialysis
* serious uncontrolled 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 (e.g., early stage breast or prostate cancer)
* uncontrolled hypertension at rest (systolic \> 180 mmHg and/or diastolic \> 100 mmHg)
* bone fracture in the previous year
* hip joint replacement or metal implants that would preclude accurate assessment of lean mass or hip bone mineral density
* Use of any pharmacologic agents (within past 180-days) that are known to influence BMD, including anti-resorptive or bone anabolic therapies, any compounded or over-the-counter androgenic hormone or androgen precursor, clomiphene, aromatase inhibitors, anti-estrogen or estrogen treatment, or growth hormone?
* Chronic use of systemic glucocorticoids \>7.5 mg/d prednisone equivalent (e.g., hydrocortisone 30 mg, methylprednisolone 6 mg, or dexamethasone 1.2 mg)
* current participation in physical therapy for lower extremity function
* current enrollment in a clinical trial that might jeopardize safety or scientific integrity of either trial
* unable to communicate sufficiently with study personnel, and/or non-English speaking
* other medical conditions other that would adversely affect balance, cognition, or oculomotor function.
* clinical judgment of investigative team regarding safety or non-compliance.
