Intensity matters: Therapist-dependent dose of spinal transcutaneous electrical nerve stimulation

Abstract

The intensity used during transcutaneous electrical nerve stimulation (TENS) in both, clinical practice and research studies, is often based on subjective commands such as “strong but comfortable sensation”. There is no consensus regarding the effectiveness dose of TENS. The objective was to determine the difference in the effect of spinal TENS on soleus H-reflex modulation when applied by two therapists instructed to apply the stimulation at a “strong but comfortable” intensity. Twenty healthy volunteers divided into two groups: Therapist 1 (n = 10) and Therapist 2 (n = 10). Both therapist applied spinal TENS and sham stimulation at the T10–12 spinal level for 40min in random order to each subject, at an intensity designed to produce a “strong but comfortable” sensation. To avoid habituation, the intensity was adjusted every 2min. Soleus H-reflex was recorded before, during, and 10min after TENS by an observer blinded to the stimulus applied. Despite the instruction to apply TENS at a “strong comfortable” level, a significant difference in current density was identified: Therapist 1 (0.67mA/cm2, SD 0.54) applied more than Therapist 2 (0.53mA/cm2, SD 0.57; p<0.001) at the onset of the intervention. Maximal peak-to-peak H-reflex amplitude was inhibited significantly more 10min following TENS applied by Therapist 1 (-0.15mV, SD 0.16) compared with Therapist 2 (0.04mV, SD 0.16; p = 0.03). Furthermore, current density significantly correlated with the inhibitory effect on peak-to-peak Soleus H-reflex amplitude 10 min after stimulation (Rho = -0.38; p = 0.04). TENS intensity dosage by the therapist based on the subjective perception of the participants alone is unreliable and requires objective standardization. In addition, higher current density TENS produced greater inhibition of the Soleus H-reflex.

Transcutaneous Spinal Cord Stimulation and Motor Rehabilitation in Spinal Cord Injury: A Systematic Review

“…Rather than setting the stimulation intensity objectively, most of the studies that met the inclusion criteria for review instead expressed it using subjective and ambiguous terms of perception of the current by participants or observations by researchers. Recently, Serrano et al 43 showed that, when therapists set the intensity of the current on the basis of subjective terms of perception of the stimulus, the defined stimulus intensity is highly variable, which in turn has an impact on the therapeutic effect. Despite the lack of sufficient data to perform a correlational analysis between the stimulus intensity and motor effect, computational models 17,42 indicate that the intensity needed to depolarize the dorsal root afferent fibers, at the dorsal root entry zone, should be ≥17.6 V because it is estimated that the amount of current that penetrates this region through the intervertebral tissues is only 9%.…”

Effect of high-frequency alternating current transcutaneous stimulation over muscle strength: a controlled pilot study

“…The intensity was gradually increased until a minimally visible contraction was observed and subsequently decreased until it disappeared, and this sensation remained throughout the session. To avoid habituation to the stimulus, participant were asked every 2 minutes to corroborate the perceived sensation, and the intensity was increased if requested [ 6 , 7 ]. The same procedure was performed with the 5 kHz stimulation.…”

20-kHz alternating current stimulation: effects on motor and somatosensory thresholds

“…The intensity was gradually increased until a minimal visible contraction was observed and then subsequently decreased until the muscle contraction disappeared, with the same intensity maintained throughout the stimulation. To avoid habituation to the stimulus, participants were asked to corroborate the perceived stimulation sensation every 2 min, so that the stimulation intensity could be increased if requested [21,22].…”