Chronic activity patterns such as strength training produce
marked adaptations not only in the muscular but also in the
nervous system (for reviews, see McDonagh & Davies, 1984;
Sale, 1988; Enoka, 1996). One line of evidence that
substantiates the significant role played by the neural
mechanism during training is the increase in strength that
is often greater than can be accounted for by only a change
in the muscle cross-sectional area (Jones & Rutherford,
1987). In addition to an increase in muscle force generating
capacity as the result of heavy resistance training (Duchateau
& Hainaut, 1984), another common observation is an
enhanced electromyographic (EMG) level of muscle activity
(Hakkinen & Komi, 1983; Davies et al. 1985; Narici et al.
1989). In the absence of electrically induced EMG (M wave)
modification (Duchateau & Hainaut, 1984) this latter change,
which usually precedes muscle enlargement, suggests that the
neural drive is intensified by training and so contributes to
the increase in force.
Training with small loads at maximal movement velocity
also induces muscular and neural adaptations. This method,
usually called dynamic or ‘explosive’ training, involves
ballistic contractions, characterized by short times to peak
tension, high rates of tension development and high single
motor unit discharge frequencies (Desmedt & Godaux,
1977). It has previously been shown that dynamic training
induces changes in the contractile properties of the human
adductor pollicis muscle (Duchateau & Hainaut, 1984), an
increase in maximal force and rate of tension development,
and shortening of the twitch time to peak of single motor
units, without any change in the order of recruitment
(Hainaut et al. 1981). Hakkinen et al. (1985) investigated
the influence of dynamic training on the time course of the
Journal of Physiology (1998), 513.1, pp.295—305
295
Changes in single motor unit behaviour contribute to the
increase in contraction speed after dynamic training in
humans
Micha
‡
el Van Cutsem, Jacques Duchateau and Karl Hainaut
Laboratory of Biology, Universit
‹
e Libre de Bruxelles, 28 avenue P. H
‹
eger, CP 168,
1000 Brussels, Belgium
(Received 25 March 1998; accepted after revision 17 August 1998)
1. The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in
the tibialis anterior in response to 12 weeks of dynamic training were studied in five human
subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a
week, against a load of 30—40% of the maximal muscle strength.
2. Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the
speed of voluntary ballistic contraction. This last enhancement was mainly related to neural
adaptations since the time course of the muscle twitch induced by electrical stimulation
remained unaffected.
3. The motor unit torque, recorded by the spike-triggered averaging method, increased
without any change in its time to peak. The orderly motor unit recruitment (size principle)
was preserved during slow ramp contraction after training but the units were activated
earlier and had a greater maximal firing frequency during voluntary ballistic contractions.
In addition, the high frequency firing rate observed at the onset of the contractions was
maintained during the subsequent spikes after training.
4. Dynamic training induced brief (2—5 ms) motor unit interspike intervals, or ‘doublets’.
These doublets appeared to be different from the closely spaced (±10 ms) discharges usually
observed at the onset of the ballistic contractions. Motor units with different recruitment
thresholds showed doublet discharges and the percentage of the sample of units firing
doublets was increased by training from 5·2 to 32·7%. The presence of these discharges was
observed not only at the onset of the series of spikes but also later in the electromyographic
(EMG) burst.
5. It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing
rate contribute to the increase in the speed of voluntary muscle contraction after dynamic
training.
8050
Keywords: Skeletal muscle, Motor unit, Electromyogram

isometric force and EMG activity in the leg extensor muscles
and observed a greater rate of tension development
associated with enhanced EMG activity in the early stage of
the onset of contraction. More recently, Behm & Sale (1993)
suggested that it is the intended movement velocity rather
than an actual ballistic movement that determines the
degree of adaptation in the velocity of a mechanical response.
Despite ample evidence of a significant role for neural
mechanisms in the neuromuscular adaptations associated
with training, there has been less progress in identifying
the specific mechanisms responsible for these adaptations.
The most commonly proposed explanation is that the
improvement in force or power is due to (1) a change in
muscle synergy andÏor an agonist—antagonist activation
pattern; (2) an increased activation of the muscle as a result
of changes in motor unit recruitment or firing patterns
(Hakkinen & Komi, 1983; Rutherford & Jones, 1986;
Carolan & Cafarelli, 1992; Moritani, 1993; Zehr & Sale,
1994). These interesting suggestions, however, are rather
speculative in view of the lack of experimental data.
The purpose of the present study was to analyse the effects
of 3 months of dynamic training on the neuromuscular
adaptations of the ankle dorsiflexor muscles, and particularly
in connection with the rate of tension development during
voluntary contractions. In these muscles, we analysed the
maximal voluntary contraction (MVC; i.e. the maximal
voluntary torque), the speed of ballistic contractions, the
EMG activity during MVC and voluntary ballistic
contractions and the electrically induced twitch in order to
distinguish neural adaptations from contractile changes.
