ANNALS OF HUMAN BIOLOGY MAY–JUNE 2003, VOL. 30, NO.
3, 279–294
The influence of gender and body characteristics on upright stance I. Farenc, P. Rougier and L. Berger Laboratoire de Mode´lization des Activite´s Sportives, UFR CISM, Universite´ de Savoie, Domaine Universitaire de Savoie-Technolac, F73376 Le Bourget du Lac cedex France Received 24 June 2002; in revised form 18 September 2002; accepted 13 November 2002
Summary. Background: Morphologic characteristics such as height and body weight determine body inertia, an important factor related to postural stability. However, whilst investigations have classically analysed these parameters separately, global morphology has been poorly researched. Secondly, the influence of gender on postural stability demonstrates opposing trends, some authors observing that men sway less than women, and others noting the contrary. Aim: The aims of this study are to evaluate morphology and gender effects on healthy subjects during postural maintenance. Subjects and methods: The studied subjects were categorized through the Livi index. A method associating frequential and Brownian parameters characterized the horizontal displacements of the centre of gravity (CGh) and those of the difference between the centre of pressure (CP) and the vertical projection of the centre of gravity (CP-CGv) separately. Moreover, the moments of body inertia (MI) and natural body frequency (NBF) were also used to determine the influence of morphology. Results and conclusions: The results reveal that thinner subjects have larger CGh displacements than normal or corpulent subjects. Morphologic characteristics (NBF) can explain these behavioural differences. On the other hand, men have a larger sway amplitude for CGh motions than women. This can be explained by both morphologic (MI, NBF) and physiological (architectural properties of the soleus muscle) characteristics.
1.
Introduction The study of instantaneous upright stance is of particular interest since it corresponds to a very specific and fundamental attitude in humans. The human body, in this posture, can be modelled as an inverted pendulum. Biomechanically, the centre of gravity (CG) and the centre of pressure (CP) displacements can be used to describe undisturbed upright stance. By definition, CG is the barycentre of the centres of mass of body segments and CP is the application point of the resultant of the ground reaction forces, which results from the ankle and hip muscular torques, which, by definition, cannot be constant over time. The inertia of the human body intervenes in this task by generating a horizontal lag between CP and CG motions and the relative horizontal displacements of CG (CGh) motions induced by CP displacements. Consequently, a vertical difference between these CP and CG motions (CP-CGv) is necessarily present even though a vertical alignment can be seen for transient moments. The horizontal accelerations communicated to the CG motions, as initially demonstrated by Brenie`re et al. (1987), explains that the body is swaying continuously during postural maintenance. The body inertia is determined by the morphology of the subject, and can be deduced from height and weight (Ledebt and Brenie`re 1994). The influence of these height and weight variables on postural balance has been studied by several authors (Bessineton et al. 1976, Berger et al. 1992). To our knowledge, only Allard et al. (2001) have considered height and weight as a global morphologic factor. Annals of Human Biology ISSN 0301–4460 print/ISSN 1464–5033 online # 2003 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/0301446031000068842
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On the other hand, several authors have debated the influence of gender in postural stability. Some research in undisturbed upright stance (Black et al. 1982, Maki et al. 1990, Hageman et al. 1995) reported no gender difference in either the eyes open (EO) and eyes closed (EC) condition. Conversely, others (Stribley et al. 1974, Juntunen et al. 1987, Ekdahl et al. 1989, Kollegger et al. 1992) reported differences between men and women in postural control. Juntunen et al. (1987) and Kollegger et al. (1992) suggest that these gender differences may be explained by the stabilizing part of vision, which seems be more significant for men. For these reasons, our subjects were tested in both EO and EC conditions in order to assess the respective roles of visual and somatosensorial information. In the EO condition, vision constitutes the main sensorial input whereas it is recognized that postural control relies principally upon somaesthetic information in the EC condition (Diener et al. 1984, Nakagawa et al. 1993). Lastly, men and women can also be characterized by differences in muscle architectural properties such as fibre size and angles of pennation (Chow et al. 2000). The above-mentioned studies suggest that morphology and gender characteristics may modify postural organization. To test the plausibility of this influence we chose a discriminating method based on the CP trajectory measured through a force platform. By modelling the CP trajectory as fractional Brownian motion (fBm), Collins and De Luca (1993) and Rougier (1999a) have demonstrated that this CP assumes two different and quite opposite tasks. On the one hand, it remains within the base of support and appears to contain the CG in its position. On the other hand, the CP is, at times, used to displace the CG in order to make it return to a position more compatible with equilibrium principles. Therefore, it seems logical to dissociate the CP into two elementary components: CGh and CP-CGv (Rougier and Caron 2000). The CGh motions, recognized as