JOURNALOF NEUROPHYSIOLOGY Vol. 67, No. 6, June 1992. Printed
in U.S.A.
Coordination Between Equilibrium and Head-Trunk Orientation During Leg Movement: A New Strategy Built Up by Training L. MOUCHNINO, R. AURENTY, J. MASSION, AND A. PEDOTTI Laboratoire de Neurobiologie et Mouvements, Centre National de la Recherche ScientiJique, 13402 Marseille, France, and Centro di Bioingegneria, PolitecnicoFondazione Pro Juventute Don Gnocchi, Via Gozzadini, 7, Milan, Italy control of equilibrium and head-trunk orientation was signifi1. During unilateral leg movementsperformedwhile standing, cantly lower. it is necessary to displacethe centerof gravity toward the other leg to maintain equilibrium. In addition, the orientation of particular segments, suchasthe headand trunk, which are usedasreference INTRODUCTION valuesfor organizing the motor act, needsto be preserved.The Performing a leg movement while standing raises specific aim of the presentstudy was to investigatethe coordination betweenmovement, equilibrium, and local posture. problems because of the fact that the limb to be moved is 2. Experimentswerecarried out on standingsubjectswho were involved in supporting the body. In the quadruped, the oninstructedto raiseone leglaterally to an angleof 45Oin responseto set of the movement is preceded by a displacement of the a light. Two sourcesof light placedin front of the subjectindicated center of gravity (CG) toward the center of a triangle the sideon which the movementwasto be performed.Three main formed by the three remaining supporting limbs (Dufosse aspectsof the posturokinetic sequencewere investigatedin two et al. 1982), whereas in man, the CG moves toward the populations,naive subjectsand dancers: et al. 199 1; Rogers and Pai 1) The body weight transfer toward the supporting leg was supporting leg (Mouchnino sefound to havetwo components:first, a “ballistic” one,initiated by 1990). The motor act is therefore a posturokinetic a thrust exertedby the moving leg; and second,an “adjustment” quence during which two goals are reached: first, displacing componentduring which the displacementof the centerof gravity the CG toward a new position compatible with equilibrium (CG) reachesa final position (steady state). An early burst in the maintenance during the leg movement and second, moving gastrocnemius medialisof the moving legoften precedesthe onset the leg toward a new position. of the center of pressurechange.Two differencesbetweennaive In the present series of experiments, we wanted to investisubjectsanddancerswereobserved:first, the newCG positionwas gate three main aspects of the posturokinetic sequence. almost reachedin one step very near to the end of the ballistic With each of these aspects, we wanted to compare a populacomponent and required only a short adjustment in dancers, of dancers who whereasin naive subjectsit wasreachedin two steps,including a tion of naive subjects with a population have undergone specific training in equilibrium and movemuch longer adjustmentcomponent. Second,the dancerswere ment control. The first aspect concerns the way in which ableto minimize the CG displacementtoward the supportingside; this might bebecausethey form a better internal representationof the displacement of the CG position is controlled. It has the biomechanicallimits of stability becauseof their long training. been previously reported both in quadrupeds and in man 2) The onset of the lateral displacementof the malleolus that, in fast sequences, the CG displacement is initiated by a marker of the moving legalwaysoccurred when the body weight thrust of the moving leg that pushes the trunk toward the had almostcompletedits transfer to above the support foot. This supporting leg (Birjukova et al. 1989; Rogers and Pai showsthat the positioningof the CG in a new positioncompatible 1990). The following questions are addressed here: is this with equilibrium maintenancewasa prerequisitefor the legmove- initial thrust responsible for the displacement of the CG ment to be performed. The relative timing of events during the toward its final position in a ballistic way, or is there an posturokineticsequencewasfairly fixed in the dancers,whereasit additional adjustment before reaching the final position? varied from one trial to another in the naive subjects. 3) The coordination between movement, equilibrium, and And, does specific training change the way in which the CG head-trunk orientation involves two control strategies.An “incli- is transferred? The second aspect concerns the onset of the nation” strategywasusedby the naive subjects;this consistedof movement with respect to the CG displacement. Does the an external rotation of the supportingleg around the anteroposte- movement onset start before the CG is displaced toward the rior anklejoint axis. A counter-rotation at the neck level ensured other leg, or is the movement onset delayed until a new CG the stability of the interorbital line in the horizontal plane. A position is reached? Here again, comparisons between un“translation” strategywasusedby the dancers;the external rota- trained subjects and dancers helped to establish whether tion of the supporting leg around the ankle joint was associated training had any effect on the subjects’s organization of the with a feedforwardcounter-rotation of the trunk aroundthe coxosequence. The third aspect concerns the maintenance of the femoraljoint which maintainedthe verticality of the whole headtrunk axis. Becausethe naive subjectswereunableto perform the position of the head and/or the trunk axis during the CG In fact, the body is a multilink structure counter-rotation of the trunk in a feedforward manner, this new displacement. coordination musthave resultedin the dancersfrom long training where the positions of the various segments can be reguand indicatesthat a new motor program waselaboratedby these lated on the basis of specific reference positions. The head subjects.The number of degreesof freedom in this coordinated axis, for example, can be stabilized either with respect to the SUMMARY