Single motor unit properties and behaviour pattern were
also investigated in the tibialis anterior with special
emphasis on a possible change in discharge frequency during
ballistic contractions.
METHODS
Subjects and experimental design
Five subjects (3 female and 2 male) aged between 18 and 22 years
took part in this investigation and were tested on at least three
occasions before and also after training. All the subjects were
familiar with the experimental procedures. Their mean ± s.d. height
and weight were 168 ± 7·1 cm and 62·4 ± 11·7 kg, respectively. A
control group of five subjects (1 female and 4 male; height,
178·2 ± 5·5 cm; weight, 73·0 ± 9·8 kg) did not train and were
retested after 1 and 6 weeks in order to assess the reliability of the
observations. The dorsiflexor muscles of the left leg (the non-
dominant side) were tested. This study was approved by the
University Ethics Committee and the subjects gave their informed
consent prior to participation in the investigation. All the
experimental procedures were performed in accordance with the
Declaration of Helsinki.
Training programme
The subjects trained the dorsiflexor muscles of one leg for a period
of 12 weeks at a frequency of five sessions per week. During each
session they executed ten sets of ten fast dorsiflexion contractions
(from 90 deg to full dorsiflexion) against a load that was 30—40% of
the maximum that could be lifted once (one repetition maximum;
1 RM). The subjects were instructed to attempt to move the
footplate as fast as possible at each repetition. To avoid fatigue, a
2—3 s rest period was interposed between the individual contractions
of a set, with a 2—3 min rest period between sets. The maximal
force (1 RM) was tested every month and the training load was
adjusted accordingly. At the end of the training session the
average load lifted by the subjects had increased by 34·4 ± 3·5%
(mean ± s.e.m.).
EMG recordings
Motor unit action potentials were recorded by a selective electrode
made up of 50 ìm diamel-coated nichrome wires inserted into the
muscle by means of a hypodermic needle (Duchateau & Hainaut,
1990). The electrode was inserted through the skin in the mid part
of the muscle belly. During each experimental session the same
needle electrode was inserted in at least three separate locations
and at each location it was manipulated to different depths and
angles to record the action potentials from different motor units.
The signals were amplified by a custom-made differential amplifier
and filtered (100 Hz to 10 kHz) before being displayed on a
Tektronix TAS 455 oscilloscope. The surface EMG was recorded by
means of two disk electrodes (8 mm in diameter) placed 2—3 cm
apart on either side of the needle electrode. The surface EMG signal
was amplified (²1000) and filtered (10 Hz to 5 kHz) by a custom-
made differential amplifier.
Mechanical recordings
During the pre- and post-training recordings the subject sat on a
chair in a slightly reclining position with one foot strapped to a
footplatewhichwasinclinedatanangleof45degtothefloor.The
ankle and knee angles were set at about 90 and 110 deg,
respectively. The foot was tightly attached to the plate by means of
two straps and held in place by a heel block. One strap was placed
around the ankle and the other around the foot 1—2 cm proximal to
the metatarsophalangeal joint. Under these conditions the force
measurement was nearly isometric and contamination by other
extensor muscles (extensor digitorum longus and extensor hallucis
longus) reduced to a minimum. The isometric force developed by
the dorsiflexor muscles was measured by connecting the footplate to
a strain gauge transducer (TC 2000, Kulite, Basingstoke, UK) and
the signal was amplified (AM 502, Tektronix, Beaverton, OR,
USA; bandwidth DC − 300 Hz). The transducer was attached at
the level of the metatarsophalangeal joint of the big toe (lever arm,
17 cm). The sensitivity of the force transducer was 30 mV N¢
(linear range, 0—500 N). The force signal was then high-gain-AC
amplified and filtered (1 Hz to 100 Hz) by means of an AM 502
plug-in Tektronix amplifier to obtain the mechanical contribution
of single motor units.
Muscle twitch torque was induced by a rectangular supramaximal
(10—20% above torque responses) electrical pulse (0·1 ms in
duration) delivered by a custom-made stimulator triggered by a
digital timer (model 4030, Digitimer Ltd, Welwyn Garden City,
UK). The peroneal nerve was stimulated by two electrodes (silver
disks 8 mm in diameter), with the cathode being placed over the
proximal border of the tibialis anterior while the anode was
fastened to the fibular head. In order to avoid the activation of the
peroneal muscles the nerve was stimulated beyond their
innervation collateral and the absence of any peroneal muscle
activity was assessed by means of the EMG recording.
The mechanical recording of the single motor units was assessed by
the spike-triggered averaging method (Milner-Brown et al. 1973).
Briefly, this method consists of triggering the sweep of an averager
M. Van Cutsem, J. Duchateau and K. Hainaut
J. Physiol. 513.1
296