the controlled variable, represent the net performance and are exclusively controlled by the CP displacements (Winter et al. 1996). Complementarily, it can be demonstrated that CP-CGv amplitudes present a certain proportionality with the horizontal CG acceleration (Brenie`re et al. 1987). Thus, the larger the CP-CGv, the greater the acceleration of the CGh. Winter et al. (1998) have also specified that CP-CGv represents the difference between the stiffness torque and the gravitational torque. The effective stiffness appears proportional to the body’s natural frequency, which can be estimated from the CP-CGv frequential spectrum. The respective contributions of CGh and CP-CGv motions in complex CP displacements can be obtained through a frequential and fBm analyses. The purpose of this study is to evaluate the effects of morphology and gender of healthy subjects on postural control. First, a distribution was made with the Livi index in order to evaluate the morphologic effects on postural control. This index is classically used to observe the effects of morphology (Carlberg et al. 1983). Secondly, a comparison according to gender was performed in order to highlight some of the differences between men and women. Finally, the moments of body inertia (MI) and natural body frequency (NBF) were used to investigate the dynamic implications of morphologic characteristics in the postural control process. 2. Materials and methods 2.1. Population A total of 57 individuals, 22 women (age 26 9 years; body mass 58.76 4.76 kg; height 167 0.06 cm) and 35 men (age 29 11 years; body mass 71.35 5.73 kg; height 178 0.05 cm), ranging in age from 20 to 60 years were tested. These
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volunteers were healthy and physically active with no known musculoskeletal impairments or neurological disorders that might have affected their sense of balance. These subjects were categorized by the index of Livi (I) defined by the following ratio: pffiffiffiffiffiffiffiffiffiffiffi 100 3 mass I¼ height The classification of subjects was based on the following criteria: . Group of Thin individuals (GT): 22 4 I 4 22.9 . Group of Normal individuals (GN): 23 4 I 4 23.9 . Group of Corpulent individuals (GC): 24 4 I 4 24.9 From this index, three morphologic groups were formed: 18 subjects belonging to GT (six women and 12 men), 27 to GN (12 women and 15 men) and 10 to GC (four women and seven men). 2.2. Experimental procedure The subjects stood barefoot on a force platform (Equi+, model PF01) in the standard position (feet abducted at 308, heels separated by 3 cm) and were asked to sway as little as possible, with their arms at their sides. The signals from these load cells were amplified and converted from analogue to digital form and recorded on a personal computer. The CP trajectory was subsequently processed through a specific software program, Equi+Prog01. The coordinates system defines a horizontal plane in which ML and AP characterize Medio-Lateral and Antero-Posterior directions, respectively. Two experimental conditions were randomly tested in the Eyes Open (EO) condition, where subjects were instructed to stare at a plumb line, and the Eyes Closed (EC) condition, the latter being used as reference. The experiment included five trials of 64 s with a 64 Hz sample frequency. Rest periods of a similar duration (64 s) were allowed between each trial and a 10 min rest was granted between each condition. 2.3. CGh estimation Using the CP trajectories, computed from the force platform data, the CGh and consequently the CP-CGv motions were calculated. The latter motions are determined through an amplitude ratio varying as a function of frequency between the vertical projection of the centre of gravity and CP motions. Since the task of upright stance generates reduced body sway, the human body can theoretically be modelled as an inverse pendulum where CG angular oscillations corresponding to the CGh positions are considered as a simple periodic function in phase with the CP. This method is derived from a mathematical model, initially proposed in the study of gait initiation (Brenie`re 1996) and then extended to undisturbed stance (Caron et al. 1997). This method is given by the following formula: CGh =CP ¼ 20 =ð 20 þ 2 Þ where ¼ 2f is the pulsation (rad s–1) and 0 ¼ ðmgh=ðIG þ mh2 ÞÞ1=2 (Hz), termed natural body frequency, is a biomechanical constant relative to the anthropometry of the subject (m, g, h, IG are, respectively, mass of the subject, gravity acceleration, distance from CG to the ground, and moment of body inertia in the ML and AP direction with respect to the CG).
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CP-CGv
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Figure 1. Synoptic of the different stages in the determination of CP-CGv and CGh motions in one direction. One trajectory representing one direction is presented according to time (part A). In order to obtain CP-CGv motions, we use a mathematical low-pass filter expressing an amplitude ratio between CG and CP as a function of the movement frequency. The CP displacements are then processed through a Fast Fourier Transform (FFT) in order to obtain the amplitude distribution as a function of the frequency. Once this CP spectrum (part B) is obtained, a multiplication with the aforementioned filter will give the CGh spectrum (part C) and a subtraction will give the CP-CGv spectrum (part D). At this stage, through an inverse FFT (IFFT), it is possible to return to the temporal domain and obtain CGh (part E) and consequently CP-CGv (part F) motions. The latter motion can also be obtained by applying IFFT to the CP-CGv spectrum (arrow with a broken line).
