• Transcutaneous electrical nerve stimulation (TENS) is a frequently applied therapy in chronic pain. Although evidence regarding the effectiveness of TENS in chronic pain is inconclusive many patients continue using TENS on long term in daily practice. An important question is therefore why some patients respond well and others don't at all. In chronic pain evidence for abnormal pain processing (sensitization) has been found in several patients groups. The process of sensitization might influence effectiveness of TENS as the theoretical working mechanism of TENS is based on the modulation of the transmission of nociceptive impulses form peripheral receptors throughout spinal nerve system into the brain. In this study we want to study the effect of TENS on pain transmission, as measured by contact heat evoked potentials (CHEPS), between 'responders' and non-responders' after a two weeks TENS treatment. 'Responders' will be defined as patients with a pain reduction of ≥ 30% on a VAS after a two week treatment period with TENS. Non-responders are patients with a pain reduction < 15%.
• Objective: a) Is pain reducing effect TENS in responders based on modulation of pain transmission and perception, as measured by CHEPS? b) Is the ability of TENS to modulate pain transmission and perception influenced by abnormal pain processing?
• Prospective cohort study
• Patients with chronic non specific pain (duration > 6 months), above 18 year, will be included. Patients are referred from the Pain Clinic of the University Hospital Maastricht (MUMC). Exclusion criteria are: a) pain due to cancer, b) the use of a cardiac pacemaker, c) pregnancy, d) neurological sensory deficits, e) language and/or cognitive inability to complete the health assessment questionnaires f) previous TENS for pain relief.
• Patients receive a two week treatment period with TENS-treatment, as regular, at home after instruction. The frequency is set at 100 Hz and pulse duration at 250 μ sec. Patients have to use the TENS daily (minimal 4 times a day for 30 minutes).

Main study parameters/endpoints are decrease in amplitude of CHEPS in responders versus non-responders.

Inclusion Criteria:

• referred for TENS treatment by a pain physician,
• duration of pain > 6 months,
• age above 18,
• no current other treatment for pain besides pain medication. Patients are allowed to continue their regular pain medication (as described by physician) prior to the TENS treatment

Exclusion Criteria:

• pain due to cancer,
• the use of a cardiac pacemaker,
• pregnancy,
• neurological sensory deficits,
• language and/or cognitive inability to complete the health assessment questionnaires
• previous TENS for pain relief.

Patients with chronic musculoskeltal pain will be recruted from the Pain Clinic of the University Hospital Maastricht (MUMC)