AND
CONCLUSIONS
0022-3077/92 $2.00Copyright0 1992TheAmericanPnysiological Society
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1588
L. MOUCHNINO,
R. AURENTY
vertical geocentric reference or with respect to the trunk axis (Assaiante and Amblard 1990; Pozzo et al. 1989). The change in CG position occurring during lateral leg raising is achieved by rotating the supporting leg externally around the anteroposterior ankle joint axis. This should laterally incline the trunk and head axes together with the leg. These postural changes might be compared with the behavior of an inverted pendulum (Gurfinkel et al. 1988; Horak and Nashner 1986; Nashner and McCollum 1985 ) . As a consequence of the body inclination, the initial vertical orientation of the head and trunk in space should be lost. The following question then arises: does a specific coordinated control take place that permits the CG displacement to be accomplished while the head and/or trunk axis is maintained vertical? And does training have any specific effects on subjects’ ability to organize this coordinated control? The main results obtained support two conclusions: first, that the CG displacement is performed in a ballistic way in dancers, where the new CG position is almost reached in a single step; and second, that the movement onset starts before that the new CG position is reached in naive subjects, whereas in dancers it occurs at the completion of the CG displacement. Finally, two kinematic strategies are described: the “inclination” strategy in naive subjects and the “translation” strategy in dancers, both of which result in the maintenance of the verticality of the head and the head and trunk axes, respectively, during the lateral inclination of the supporting leg associated with the CG displacement.
J. MASSION,
AND
A. PEDOTTI
askedto raiseoneleglaterally with no flexion of the knee,asfastas possible,to an angleof 45” (moving leg) and to maintain the final position for a few seconds.During eachsession, the subjecthad to perform four trials with eachleg, and the order in which the legs had to be lifted wasdeterminedby a random sequence.This protocol wasrun with five naive subjectsand five dancers. PARADIGM 2. This paradigmwasidentical to paradigm 1,except that the subjectwasaskedto keep his or her trunk vertical. This additional instruction wasgiven to four naive subjects.
Recording and data analysis
Ground reaction force, kinematic, and electromyographic (EMG) recordingswereperformedfor 2,500 msfrom the onsetof the diode light. The ground reaction forceswere recordedfrom a Kistler force platform on which the subjectwasstandingbarefoot. The horizontal displacementof the center of pressure(CP) wasdeterminated from the vertical ground reaction forcesand displayedin both frontal and sagittalplanes. The kinematic analysiswas performed by meansof an automatic TV image processor(EL.I.TE. system,Fezzigno and Pedotti, 1985) . Fourteen hemisphericalretroreflexive markers ( 7 mm diam) gluedonto the skin wereplacedsymmetricallyin pairs at the following sites:infraorbital margin, acromion, anterosuperior iliac spine,trochanter, middle of the internal border of the tibia1plate, medialmalleolusand first metatarsaljoint (Fig. 1). A cameraplaced5 m in front of the subjectrecordedthe displacementsof the markers.Becausethe movementswereperformedin the frontal plane, only this plane wasconsideredfor the analysis. From marker recording,a stick diagramof the body segments was constructedand displayed.Velocity curveswerecalculatedon the basisof the first derivative of the correspondingdisplacement.In addition, three lineswereconsidered:the line joining the infraorMETHODS bital markers(interorbital line), the linejoining the two acromion markers(shoulder line), and the line joining the two iliac crest Subjects and experimental paradigms markers(hip line). Experimentswere performed on 14 healthy subjectsof both From the positionsof the various markers,the following four sexes(7 male, 7 female) agedfrom 22 to 50 yr (mean age29 yr). angleson the frontal plane wereanalyzed (Fig. 1): Five subjectswere experiencedin modern dance techniquesand 1) Alpha angle( CW), betweenthe leg segment(line joining the had been trained for 2 15 yr; nine other subjectswere “naive,” trochanter marker to the malleolusmarker) and the horizontal becausethey practicedno regularsport and had no previousdance plane,giving the position of the supportinglegwith respectto the training. ground. PARADIGM 1. The subjectstoodbarefoot,with the heelsjoined at 2) Beta angle(,Q, formed by the trunk segment(line joining an angleof 90”, the handsbehind the back, the eyesgazinghori- the trochanter marker to the acromion marker) and the leg segzontally at two electroluminescentdiodesplacedsymmetrically 5 ment, giving the positionof the trunk relative to the position of the m in front of the subject’seyes.The illumination of one of the supportingleg. diodes constituted the movement onset signal and indicated 3) Gamma angle( r ), definedbetweenthe shoulderline (joinwhich leg shouldbe moved. At the signalonset, the subjectwas ing the two acromionmarkers)and perpendicularto the interorbi, Infraorbital
margin ,.** ..*.