The main stages of this method are presented in figure 1. Note that this method was described in detail in a previous study (Rougier and Farenc 2000). 2.4. Method Two different approaches were taken to study the CGh and CP-CGv elementary motions: (1) frequential and (2) fractional Brownian motion (fBm). The former is a global analysis that highlights the main modifications induced by control mechanisms in order to maintain equilibrium. By definition, a given movement can be characterized by an oscillatory process operating over a specific frequency bandwidth. The latter (fBm modelling) provides insight into the nature and the temporal organization of the control mechanisms called into play. Finally, the moment of body inertia (MI) and the natural body frequency (NBF) were calculated for each subject in order to determine the role played by body parameters (weight and height) in upright stance. 2.5. Frequential analysis The frequency analysis provides mean spectral decompositions of various motions on specific bandwidths (0–0.5 Hz for CGh; 0–3 Hz for CP-CGv). To quantify the various spectra, several parameters are used. The root mean square (RMS), which measures the total spectrum energy, characterizes the amplitudes independent of the
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frequency distribution. On the other hand, the median frequency (MF) divides the spectrum into two parts with similar energy. 2.6. Fractional Brownian motion (fBm) modelling fBm modelling can determine the respective contribution of stochastic and deterministic processes in postural control mechanisms. It is for this reason that the concept of fBm modelling is particularly well adapted to the study of non-stationary signals (Riley et al. 1997). Initially used to study the global CP trajectories (Collins and De Luca 1993, Rougier 1999a), it was recently extended to CP-CGv and CGh motions (Rougier and Caron 2000). One interesting result of this latter study is that partially deterministic controls successively operate initially on CP-CGv and then on CGh motions. In addition, when one elementary motion is controlled, the other behaves stochastically. This approach provides insight into of the nature of the control mechanisms called into play and the temporal organization. The variograms provide information that graphically expresses the mean square displacement hx2 i as a function of increasing time intervals t for both ML and AP directions. Given that two straight line portions generally characterize variograms for undisturbed upright stance (one quite flat one preceding or succeeding a steeper one), one stage in the process involves the determination of the transition point, i.e. the point corresponding to the slope inflection, using an automatic method (Rougier 1999b). The transition point represents the spatio-temporal threshold from which the corrective mechanism is initiated. By definition, CP and CG displacements being in phase, the temporal coordinate of the CP trajectories will also be that of the CGh and CP-CGv motions. On the other hand, the scaling exponents H (corresponding to the variogram’s half slopes) indexed as short and long latencies Hsl and Hll, characterize the degree of control of each trajectory for the shortest and longest time intervals, respectively. Thus, for each of the two CGh and CP-CGv motions investigated and each ML and AP component, two scaling exponents (indexed as short and long latencies: Hsl and Hll) as well as the coordinates of the transition point were extracted. 2.7. The moment of inertia (MI) and the natural body frequency (NBF) It is believed that the body itself imposes its own constraints. Ledebt and Brenie`re (1994) have investigated the dynamic implications of anatomical parameters in the gait initiation process. This has led to the establishment of the moments of body inertia in both the AP (MIAP) and ML direction (MIML) estimated by the following relationships: MIML ¼ 0:0533 m H 2 MIAP ¼ 0:0572 m H 2 where m and H represent the body mass and the height of the subject, respectively. As explained above, the oscillations of subjects are assimilated to those of an inverse pendulum. In this case, the moment of body inertia against the ankle is estimated as constant (Winter et al. 1996). In upright quiet stance, the human body, considered as an oscillating system, is characterized by specific parameters such as natural frequency. Brenie`re (1996) has demonstrated that the human body has a natural body frequency (NBF) which
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depends mainly on the body weight and moment of inertia. The NBF is defined by the following relationship: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi NBF ¼ ðm g hCG Þ=ðMI þ m þ h2CG Þ where m, g, hCG and MI represent the body mass, the acceleration of the gravity, the CG height and the moment of body inertia for each direction, respectively. 2.8. Statistical analysis To compare the postural control of morphologic groups (GT, GN, GC), a oneway ANOVA was performed on each of the frequential and mBf parameters. In addition, a two-way gender (women vs men) visual conditions (EO vs EC) ANOVA was performed on frequential and mBf parameters. Finally, to test the hypothesis that standing posture is related to anthropometric characteristics, a two-way gender (women vs men) morphologic groups (GT, GN, GC) ANOVA was performed on MI and NBF parameters for both ML and AP directions. Post hoc LSD tests were carried out to identify the groups that were statistically different from each other. The first level of significant differences was set at p < 0.05. 3. Results 3.1. Differences according to body morphology 3.1.1. Frequential parameters. Some significant differences of RMS for both CP-CGv and CGh motions in the ML direction are observed (CP-CGv F(2,103) ¼ 7.92, p < 0.001; CGh F(2,103) ¼ 5.57, p < 0.01). The RMS of CP-CGv motions demonstrates that the GT group elicit larger initial horizontal accelerations from their CG than the GN and GC groups. Indeed, the amplitudes of spectra (RMS) of CP-CGv motions are larger in the ML direction for GT when compared to those of GN (p < 0.001) and GC (p < 0.01). In the ML direction, GT display larger CGh motions than GN (p < 0.01) and GC (p < 0.01), as indicated by the RMS. These results are illustrated in figure 2. 3.1.2. fBm modelling. The main differences between the various morphologic groups (GT, GN, GC) concern the scaling exponents of short latencies (Hsl) of the CP-CGv motions in ML and AP directions (ML F(2,103) ¼ 6.036, p < 0.01; AP F(2,103) ¼ 10.02, p < 0.001). As seen in figure 3, Hsl demonstrates that the CP-CGv trajectories are more controlled for the GC group in both ML and AP directions. Indeed, significant decreases in the slopes of the initial line portions (Hsl) are observed for CP-CGv motions in the ML direction for GN when compared to GC (p < 0.01), and for GN when compared to GT (p < 0.05). In the AP direction, similar results are observed for GT when compared to GC (p < 0.01), and for GN when compared to GC (p < 0.001). In addition, the spatio-temporal coordinates of the transition point, i.e. the time interval t and the mean square distance (hx2 i) covered at the onset of the corrective process, are also modified according to the morphologic group (hx2 i F(2,103) ¼ 6.49, p < 0.01; t F(2,103) ¼ 3.10, p < 0.05) (figure 3). Indeed, in the ML direction GN elicits larger delays than GT in setting the corrective process (p < 0.05). Moreover, the hx2 i in the ML direction demonstrates a larger increase in CP-CGv motions for the GT when compared to GN (p < 0.01) and GC (p < 0.01). Note that no significant difference is observed for CGh motions.
