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Università degli Studi di Milano, 1Istituto di Fisiologia Umana II, 20132 Milano; and 2Dipartimento di Medicina, Chirurgia e Odontoiatria, 20142 Milano, Italy
Submitted 13 January 2003; accepted in final form 9 March 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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,
1991,
2000;
Carson et al. 1995;
Jeka and Kelso 1995;
Kelso and Jeka 1992;
Serrien and Swinnen 1998;
Swinnen et al. 1995). The term nervous constraint, which is usually referred to factors, or situations, which limit the coupling repertoire (e.g., hindering or impeding nonisodirectional coupling of ipsilateral limbs), may however conceal, not a limit, but rather an obligation to produce a certain behavior. For instance, the clear-cut preference for isodirectional (in-phase) coupling of ipsilateral limbs may be sustained by nervous mechanisms that compel the limbs to "imitate" each other when they are moved simultaneously and, consequently, discourage other types of coupling, for instance, in-phase opposition.
In favor of this view, it was recently found that during the voluntary
rhythmic flexion-extension movement of the ankle the H-reflex excitability in
the resting forearm undergoes cyclic modulation
(Baldissera et al. 1998). In
the Flexor Carpi Radialis (FCR), with the forearm in prone position,
modulation is characterized by an increase of the H-reflex amplitude during
foot plantar flexion and a reduction during dorsi-flexion, whereas it is
opposite in-phase when the forearm is held supine
(Baldissera et al. 2003). It
was therefore argued that, when the two extremities are moved together, this
pattern may favor the preferred in-phase (isodirectional) coupling (Baldissera
et al. 1982,
2000) of the hand (either prone
or supine) and foot.
To account for these findings, it could be postulated that afferent signals
generated by the foot movement influence the reflex excitability in the
cervical spinal segments. Further investigations, however, uncovered that,
during oscillations of the foot, the cortical motor area innervating the
relaxed forearm muscles undergoes excitability fluctuations parallel to those
occurring in forearm motoneurones
(Baldissera et al. 2002) and
that the former are possibly the cause of the latter. On this basis, it was
proposed that centers elaborating the motor program for limb oscillations send
parallel projections to the motor pathways to both limbs even when only one
(e.g., the foot) is moved. In this contingency, the command directed to the
foot is strongly activated while the collateral component directed to the
resting hand is weakly excited. This would result in the movement of the foot
and in the subliminal activation of the motor pathways directed to the resting
hand. An alternative possibility, however, must be considered (i.e., that the
spinal and cortical excitability changes may originate from kinesthetic
afferent signals generated by the foot movements and fed into the motor
pathways directed to resting forearm). Indeed such a feedback action seems to
be involved in the control of hand-foot coupling, when they both move, as a
means to compensate for mechanical disparities between the two limbs. The
latter control system apparently operates by anticipating muscular activation
of the segment with greater inertia
(Baldissera and Cavallari
2001). If a similar afferent feedback were also responsible for
the excitability changes recorded in the forearm, then the H-reflex modulation
should be tightly bound to the time course of foot movement. Conversely, the
hypothesis of a central common origin would predict that H-modulation is
temporally linked to foot electromyographic (EMG) activity (i.e., the motor
command) not to foot position.
Experiments aiming at modifying the time (phase) relations between the
motor commands and the subsequent movements of the foot can put the above
predictions to the test and help to reveal to which of the two events
(muscular activation or movement) the modulation is linked. In the present
experiments we modified such phase relations by applying an external load to
the foot and/or by changing the frequency of its oscillations (see
Baldissera et al. 2001). With
these experimental manipulations and following our standard protocol for
testing the H-reflex modulation (Baldissera et al.