.. . . Interorbitnl
Shoulders
-cc--
Antero-posterior
iliac spine
liue
Trunk segment . ..*..
Trochanter
line
..................
P
.‘.........
. Hip line
FIG. 1. Experimental set-up: lines, segments and angles. Left: subject is standing on a force platform; location of the markers is shown by dots. Right: diagram of the lines, segments and angles.
Tibia1 plate
Medial malleolus
1st
metatarsal
joint
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on September 18, 2018. Copyright © 1992 American Physiological Society. All rights reserved.
STRATEGIES
FOR BALANCE
1589
CONTROL
tal line (joining the two infraorbital markers), giving the position of the headrelative to the trunk. 4) Delta angle(6)) betweenthe interorbital line and the horizontal plane, giving the position of the headin space. EMG recordingswere madefrom eight musclesof eachsubject by meansof bipolar surfaceelectrodesspaced2 cm apart, placed on the right sideof the subject.Preamplifierswereplacednext to the recording electrodes.The EMG signalswere amplified by a factor of 1,000,band-passfiltered from 30 Hz to 1kHz, and rectified. The EMG measurementsfrom a total of 12 muscleswere studied. Becausea maximum of eight EMG channelscould be used,the combination of musclesexplored varied amongthe subjects. The musclesstudiedwerethe following: erector spinaeat L4 level (trunk extensor); rectus abdominis (trunk flexor); gluteus maximus( GlM, hip extensorand external rotator); tensor faciae latae (TFL, hip flexor, internal rotator, and legabductor); vastus lateralis and vastus medialis (VL, VM, knee extensor and hip flexor) ; bicepsfemoris( Bi, kneeflexor and hip extensor); gastrocnemiusmedialis(GM), gastrocnemiuslateralis,and soleus(Sol; ankleextensorand plantarflexor); and tibialis anterior and peroneuslateralis(TA, PL, ankle dorsiflexor).
of 534 t 162 ms in naive subjects and 559 t 169 ms in dancers. The duration was quite variable from trial to trial in naive subjects, as indicated by the lack of correlation between times tI and t2 at the start and end of the transfer phase, respectively, whereas in dancers the duration was fairly fixed in each subject, as shown by the high degree of correlation between t, and t2 (r > 0.80; P < 0.05). LATENCIES OF CP CHANGES. The mean latency between the stimulus onset and the first CP change was 18 1 t 82 ms in naive subjects and 2 17 t 9 1 ms in dancers; the difference between the two populations was not statistically significant. Analysis of changes in the CP curve in the frontal plane showed that the CP first moved laterally toward the moving leg and then toward the supporting leg. The early lateral displacement of the CP preceded the earliest motion of any markers by 226 t 96 ms in naive subjects and 2 16 t 55 ms in dancers. Moreover, the earliest displacement of any body segment (trunk) took place only after the CP curve had reached its maximal amplitude toward the moving leg Statistical analysis (time interval between maximal CP thrust and onset of lateral body displacement: 38 t 39 ms in naive subjects and The parametersrecordedin individual trials werethe latency of onsetand the amplitude of the displacementsof the markers.Sta- 26 t 68 ms in dancers (Fig. 3). The onset of the lateral CP displacement always preceded tistical analysiswasperformed on individuals and groups(naive subjectsand dancers). Relationshipsamong pairs of variables the body motion. Because its initiation is accompanied by were evaluatedby linear regressionanalysis.The meanand vari- an increase of the vertical forces of 9.7 t 6 N in both naive
anceof pairedvariableswerecomparedwith the useof Student’st test, and the two populationsasa whole werealsocomparedwith the useof Student’st test.
