Physiological Responses to Upper Limb NMES

Exploring Local Physiological Changes in the Arm During NMES Using the Firefly® Device

The primary objective of this study is to explore the application of the Firefly® device for delivering NMES to anatomically defined regions of the upper limb and to observe associated physiological changes. Specifically, the study will assess alterations in arterial and venous blood flow, as well as local muscle oxygen saturation, as potential indicators of muscle perfusion during NMES.

Participants will undergo NMES using the Firefly® device, applied to two predefined regions of the upper limb: one targeting the anterior arm nerve bundle (a collective term referring to motor and sensory nerves within the anterior and medial compartments of the upper arm, including the musculocutaneous and adjacent (median, medial cutaneous, ulnar and radial) nerves, and the other targeting the median nerve in the proximal forearm.

Vascular responses will be assessed at two sites, the brachial artery and axillary vein, using Doppler ultrasound. For each nerve-vascular site pairing, NMES will alternate between stimulation OFF and ON conditions (10 OFF and 10 ON cycles), comprising one stimulation block of approximately 20 minutes. Each participant will complete four stimulation blocks in total.

During both ON and OFF phases, blood flow volume and haemodynamic parameters, including peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive index (RI), will be recorded. Additionally, local muscle oxygen saturation (SmO₂) will be continuously monitored using functional near-infrared spectroscopy (fNIRS) to assess tissue-level physiological changes.

The total study participation time per individual is approximately 2 hours.

Inclusion Criteria:

• Aged 18 to 45 years (inclusive)
• No known history of vascular or neurological conditions affecting the upper extremities
• Able to read, understand, and communicate in English to ensure informed consent and safe participation in the study

Exclusion Criteria:

• Presence of any condition that contraindicates the use of neuromuscular electrical stimulation (e.g., implanted pacemaker, epilepsy)
• Current or recent injury to the upper limbs that may interfere with stimulation or measurement
• Anatomical variations in the brachial artery or axillary vein, specifically vascular branching outside expected anatomical locations, as identified by ultrasound, which may impair accurate blood flow measurement or device placement
• Currently participating in another interventional or physiological research study or having done so within the past 30 days.
• Currently pregnant