1998,
2002), we explored whether the
modulation time course is phase-linked either to movement or to muscular
activation.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Subjects followed a general experimental protocol (see
Baldissera et al. 2002), in
which they were asked to perform sequences of four to five flexion-extension
cycles of the foot about the ankle, starting at their own will and following a
tempo of 1.66 Hz (600-ms period), imposed by a metronome. A beeping signal
allowed the subject to start a new oscillations sequence. The interval between
sequences lasted ≥8 s.
During each movement sequence, transit of the foot in front of a photocell generated a signal that was fed into a PC which, at the third signal, triggered the stimulator to elicit an H-reflex at one of five different delays during the third movement cycle. At the same time, the potentiometric signal giving the foot angular position and the EMGs from the foot movers TA and Sol were recorded, A/D converted (sampling rate 250 Hz for the potentiometer signal), and stored for further analysis.
In the different experiments this protocol was repeated after modifying the experimental setup and/or the procedures in the two following ways: 1) a metal disk (weight 10 kg, radius 14 cm, inertial momentum 98 g2) was applied concentrically to the axis of rotation of the foot platform; and 2) oscillations were repeated at different frequencies between 1.0 and 2.0 Hz. Both modifications were utilized with the aim of altering the phase relations between muscle activity in foot movers and the resulting foot oscillations.
A series of 25 sequences was delivered in each trial. The corresponding 25 reflexes were divided into five groups. Within each group, the reflexes were evoked in a random sequence at five different delays from the photocell trigger, dividing in even parts the imposed oscillation period (e.g., 0, 120, 240, 360, and 480 ms for a 600-ms period). H-reflex responses were amplified, filtered (pass-band 10 to 3,000 Hz), and digitally converted (sampling rate 5 kHz). To attenuate long-term variability independent of the stimulus position in the cycle, peak-to-peak amplitude of each response was measured and expressed as the deviation (in µV) from the mean of the five responses of its own group. This value was then averaged with those obtained at the same delay in the other groups of reflexes.
To obtain a more effective modulation linked with foot movement, in all
experiments, except those describing the effects of changing movement
frequency, the FCR H-reflex was facilitated by short-latency conditioning by
transcranial magnetic stimulation, a procedure that enhances the modulation
amplitude but does not affect its time course
(Baldissera et al. 2002;
Fig. 3). For this purpose, the
subject's head was restrained by a fitted support and a stereotactic apparatus
held an 8-shaped coil, connected to a magnetic stimulator (Magstim 200,
maximal power 2.2 T), over the cortical focus for transcranial magnetic
stimulation (TMS) activation of forearm muscles. To induce facilitation of the
H-reflex in FCR (Baldissera and Cavallari
1993; Gracies et al.
1994), TMS was delivered 2–3.5 ms after median nerve
stimulation (i.e., during the facilitation rising-phase, as determined in each
subject by testing 3–4 conditioning-test intervals). The TMS intensity
was just below (80–95%) the threshold for evoking cortical muscle action
potentials (CMAPs) at rest (usually 50–60% of maximal output).
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). For each experimental condition, three to four trials were repeated in each subject. During each recorded (third) cycle, movement period was measured and found to never deviate between cycles by more than 5%. The ensemble-average of the (third) foot oscillations of all trials was then calculated and fitted by a four-parameter sine-wave function, whose parameters were calculated by minimizing the sum of the squared differences between the observed and predicted values (Marquardt-Levenberg algorithm, SigmaPlot), and its period was estimated.
Starting from the position data, collected with a sampling interval
(t) of 4 ms, movement velocity, and acceleration were calculated as
the first and second derivatives of the position (y)
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Significance of H-reflex modulation within each subject and condition (loading or frequency) was evaluated by one-way analysis of variance (ANOVA). Significance level for all tests was P < 0.05. To allow immediate phase matching between the excitability changes occurring in forearm motoneurones and foot movement (and its derivatives), the reflex data were then fitted by a sine-wave function with the same period as that of the movement. To attenuate the influence of background variability on the sinusoidal correlation with the independent variable (delay in the period), the best-fit determination coeffi-cient, R2, was calculated on the mean reflex values for each delay.