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Figure 2. Upper part: the spectral decomposition characterizing CP-CGv motions and histograms showing group means and standard deviations for frequential parameters for the three morphologic groups (GT, GN, GC) in the ML direction. Lower part: the spectral decomposition characterizing CGh motions and histograms for frequential parameters for GT, GN and GC, which characterize these spectra decompositions for the ML direction. There are significant results on the amplitudes of spectra (RMS) for both CP-CGv and CGh in the ML direction. The significant effects assessed by the ANOVA are presented above the histograms whilst those relative to the simple effects are displayed below (***p < 0.001; **p < 0.01; *p < 0.05).
3.2. Differences between women and men 3.2.1. Frequential parameters. Firstly, it should be noted that the observed gender differences appear only in the AP direction. The frequential distribution, expressed by MF calculation for CP-CGv motions, displays a shift towards lower frequencies for women (figure 4, upper part) when compared to men (F(1,105) ¼ 4.95, p < 0.05). As seen in the lower part of figure 4, the women’s mean amplitudes (RMSAP) are smaller for CGh motions (CGh: F(1,105) ¼ 6.69, p < 0.05) whereas the MF for CGh is larger than that for men (F(1,105) ¼ 4.79, p < 0.05). 3.2.2. fBm parameters. The main differences between women and men concern the scaling exponents of short latencies (Hsl) of the CP-CGv motions. A significant
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Figure 3. Upper part: the variograms characterizing CP-CGv motions for the three morphologic groups (GT, GN, GC) and histograms showing group means and standard deviations for fBm parameters, which characterize these variograms for the ML direction. Lower part: variograms and histograms characterizing CP-CGv motions in the AP direction. Some significant differences are observed for CP-CGv motions in both ML and AP directions between the three morphologic groups. Note that the differences are more pronounced in the ML direction. As in figure 2, the statistical results are expressed above the histograms (***p < 0.001; **p < 0.01; *p < 0.05).
decrease in the slopes of the initial line portions demonstrates that the CP-CGv trajectories are less controlled in the case of the men in the AP direction. The scaling exponents corresponding to the shorter t illustrated by the variogram and histograms in figure 5 (lower part) confirm this. Indeed, the decreases observed in Hsl for CP-CGv motions in the AP direction appear significant for men when compared to women (F(1,105) ¼ 7.30, p < 0.01). It is worth noting that no significant difference is observed for hx2 i and t for both CP-CGv and CGh motions. Note that no differences between women and men are observed for both frequential or fBm parameters.
3.3. Differences according to morphologic characteristics, moment of body inertia (MI) and natural body frequency (NBF) Some significant differences in morphologic characteristics are observed for both gender (women vs men) and morphologic groups (GT, GN, GC), as illustrated in figure 6. Indeed, as expected, differences arising due to the subjects’ height are observed for morphologic groups (F(2,52) ¼ 7.64, p < 0.01): GT are taller than GN
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Frequency (Hz) Figure 4. Upper part: the spectral decomposition characterizing the whole sample population (mean in continuous lines SD in dashed thin lines) for CP-CGv motions for women and men, and histograms for frequential parameters which characterize these spectra decompositions in the AP direction. Lower part: spectral decomposition and histograms for CGh motions in the AP direction. On the one hand, the women’s MFAP for CP-CGv motions is indeed lower than that of the men. On the other hand, the women’s mean amplitudes (RMSAP) are smaller for CGh motions whereas the MF for CGh is larger than for the men (***p < 0.001; **p < 0.01; *p < 0.05).
(p < 0.001) and GC (p < 0.01) (figure 6, lower part). Concerning gender groups, men are taller (F(2,52) ¼ 43.10, p < 0.001) and bigger (F(2,52) ¼ 41.95, p < 0.001) than women (figure 6, upper part). MI and NBF results demonstrate some differences for both gender (women vs men) and morphologic groups (GT, GN, GC). Concerning gender groups, women have smaller MI than men in both ML and AP directions (MIML F(1,53) ¼ 79.81, p < 0.001; MIAP F(1,53) ¼ 79.81, p < 0.001). On the other hand, women sway at a higher frequency than men in both ML and AP directions, as indicated by the NBF parameter (ML F(1,53) ¼ 57.36, p < 0.001; AP F(1,53) ¼ 57.56, p < 0.001) (figure 7, upper part).
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Figure 5. Upper part: the variograms for CP-CGv motions characterizing the whole sample population (mean in largest lines SD in thin lines) for both women and men, and histograms for fBm parameters, which characterize these variograms in the ML direction. Lower part: variograms and histograms of CP-CGv motions in the AP direction. The main result concerns the scaling exponents corresponding to the shorter t: some significant decreases in Hsl are observed for CP-CGv motions in the AP direction for men when compared with women (***p < 0.001; **p < 0.01; *p < 0.05).
