Light Finger Touch on the Upper Legs Reduces Postural Sway During Quasi-Static Standing

Transcription

1 Motor Control, 2006, 10, Human Kinetics, Inc. Light Finger Touch on the Upper Legs Reduces Postural Sway During Quasi-Static Standing Akinori Nagano, Shinsuke Yoshioka, Dean Charles Hay, and Senshi Fukashiro The purpose of this study was to test whether a light finger touch on one s own body (upper legs) reduces postural sway. Ten healthy males participated. In the first part of the study, the participants stood upright with their eyes closed on a force platform while ground reaction force data were collected. Two conditions differing in the placement of the arms and fingers were tested. In the no-touch condition, the participants kept their hands in loose fists. In the finger-touch condition, the participants lightly touched the lateral sides of the upper legs with all fingers. Postural sway measures were calculated from the ground reaction force data. In the second part of the study, the participants stood upright on a pneumatic balance disk while ground reaction force data were collected. Experimental and measurement protocols were identical to those used in the first part of the study. The results showed that light finger touch on the upper legs significantly reduced postural sway on the balance disk up to ~7%. The data from this study suggest that decreased postural sway due to finger contact may improve balance control during other standing tasks. Key Words: balance disk, posture, sway, contact Improvement and maintenance of quality of life (QOL) is an increasingly important issue in societies with aging populations. Independent mobility is a key element of QOL, requiring good balance control to avoid the incidence of falls during locomotion (Fernie, Gryfe, Holliday, & Llewellyn, 1982; Maki, Holliday, & Topper, 1991, 1994). Given that this is an important issue, researchers have made efforts to improve balance control through various procedures. It has been found that postural sway decreases most notably with a light touch sensation of the fingers. Holden and coworkers (Holden, Ventura, & Lackner, 1994) reported stabilization of posture by precision contact of the index finger. They studied how sensory-motor information about body displacement, provided by contact of the index finger with a stationary bar fixed to the floor, helps stabilize posture in the Nagano and Yoshioka are with the Computational Biomechanics Unit, RIKEN Hirosawa 2-1, Wako, Saitama, Japan. Yoshioka, Hay, and Fukashiro are with the Dept of Life Sciences (Sports Sciences), the University of Tokyo, Meguro, Tokyo, Japan. Hay is also with the Motor Control Section, Research Institute of National Rehabilitation Center for the Disabled, Tokorozawa, Saitama, Japan. 348

2 Finger Touch on Leg Reduces Postural Sway 349 absence of vision. They found that the resultant stabilization was equivalent to that conferred by vision, but at contact force levels insufficient to provide mechanical support of the body (less than 1 N). Similarly, Jeka (1997) reviewed the use of light-touch contact as a postural aid. The author summarized several research results (Jeka, Easton, Bentzen, & Lackner, 1996; Jeka & Lackner, 1994, 1995) that suggested that light somatosensory contact cues at the fingertips and hand reduces postural sway in individuals without balance problems, in individuals with bilateral vestibular loss, and in individuals with congenital blindness. In these studies, the finger(s) made contact with an object fixed to the floor, or by holding a cane in contact with the ground in one hand. Similar effects have been reported for a finger contact with mobile external supports. Krishnamoorthy and coworkers (Krishnamoorthy, Slijper, & Latash, 2002) used a 3 kg mass attached to a dynamic pulley system as the point of finger contact, while Riley and coworkers (Riley, Stoffregen, Grocki, & Turvey, 1999) used a cloth curtain. Many researchers have used similar experimental settings (Lackner, Rabin, & DiZio, 2001; Rabin, Bortolami, DiZio, & Lackner, 1999; Rogers, Wardman, Lord, & Fitzpatrick, 2001) that yielded consistent conclusions: light finger touch on an external support reduces postural sway. These findings are not only interesting but also useful, as they may suggest a promising approach to improving human balance. Generally, the light touch of an external object will provide additional information regarding the alignment of the finger, hand, and arm with respect to the environment. However, to the best of our knowledge, the effects of light touch on one s own body have not been thoroughly examined. Light touch on one s own body primarily provides information about the relative movement of body segments. This adds to the information obtained by proprioceptors. For example, when standing upright, a light finger touch on the upper legs helps one sense movement of the trunk, arms, and legs, relative to one another. We hypothesized that this additional information would contribute to reduced postural sway while standing. Therefore, the purpose of this study was to evaluate the effects of light contact between the fingers and body on postural sway. We focused on finger touch of the upper legs, as this touch provides information about the relative movements of the trunk, arms, and legs without causing a large change in the location of the body s center of mass. Methods Ten healthy, active males with no known history of neural, muscular, or skeletal disorders took part in this study. Informed consent was obtained from each participant under the approval of the ethics committee of the University of Tokyo. The mean age, body height, and body mass of the participants were 27.2 (± 2.9) years, (± 3.9) cm, and 72.9 (± 5.5) kg, respectively. During the study, the participants wore their normal daily attire, which included trousers and long-sleeved shirts. The participants did not report any pain, fatigue, or other problems during the course of the experiment. In the first part of the experiment, the participants stood upright on a force platform (model 9281B, Kistler Instrumente AG, Winterthur, Switzerland). Each

