i
e
i
ivity
d fo
Abstract
not fully understood, but has usually been attributed to dis-
turbance of excitation–contraction (EC) coupling after
eccentric actions (Warren et al., 2001), however, failure
2004). EC coupling could be disturbed due to (a) defect
is often found to be damaged after eccentric actions in ani-
mal studies (Friden et al., 1981; Komulainen et al., 1998),
but not always in human studies (Yu et al., 2002).
Recently, Koskinen et al. (2006) have observed some loss
of dystrophin also in some human subjects after eccentric
actions. This is an important finding, since any damage
* Corresponding author. Tel.: +358 14 260 4627; fax: +358 14 260 2071.
E-mail address: harri.piitulainen@sport.jyu.fi (H. Piitulainen).
Available online at www.sciencedirect.com
Journal of Electromyography and KineIt is well known in the literature that eccentric exercise is
accompanied by a prolonged loss of muscle force (Davies
and White, 1981), muscle pain (Armstrong, 1984),
increased joint stiffness (Stauber et al., 1990), muscle swell-
ing (Clarkson et al., 1992), a shift in optimal joint angle for
torque generation (Jones et al., 1997), muscle protein deg-
radation and efflux to circulation (Belcastro et al., 1998;
Clarkson et al., 1986). All these effects are well known
symptoms of exercise induced muscle damage (EIMD)
(Allen, 2001). The cause for the loss of muscle strength is
of Ca2+ kinetics in the sarcoplasmic reticulum (SR) (War-
ren et al., 2001), (b) sarcomere level damage in the contrac-
tile machinery (Friden et al., 1981; Roth et al., 1999) and
(c) ultrastructural selective damage of force bearing pro-
teins such as subsarcolemmal dystrophin (Komulainen
et al., 1998; Koskinen et al., 2006) or the intermediate fila-
ment desmin (Barash et al., 2002; Friden et al., 1981).
Selective damage of force bearing proteins may disturb lat-
eral force transmission in the sarcolemma, and thus impair
muscle strength (Monti et al., 1999). The sarcolemma itselfIt has been shown that intensive eccentric muscle actions lead to prolonged loss of muscle force and sarcolemmal damage. This may
lead to a reduction in the excitability of the sarcolemma and contribute to the functional deficit. Experiments were carried out to test
sarcolemmal excitability after eccentric elbow flexor exercise in humans. Electrically elicited surface compound muscle action potential
(M-wave) properties from 30 s stimulation trains (20 Hz) were analyzed in biceps brachii muscle immediately after, 1 h and 48 h after the
exercise. M-wave area, amplitude, root mean square and duration were reduced immediately after the eccentric exercise. However, no
such reduction could be observed 48 h after the exercise, although the maximal voluntary isometric and eccentric torques were still
depressed by 12.2 ± 9% (P < 0.001) and 17.7 ± 9% (P < 0.001), respectively. Acute increase in plasma concentrations of K+ and Ca2+
were also observed after the eccentric exercise. These findings suggest that eccentric exercise may acutely decrease sarcolemmal excitabil-
ity, which seems to be partially related to increased extracellular ion concentrations. However, disturbance of sarcolemmal excitability is
not the major factor determining eccentric exercise induced prolonged loss of muscle strength, because no prolonged impairment was
observed in any of the studied M-wave parameters.
 2007 Elsevier Ltd. All rights reserved.
Keywords: M-wave; Electromyogram; Muscle fatigue; Delayed-onset muscle soreness; Sarcolemma
1. Introduction of activation has also been suggested (Linnamo et al.,Sarcolemmal excitability as
after eccentric ex
Harri Piitulainen *, Paavo Kom
Neuromuscular Research Center, Department of Biology of Physical Act
Received 4 September 2006; received in revise1050-6411/$ - see front matter  2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jelekin.2007.01.004nvestigated with M-waves
rcise in humans
, Vesa Linnamo, Janne Avela
, University of Jyva¨skyla¨, P.O. Box 35, FIN-40014, Jyva¨skyla¨, Finland
rm 9 January 2007; accepted 9 January 2007
siology 18 (2008) 672–681
www.elsevier.com/locate/jelekin

yogrin the excitable membranes of the muscle cell (sarcolemma
or T-tubular system) could lead to a failure in action poten-
tial (AP) generation and/or propagation, and thus impair
EC coupling (Allen, 2001).