Concerning the morphologic groups, some differences are observed between GT, GN and GC for NBF parameters in both ML and AP directions (ML F(2,52) ¼ 7.45, p < 0.01; AP F(2,52) ¼ 7.45, p < 0.01). Indeed, GT have lower NBF than GN (ML p < 0.01; AP p < 0.01) and GC (ML p < 0.001; AP p < 0.001) in both ML and AP directions (figure 7, upper part). Note that we observe no significant differences between morphologic groups for the MI parameter. On the other hand, the NBF parameter is sensitive to both gender and morphologic groups in both ML and AP directions (ML F(2,49) ¼ 3.85, p < 0.05; AP F(2,49) ¼ 3.65, p < 0.05). Indeed, thin women sway at a lower frequency than normal (NBFML p < 0.001; NBFAP p < 0.001) and corpulent women (NBFML p < 0.001; NBFAP p < 0.001) in both ML and AP directions (figure 7, lower part). The same tendency is observed for men: thin men sway at a lower frequency than normal (NBFML p < 0.001; NBFAP p < 0.001) and corpulent men (NBFML p ¼ 0.051; NBFAP p ¼ 0.052) (figure 7, lower part). The totality of MI and NBF results for gender and morphologic groups are presented in figures 5 and 6.
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Figure 6. Histograms of moment of body inertia (MI) and morphologic characteristics (body mass, height) show group means and standard deviations for both gender and morphologic groups. Men are taller and bigger than women. Moreover, women have smaller MI than men in both ML and AP directions. On the other hand, the GT are taller than GN and GC (***p < 0.001; **p < 0.01; *p < 0.05).
4. Discussion 4.1. Morphologic influence on the postural control Classically, height and body weight constitute two investigated parameters that have a significant effect on body inertia and consequently postural control. To our knowledge, only Allard et al. (2001) have considered morphologic somatotypes in studying the standing posture equilibrium in able-bodied girls. Through this somatotype determination, it was shown that ‘the endomorphs’ (endomorphy: the relative fatness of the body) display a better postural stability than ‘the ectomorphs’ (ectomorphy: ratio of height to weight depicting the relative linearity of the body). Indeed, the surface area of CP displacements of ectomorphs was larger than that of endomorphs. To explain such an effect, it is suggested that the ectomorphs present a relatively low muscle component, a high height–weight ratio and an elevated position of the body’s centre of mass. These results quantify the postural performance but are nevertheless insufficient when it comes to providing an accurate explanation for postural control. Indeed, without the CP decomposition in CGh and CPCGv elementary motions, it is rather difficult to disentangle the various postural strategies. For a similar performance in terms of CP trajectory, different strategies aimed at controlling CGh and CP-CGv motions can theoretically be utilized by the subject. For instance, various investigations on blindness and the forward body leaning effects on postural control (Rougier and Farenc 2000, Rougier et al. 2001) have demonstrated the importance of focusing the analysis on CGh and CP-CGv motions.
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Figure 7. Histograms of the NBF parameter show group means and standard deviations for morphologic (GT, GN, GC) and gender (women vs men) groups in both ML and AP directions. On the one hand, the thin group has a lower frequency than the normal and corpulent group in both ML and AP directions. On the other hand, women sway at a higher frequency than men in both ML and AP directions. Finally, thin women sway at a lower frequency than normal and corpulent women in both ML and AP directions. The same tendency is observed for men (***p < 0.001; **p < 0.01; *p < 0.05).
In the present study, some differences are observed according to various morphologies (GT, GN, GC) defined by Livi’s index. The main result is that the thin subjects (GT) demonstrate larger CGh displacements, mainly in the ML direction, when compared to normal (GN) and corpulent (GC) subjects. According to the RMS results of CP-CGv motions, the thin subjects display an increase in CP-CGv motion amplitudes, which, from a biomechanical point of view, cause an increase in the horizontal accelerations communicated to the CG (Brenie`re et al. 1987). According to Winter et al. (1996), an MF increase for CP-CGv motions in the ML direction involves an increase of abductor–adductor muscles in the thin subjects. This increased level of muscular contraction would be due to a larger recruitment of motor units. Indeed, since the pioneering work by Henneman et al. (1965), it is well established that an increase in strength is generated by recruiting additional motor units, inducing a tendency for the MF parameter to be shifted towards higher frequencies. Bearing in mind the amplitude ratio between CP and CG motions, and due to this recruitment principle, the displacements of the CP over higher frequencies theoretically produce reduced CG motions and consequently larger CP-CGv motions. Complementarily, the fBm modelling demonstrates that the increase in the CPCGv amplitudes of thin subjects can be explained by a lesser control in both ML and AP directions (significant decrease of Hsl). On the other hand, the distances covered
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by the trajectories before a corrective mechanism intervenes are longer for the thin subjects when compared to normal and corpulent subjects, mainly in the ML direction (significant increase of hx2 i for CP-CGv motions). Consequently, the horizontal accelerations communicated to the CG when a corrective mechanism does intervene are augmented, thus making the return of the CG to a position more compatible with equilibrium principles more difficult. A biomechanical analysis shows that thin subjects sway spontaneously at lower frequencies than normal and corpulent subjects. This result may appear surprising at first. Nevertheless, it can be explained by the key parameter for the NBF calculation: the height of the subject. These results of natural body frequency (NBF) demonstrate that weight and mainly height characteristics constitute determinant parameters in postural control, especially in a gravitational physical environment. 