I Moving
leg +
t1
RESULTS
Phases of the motor act Three different phases were clearly identified during the motor act on the basis of kinetic and kinematic analysis: a “transfer phase,” a “movement phase” and a “position maintenance phase” (Fig. 2). During the transfer phase there is a transfer of the body weight toward the supporting leg between times tI and t2. The time tI can be identified from the kinetic analysis and was defined by the onset of the CP change, which corresponds to the onset of the first loms time interval where the amplitude of the CP change is > 1 mm, provided that it is followed by two steps of IO-ms time interval where an increase of 2 1 mm occurs. The time t2 is the onset of the lateral displacement of the malleolus marker, which occurs on the kinematic analysis and corresponds to the onset of the first 20-ms time interval where the amplitude of the lateral malleolus displacement of the moving leg is r2 mm, provided that it is followed by two steps of 20-ms time interval where an increase of 22 mm occurs. The movement phase starts at time t2 and ends at time t,, which corresponds to the end of the leg raising and the onset of the position maintenance. The position maintenance phase starts at time t3, at the point where the vertical velocity curve of the malleolus marker is entering the steady-state region ~0 t 5% of the maximal velocity. Transfer phase: kinetic analysis The present study will focus on the analysis of the transfer phase. The duration of this phase had a mean value
Malleolue LATERAL
of Moving Leg DISPLACEMENT
I !
I
200mm
Malleolus of VERTICAL
FIG. 2. Phases of the motor act. Recordings on a naive subject. Reference times were measured from 3 different curves. Top: CP curve recorded during the motor act. The start signal (diode light onset) occurred at the beginning of the curve. The reference times plotted on this curve are as follows: ti (onset of the CP change), tba,(end of the ballistic component of the CP change), and tadj (end of the adjustment component of the CP change). Middle: lateral displacement of the left internal malleolus marker on the moving leg. t2 corresponds to the onset of the lateral displacement of this marker (onset of movement). Bottom: vertical velocity curve ( 1st derivative of the displacement curve) of the left internal malleolus marker on the moving leg. t3corresponds to the end of the movement phase (vertical velocity curve) and to the onset of the position maintenance phase.
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1590
L. MOUCHNINO, 600 -
600
-
400
-
200
-
N A I s v u E B J E C T S
UY E & z
R. AURENTY,
4
-s+8
Onset of CP change 01) Latency of the peak CP thrust Onset of body lateral displacement
II
A N C E R S
800 t
E > 05 t4
600
-
400
-
200
-
0
IINDIVIDUAL
J. MASSION,
AND
A. PEDOTTI
tained throughout the movement (steady-state position). The tadj was measured when the CP curve reached a level corresponding to the mean steady state (measured during the position maintenance phase) plus 5% of the difference between the initial CP position and its final position, to take into account the slight oscillations of the CP curve that were observed throughout the movement. From t,,j to the end of the recording, the CP position remained quite stable and equal to the CG position. During the adjustment component, the gradual shift of the CP completes the transfer of the body weight onto the supporting leg. At this point, two questions can be raised. 1) Do the two components of the body weight transfer differ in duration between naive subjects and dancers? 2) Does the lateral leg raising occur before or after the end of the body weight transfer toward the supporting leg? First, the duration of the ballistic component did not differ significantly between naive subjects ( 532 t 129 ms) and dancers (547 t 138 ms). The adjustment component duration was statistically (P < 0.00 1) longer, however, in naive subjects ( 163 t 150 ms) than in dancers (57 t 54 ms); these data suggest that the dancers were more efficient at reaching the steady-state position at the end of the ballistic component than the naive subjects. Second, the temporal relationships between the move-
TRIALS
3. Body weight transfer: kinetic and kinematic changes. The onset of CP change ( tl ), the latency of the peak CP thrust, and the onset of the lateral displacement of the body (earliest lateral displacement of any marker) were compared in each trial. Note that the latency of the peak CP thrust and the onset of the lateral displacement of the body were timelocked in both naive subjects and dancers. FIG.
;
l
120
n
NAIVE SUBJECTS r -0.81 (P These statistical data point to the conclusion that the movement was initiated at the end of the ballistic component in both naive subjects and dancers. At the movement onset, the final position (steady state) is about to be reached in dancers, whereas in naive subjects, it is only reached after a final adjustment. AMPLITUDE OF THE CG DISPLACEMENT. we measured the difference between the initial position of the CP at the stimulus onset and its value during the final position. These two values are measured under static conditions in both the frontal and the sagittal planes. Because under static conditions the CP position corresponds to the CG projection onto the ground, the measured differences correspond to the CG displacement. Very little displacement occurred in the sagittal plane (individual mean value
in U.S.A.
