RESEARCH ARTICLE
F. Dominici Æ T. Popa Æ F. Ginanneschi Æ R. Mazzocchio
A. Rossi
Cortico-motoneuronal output to intrinsic hand muscles is differentially
influenced by static changes in shoulder positions
Received: 10 November 2004 / Accepted: 21 December 2004 / Published online: 10 May 2005

Springer-Verlag 2005
Abstract We investigated whether shoulder position
influenced the recruitment properties of the abductor
digiti minimi muscle (ADM) and first dorsal interos-
seous muscle (FDI). ADM and FDI motor evoked
potentials (MEPs) in response to transcranial magnetic
stimulation (TMS) were obtained in seven healthy vol-
unteers at two different static positions of the shoulder
joint (30 adduction vs 30 abduction) while the arm was
passively supported at shoulder level (90 in the hori-
zontal plane) and the elbow joint was fixed at 90. ADM
and FDI voluntary activity was also examined during (1)
externally paced finger abductions at 2 Hz in the two
different shoulder positions (EMGADM and EMGFDI
was back-averaged time-locked to the end of finger
abduction) and (2) maximal voluntary abduction of the
little finger and the index finger. Maximal EMG power
and force were analysed in the two shoulder positions.
H-reflexes from ADM and FDI were also obtained in
two subjects. The ADM stimulus-response curve to
TMS showed that the slope and plateau level were sig-
nificantly reduced with the shoulder at 30 abduction. In
contrast, the FDI stimulus-response curve to TMS was
not influenced by shoulder position. The back-averaged
EMGADM showed a significant decrease in peak ampli-
tude and area with the shoulder at 30 abduction, while
no change in EMGFDI was observed under the same
condition. Similarly, maximal EMGADM and force
exertion by the little finger were significantly reduced
with the shoulder at 30 abduction, while no such effect
was observed for FDI. ADM H-reflex, but not FDI, was
also decreased with shoulder abduction. These results
indicate that the corticospinal pathway to ADM is less
accessible to TMS and to voluntary command when the
shoulder is placed at 30 abduction. In contrast, acti-
vation of FDI, whether by TMS or by volition, is not
influenced by shoulder position. This finding suggests
that there are differences in the corticospinal innervation
to ADM and FDI, possibly due to the different role of
these muscles in hand function.
Introduction
Coactivation of distal and proximal muscles is com-
monly observed in the reach and prehension task (Kal-
aska et al. 1997; Scott 2000) or in the pointing task
(Devanne et al. 2002). It is possible that the neural
control of the hand is influenced by activity originating
at proximal joints (Scott and Kalaska 1997). In partic-
ular, sensory information from cutaneous, propriocep-
tive and visual afferents may differentially modulate the
cortical outflow in relation to the starting posture or
geometry of the arm.
We have recently examined the effects of changes in
static position of the shoulder on the cortico-motoneu-
ronal excitability of one intrinsic hand muscle, the
abductor digiti minimi muscle (ADM), as tested by
transcranial magnetic stimulation (TMS) under resting
conditions (Ginanneschi et al. 2005). When the shoulder
joint is placed at 30 abduction in the horizontal plane,
the corticospinal activation of ADM to TMS is signifi-
cantly reduced. This may result in lesser accessibility of
the corticospinal system to voluntary activation with
important functional implications.
The purpose of this study was to evaluate whether
static changes in shoulder position influenced the
recruitment properties of intrinsic hand muscles other
than ADM. The possibility that the voluntary drive to
intrinsic hand muscles may change in relation to
shoulder position and that, as a result, it may affect their
functional capacity was also investigated.