Comparisons of phase delays between different conditions were performed by paired-sample t-test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Figure 1 illustrates, for one of the six subjects, an example of the H-reflex modulation (mean change of the H-reflex amplitude at 5 delays during the foot movement cycle, see METHODS) occurring in the resting FCR during oscillations of the ipsilateral foot at about 1.6 Hz. In this figure, the ensemble-average of the foot position (pos), its first and second derivatives (vel and acc), the integrated EMGs from the foot movers (TA and Sol), as well as the five delays at which the H-reflex was tested are all displayed on the same abscissa and normalized to the period of the movement cycle.
Note that the rising phase of the H-reflex modulation coincided with the EMG burst in Sol and the declining phase with the EMG burst in TA. The reflex data were significantly fitted (Fig. 1A, dotted line, P < 0.001) by a sine-wave function with the same period as that fitting the movement (Fig. 1B, dotted line). Determination coefficient (R2) for the best-fit of the five mean values was 0.96.
The sinusoidal nature of both the foot movement and the associated
modulation of the H-reflex allowed us to easily estimate the phase relation
existing between the modulation and the parameters of movement dynamics
(position, velocity, and acceleration). In this subject, the sine function
fitting the modulation led the movement sine-wave (flexion positive) by a
phase angle (
) of 74°, thus almost approaching the velocity
sine-wave (dashed line). The rising phase of the H-reflex modulation is
therefore temporally related to both the rising phase of movement velocity and
the period of Sol EMG activation. Similar results were obtained in all six
subjects. On one hand, the relation with velocity may suggest that modulation
is produced by the discharge of receptors signaling this parameter of foot
movement. On the other hand, however, the linkage to Sol activation may
support the "central" hypothesis (i.e., that the reflex modulation
originates from a collateral action to the resting
"isodirectional" muscle FCR during Sol contraction).
To have further elements that help clarify this point, we verified whether the link between H-modulation and foot movement described above persists when the relations between the movement (which remains the same) and its motor command are modified.
Modifications of the phase relation between muscular activation and foot movement
Application of a rotating inertial load to a voluntarily oscillating limb
increases the delay with which movement follows muscular activation. Moreover,
the delay is frequency-dependent and increases when the oscillation frequency
increases (Fig. 2E,
see also Baldissera and Cavallari
2001). The changes in the phase relations between the TA EMG and
the foot movement produced either by loading the foot with a rotating mass or
by modifying the oscillation frequency are exemplified in
Fig. 2. In this example, the
phase difference between the onset of the TA EMG and the onset (bold arrow) of
the foot dorsi-flexion increased by about the same amount either when, at 1.66
Hz, the foot was loaded with a balanced rotating mass (inertial momentum 98
g2) (compare B and D) or when, with the foot
loaded, the frequency was changed from 1.0 to 1.66 Hz (compare C and
D). Frequency-related changes of the EMG-movement delay between 0.8
and 3 Hz are illustrated in Fig.
2E, for both the unloaded and the loaded conditions (open
and filled circles, respectively). To be consistent with the phase
measurements regarding the H-reflex modulation (see Figs.
1 and
5), here we measured the phase
difference between movement and EMG (') not from the
movement onset, as previously done
(Baldissera and Cavallari
2001), but from the oscillation midpoint (vertical dotted line in
A).
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On the basis of the above effects of frequency and loading on coupling between muscular activation and movement, the H-reflex modulation was measured after connecting a rotating mass to the foot support and different rhythms of foot oscillations were examined between 1 and 2 Hz.