Coordination Between Equilibrium and Head-Trunk Orientation During Leg Movement: A New Strategy Built Up by Training L. MOUCHNINO, R. AURENTY, J. MASSION, AND A. PEDOTTI Laboratoire de Neurobiologie et Mouvements, Centre National de la Recherche ScientiJique, 13402 Marseille, France, and Centro di Bioingegneria, PolitecnicoFondazione Pro Juventute Don Gnocchi, Via Gozzadini, 7, Milan, Italy control of equilibrium and head-trunk orientation was signifi1. During unilateral leg movementsperformedwhile standing, cantly lower. it is necessary to displacethe centerof gravity toward the other leg to maintain equilibrium. In addition, the orientation of particular segments, suchasthe headand trunk, which are usedasreference INTRODUCTION valuesfor organizing the motor act, needsto be preserved.The Performing a leg movement while standing raises specific aim of the presentstudy was to investigatethe coordination betweenmovement, equilibrium, and local posture. problems because of the fact that the limb to be moved is 2. Experimentswerecarried out on standingsubjectswho were involved in supporting the body. In the quadruped, the oninstructedto raiseone leglaterally to an angleof 45Oin responseto set of the movement is preceded by a displacement of the a light. Two sourcesof light placedin front of the subjectindicated center of gravity (CG) toward the center of a triangle the sideon which the movementwasto be performed.Three main formed by the three remaining supporting limbs (Dufosse aspectsof the posturokinetic sequencewere investigatedin two et al. 1982), whereas in man, the CG moves toward the populations,naive subjectsand dancers: et al. 199 1; Rogers and Pai 1) The body weight transfer toward the supporting leg was supporting leg (Mouchnino sefound to havetwo components:first, a “ballistic” one,initiated by 1990). The motor act is therefore a posturokinetic a thrust exertedby the moving leg; and second,an “adjustment” quence during which two goals are reached: first, displacing componentduring which the displacementof the centerof gravity the CG toward a new position compatible with equilibrium (CG) reachesa final position (steady state). An early burst in the maintenance during the leg movement and second, moving gastrocnemius medialisof the moving legoften precedesthe onset the leg toward a new position. of the center of pressurechange.Two differencesbetweennaive In the present series of experiments, we wanted to investisubjectsanddancerswereobserved:first, the newCG positionwas gate three main aspects of the posturokinetic sequence. almost reachedin one step very near to the end of the ballistic With each of these aspects, we wanted to compare a populacomponent and required only a short adjustment in dancers, of dancers who whereasin naive subjectsit wasreachedin two steps,including a tion of naive subjects with a population have undergone specific training in equilibrium and movemuch longer adjustmentcomponent. Second,the dancerswere ment control. The first aspect concerns the way in which ableto minimize the CG displacementtoward the supportingside; this might bebecausethey form a better internal representationof the displacement of the CG position is controlled. It has the biomechanicallimits of stability becauseof their long training. been previously reported both in quadrupeds and in man 2) The onset of the lateral displacementof the malleolus that, in fast sequences, the CG displacement is initiated by a marker of the moving legalwaysoccurred when the body weight thrust of the moving leg that pushes the trunk toward the had almostcompletedits transfer to above the support foot. This supporting leg (Birjukova et al. 1989; Rogers and Pai showsthat the positioningof the CG in a new positioncompatible 1990). The following questions are addressed here: is this with equilibrium maintenancewasa prerequisitefor the legmove- initial thrust responsible for the displacement of the CG ment to be performed. The relative timing of events during the toward its final position in a ballistic way, or is there an posturokineticsequencewasfairly fixed in the dancers,whereasit additional adjustment before reaching the final position? varied from one trial to another in the naive subjects. 3) The coordination between movement, equilibrium, and And, does specific training change the way in which the CG head-trunk orientation involves two control strategies.An “incli- is transferred? The second aspect concerns the onset of the nation” strategywasusedby the naive subjects;this consistedof movement with respect to the CG displacement. Does the an external rotation of the supportingleg around the anteroposte- movement onset start before the CG is displaced toward the rior anklejoint axis. A counter-rotation at the neck level ensured other leg, or is the movement onset delayed until a new CG the stability of the interorbital line in the horizontal plane. A position is reached? Here again, comparisons between un“translation” strategywasusedby the dancers;the external rota- trained subjects and dancers helped to establish whether tion of the supporting leg around the ankle joint was associated training had any effect on the subjects’s organization of the with a feedforwardcounter-rotation of the trunk aroundthe coxosequence. The third aspect concerns the maintenance of the femoraljoint which maintainedthe verticality of the whole headtrunk axis. Becausethe naive subjectswereunableto perform the position of the head and/or the trunk axis during the CG In fact, the body is a multilink structure counter-rotation of the trunk in a feedforward manner, this new displacement. coordination musthave resultedin the dancersfrom long training where the positions of the various segments can be reguand indicatesthat a new motor program waselaboratedby these lated on the basis of specific reference positions. The head subjects.The number of degreesof freedom in this coordinated axis, for example, can be stabilized either with respect to the SUMMARY
AND
CONCLUSIONS
0022-3077/92 $2.00Copyright0 1992TheAmericanPnysiological Society
1587
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on September 18, 2018. Copyright © 1992 American Physiological Society. All rights reserved.
