The effect of three electrotherapeutic modalities
upon peripheral nerve conduction and mechanical
pain threshold
J. Alves-Guerreiro, J. G. Noble, A. S. Lowe and D. M. Walsh
Rehabilitation Sciences Research Group, School of Rehabilitation Sciences, University of Ulster at Jordanstown, Newtownabbey, UK
Received 13 March 2001; accepted 30 May 2001
Correspondence: Dr Deirdre Walsh, Rehabilitation Sciences Research Group, School of Rehabilitation Sciences, University of Ulster at Jordanstown,
Newtownabbey, Co. Antrim BT37 0QB, UK
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Summary
The current study was designed to examine the
neurophysiological and hypoalgesic effects of three
types of electrical stimulation. Following approval by
the University’s Research Ethical Committee, healthy
volunteers (n ˆ 40; 20 males and 20 females; age 20–40
years; mean age 26Æ18 years) were recruited and
screened for contraindications. Subjects were ran-
domly allocated in equal numbers to the following
groups: control, transcutaneous electrical nerve
stimulation (TENS; 150 Hz, 125 ls), interferential
therapy (IFT; 150 Hz, 125 ls) or action potential
stimulation therapy (APS; 153 Hz, 6Æ4 ms). All treat-
ments were applied under double-blind conditions for
15 min over the course of the median nerve in the
subject’s right forearm. Antidromic median nerve
compound action potentials (CAPs) were recorded
pretreatment, immediately post-treatment (i.e. at
15 min) and then at 25, 35 and 45 min. Immediately
following CAP recording, mechanical pain threshold
(MPT) was recorded from two sites on the palmar
surface of the right hand. Statistical analysis showed
significant differences between groups for peak to
peak amplitude (PPA) at 25, 35 and 45 min (Kruskal–
Wallis: P ˆ 0Æ01, 0Æ01 and 0Æ02). Mann–Whitney
U-tests indicated a significant increase in PPA in the
IFT group compared with all other groups at 25 and
35 min and compared with the TENS and APS
groups at 45 min. No significant differences were
found for the MPT data. This study has therefore
demonstrated that none of the aforementioned
modalities produced a significant hypoalgesic effect;
however, IFT produced a significant change in PPA
compared with TENS and APS.
Keywords: electrical current, experimental pain,
nerve conduction, stimulation parameters.
Introduction
The use of electrical stimulation for pain relief dates
back as far as the Egyptian Fifth Dynasty. In the
1960s, the publication of the pain gate theory
(Melzack & Wall, 1965) acted as a ‘catalyst’ for
electroanalgesia by providing the theoretical basis for
the use of electrical currents for pain relief. Recent
surveys have indicated the popularity of electrical
currents in the treatment of pain of various aetiologies
(Pope et al., 1995; Robertson & Spurritt, 1998).
Transcutaneous electrical nerve stimulation (TENS),
interferential therapy (IFT) and action potential
stimulation (APS) therapy are examples of three
different types of electrical currents currently used
for pain relief in clinical practice. However, despite
the popularity of such modalities, there is ongoing
debate regarding their hypoalgesic and neurophysio-
logical effects.
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704 Ó 2001 Blackwell Science Ltd • Clinical Physiology 21, 6, 704–711

TENS has been extensively used in the manage-
ment of pain for a wide variety of clinical conditions
(Sjo¨ lund, 1988; Akyu
¨
z et al., 1995; Walsh, 1997);
although the clinical success of TENS is documented,
the associated neural mechanisms that modulate pain
are not well understood (Johnson, 1998; McDowell
et al., 1999). TENS and APS therapy are regarded as
low frequency currents (<250 Hz) while IFT com-
prises two medium frequency currents (usually
4 kHz) one of which is amplitude modulated to
produce a low frequency current (0–250 Hz). It is
claimed that IFT has an advantage over other
electrical currents in that its carrier frequency is
associated with relatively lower skin resistance while
still producing low frequency effects within the tissues
(Kloth, 1987; Martin, 1996). Despite the increasing
acceptance of this electrotherapeutic modality
amongst clinicians (Lindsay et al., 1995; Pope et al.,
1995; Robertson & Spurritt, 1998), literature report-
ing the efficacy of IFT is deficient, with several
authors pointing to the scarcity of empirical evidence
for this modality (Schmitz et al., 1997; Johnson, 1999;
Palmer et al., 1999; Noble et al., 2000). APS therapy
is a novel electrical stimulation device that delivers a
monophasic exponential decaying waveform with a
fixed frequency (153 Hz). This device was developed
in the early 1990s in South Africa. At the present time
there is an obvious scarcity of published literature on
this form of electrical stimulation.
Previous work at this centre has investigated the
peripheral effects of electrical currents, e.g. TENS
and H-wave therapy, by observing alterations in
compound action potential (CAP) characteristics
before and after the application of the treatment
(McDowell et al., 1996; Walsh et al., 1995, 1998). In
addition, mechanical pain threshold (MPT) record-
ings have been used successfully to assess the
hypoalgesic effects associated with such neurophysio-
logical changes (Walsh et al., 1995, 1998). An increase
in negative peak latency (NPL) is an indicator of a
decrease in the nerve conduction velocity and has
been shown to highly correlate with changes in MPT
(Walsh et al., 1995, 1998). Walsh et al. (1998) repor-
ted parameter-specific effects of TENS on conduc-
tion in the superficial radial nerve and on MPT
recorded within the distribution of this nerve; an
increase in NPL was associated with an increase in
MPT. Other authors have similarly reported that
TENS induced alterations in nerve conduction
velocity (Campbell & Taub, 1973; Torebjo
¨
rk &
Hallin, 1974; Ignelzi et al., 1981; Sjo¨ lund, 1988),
while Golding et al. (1986) and Cox et al. (1993) did
not report any such significant changes. In contrast,
few studies have examined the neurophysiological or
hypoalgesic effects of IFT or APS.
As an extension of previous work, the current study
was designed to compare the peripheral neuroph-
ysiological effects of three different electrical currents
(TENS, IFT and APS) by recording nerve conduc-
tion in the median nerve; in addition, local hypoal-
gesic effects were assessed using MPT recorded
within the distribution of the median nerve.
Methods
Following approval by the University of Ulster’s
Research Ethical Committee, 40 healthy volunteers
(n ˆ 40; 20 males and 20 females; age 20–40 years;
mean age 26Æ18 years) were recruited from the staff
and students of the University, and screened for
medical history or current signs/symptoms of neuro-
muscular disorders including peripheral neuropathy.
The experimental procedure was explained to subjects
who were then asked to sign a consent form and
randomly assigned in equal numbers to one of four
experimental groups: control; TENS (150 Hz); IFT
(150 Hz); APS (153 Hz); n ˆ 5 males and 5 females
per group. Subjects remained supine for the duration
of the experiment. The anterior surface of the right
forearm and hand was prepared using alcohol, and the
stimulation and recording sites at the elbow and
second digit, respectively, were cleaned with a
colloidal abrasive in order to decrease skin resistance.
All data were recorded and statistically analysed by
the primary investigator who was blind to the
treatments applied by a second investigator.
Recording of compound action potentials
In order to record antidromic CAPs, a bipolar muscle
stimulator was used to identify the right median nerve
at the elbow. A monopolar muscle stimulator was
further used to map the course of the nerve in the
right forearm. A surface bar stimulation electrode was
attached at the right elbow and two digital ring
electrodes were attached to the second digit, with the
J. Alves-Guerreiro et al. • Peripheral effects of electrical stimulation
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Clinical Physiology 21, 6, 704–711 • Ó 2001 Blackwell Science Ltd 705