Effect of foot loading on H-reflex modulation
Figure 3, A–D, illustrates, for a representative subject, the change induced by foot loading on the phase relation between the modulation of the FCR H-reflex and the foot movement. In both conditions H-reflex modulation was significantly fitted (A: P < 0.04; B: P < 0.0001) by a sine-wave function (R2 = 0.81 in A and 0.97 in B). However, while in the unloaded condition the rising phase of the H-modulation started just before plantar flexion (A), after foot loading it was shifted forward, starting at the beginning of dorsi-flexion (B). In this subject, the advance of the H-modulation on movement was thus increased by 76°, from 53° (foot unloaded) to 129° (foot loaded). Note however that, in parallel with the shift of the H-reflex modulation, loading had produced a comparable forward shift of the TA EMG with respect to the movement, so that the modulation ascending phase still coincided with the period free of TA activity (i.e., the period of Sol contraction). Similar results were obtained in all six subjects, shown cumulatively in Fig. 3, E–F. Sinusoidal fitting of the individual reflex data were always significant, P ranging between <0.001 and <0.05. Determination coefficient, R2, was 0.81, 0.96, 0.75, 0.79, 0.92, 0.78; and 0.97, 0.92, 0.88, 0.57, 0.92, 0.74 for the loaded and unloaded conditions, respectively. Sinusoidal fitting of the cumulative reflex data was highly significant (P < 0.0001, determination coefficient, R2, 0.78 for unloaded and 0.62 for loaded data). The overall advance of the H-modulation on movement was increased by 90°, from 62° (foot unloaded) to 152° (foot loaded).
Whatever the reason for the phase advance observed in the H-reflex modulation after loading, its mere occurrence implies that it is not linked to movement or its derivatives, which are not modified by load application. Therefore modulation could not be attributed to any specific feedback mechanism based on kinesthetic afferences, monitoring the foot position, velocity, or acceleration.
The phase difference () between the best-fit sine-waves of the
H-modulation and the foot movement (H Mod-Mov delay) was measured as indicated
in Fig. 4A. Individual
values are plotted in Fig.
4C by small symbols (open = foot unloaded, filled = foot
loaded) and their means plotted by the large upper symbols. Mean values are
significantly different from each other (P < 0.001, paired
t-test) changing from 63 ±15° SD (foot unloaded) to 141
±41° (foot loaded). Having been obtained by averaging the
individual s, these mean values are slightly different from
s obtained by fitting the cumulative data points
(Fig. 3,
E–F).
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Figure 4 also illustrates
the phase relations between H-reflex modulation and activation onset in TA and
Sol muscles, before and after loading. These relations were measured as the
phase delay ('') between the H-modulation and the EMG onset,
as illustrated for the TA EMG in Fig.
4B (H Mod-TA delay). Their individual and mean values are
plotted in Fig. 4D. It
is apparent that values of '' for either Sol or TA are mostly
the same after loading (filled symbols) as they were before loading (open
symbols) and that the mean values of the data in the two conditions (large
upper symbols) are not different from each other ('' TA:
unloaded = 136° ± 24° SD; loaded = 142° ± 30°;
P > 0.44, paired t-test. '' Sol:
unloaded = –10° ± 29°; loaded = –12° ±
24°; P > 0.75).
In conclusion, loading of the foot modifies the phase relations of the H-reflex modulation with foot movement, while its relations with TA and Sol muscle contractions remains unmodified.
