The goal of this clinical investigation is to evaluate the performance of three neuromuscular electrical stimulation (NMES) foot pad programmes in healthy adults. It will also learn about how well the NMES programmes are tolerated.

The main questions it aims to answer are:

• Do two newly-developed NMES programmes ('P1' and 'PM7') increase lower limb blood flow more than the existing 'Medic' NMES programme during use?
• How do the programmes compare in terms of user discomfort and muscle oxygenation during use?

Researchers will compare the P1 and PM7 programmes with the existing Medic programme to determine whether the new programmes produce greater improvements in lower limb haemodynamics during device use.

Participants will:

• Attend four study visits at a single research site.
• Complete 1 familiarisation visit and 3 testing visits, with 12 hours to 7 days between visits.
• Use all three NMES programmes once in a randomised order.
• Receive 30 minutes of NMES during each testing visit.
• Undergo blood flow and muscle oxygenation measurements during each test.
• Complete questionnaires about discomfort and their experience using the device.

The TRI-HEX study is a single-centre, randomised, double-blind, crossover clinical investigation evaluating the haemodynamic performance of three neuromuscular electrical stimulation (NMES) foot pad programs delivered through an investigational version of the Revitive device platform. The study will recruit 24 healthy adult participants in the United Kingdom.

Revitive is a non-invasive NMES device designed to improve blood circulation in the lower limbs during use. The investigational device (14065AT) is derived from the commercially available Revitive Medic Coach device and incorporates firmware modifications that enable delivery of additional predefined stimulation programmes for research purposes. The modification does not alter the device's intended purpose, mode of action, hardware, materials, or electrical output limits.

The study will evaluate three NMES programmes:

Medic - the currently marketed Revitive NMES foot pad programme. Programme 1 (P1) - a newly developed stimulation program designed to minimise muscle adaptation during use.

Personalised Medic v7 (PM7) - a stimulation programme that incorporates a personalised modulation function intended to optimise haemodynamic response.

The primary objective is to compare the effect of P1 and PM7 against the Medic programme on lower limb blood volume flow during a single 30-minute NMES session. Blood volume flow will be measured at the superficial femoral artery using Doppler ultrasound.

Secondary objectives are to compare:

User discomfort during stimulation using a visual analogue scale (VAS). Changes in muscle oxygen saturation (SmO2) measured by near-infrared spectroscopy (NIRS).

Exploratory objectives include assessment of:

Time averaged mean velocity (TAMV) at the superficial femoral artery. Changes in superficial femoral artery diameter. Post-stimulation recovery of muscle oxygenation. Relationships between haemodynamic outcomes and device-recorded accelerometry data.

Relationships between haemodynamic outcomes and skin hydration measurements.

The study uses a double-blinded crossover design, where each participant receives all three interventions in a randomised sequence and both participants and the Chief Investigator remain blinded to intervention allocation. Randomisation is performed using a predefined crossover allocation schedule, and an independent unblinded assessor is responsible for treatment sequence management.

Participants will attend four visits:

Visit 1: eligibility confirmation, informed consent, device familiarisation, collection of baseline demographic and lifestyle information, and a motion analysis baseline assessment.

Visits 2-4: completion of one 30-minute NMES session per visit using one of the three study programmes, with a washout period of 12 hours to 7 days between sessions.

During each testing visit, participants will undergo a 10-minute baseline measurement period, followed by a 30-minute NMES session and 8-minute post-stimulation monitoring. Doppler ultrasound and NIRS measurements will be collected throughout the assessment period. Participants will also complete questionnaires regarding discomfort and device experience.

The study is designed as an exploratory clinical investigation to generate haemodynamic performance data on newly developed NMES stimulation programmes under controlled conditions. Results will be used to assess differences in haemodynamic performance between the investigational programmes and to support future product development and evidence generation activities.

Inclusion Criteria:

• Healthy
• Age 18 or over
• Ability to communicate in English and provide informed consent

Exclusion Criteria:

• Lack of capacity to provide informed consent
• Electronic implanted device (e.g. cardiac pacemaker)
• Pregnancy or intending to become pregnant during the study period
• Currently being treated for or have symptoms of an existing deep vein thrombosis (DVT)
• Diagnosis of vascular disease or disorder
• Terminal illness
• Neurological disorders (e.g. epilepsy)
• Musculoskeletal injury/disorders (e.g. recent injury or surgery)
• Skin conditions (e.g. eczema or open wounds near plantar surface of the foot)
• Cardiovascular conditions (e.g. arrhythmia or uncontrolled hypertension)
• Diabetes mellitus with neuropathy.

Individual participant data (IPD) will not be shared because this is an early feasibility, manufacturer-sponsored exploratory investigation with a small sample size (n=24). Due to the limited study population and the potential risk of participant re-identification, de-identified participant-level datasets will not be made publicly available. Aggregate study results will be reported through ClinicalTrials.gov and any resulting scientific publications.