Patients in Arm 1 receive standard of care rehabilitation protocol using a recumbent or Nu-step bike during warm-up, followed by individualized therapeutic exercise, and cool-down protocols. The warm-up phase in the study refers to therapeutic exercise. The therapeutic exercise aims to condition and prepare patients for subsequent functional or therapeutic activities. The active comparator (Arm 1) is to ask participants to use modalities, such as a recumbent bike or a Nu-step bike during the warm-up phase of an outpatient physical therapy following a single total knee replacement.

Patients in Arm 2 will use an anti-gravity treadmill (AlterG) during warm-up, followed by individualized therapeutic exercise and cool-down protocols.

Patients in Arm 3 will use a recumbent or Nu-step bike along with the PENS neuromuscular stimulation modality during warm-up, followed by individualized therapeutic exercise and cool-down protocols.

Patients in Arm 4 will use both an anti-gravity treadmill (AlterG) and the PENS neuromuscular stimulation modality during warm-up, followed by individualized therapeutic exercise and cool-down protocols.

The intervention is to ask study participants to ambulate using an anti-gravity treadmill that integrates patented, NASA Differential Air Pressure (DAP) technology -- a precise air calibration system -- to uniformly reduce gravitational load and body weight during the warm-up phase of an outpatient physical therapy following a single total knee replacement.

The primary outcome measure is the AM-PAC Basic Mobility score. AM-PAC is a patient-reported instrument to measure functional level in 3 domains: basic mobility, daily activity, and applied cognition. For the purpose of the study, only the basic mobility domain was measured. Using item-response theory, the AM-PAC program selects the most representative questions from its extensive item bank to ask when measuring a patient's functional level. This study used the AM-PAC basic mobility paper short form designed for outpatient settings. The short-form consists of 18 questions and produces a raw score (18 to 72) transformed into a score ranging from 29.41 to 80.30 based on item-degree of difficulty. Higher transformed scores denote higher functional mobility: 34 - 51.9 = limited indoor mobility; 52 - 65.9 = enhanced indoor mobility; 66 and above = outdoor mobility.

The primary outcome measure is the AM-PAC Basic Mobility score upon discharge from outpatient therapy. AM-PAC is a patient-reported instrument to measure functional level in 3 domains: basic mobility, daily activity, and applied cognition. For the purpose of the study, only the basic mobility domain was measured. Using item-response theory, the AM-PAC program selects the most representative questions from its extensive item bank to ask when measuring a patient's functional level. This study used the AM-PAC basic mobility paper short form designed for outpatient settings. The short-form consists of 18 questions and produces a raw score (18 to 72) transformed into a score ranging from 29.41 to 80.30 based on item-degree of difficulty. Higher transformed scores denote higher functional mobility: 34 - 51.9 = limited indoor mobility; 52 - 65.9 = enhanced indoor mobility; 66 and above = outdoor mobility.