Effects of changing movement frequency
The FCR H-reflex modulation was measured at four different oscillation
frequencies, ranging between 1 and 2 Hz, in five subjects. In this range, the
effects of frequency on the EMG-movement delay are much larger after loading
than before (cf. Fig. 2); thus,
the tests were performed in the loaded condition. In the subject of
Fig. 5, the four-step increase
of the oscillation frequency (A to D) induced a progressive
forward shift of both the modulation curve and the onset of TA and Sol EMGs
with respect to movement, so that the phase relations between H-reflex
modulation and EMG onsets remained virtually unvaried. The phase delays
(H Modulation-Movement delay), ' (Movement-TA
EMG delay), and '' (H Modulation-TA EMG and -Sol EMG delays)
were measured between the four reference points indicated by vertical arrows
in A of Fig. 5. The
respective values are plotted in Fig. 6,
A to C, for five subjects: individual values are
shown by filled triangles and dashed lines and their means are shown by large
open symbols. Plot A shows how the phase delay between the foot oscillation
and the H-modulation () increased significantly with frequency,
while plot B illustrates the parallel decrease of ' [i.e.,
the phase delay between mid-flexion (movement reference point) and the onset
of the TA EMG]. The simultaneous reciprocal changes of the two above delays
results in the lack of change of both ''TA and
''Sol (i.e., the phase differences between the H-modulation
and the onsets of TA and Sol muscle activation, respectively)
(Fig. 6C, circles and
squares).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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), which
showed that the modulation was larger when the H-reflex was facilitated by TMS
(at short latency) than when it was unconditioned and that the modulation
disappeared when the reflex was evoked during the cortical "silent
period" induced by TMS. All these results combined suggest a cortical
origin for FCR H-reflex modulation, which would depend on activity
fluctuations occurring in the cortical motor area to the forearm motoneurones
during voluntary foot oscillations.
However, an afferent contribution to the H-reflex modulation cannot be
completely excluded: being correlated with muscle contraction, modulation
might be produced by discharges from Golgi tendon organs that, through their
cortical projections (McIntyre et al.
1984), may induce the excitability changes in the motor cortex
relayed to forearm motoneurones. In this case too, however, modulation of
forearm motor excitability would reflect the time course of the motor command
to the foot, not that of the foot movement.
The action of muscle spindles could instead be ruled out, since recordings
from their afferent fibers during voluntary movement in man
(Jones et al. 2001) showed
that their discharge was linked to lengthening of the parent muscle, not to
-drive, and well encoded the joint position. Thus being correlated with
movement kinetics, signals from spindles should not be involved in the
generation of the observed H-reflex modulation.
Other considerations contrast an afferent origin. Assuming that the H-reflex modulation is produced by a composite afferent signal, including position, velocity, and acceleration components, either from a single receptor type or from different kinds of receptors, it might be postulated that the three components ought to be affected to a different extent by the movement manipulations we employed. For example, if the reflex modulation is partly dependent on a velocity signal, then a forward phase shift of modulation on movement is expected (and actually occurs) when the movement frequency increases. Conversely, no similar effect is expected after loading, since loading leaves the movement period unchanged and, if anything, it smoothes the movement profile, thus filtering local velocity and acceleration components. Nevertheless, after both frequency increase and loading, H-reflex modulation is markedly shifted forward.
A further line in support of the hypothesis of a central origin for the
forearm excitability modulation comes from a comparison with results of
experiments in which hand–foot-coupled movements were perturbed by
loading the hand. In this condition, an afferent feed-back mechanism, likely
based on measuring the hand-foot asynchrony induced by loading, compensates
for the added inertia and helps to maintain coupling of limb oscillations
(Baldissera and Cavallari
2001). Since this compensation is mainly implemented by
anticipating the EMG onset in one limb with respect to the other, it cannot be
responsible for excitability modulation described here in the forearm, which
instead remains strictly linked to the activation of the foot EMG. In the
present experiments loading of the foot was indeed ineffective in modifying
the phase relations between muscular activation in the leg and excitability
changes in the forearm.
As suggested in INTRODUCTION, the time course of H-modulation
would favor isodirectional coupling when the hand and foot are oscillated
together. Should this view be correct, one would expect the excitability
modulation in resting forearm motoneurones to match the requirements of
isodirectional coupling. In particular, 1) a parallel modulation, but
opposite in-phase, should be present in the hand extensors; and 2)
modulation in FCR should reverse in-phase when the hand position is changed
from prone to supine. Both these predictions have been confirmed
experimentally (Baldissera et al.
2000,
2003).