4.2. Women and men: a different postural stability? Several authors have highlighted the influence of gender in postural stability for healthy adults. Juntunen et al. (1987) and Kollegger et al. (1992) found that men swayed significantly more (larger total length of CP motions) than women. The results of the present study encourage the differentiation of gender. Indeed, several statistically significant differences between men women are observed for frequential parameters. It should be emphasized that the observed differences between men and women appear only in the AP direction. Men display larger sway amplitudes (RMSAP) for CGh motions than women under both conditions. Although the RMS parameter used in the present study is different from total length, which is the classical parameter used by Juntunen et al. (1987) and Kollegger et al. (1992), there are similarities in the way global postural performance is quantified. Consequently, the present results are in accordance with the findings of Juntunen et al. (1987) and Kollegger et al. (1992). In particular, the latter explain these gender differences by the stabilizing part of vision, which would be more significant for men than for women. Should this hypothesis be true, the differences observed in postural control between men and women would be more significant, particularly in the EC condition. This is not seen in the present study. Consequently the hypothesis that vision would be a stabilizing factor in the postural control of men is not confirmed. On the other hand, the displacement of frequential distribution towards higher frequencies for CGh spectra (larger MFAP) in women in the AP direction could be linked to a decrease of the distances covered by the trajectories before the correction process operates. The fBm modelling suggests that the mechanisms (Hsl AP) involved in the control of the CP-CGv trajectories of women are endowed with more determinism than those of men. Indeed, women have a better control of these trajectories during the shorter t than men under both EC and EO conditions. The moment of body inertia (MI), larger for men, and natural body frequency (NBF), higher for women, can also be used to explain the differences observed according to gender. The greater MI, generating a greater inertia of CG motion, by definition induces an increased gap between CP and CGh motions. Indeed, women have larger amplitudes for the lowest frequencies of CP-CGv motions when compared to men, as illustrated by figure 4, and demonstrate a better control than men, as illustrated by figure 5. Interestingly, Brenie`re (1999) did not observe any change in NBF values from one adult to another, despite large variations in anthropometric characteristics. On the other hand, the NBF results of the present study indicate that women sway spontaneously at higher frequencies than men. This
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difference between women and men adults is very close, in a way, to the difference between adults and children, as observed by Brenie`re (1999). Indeed, children demonstrate higher NBF values than adults. The common factor between women and children is that they are smaller and lighter than men. These results highlight the fact that body characteristics influence postural control. However these behavioural differences between women and men cannot be attributed solely to anthropometric characteristics, since some physiologic differences also play a role. Physiologically, the soleus muscle, an antigravity muscle, is permanently recruited during quiet standing (Okada and Fujiwara 1984). It is recognized that soleus muscles of women have longer fibres, smaller angles of pennation and are not as thick as male muscle (Chow et al. 2000). These differences in muscle architectural properties have, in turn, significant implications with respect to force and velocity. These muscular properties would contribute to a greater force generation for men (Chow et al. 2000). These physiologic explanations are in accordance with our results. Indeed, women demonstrate lesser ankle stiffness than men, according to the MF results of CP-CGv motions. Consequently, the current results bring us to the conclusion that the lessened sway exhibited by women can be explained by both physiologic and morphologic characteristics. To conclude, the results of the present study encourage the consideration of subject morphology as well as the differentiation of gender. Indeed, the fact that thin subjects display larger CGh motions and different postural strategies than corpulent ones demonstrates the influence of global morphology on postural control. On the other hand, some differences between women and men have been highlighted, suggesting the need to differentiate between men and women’s postural behaviours. Interestingly, the complete set of these results could constitute normative data of healthy adults and consequently could be used as referent behaviour for clinical studies. Acknowledgements The authors would like to acknowledge S. Camaret for helping with the statistical analyses and D. Goodhew for correcting the English text. References Allard, P., Nault, M. L., Hinse, S., Leblanc, R., and Labelle, H., 2001, Relationship between morphologic somatotypes and standing posture equilibrium. Annals of Human Biology, 28, 624– 633. Berger, W., Trippel, M., Discher, M., and Dietz, V., 1992, Influence of subjects’ height on the stabilization of posture. Acta Otolaryngologica (Stockholm), 112, 22–30. Bessineton, J. C., Bizzo, G., Pacifici, M., and Baron, J. B., 1976, Statokine´sigramme, taille, poids, sexe, reproductibilite´. Agressologie, 17, 49–54. Black, F. O., Wall, C., Rockette, H. E., and Kitch, R., 1982, Normal subject postural sway during the Romberg test. American Journal of Otolaryngology, 3, 309–318. BreniE`re, Y., 1996, Why we walk the way we do? Journal of Motor Behavior, 28, 291–298. BreniE`re, Y., 1999, How locomotor parameters adapt to gravity and body structure changes during gait development in children. Motor Control, 3, 186–204. BreniE`re, Y., Do, M. C., and Bouisset, S., 1987, Are dynamic phenomena prior to stepping essential to walking? Journal of Motor Behavior, 19, 62–76. Carlberg, K. A., Buckman, M. T., Peake, G. T., and Riedesel, M. L., 1983, Body composition of oligo/amenorrheic athletes. Medicine and Science in Sports and Exercise, 15, 215–217. Caron, O., Faure, B., and Brenie' re, Y., 1997, Estimating the centre of gravity of the body on the basis of the centre of pressure in standing posture. Journal of Biomechanics, 30, 1169–1171.
