Rehabilitation is one of the few areas of medicine where the patient does most of the work and the device mostly has to not get in the way. That sounds easy and it is not. A physical therapy device applies real mechanical force to an injured body, is used by a person whose strength and coordination are impaired, is operated by a therapist with fifteen minutes per patient, and increasingly has to work unsupervised in a living room. Getting it wrong causes reinjury; getting the business model wrong is even more common.
Is it a medical device, or exercise equipment?
The first fork is regulatory and it is decided almost entirely by your claims. A resistance band or a stationary bike sold for general fitness is not a medical device. The same hardware sold with a claim that it treats a condition, restores function after surgery, or is prescribed by a clinician usually is. Marketing copy, not mechanics, moves the product across the line, and teams cross it accidentally in a launch video. Our article on whether your product is a medical device covers the FDA definition test.
Choose deliberately. A general wellness positioning reaches market fast with no submission, but locks you out of clinical channels, reimbursement, and any therapeutic claim. A device positioning costs time and money up front and opens the only durable market in this space.
The regulatory map for common rehab categories
| Device type | Typical FDA class | Usual route |
|---|---|---|
| Powered exercise equipment, therapeutic massagers | Class I or II | Often exempt or straightforward 510(k) |
| Continuous passive motion devices | Class II | 510(k) with a predicate |
| TENS and powered muscle stimulators | Class II | 510(k), plus IEC 60601-2-10 testing |
| Therapeutic ultrasound units | Class II | 510(k), plus acoustic output characterization |
| Powered lower-limb exoskeletons | Class II | Established via De Novo, now with predicates |
| Prescribed home rehab software with therapy claims | Class II | Treated as software as a medical device |
Most rehab devices are Class II and reach the market by demonstrating substantial equivalence to a predicate. That makes predicate selection the single highest-leverage decision in the program, since the predicate defines the performance testing you will owe. Our guides to FDA medical device classes and the 510(k) submission process cover the mechanics. If your concept has no reasonable predicate, expect a De Novo request and a considerably longer timeline.
Engineering around a patient who cannot protect themselves
The defining hazard in rehab hardware is that the device can move a limb further or harder than the tissue tolerates, and the patient may not be able to stop it. That drives an architecture with layered protection rather than a single software limit:
- Mechanical hard stops that physically bound range of motion, set independently of any firmware value.
- Torque limiting through a slip clutch, series elastic element, or current limit enforced in hardware, not only in the control loop.
- A patient-accessible stop that removes power and releases the limb, operable with limited grip strength.
- Back-drivability so a therapist can move the mechanism by hand if power is lost, which rules out high-ratio worm gearing in most joints.
- Fail-safe behavior where power loss releases rather than clamps.
Sizing the actuation is where prototypes most often disappoint. Peak torque must account for the limb's mass and moment arm, spasticity or involuntary resistance, mechanism friction, and a margin for the heaviest patient in your indicated population. Then check duty cycle, because a motor sized for peak torque frequently overheats across a forty-minute session. Our guide to calculating the motor torque your product needs walks through the arithmetic.
Cycle life is the other quiet killer. A clinic device may see twenty patients a day for years, so bearings, cables, straps, and upholstery need life testing at realistic loads, and wear parts should be swappable by clinic staff without a service call.
Electrical safety, applied parts, and stimulation
Anything mains-powered that touches a patient falls under IEC 60601-1: applied part classification, leakage currents, creepage and clearance, and mechanical hazards including entrapment and crushing. Our overview of IEC 60601 electrical safety testing covers the general standard.
Stimulation devices add IEC 60601-2-10, constraining output waveform, charge per pulse, current density at the electrode, and behavior when an electrode lifts. Skin burns under stimulation electrodes are a recurring adverse-event theme, so electrode area, contact impedance monitoring, and automatic shutdown on poor contact belong in the design inputs. Ultrasound therapy devices need verified acoustic output with a documented effective radiating area.
The clinic and the living room are different products
A clinic unit is used by a trained therapist, wiped down between patients, plugged into a grounded outlet, and expected to run all day. A home unit is used by a patient alone, cleaned rarely, powered from an unknown outlet, and stored in a closet. It has to survive misuse, guide setup without a therapist present, detect an incorrectly performed session, and meet the harsher home-healthcare EMC levels. Our article on home-use medical devices covers what changes when the operator is not a clinician.
Usability validation should reflect that. Run summative testing under IEC 62366 with people who match your indicated population, including the range of impairment you claim to serve, not with healthy staff. Related considerations for permanent rather than temporary limitations are in our piece on assistive device development.
Reimbursement is the business model
In rehab, the sale is usually to a clinic, a hospital system, or a payer, and the question they ask is not whether the device is clever but whether it gets paid for. Three routes exist and they behave very differently:
- Capital purchase by a clinic, justified by throughput. If one therapist can supervise two patients instead of one, that is quantifiable payback and the easiest argument to make.
- Durable medical equipment dispensed to a patient, requiring an appropriate HCPCS code and a payer policy that covers it. Establishing a new code is slow and should never be assumed.
- Cash pay or subscription, which avoids coding entirely but caps the market to what patients fund themselves.
Decide the route before you finish the design, because it changes the feature set. A throughput argument needs session logging and multi-patient management; a payer argument needs objective outcome data captured by the device. Retrofitting either after the evidence is collected is expensive.
Projects House develops rehabilitation and physical therapy devices from mechanism concept through IEC 60601 testing and FDA submission, including the actuation sizing and human factors work that decides whether a clinic keeps using the product. Tell us what you are building through our contact form.