1588
L. MOUCHNINO,
R. AURENTY
vertical geocentric reference or with respect to the trunk axis (Assaiante and Amblard 1990; Pozzo et al. 1989). The change in CG position occurring during lateral leg raising is achieved by rotating the supporting leg externally around the anteroposterior ankle joint axis. This should laterally incline the trunk and head axes together with the leg. These postural changes might be compared with the behavior of an inverted pendulum (Gurfinkel et al. 1988; Horak and Nashner 1986; Nashner and McCollum 1985 ) . As a consequence of the body inclination, the initial vertical orientation of the head and trunk in space should be lost. The following question then arises: does a specific coordinated control take place that permits the CG displacement to be accomplished while the head and/or trunk axis is maintained vertical? And does training have any specific effects on subjects’ ability to organize this coordinated control? The main results obtained support two conclusions: first, that the CG displacement is performed in a ballistic way in dancers, where the new CG position is almost reached in a single step; and second, that the movement onset starts before that the new CG position is reached in naive subjects, whereas in dancers it occurs at the completion of the CG displacement. Finally, two kinematic strategies are described: the “inclination” strategy in naive subjects and the “translation” strategy in dancers, both of which result in the maintenance of the verticality of the head and the head and trunk axes, respectively, during the lateral inclination of the supporting leg associated with the CG displacement.
J. MASSION,
AND
A. PEDOTTI
askedto raiseoneleglaterally with no flexion of the knee,asfastas possible,to an angleof 45” (moving leg) and to maintain the final position for a few seconds.During eachsession, the subjecthad to perform four trials with eachleg, and the order in which the legs had to be lifted wasdeterminedby a random sequence.This protocol wasrun with five naive subjectsand five dancers. PARADIGM 2. This paradigmwasidentical to paradigm 1,except that the subjectwasaskedto keep his or her trunk vertical. This additional instruction wasgiven to four naive subjects.
Recording and data analysis
Ground reaction force, kinematic, and electromyographic (EMG) recordingswereperformedfor 2,500 msfrom the onsetof the diode light. The ground reaction forceswere recordedfrom a Kistler force platform on which the subjectwasstandingbarefoot. The horizontal displacementof the center of pressure(CP) wasdeterminated from the vertical ground reaction forcesand displayedin both frontal and sagittalplanes. The kinematic analysiswas performed by meansof an automatic TV image processor(EL.I.TE. system,Fezzigno and Pedotti, 1985) . Fourteen hemisphericalretroreflexive markers ( 7 mm diam) gluedonto the skin wereplacedsymmetricallyin pairs at the following sites:infraorbital margin, acromion, anterosuperior iliac spine,trochanter, middle of the internal border of the tibia1plate, medialmalleolusand first metatarsaljoint (Fig. 1). A cameraplaced5 m in front of the subjectrecordedthe displacementsof the markers.Becausethe movementswereperformedin the frontal plane, only this plane wasconsideredfor the analysis. From marker recording,a stick diagramof the body segments was constructedand displayed.Velocity curveswerecalculatedon the basisof the first derivative of the correspondingdisplacement.In addition, three lineswereconsidered:the line joining the infraorMETHODS bital markers(interorbital line), the linejoining the two acromion markers(shoulder line), and the line joining the two iliac crest Subjects and experimental paradigms markers(hip line). Experimentswere performed on 14 healthy subjectsof both From the positionsof the various markers,the following four sexes(7 male, 7 female) agedfrom 22 to 50 yr (mean age29 yr). angleson the frontal plane wereanalyzed (Fig. 1): Five subjectswere experiencedin modern dance techniquesand 1) Alpha angle( CW), betweenthe leg segment(line joining the had been trained for 2 15 yr; nine other subjectswere “naive,” trochanter marker to the malleolusmarker) and the horizontal becausethey practicedno regularsport and had no previousdance plane,giving the position of the supportinglegwith respectto the training. ground. PARADIGM 1. The subjectstoodbarefoot,with the heelsjoined at 2) Beta angle(,Q, formed by the trunk segment(line joining an angleof 90”, the handsbehind the back, the eyesgazinghori- the trochanter marker to the acromion marker) and the leg segzontally at two electroluminescentdiodesplacedsymmetrically 5 ment, giving the positionof the trunk relative to the position of the m in front of the subject’seyes.The illumination of one of the supportingleg. diodes constituted the movement onset signal and indicated 3) Gamma angle( r ), definedbetweenthe shoulderline (joinwhich leg shouldbe moved. At the signalonset, the subjectwas ing the two acromionmarkers)and perpendicularto the interorbi, Infraorbital
margin ,.** ..*.
