An Investigation Into the Magnitude of the Current Window and Perception of Transcutaneous Electrical Nerve Stimulation (TENS) Sensation at Various Frequencies and Body Sites in Healthy Human Participants Nicola Hughes, MB,*w Michael I. Bennett, MB ChB, MD, FRCP, FFPMRCA,zy and Mark I. Johnson, PhD, BSc*z Introduction: Strong nonpainful transcutaneous electrical nerve stimulation (TENS) is prerequisite to a successful analgesic out- come although the ease with which this sensation is achieved is likely to depend on the magnitude of current amplitude (mA) between sensory detection threshold (SDT) and pain threshold, that is, the current window. Objectives: To measure the current window and participant’s per- ception of the comfort of the TENS sensation at different body sites. Methods: A repeated measure cross-over study was conducted us- ing 30 healthy adult volunteers. Current amplitudes (mA) of TENS [2 pulses per second (pps); 30 pps; 80 pps] at SDT, pain threshold, and strong nonpainful intensities were measured at the tibia (bone), knee joint (connective tissue), lower back [paraspinal (skeletal) muscle], volar surface of forearm (nerve) and waist (fat). The am- plitude to achieve a strong nonpainful intensity was represented as a percentage of the current window. Data were analyzed using repeated measures analysis of variance. Results: Effects were detected for body site and frequency for SDT (P < 0.001, P = 0.018, respectively), current window (P < 0.001, P < 0.001, respectively), and strong nonpainful TENS as a percent- age of the current window (P = 0.002, P < 0.001, respectively). The current window was larger for the knee joint compared with tibia (difference [95% confidence interval] = 12.76 mA [4.25, 21.28]; P = 0.001) and forearm (10.33 mA [2.62, 18.40]; P = 0.006), and for the lower back compared with tibia (12.10 mA [1.65, 22.52]; P = 0.015) and forearm (9.65 mA [1.06, 18.24]; P = 0.019). The current window was larger for 2 pps compared with 30 pps (P < 0.001) and 80 pps (P < 0.001). Participants rated strong non- painful TENS as most comfortable at the lower back (P < 0.001) and least comfortable at the tibia and forearm (P < 0.001). Conclusions: TENS is most comfortable and easiest to titrate to a strong nonpainful intensity when applied over areas of muscle and soft tissue. Key Words: transcutaneous electric nerve stimulation (TENS), pain measurement, sensory detection threshold, pain threshold, hypo- algesia, nontherapeutic human experimentation (Clin J Pain 2013;29:146–153) T ranscutaneous electrical nerve stimulation (TENS) is a noninvasive, inexpensive, and self-administered analgesic technique. 1,2 In clinical practice TENS is administered to selectively activate low-threshold peripheral afferent nerves by titrating intensity to produce a strong yet nonpainful TENS sensation at the site of pain (ie, conventional TENS). A meta-analysis of 21 randomized controlled trials demon- strated superiority of strong nonpainful TENS administered at the site of postoperative pain compared with TENS ad- ministered at insufficient intensities away from the pain. 3 Strong nonpainful TENS raises pain threshold (PTh) to a greater extent compared with sensory threshold TENS or no current control for experimentally induced mechanical stimuli 4–6 and CO 2 laser stimuli. 7 Animal studies demonstrate that TENS needs to be delivered in the receptive field to reduce ongoing transmission of central nociceptive neurons 2,8 and human studies show that TENS at ipsilateral sites is superior to TENS at contralateral sites. 5,9 In clinical practice TENS produces pain relief for some patients and not for others, and this has led to uncertainty about efficacy and effectiveness. 9 Inadequate pain relief during TENS may be due to difficulty in titrating current amplitude to achieve a strong nonpainful TENS sensation. This depends on the range of the current (mA) between sensory detection threshold (SDT; the point at which a user perceives the first sensation of TENS) and PTh (the point at which a user perceives TENS to be painful), that is, the therapeutic current window. The precision of titrating current amplitude to a strong nonpainful level will be re- duced if the current window is small. Studies using quan- titative sensory testing procedures have demonstrated that sensitivity and discriminative ability toward different stim- uli vary according to body site. 10–12 There has been much research on the effect of TENS frequency, pattern, width, and waveform using healthy humans exposed to experimentally induced pain. 13–15 Ap- propriate electrode placement is also necessary to maximize TENS effects; however, studies that investigate electrode placement are few and tend to find no difference in pain outcome between sites. 16–18 Recently, Moran et al 6 meas- ured pressure pain threshold in healthy human volunteers receiving TENS at strong nonpainful (mean ± SD = 39.13 ± Received for publication March 4, 2011; revised October 11, 2011; accepted March 27, 2012. From the *Faculty of Health and Social Sciences, Leeds Metropolitan University; wSt James’s Institute of Oncology, St James’s University Hospital; zLeeds Pallium Research Group; and yLeeds Institute of Health Sciences, School of Medicine, University of Leeds, Leeds, UK. This work was carried out in the Faculty of Health and Social Sciences at Leeds Metropolitan University, UK. No commercial party has or will confer a benefit upon the authors or upon any organization with which the authors are associated. M.I.J. has delivered occasional workshops on TENS that have been sponsored by TENS and pharmaceutical companies. The remaining authors declare no conflict of interest. Reprints: Mark I. Johnson, PhD, BSc, Faculty of Health and Social Sciences, Leeds Metropolitan University, Civic Quarter, Leeds LS1 3HE, UK (e-mail: m.johnson@LeedsMet.ac.uk). Copyright r 2013 by Lippincott Williams & Wilkins ORIGINAL ARTICLE 146 | www.clinicalpain.com Clin J Pain  Volume 29, Number 2, February 2013