The purpose of this study is to determine the antalgic of surface electrical stimulation with pulse frequencies and pad sizes on blunting pressure pain on healthy human due to various temporal and spatial summations of the stimulation.

Potential participants expressing interest in the study will be given to read the contraindications for the pain tests and the stimulation intervention, which has been documented on the consent form. All participants meeting entry requirements will be invited to participate in the experiments which take place on a separate occasion.

Each participant will sign the consent form and will be told again that they can withdraw from the study at any time and without explanation. Each participant will be required to experience a 10-minute pre-stimulation period, a 30 minute TENS intervention, a 30 minute follow-up period.

Participants will be seated on a chair with arm support provided by a side table. The investigator will be then informed the stimulation intervention and used pad size on the stimulation periods in written form contained in a sealed envelope. After the participant's verbal permission is obtained, the investigator will apply the pads on the dorsal forearm area.

Either the large size pad, 5cm x 10cm, or the small size,2.5cm x 2.5cm, will be choose to place on the dorsal side of the forearm to straddle the site for blunt pressure pain. The muscle bellies of the forearm are mainly located on the proximal half part. The proximal pad will be attached to the lateral epi-condyle of the elbow, and the distal electrode will be placed 3cm distal to the proximal one along the line between the lateral part of elbow and the mid-point of the wrist. The pads will be then connected to the intervention stimulator. The investigator will relay instructions to the participants using cue cards displayed 30 seconds before any required action.

Before each blunting pressure pain test, the participant will sit comfortably and place their right upper extremity on a side table with the shoulder abduction, elbow flexion, the forearm pronation. The test will be induced using a pressure algometer with a flat circular probe, measuring 1.1cm in diameter (1cm2 on skin surface). Force will be displayed digitally in increments of 0.1 Newton. The algometer will be applied to the mid-point between the intervention pads. The rate of the pressure algometer application will be around 6 Newtons per second on average. The participants will be asked to concentrate on the sensations in the forearm. When the pressure becomes definitely painful, the participant will state "Pain!" and the force will be stopped. The pain threshold will be taken twice in a minute for one test and each measurement will take no more than 30 seconds.

There will be seven 1-minute tests performed in this experiment. The first test in pre-stimulation period. The second and third tests were administered and started at 9, 19, 29 minute time points in the stimulation-on period. A 30 minute post stimulation period will be then followed. Another 3 tests will be performed in this period in ten-minute intervals at 39, 49, 59 minute time points after the stimulation start.

Inclusion Criteria:

• Healthy and aged 18 years or over

Exclusion Criteria:

• Any open wound around the arm or hand area, including edema or inflammation;
• Cardiac or vascular problems (such as coronary artery disease, coronary thrombosis, or angina pain, high blood pressures, phlebitis, and pacemaker implantation);
• Any dermatological problems or infections around the hand, forearm, or arm area (such as dermatitis or eczema, bacterial or fungal infections);
• Any history of neoplasm or malignancy;
• Bleeding diseases on the arm or hand (such as haemorrhage); Menstruation or pregnancy;
• Abnormal neurological signs in the upper limbs (such as altered skin sensations);
• Hypersensitivity or phobia to electrical application;
• Any current medication regime.

The group allocation for each participant was sealed in an envelope prepared by an investigator. Just before the start of the second visit, a second investigator opened the envelope to confirm the allocation of TENS groups for individual participants and applied the TENS. For better blinding, a third investigator assessed the blunt pressure pain threshold (BPPT) for the baseline value.