.. . . Interorbitnl
Shoulders
-cc--
Antero-posterior
iliac spine
liue
Trunk segment . ..*..
Trochanter
line
..................
P
.‘.........
. Hip line
FIG. 1. Experimental set-up: lines, segments and angles. Left: subject is standing on a force platform; location of the markers is shown by dots. Right: diagram of the lines, segments and angles.
Tibia1 plate
Medial malleolus
1st
metatarsal
joint
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (192.236.036.029) on September 18, 2018. Copyright © 1992 American Physiological Society. All rights reserved.
STRATEGIES
FOR BALANCE
1589
CONTROL
tal line (joining the two infraorbital markers), giving the position of the headrelative to the trunk. 4) Delta angle(6)) betweenthe interorbital line and the horizontal plane, giving the position of the headin space. EMG recordingswere madefrom eight musclesof eachsubject by meansof bipolar surfaceelectrodesspaced2 cm apart, placed on the right sideof the subject.Preamplifierswereplacednext to the recording electrodes.The EMG signalswere amplified by a factor of 1,000,band-passfiltered from 30 Hz to 1kHz, and rectified. The EMG measurementsfrom a total of 12 muscleswere studied. Becausea maximum of eight EMG channelscould be used,the combination of musclesexplored varied amongthe subjects. The musclesstudiedwerethe following: erector spinaeat L4 level (trunk extensor); rectus abdominis (trunk flexor); gluteus maximus( GlM, hip extensorand external rotator); tensor faciae latae (TFL, hip flexor, internal rotator, and legabductor); vastus lateralis and vastus medialis (VL, VM, knee extensor and hip flexor) ; bicepsfemoris( Bi, kneeflexor and hip extensor); gastrocnemiusmedialis(GM), gastrocnemiuslateralis,and soleus(Sol; ankleextensorand plantarflexor); and tibialis anterior and peroneuslateralis(TA, PL, ankle dorsiflexor).
of 534 t 162 ms in naive subjects and 559 t 169 ms in dancers. The duration was quite variable from trial to trial in naive subjects, as indicated by the lack of correlation between times tI and t2 at the start and end of the transfer phase, respectively, whereas in dancers the duration was fairly fixed in each subject, as shown by the high degree of correlation between t, and t2 (r > 0.80; P < 0.05). LATENCIES OF CP CHANGES. The mean latency between the stimulus onset and the first CP change was 18 1 t 82 ms in naive subjects and 2 17 t 9 1 ms in dancers; the difference between the two populations was not statistically significant. Analysis of changes in the CP curve in the frontal plane showed that the CP first moved laterally toward the moving leg and then toward the supporting leg. The early lateral displacement of the CP preceded the earliest motion of any markers by 226 t 96 ms in naive subjects and 2 16 t 55 ms in dancers. Moreover, the earliest displacement of any body segment (trunk) took place only after the CP curve had reached its maximal amplitude toward the moving leg Statistical analysis (time interval between maximal CP thrust and onset of lateral body displacement: 38 t 39 ms in naive subjects and The parametersrecordedin individual trials werethe latency of onsetand the amplitude of the displacementsof the markers.Sta- 26 t 68 ms in dancers (Fig. 3). The onset of the lateral CP displacement always preceded tistical analysiswasperformed on individuals and groups(naive subjectsand dancers). Relationshipsamong pairs of variables the body motion. Because its initiation is accompanied by were evaluatedby linear regressionanalysis.The meanand vari- an increase of the vertical forces of 9.7 t 6 N in both naive
anceof pairedvariableswerecomparedwith the useof Student’st test, and the two populationsasa whole werealsocomparedwith the useof Student’st test.
