INCLUSION AND EXCLUSION CRITERIA  All potential subjects will be evaluated by research staff in order to match them to the inclusion and exclusion criteria that has been established - see below:  Inclusion Criteria - Able bodied Subjects:  Healthy, able-bodied subject Age constrained from 18-70 years old No injury to either upper or lower extremity or history of back pain English speaking.  Exlusion Criteria - Able bodied Subjects:  Waist circumference greater than 125 cm Pregnant women (status determined by self-reporting) Co-morbidity that interferes with the study (e.g. significant arthritis or joint problems, history of back injury, neuromuscular disorders, stroke, epilepsy, etc.) Individuals currently on anti-coagulants. Inactive, physically unfit Severe Osteoporosis (status determined by self-reporting) Non-English speaking Cognitive deficits or visual impairments (MMSE score <17) that would impair their ability to give informed consent or impair their ability to follow simple instructions during the experiments  Inclusion Criteria - Fall Risk Subjects:  Ages between 18-85 years old Individuals diagnosed with a Stroke (> 6 months post), Parkinson's disease, aging elderly (ages 60-85)or lower-limb amputee with at least one self-reported fall in the last six months. Able to sit unsupported, walk at least with an assistive device and be able to follow a three-step command. For individuals with Parkinson's disease, Scoring 1 or higher on questions in Section II (Activities of Daily Living) and Section III (Motor examinations) on the Unified PD Rating Scale (UPDRS), be able to walk at least with an assistive device and be able to follow a three-step command. Waist circumference between 90 and 125 cm Either homebound or community ambulators. Willing to carry and use a smartphone and Airbag device. Willing to wear the airbag system as directed by the research personnel. English speaking Able and willing to give written consent and comply with study procedures.  Exclusion Criteria - Fall Risk Subjects  Serious cardiac conditions, any musculoskeletal disorder, or other comorbidities that would interfere with participation in this minimal risk study. Non-healing ulcers of a lower extremity, Renal dialysis or end-stage liver disease, Legal blindness or severe visual impairment, a history of significant psychiatric illness. Subjects reporting a head injury from exposure to a blast/concussion injury with one or more of the following symptoms: dizziness, vertigo, headache, migraine, oscillopsia, movement induced vertigo, imbalance. Individuals who use a wheelchair for mobility both outdoors and indoors. Waist circumference greater than 125 cm Non-English speaking individuals Severe Osteoporosis (status determined by self-reporting,medical records) The subject is pregnant, nursing or planning a pregnancy. Individuals currently on anti-coagulants. Cognitive deficits or visual impairments (MMSE score < 17) that would impair their ability to give informed consent or impair their ability to follow simple instructions during the experiments