As for the neural substrates of isodirectional coupling, recent findings
have shown strong functional interactions between the foot and hand areas of
the motor cortex during coupled movements of the two limbs
(Liepert et al. 1999). These
intracortical connections could in principle explain the link observed between
the hand and the foot, but they still do not clarify how the principle of
isodirectionality is maintained when the hand position is changed from prone
to supine. Interestingly, Kakei and colleagues described "extrinsic-like
neurons," principally in the ventral premotor cortex but also in the
primary motor cortex of the macaque, so called because they appear to encode
the movement of the wrist in a frame of extrinsic spatial coordinates,
independently of the forearm position (Kakei et al.
1999,
2001). Neurons with these
properties, inserted in the network responsible for hand-foot coupling, would
indeed encode the movements of the two limbs in absolute spatial coordinates
irrespectively to the pattern of muscle activation.
Modulation of reflex excitability in one limb during voluntary movements of
another limb has been described by several authors in recent years, though in
contexts different from the one considered here. During active pedaling with
one leg, the soleus H-reflex in the contralateral resting limb undergoes a
profound modulation (Brooke et al.
1992) which is absent during passive pedaling movements, a
condition in which the Sol H-reflex is tonically depressed
(Cheng et al. 1998;
Collins et al. 1993;
McIlroy et al. 1992).
Voluntary oscillations of the whole arm
(Hiraoka 2001) as well as
passive oscillations of the forearm
(Hiraoka and Nagata 1999) have
both been reported to modulate the Sol H-reflex excitability in a
phase-dependent way. Further, elevation of the arm is preceded and
accompanied, in most subjects, by a burst in Biceps Femoris EMG and a complete
silence in Soleus EMG, the latter being accompanied by a strong depression of
the Sol H-reflex (Kasai and Komiyama
1996) ascribed to a mixture of central and peripheral mechanisms
(Kawanishi et al. 1999). These
influences of arm movements on the leg muscles were explicitly discussed as
being "anticipatory postural activities"
(Bouisset and Zattara 1987;
Cordo and Nashner 1982;
Marsden et al. 1978,
1981;
Zattara and Bouisset 1988), a
category to which also the crossed inter-leg relation quoted above
(Brooke et al. 1992) may
possibly be ascribed, aimed at realizing a fixation chain for trunk
stabilization during one-leg pedaling. Finally, modulation of the FCR H-reflex
has been reported to occur during voluntary oscillations of the contralateral
hand (Carson et al. 1999) and
this may also be expression of postural activities, for instance, aimed at
stabilizing objects during bimanual manipulation.
Anticipatory postural activities are aimed to prepare a fixation chain
connecting the moving segment to a firm support, or to produce a
counter-movement that contrasts the postural unbalance produced by the main
body action. When explicitly manifest, they are characterized by the parallel
activation of muscles in different body segments, are scaled with the
intensity of the prime movement (Aruin and
Latash 1996), and can be reduced or abolished when the
biomechanical context is modified (Aruin et
al. 1998). Further, their timing and spatial distribution may vary
when the surrounding conditions or some feature of the movement (e.g.,
direction) is changed (Aruin and Latash
1995; Nashner and Forssberg
1986). This allows one to imagine that, even when a manifest
intervention of the anticipatory postural activities is not required,
subthreshold effects may nevertheless take place. In this view, the positive
and negative constraints characterizing ipsilateral limb coupling might indeed
be the expression of some underlying postural mechanism.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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FOOTNOTES |
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Address for reprint requests: F. Baldissera, Istituto di Fisiologia Umana II, Via Mangiagalli 32, I-20133 Milano, Italy (E-mail: fausto.baldissera{at}unimi.it).
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) and loaded (
) foot. In each subject, besides period
normalization and alignment to the midpoint of the movement cycle, data were
normalized in size to the amplitude of the respective best-fit sine wave. Note
the change in 
= 532 ms) was utilized for normalizing to 1 cycle the time-scale of all
parameters. The phase of the best-fit sinusoids for the movement and for the
H-reflex modulation was measured and their difference (