I Moving
leg +
t1
RESULTS
Phases of the motor act Three different phases were clearly identified during the motor act on the basis of kinetic and kinematic analysis: a “transfer phase,” a “movement phase” and a “position maintenance phase” (Fig. 2). During the transfer phase there is a transfer of the body weight toward the supporting leg between times tI and t2. The time tI can be identified from the kinetic analysis and was defined by the onset of the CP change, which corresponds to the onset of the first loms time interval where the amplitude of the CP change is > 1 mm, provided that it is followed by two steps of IO-ms time interval where an increase of 2 1 mm occurs. The time t2 is the onset of the lateral displacement of the malleolus marker, which occurs on the kinematic analysis and corresponds to the onset of the first 20-ms time interval where the amplitude of the lateral malleolus displacement of the moving leg is r2 mm, provided that it is followed by two steps of 20-ms time interval where an increase of 22 mm occurs. The movement phase starts at time t2 and ends at time t,, which corresponds to the end of the leg raising and the onset of the position maintenance. The position maintenance phase starts at time t3, at the point where the vertical velocity curve of the malleolus marker is entering the steady-state region ~0 t 5% of the maximal velocity. Transfer phase: kinetic analysis The present study will focus on the analysis of the transfer phase. The duration of this phase had a mean value
Malleolue LATERAL
of Moving Leg DISPLACEMENT
I !
I
200mm
Malleolus of VERTICAL
FIG. 2. Phases of the motor act. Recordings on a naive subject. Reference times were measured from 3 different curves. Top: CP curve recorded during the motor act. The start signal (diode light onset) occurred at the beginning of the curve. The reference times plotted on this curve are as follows: ti (onset of the CP change), tba,(end of the ballistic component of the CP change), and tadj (end of the adjustment component of the CP change). Middle: lateral displacement of the left internal malleolus marker on the moving leg. t2 corresponds to the onset of the lateral displacement of this marker (onset of movement). Bottom: vertical velocity curve ( 1st derivative of the displacement curve) of the left internal malleolus marker on the moving leg. t3corresponds to the end of the movement phase (vertical velocity curve) and to the onset of the position maintenance phase.
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Onset of CP change 01) Latency of the peak CP thrust Onset of body lateral displacement
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tained throughout the movement (steady-state position). The tadj was measured when the CP curve reached a level corresponding to the mean steady state (measured during the position maintenance phase) plus 5% of the difference between the initial CP position and its final position, to take into account the slight oscillations of the CP curve that were observed throughout the movement. From t,,j to the end of the recording, the CP position remained quite stable and equal to the CG position. During the adjustment component, the gradual shift of the CP completes the transfer of the body weight onto the supporting leg. At this point, two questions can be raised. 1) Do the two components of the body weight transfer differ in duration between naive subjects and dancers? 2) Does the lateral leg raising occur before or after the end of the body weight transfer toward the supporting leg? First, the duration of the ballistic component did not differ significantly between naive subjects ( 532 t 129 ms) and dancers (547 t 138 ms). The adjustment component duration was statistically (P < 0.00 1) longer, however, in naive subjects ( 163 t 150 ms) than in dancers (57 t 54 ms); these data suggest that the dancers were more efficient at reaching the steady-state position at the end of the ballistic component than the naive subjects. Second, the temporal relationships between the move-
TRIALS
3. Body weight transfer: kinetic and kinematic changes. The onset of CP change ( tl ), the latency of the peak CP thrust, and the onset of the lateral displacement of the body (earliest lateral displacement of any marker) were compared in each trial. Note that the latency of the peak CP thrust and the onset of the lateral displacement of the body were timelocked in both naive subjects and dancers. FIG.
;
l
120
n
NAIVE SUBJECTS r -0.81 (P These statistical data point to the conclusion that the movement was initiated at the end of the ballistic component in both naive subjects and dancers. At the movement onset, the final position (steady state) is about to be reached in dancers, whereas in naive subjects, it is only reached after a final adjustment. AMPLITUDE OF THE CG DISPLACEMENT. we measured the difference between the initial position of the CP at the stimulus onset and its value during the final position. These two values are measured under static conditions in both the frontal and the sagittal planes. Because under static conditions the CP position corresponds to the CG projection onto the ground, the measured differences correspond to the CG displacement. Very little displacement occurred in the sagittal plane (individual mean value
