"Physiological Responses to Manual Pressure in Healthy Adults"

This prospective randomized experimental study will be conducted in healthy adults to characterize acute physiological responses to different intensities and patterns of manual pressure applied to the upper trapezius muscle. Participants will be randomly allocated to one of three experimental groups:

• Group A - Graded sustained pressure: a personalized pressure scale is defined based on the individual pressure pain threshold (PPT) measured with an algometer. Five pressure levels are set at 0% (contact without pressure), 25%, 50%, 75% and 95% of PPT, and applied as sustained pressure at a single point over the upper trapezius.
• Group B - Graded longitudinal kneading: the same personalized pressure scale (0, 25, 50, 75 and 95% of PPT) is applied as longitudinal kneading distributed over a predefined area of the upper trapezius.
• Group C - Manual therapy protocols: each participant receives three standardized manual therapy protocols in separate sessions, with randomized order: manual lymphatic drainage (approx. 40 minutes at ~40 mmHg), light-pressure massage (0.5-0.8 N/cm², 20-30 minutes) and moderate-pressure massage (2-3 N/cm², 20 minutes), all targeting the upper trapezius and related regions.

In groups A and B, the five pressure levels are each applied for 2 minutes in a single experimental session, in randomized order, with rest periods of at least 5 minutes between levels (extended if needed until heart rate returns within ±10% of baseline). In group C, each manual therapy protocol is applied in a separate session with at least one day between sessions. In all groups, heart rate and heart rate variability are recorded continuously with a chest strap, and skin temperature and skin conductance are continuously monitored with surface sensors. Blood pressure, mechanical pain thresholds (von Frey filaments at predefined trapezius and forearm sites), and capillary blood samples for cortisol are obtained at baseline and immediately after each pressure level or massage protocol. In group C, VEGF is also measured in capillary blood.

In group C only, duplex ultrasound of the left common carotid artery is performed under standardized conditions (semi-recumbent position, head rotated 45° to the right) before the manual therapy protocol and 10 minutes after its completion, to assess luminal diameter, intima-media thickness and peak systolic velocity. All pressure applications in all groups are monitored with a pressure-sensing system (Loadpad) placed on the therapist's hand.

A separate visit includes whole-body dual-energy X-ray absorptiometry (DEXA) to characterize body composition (total and segmental lean mass and fat mass). The main outcome is a heart rate variability index (e.g., RMSSD), while hormonal (cortisol, VEGF in group C), sensory (pain thresholds), hemodynamic (blood pressure) and vascular (carotid ultrasound in group C) measures are considered secondary outcomes. The study is powered assuming a large effect size on HRV based on previous massage research, with a planned sample size of 30 participants per group (N = 90).

Inclusion criteria

• Age 18-65 years.
• Healthy volunteers without diagnosed cardiovascular, neurological or severe musculoskeletal disorders.
• No significant pain or pathology in the cervical, shoulder or upper back region that would interfere with pressure application.
• Ability to understand the study procedures and to provide written informed consent.
• Availability to attend the experimental session and the DEXA visit.

Exclusion criteria

• History of major cardiovascular disease (e.g., ischemic heart disease, heart failure, clinically significant arrhythmias, uncontrolled hypertension, stroke or transient ischemic attack).
• Neurological disorders affecting pain perception or autonomic function.
• Acute or chronic musculoskeletal conditions in the cervical or shoulder region that contraindicate manual pressure on the upper trapezius.
• Coagulation disorders or high-dose anticoagulant/antiplatelet therapy that increases bleeding risk.
• Dermatologic conditions or skin lesions at sensor placement or pressure application sites.
• Pregnancy or suspected pregnancy (due to DEXA scan).
• Regular use of medications with a major impact on autonomic or cardiovascular responses (e.g., beta-blockers, antiarrhythmics) that cannot be paused according to medical judgement.
• Substance abuse or acute alcohol/drug intake in the 24 hours prior to the experimental session.
• Any other condition that, in the opinion of the investigators, may compromise safety or compliance.