The investigators seek to determine whether ankle exoskeletons can reduce metabolic energy expenditure during walking for users across the age-spectrum.

Older adults walk with greater metabolic rates than young adults. Growing evidence suggests that the greater older adult metabolic rates are related to the structural properties of their lower leg tissues. The tendons of the leg of older adults are more compliant than that of young adults. Accordingly, older adult leg tendons stretch more under a given load, such as during walking, causing their muscles to operate at shorter, less optimal lengths, and higher activity levels than the muscles of young adults - a less economical way to produces force.

Thus, the investigators seek to examine whether wearing wearable robotic boots (i.e., ankle exoskeletons) could enable muscles to produce force more economically. By adding an exoskeleton in-parallel to the ankle, the investigators hypothesize that older adults will walk with lower whole-body metabolic rate than without the exoskeleton assistance.

In this study, the investigators will have both young and older adult participants walk on a treadmill with a commercially available ankle exoskeleton set in multiple assistance modes. During these trials, the investigators will measure the metabolic cost of walking in young and older adults and also take many physiological and biomechanical measurements to help assess how exoskeletons work to reduce walking effort.

Inclusion Criteria:

• Subjects must be able to walk for 60 minutes in a 90-minute time frame.
• Subjects are apparently free of cardiovascular, metabolic, and renal disease, which includes no signs or symptoms suggestive of cardiovascular, metabolic or renal disease.
• Subjects have no current musculoskeletal injury.
• Subjects need to be either 18-45 or 65+ years old.

These criteria meet the American College of Sports Medicine's 2015 guidelines for participant health screening prior to joining a moderate or moderate-to-vigorous exercise protocol. (Riebe et al., 2015).

Exclusion Criteria:

• Have dementia or an inability to give informed consent
• Have a musculoskeletal injury or feel pain while walking
• Have a history of dizziness and/or balance problems
• Have cardiovascular, heart, metabolic, or renal disease, or respiratory problems
• Smoke cigarettes
• Asthma
• Feel pain or discomfort in the chest, neck, jaw, arms during rest or exercise
• Have orthopnea or paroxysmal nocturnal dyspnea
• Have ankle edema
• Have palpitations or tachycardia
• Have a heart murmur
• Have had a heart attack
• Have diabetes
• Have a pace maker
• Have unusual shortness of breath with usual activities
• Are <18 or 46-64 years of age
• Do not speak or understand English

4 months