The purpose of this study is to develop a new paradigm to understand how humans physically interact with each other at a single and at multiple joints, with multiple contact points, so as to synthesize robot controllers that can exhibit human-like behavior when interacting with humans (e.g., exoskeleton) or other co-robots. The investigators will develop models for a single joint robot (i.e. at the ankle joint) that can vary its haptic behavioral interactions at variable impedances, and replicate in a multi-joint robot (i.e. at the ankle, knee, and hip joints). The investigators will collect data from healthy participants and clinical populations to create a controller based on our models to implement in the robots. Then, the investigators will test our models via the robots to investigate the mechanisms underlying enhanced motor learning during different human-human haptic interaction behaviors (i.e. collaboration, competition, and cooperation. This study will be carried out in healthy participants, participants post-stroke, and participants with spinal cord injury (SCI).

The investigators will look at how the task performance and motor performance of individuals in dyadic physical interactions are affected by 1) different behavioral interaction conditions (i.e., solo task, collaboration task, competition task, or cooperation task); 2) the haptic impedance or stiffness of the virtual connection between dyadic peers (i.e., hard connection, medium connection, or soft connection); and 3) the skill level of the other partner (i.e., novice or expert). The investigators will be using both an ankle robot (M1 device) and a bilateral lower limb exoskeleton (H3/X2 device), and will collect EMG and EEG data.

For Experiment A , the investigators will recruit healthy volunteers (n = 180) to work in dyadic pairs. With the collected data, the investigators will model how humans adapt force and impedance and share roles/specialize during various dyadic interaction behaviors, and use this knowledge to develop robot controllers that mimic movement error and force adaptation for enhanced motor performance.

For Experiment B , the investigators will recruit healthy volunteers (n = 260), participants post-stroke (n = 88) and participants post-SCI (n = 88) to work in dyadic pairs within each population. The investigators will test the robot controllers following the models for mechanical adaptation and role sharing strategies between peers based on Experiment A. The investigators will also monitor single-joint and multi-joint movement error and force adaptation in regards to enhanced motor performance. The investigators will assess if the robot controllers can pass a "haptic Turing Test", rendering them indistinguishable with respect to human peers. A structural MRI will be obtained to be used for EEG source analysis.

For Experiment C, the investigators will showcase the robot controllers by interfacing with participants post-stroke (n = 4) and participants post-SCI (n = 4) with the single-joint and multi-joint assistive robots to observe motor learning and functional outcomes with 10 training sessions per robot.

Inclusion Criteria for Healthy Participants:

• Age from 18 to 80 years
• No history of a brain and/or skull lesion
• Normal hearing and vision, can be corrected
• Able to understand and give informed consent
• No neurological disorders
• Absence of pathology that could cause abnormal movements of extremities (e.g., epilepsy, stroke, marked arthritis, chronic pain, musculoskeletal injuries)
• Able to understand and speak English
• Height between 3 foot 6 inches (1.1 meters) and 6 foot 2 inches

Inclusion Criteria for Participants Post-Stroke:

• Age from 18 to 80 years
• History of unilateral, supratentorial, ischemic or hemorrhage stroke greater than 6 month
• Ability to walk >10m independently on level ground, allowed to use assistive devices or bracing as needed
• Self-selected walking speed is less than 0.8 meters/sec
• Medically stable
• No concurrent surgeries, medical treatments, participation in research or outpatient therapy
• Normal hearing and vision, both can be corrected
• Able to understand and give informed consent
• Able to understand and speak English
• Height between 3 foot 6 inches (1.1 meters) and 6 foot 2 inches

Inclusion Criteria for Participants with Spinal Cord Injury:

• Age from 18 to 80 years
• History of incomplete SCI injury (ASIA C or D) > 6 months prior
• Ability to walk >10 m independently on level ground, allowed to use assistive devices or bracing as needed
• Self-selected walking speed is less than 0.8 meters/sec
• Medically stable
• No concurrent surgeries, medical treatments, participation in research or outpatient therapy
• Normal hearing and vision, both can be corrected
• Able to understand and give informed consent
• Able to understand and speak English
• Height between 3 foot 6 inches (1.1 meters) and 6 foot 2 inches

Exclusion Criteria for Healthy Participants:

• Weight over 220 lbs
• Pregnancy (ruled out by pregnancy questionnaire)
• Current presence of wounds or pressure ulcers
• Multiple sclerosis, Parkinson's disease or other neurodegenerative disorder, severe dementia, brain injury, spinal cord injury, or cancer of the central nervous system
• History of significant head trauma (i.e., extended loss of consciousness, neurological sequelae)
• Known structural brain lesion
• Significant other disease (heart disease, malignant tumors, mental disorders)
• History or peripheral nerve injury
• History of sustained non-prescribed drug use or substance abuse (exception: current nicotine use is allowed)
• Prior neurosurgical procedures
• Inability or unwillingness to perform study-required activities
• Prisoners
• Unable to meet SRAlab "MRI Pre-Examination Screening Form" requirements

Exclusion Criteria for Participants Post-Stroke:

• Weight over 220 lbs
• Pregnancy (ruled out by pregnancy test)
• Botox (botulinum toxin) injection to lower limbs within the prior 3 months, or planned injection during study period.
• Current presence of wounds or pressure ulcers
• History of sustained non-prescribed drug use or substance abuse (exception: current nicotine use is allowed)
• Reduced cognitive function
• Severe aphasia
• Prisoners
• Co-existence of other neurological diseases (e.g., Parkinson's disease or other neurodegenerative disorder, severe dementia, brain injury, spinal cord injury, multiple sclerosis, or cancer of the central nervous system)
• History or peripheral nerve injury
• Severe hip, knee, or ankle arthritis
• Recent fracture or osteoporosis
• Significant spasticity in the lower limbs (≥3 on Modified Ashworth Scale)
• Medical (cardiac, renal, hepatic, oncological) or psychiatric disease that would interfere with study procedures
• Inability or unwillingness to perform study-required activities
• Unable to meet SRAlab "MRI Pre-Examination Screening Form" requirements

Exclusion Criteria for Participants with Spinal Cord Injury:

• Weight over 220 lbs
• Pregnancy (ruled out by pregnancy test)
• Botox (botulinum toxin) injection to lower limbs within the prior 3 months, or planned injection during study period
• Current presence of wounds or pressure ulcers
• History of sustained non-prescribed drug use or substance abuse (exception: current nicotine use is allowed)
• Prisoners
• Co-existence of other neurological diseases (e.g., Parkinson's disease or other neurodegenerative disorder, severe dementia, brain injury, spinal cord injury, multiple sclerosis, or cancer of the central nervous system)
• History of peripheral nerve injury
• Severe hip, knee, or ankle arthritis
• Recent fracture or osteoporosis
• Significant spasticity in the lower limbs ( ≥3 on Modified Ashworth Scale)
• Medical (cardiac, renal, hepatic, oncological) or psychiatric disease that would interfere with study procedures
• Inability or unwillingness to perform study-required activities
• Unable to meet SRAlab "MRI Pre-Examination Screening Form" requirements