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Chow, R. S., Medri, M. K., Martin, D. C., Leekam, R. N., Agur, A. M., and Mckee, N. H., 2000, Sonographic studies of human soleus and gastrocnemius muscle architecture: gender variability. European Journal of Applied Physiology, 82, 236–244. Collins, J. J., and De Luca, C. J., 1993, Open-loop and closed-loop control of posture: a random-walk analysis of center of pressure trajectory. Experimental Brain Research, 95, 308–318. Diener, H. C., Dichgans, J., Guschlbauer, B., and Mau, H., 1984, The significance of proprioception on postural stabilization as assessed by ischemia. Brain Research, 296, 103–103. Ekdahl, C., Jarnlo, G. B., and Andersson, S. I., 1989, Standing balance in healthy subjects. Scandinavian Journal of Rehabilitation Medicine, 21, 187–195. Hageman, P. A., Leibowitz, J. M., and Blanke, D., 1995, Age and gender effects on postural control measures. Archives of Physical Medicine and Rehabilitation, 76, 961–965. Henneman, E., Somjen, G., and Carpenter, D. O., 1965, Functional significance of cell size in spinal motoneurons. Journal of Neurophysiology 28, 560–580. Juntunen, J., Ylikoski, J., and Ojala, M., 1987, Postural body sway and exposure to high energy impulse noise. Lancet, ii, 261–264. Kollegger, H., Baumgartner, C., W˛ber, C., Oder, W., and Deecke, L., 1992, Spontaneous body sway as a function of sex, age, and vision: posturographic study in thirty healthy adults. European Neurology, 32, 253–259. Ledebt, A., and Breni E`re, Y., 1994, Dynamical implication of anatomical and mechanical parameters in gait initiation process in children. Human Movement Science, 13, 801–815. Maki, B. E., Holliday, P. J., and Fernie, G. R., 1990, Aging and postural control: a comparison of spontaneous and induced sway balance tests. Journal of American Geriatrics Society, 3, 1–9. Nakagawa, H., Ohashi, N., Watanabe, Y., and Mizukoshi, K., 1993, The contribution of proprioception to posture control in normal subjects. Acta Otolaryngologica (Stockholm) Supplement, 504, 112–116. Okada, M., and Fujiwara, K., 1984, Relation between sagittal distribution of the foot pressure in upright stance and relative EMG magnitude in some leg and foot muscles. Journal of Human Ergonomics, 13, 97–105. Riley, M. A., Wong, S., Mitra, S., and Turvey, M. T., 1997, Common effects of touch and vision on postural parameters. Experimental Brain Research, 117, 165–170. Rougier, P., 1999a, Influence of visual feedback on successive control mechanisms in upright stance in humans assessed by fractional Brownian motion modelling. Neuroscience Letters, 266, 157–160. Rougier, P., 1999b, Automatic determination of the transition between successive control mechanisms in upright stance assessed by modelling of the centre of pressure. Archives of Physiology Biochemistry, 107, 35–42. Rougier, P., and Caron, O., 2000, Center of gravity motions and ankle joint stiffness control in upright undisturbed stance modeled through a fractional Brownian motion framework. Journal of Motor Behavior, 32, 405–413. Rougier, P., and Farenc, I., 2000, Adaptative effects of loss of vision on upright undisturbed stance. Brain Research, 871, 165–174. Rougier, P., Burdet, C., Farenc, I., and Berger, L., 2001, Backward and forward leaning postures modelled by an fBm framework. Neuroscience Research, 41, 41–50. Stribley, R. F., Albers, J. W., Tourtellotte, W. W., and Cockrell, J. L., 1974, A quantitative study of stance in normal subjects. Archives of Physical Medicine and Rehabilitation, 55, 74–80. Winter, D. A., Prince, F., Franck, J. S., Powel, C., and Zabjek, K. F., 1996, Unified theory regarding A/P and M/L balance in quiet standing. Journal of Neurophysiology, 75, 2334–2343. Winter, D. A., Patla, A. E., Prince, F., Ishac, M., and Gielo-perczak, K., 1998, Stiffness control of balance in quiet standing. Journal of Neurophysiology, 80, 1211–1221. Address for correspondence: Farenc, Laboratoire de Mode´lisation des Activite´s Sportives, UFR CISM, Universite´ de Savoie, Domaine Universitaire de Savoie-Technolac, F73376 Le Bourget du Lac cedex, France. Email:
[email protected] Zusammenfassung. Hintergrund: Morphologische Merkmale, wie zum Beispiel Ko¨rperho¨he und Ko¨rpergewicht bestimmen Ko¨rper-Tra¨gheit, ein wichtiger Faktor in Zusammenhang mit Haltungsstabilita¨t. Wa¨hrend bisherige Untersuchungen diese Parameter klassisch separat analysiert haben, wurde die globale Morphologie du¨rftig forschte. Zweitens demonstriert der Einfluss des Geschlechtes auf die Haltungsstabilita¨t gegensa¨tzliche Tendenzen. Einige Autoren beobachteten, daß Ma¨nner weniger schwanken als Frauen, wa¨hrend andere das Gegenteil bemerkten. Ziel: Die Ziele dieser Untersuchung ist es, Morphologie und Geschlechtseffekte auf gesunden Subjekten wa¨hrend der Bewahrung der aufrechten Haltung auszuwerten. Subjekte und Methoden: Die untersuchten Subjekte wurden mit Hilfe des Livi-Indexes kategorisiert. Eine Methode, die Ha¨ufigkeits-Frequenzen und Brown’sche Parameter vereinigt, ist charakterisiert durch die separate horizontalen Verschiebungen des Schwerpunktes (CGh) und dem Unterschied zwischen dem Druckzentrum (CP) und der vertikalen Projektion des Schwerpunktes (CP-CGv). Zusa¨tzlich wurden die