F. Dominici Æ T. Popa Æ F. Ginanneschi
R. Mazzocchio (&) Æ A. Rossi
Sezione di Neurofisiologia Clinica, Dipartimento di Scienze
Neurologiche e del Comportamento, Universita` di Siena,
Policlinico ‘‘Le Scotte’’ Viale Bracci, 53100 Siena, Italy
E-mail: mazzocchio@unisi.it
Tel.: +39-0577-233409
Fax: +39-0577-40327
Exp Brain Res (2005) 164: 500–504
DOI 10.1007/s00221-005-2270-5

Materials and methods
Subjects
Seven (four females, three males) right-handed subjects
volunteered for the study (mean age: 29.5±6.8 years,
range: 25–50) which was divided into two experiments.
Informed consent was obtained from all subjects prior to
testing and ethical approval for the study was obtained
from the University of Siena Human Subjects Ethics
Committee in accordance with the Declaration of Hel-
sinki. The subjects sat in a reclining armchair with their
right arm placed in a horizontal plane with the shoulder
at 90 abduction. Right arm and forearm were inserted
and secured in arm pieces that fixed the elbow joint at
90and wrist joint in a neutral position. The shoulder
joint could be freely rotated in the horizontal plane from
30 adduction to 30 abduction with respect to neutral
position at 0 without modification of biomechanical
relations and muscle lengths at distal joints.
TMS was used to assess changes in cortico-moto-
neuronal excitation to two different hand muscles, FDI
and ADM at the two different static shoulder positions
(30 adduction vs 30 abduction). TMS response-inten-
sity curves were measured in the FDI and ADM of the
dominant hand separately in two different sessions. The
EMG was recorded using bipolar surface electrodes in a
belly/tendon montage; 100 ms of EMG data were col-
lected for each stimulus, plus an additional 20 ms prior
to stimulus onset. Focal TMS was applied over the hand
area of the dominant primary motor cortex (M1)
through a figure-of-eight coil (outer diameter of each
wing, 9 cm) connected to a Magstim 200 stimulator
(Magstim, Whitland, UK). The stimulating coil was
placed flat on the scalp with the handle pointing back-
wards and rotated 45 away from the midline. The
optimal coil position for activating the EMG target
muscle was determined as the site where TMS produced
consistently the largest motor evoked potential (MEP) at
slightly suprathreshold intensity. This site was marked
on the scalp with a pen to ensure constant coil placement
throughout the experiment. The resting motor threshold
(RMT) was determined. The magnetic stimulus intensity
(expressed in percentage of maximum stimulator output)
was progressively increased up to the maximum MEP
size. The TMS measurements were performed while the
target muscle was relaxed. MEPs were full-wave rectified
and the whole area of each response was assessed. Data
from the relation between stimulus intensity and the size
of the MEPs were fitted to the Boltzmann sigmoidal
function by the Levenberg–Marquard nonlinear, least
mean squares algorithm (Devanne et al. 1997).
We also investigated whether the voluntary recruit-
ment of ADM and FDI was affected by static changes in
shoulder positions. Subjects were asked to perform: (1)
repetitive finger movements paced by a metronome at
2 Hz over a period of 40 s in the two different shoulder
positions in a randomised order. After some training
sessions with the arm in a neutral position (shoulder
joint at 0 as illustrated in Fig. 1a) all subjects were able
Fig. 1 a Schematic representation of the arm position. Upper limb
was placed and secured on a rigid support with elbow and wrist
joint fixed at 90 and 0, respectively. Shoulder joint was 90
abducted in the vertical plane (i.e. arm and shoulder were in the
same plane) and it could be freely rotated forward (30 adduction)
and backward (30 abduction) in the horizontal plane with respect
to neutral position at 0, and vice versa. b, c Relationship between
MEP size and stimulus intensity. Grand average of MEP size
(±SE) recorded from ADM (b) and FDI (c) at 30 adduction
(empty circles) and 30 abduction (filled circles) of the shoulder
joint. The values were fitted with the Boltzmann sigmoid function.
The MEP is expressed as a percentage of the maximum response
size at 30 shoulder adduction. Intensity of stimulation is expressed
in times the threshold value (RMT) at 30 shoulder adduction
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