It could therefore be hypothesised that reduced excit-
ability of the sarcolemma could lead to a failure in gener-
ation and propagation of AP after intensive contractile
activity. This could be due to increased extracellular K+
concentration (Sejersted and Sjogaard, 2000) or direct
damage of the sarcolemma (Friden et al., 1981; Koskinen
et al., 2006). Sarcolemmal excitability is defined as the
inward current that is required to depolarize the sarco-
lemma enough to reach the threshold potential that trig-
gers a sufficient amount of Na+ channels to elicit an AP
(Sejersted and Sjogaard, 2000). If the sarcolemma is dam-
aged, as indicated by dystrophin negative immunohisto-
chemical stained fibers after eccentric actions (Koskinen
et al., 2006), the sarcolemmal ion permeability may
increase. This could possibly lead to ion concentration
disturbance over the muscle cell membrane and affect
the resting and threshold membrane potentials. This will
naturally also affect the membrane excitability. In addi-
tion to direct microscopic observations of damaged sarco-
lemma, there are changes in sarcolemmal function after
eccentric actions. McBride et al. (2000) observed that in
animals, eccentric exercise caused prolonged membrane
depolarization due to increased cation conductance via
stretch-activated ion channels in the sarcolemma. Thus,
eccentric actions may decrease the sarcolemmal excitabil-
ity due to increased sarcolemmal ion permeability and,
therefore, decrease the force production capability of the
muscle.
In muscle fatigue experiments, sarcolemmal excitability
has been studied with electrically elicited electromyography
(EMG) signals (Bellemare and Garzaniti, 1988; Michaut
et al., 2002; Milner-Brown and Miller, 1986; Pasquet
et al., 2000; Stephens and Taylor, 1972). Electrical stimula-
tion has the advantage of standardized conditions as com-
pared to voluntary muscle actions, since it controls motor
unit (MU) firing frequency, MU recruitment, eliminates
cross-talk from nearby muscles and is independent of sub-
ject motivation to perform muscular contraction (Merletti
et al., 1990). Motor point electrical stimulation recruits
only a portion of the muscle, thus accurate repositioning
of the electrode is very critical to obtain repeatable results.
In motor point electrical stimulation, all the activated MUs
are recruited at the same time. Thus, simultaneously mea-
sured EMG signals enable analysis of the M-wave, which
contains information about membrane properties of the
active MUs (Merletti et al., 1992).
Even though disturbances in EC coupling have been
suggested to be responsible at least partly for the strength
loss after eccentric actions (Warren et al., 2001), it is not
certain if sarcolemmal excitability impairment contributes
to EC coupling disturbance after eccentric actions. There-
H. Piitulainen et al. / Journal of Electromfore, the aim of the present study was to verify if sarcolem-
mal excitability is decreased during the two day post-exercise period after eccentric exercise, and whether it
could further explain the EC coupling dysfunction.
2. Methods
2.1. Subjects
One female and eleven male volunteers (age 25.4 ± 2.8 yr;
height 179.7 ± 5.8 cm; weight 76.9 ± 11.8 kg) participated in the
study. One female and two males were only included for the
reliability measurements of the electrically elicited EMG param-
eters, and nine male subjects carried out both the exercise pro-
tocol and reliability measurements. Subjects were physically active
students and had no known symptoms of neuromuscular disor-
ders. The subjects were non-smokers, did not drink caffeine rich
drinks 12 h before the measurements, and avoided strenuous
exercise two days before the first measurement and throughout
the whole study period. The study was conducted in accordance
with the Declaration of Helsinki and approved by the ethics
committee of the University of Jyva¨skyla¨. All participants were
aware of the possible risks and discomfort of the experiments and
signed a written informed consent form before inclusion.
2.2. Study protocol
The measurements consisted of three different sessions on
different days and five identical measurements. During the
familiarisation session (PRE, 1–2 day before exercise session)
subjects practised submaximal eccentric elbow flexor actions,
electrode locations were determined and measurements for reli-
ability analysis were conducted. The effect of a few eccentric
actions was checked during the familiarisation session and no
changes were observed in any of the measured variables. Baseline
measurements were performed before (BEF) the eccentric exercise
and followed up to 48 h post-exercise [immediately after (IA), one
hour (1 h) after and two days after (2D) the eccentric exercise].
The measurements were identical and had the same order in each
session. Tetanic electrical stimulation (+10 s post-exercise) was
conducted first and was followed by a collection of blood samples
(+40 s post-exercise), maximal voluntary contraction (MVC)
measurements (+60–80 s post-exercise), measurement of elbow
joint angle and assessment of subjective muscle soreness (+5 min
post-exercise).
The tetanic electrical stimulation was repeated during the PRE
measurement to analyze trial-to-trial reliability. Day-to-day reli-
ability was acquired by comparing the PRE and BEF measure-
ments. Reliability was tested to ascertain the consistency of the
used M-wave parameters. The warm-up protocol before each
session consisted of three submaximal eccentric and isometric
elbow flexions.
2.3. Exercise protocol
Subjects performed two sets of twenty maximal eccentric
contractions (2 · 20) with the elbow flexors of the right arm, on a
motorized isokinetic dynamometer (Komi et al., 2000). The
exercise protocol included a moderate number of eccentric actions
to avoid extensive swelling of the soft tissues (Hesselink et al.,
1996; Nosaka et al., 2002), but high enough to induce muscle
damage (Nosaka and Clarkson, 1996). The supinated right fore-
aphy and Kinesiology 18 (2008) 672–681 673arm was attached to a strain gauge transducer, which was fixed to
the lever arm of the dynamometer for recording of the force