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Momente von Ko¨rper-Tra¨gheit (MI) und der natu¨rlichen Ko¨rper-Frequenz (NBF) benutzt, um den Einfluß der Morphologie zu bestimmen. Ergebnisse und Schlussfolgerungen: Die Ergebnisse legen offen, daß die du¨nneren Subjekte eine gro¨ßere CGh-Verschiebungen aufweisen als normale oder korpulent Subjekte. Morphologische Merkmale (NBF) ko¨nnen diese Verhaltensunterschiede erkla¨ren. Andererseits haben Ma¨nner eine gro¨ßere Schwankungsamplitude fu¨r CGh-Bewegungen als Frauen. Dies kann sowohl durch morphologisch (MI, NBF) als auch physiologisch (architektonische Eigenschaften des Musculus soleus) Merkmale erkla¨rt werden. Re´sume´. Etat actuel des connaissances: Des caracte´ristiques morphologiques telles que la taille et le poids corporel de´terminent l’inertie corporelle, un facteur important de la stabilite´ posturale. Cependant, tandis que des investigations ont classiquement analyse´ ces parame`tres se´pare´ment, la morphologie globale a fait l’objet de peu de recherche. Par ailleurs, l’influence du sexe sur la stabilite´ posturale indique des tendances contradictoires, certains auteurs observant que les hommes oscillent moins que les femmes et d’autres notant le contraire. But: Cette e´tude a pour objet d’e´valuer les effets de la morphologie et du sexe sur des sujets en bonne sante´ pendant le maintien postural. Sujets et me´thodes: Les sujets e´tudie´s ont e´te´ classe´s sur la base de l’indice de Livi. Une me´thode associant des parame`tres de fre´quence et des parame`tres browniens caracte´risait se´pare´ment les de´placements horizontaux du centre de gravite´ (CGh) et ceux de la diffe´rence entre le centre de pression (CP) et la projection verticale du centre de gravite´ (CP-CGv). De plus, on a e´galement utilise´ les moments d’inertie corporelle (MI) et la fre´quence corporelle naturelle (NBF) pour de´terminer l’influence de la morphologie. Re´sultats et conclusions: Les re´sultats re´ve`lent que les sujets les plus minces ont des de´placements du CGh plus grands que les sujets normaux ou corpulents. Les caracte´ristiques morphologiques (NBF) peuvent expliquer ces diffe´rences de comportements. Par ailleurs, les hommes ont une amplitude d’oscillation des mouvements du CGh plus grande que celle des femmes. Ceci peut s’expliquer par des caracte´ristiques a` la fois morphologiques (MI, NBF) et physiologiques (proprie´te´s architecturales du muscle soleus). Resumen. Antecedentes: Caracterı´ sticas morfolo´gicas tales como la estatura y el peso corporal determinan la inercia del cuerpo, un importante factor relacionado con la estabilidad postural. Sin embargo, mientras que las investigaciones han analizado cla´sicamente estos para´metros por separado, la morfologı´ a global ha sido poco investigada. En segundo lugar, la influencia del sexo sobre la estabilidad postural demuestra tendencias opuestas: algunos autores observan que los hombres se balancean menos que las mujeres y otros sen˜alan lo contrario. Objetivo: Los objetivos de este estudio son evaluar los efectos de la morfologı´ a y el ge´nero sobre sujetos sanos durante el mantenimiento postural. Sujetos y me´todos: Los sujetos estudiados fueron divididos en categorı´ as mediante el ı´ ndice de Livi. Un me´todo que asociaba para´metros frecuenciales y Brownianos caracterizo´ los desplazamientos horizontales del centro de gravedad (CGh) y los de la diferencia entre el centro de presio´n (CP) y la proyeccio´n vertical del centro de gravedad (CP-CGv) de forma separada. Adema´s, tambie´n se utilizaron los momentos de inercia corporal (MI) y la frecuencia corporal natural (NBF) para determinar la influencia de la morfologı´ a. Resultados y conclusiones: Los resultados revelan que los sujetos ma´s delgados tienen desplazamientos mayores del centro de gravedad (CGh) que los sujetos normales o corpulentos. Las caracterı´ sticas morfolo´gicas (NBF) pueden explicar estas diferencias de comportamiento. Por otra parte, los hombres tienen una amplitud de balanceo mayor que las mujeres para los movimientos CGh. Esto puede explicarse tanto por las caracterı´ sticas morfolo´gicas (MI, NBF) como fisiolo´gicas (propiedades arquitecturales del mu´sculo so´leo).