3 350 Nagano et al. participant was allowed to find a comfortable placement of the feet before the measurement. The foot placement on the force platform was marked with adhesive tape, so that each participant could place his feet consistently in subsequent experiments. The participants stepped onto the force platform, aligning their feet to the marks, and closed their eyes. The participants stood quietly as upright as possible on the force platform and once the posture became stable (no motion, visual inspection) ground reaction force data collection was initiated. The participants kept their eyes closed during the measurement. A laptop computer connected to an A/D converter (PowerLab, ADInstruments, Colorado Springs, CO) recorded the data at 100 Hz. Each recording lasted 30 s. The motion of the center of pressure (COP) was calculated from the force platform data (equations are defined in the Appendix). Two hand positions were tested (Figure 1). In the first condition, the participants held their hands in loose fists. Arms were kept straight and crossed in front Figure 1 The positions of the arms used in this study. (A) In the no-touch condition, the subjects closed their hands, kept their arms straight, and crossed them in front of their body. This minimized the contact between the fingers and palms and the body. (B) In the fingertouch condition, the subjects lightly touched their upper legs with all fingers.

4 Finger Touch on Leg Reduces Postural Sway 351 of the body with the wrists near the navel. This inhibited contact between the fingers/palms and the legs, and at the same time prevented the use of arm swings for balance recovery. There was a small amount of contact between the surface of the arms and the trunk. In the second condition (finger touch), all fingertips lightly touched the lateral sides of the upper legs. The skin of the fingertips is known for its high spatial resolution and sensitivity (Weinstein, 1968). The participants were instructed to touch their legs lightly, and never to apply force for mechanical support. The participants were carefully observed by the experimenters during the trials and compliance with this instruction was confirmed. The two positions of the hands were tested in random order. Eight trials were conducted for each position, of which seven were analyzed (details are described below). The participants rested between trials to avoid the onset of fatigue. After the first part of the experiment, the participants stood upright on a pneumatic balance disk (model DK 380, Hata Sporting Goods Ind., Ltd., Japan) for 10 min. A rigid wooden board was affixed to the upper surface of the balance disk with tough-use glue (Figure 2). This change made the task of standing upright less demanding, as standing directly on the pneumatic balance disk with the eyes closed was found to be too difficult and fatiguing. The thickness of the wooden board was 17 mm. The amount of air in the balance disk was adjusted for each participant such that the thickness of the balance disk with a participant quietly Figure 2 The balance disk used in this study. A wooden board was attached to the top of a pneumatic balance disk using tough-use glue. The board was used to make the balance task possible with eyes closed.

5 352 Nagano et al. standing on it was 63 mm. Therefore the total thickness of the balance disk and the wooden board was 80 mm. This period of 10 min was a practice session to allow the participants to familiarize themselves with standing on this balance disk. The participants also found a comfortable placement of the feet in this period, which was then marked on the wooden board with adhesive tape. In the second part of the experiment, the balance disk was placed on the force platform. When the participants closed their eyes, body motion did not immediately stabilize. Therefore, recording was started about 10 s after they closed their eyes. The procedure was otherwise identical to the first part. To describe the motion of the COP, the following four variables were calculated and analyzed: average sway speed, standard deviation, maximal mediolateral range, and maximal anteroposterior range (Nagano, Yoshioka, Hay, Himeno, & Fukashiro, 2006). For average sway speed, the total excursion of the COP was calculated as the sum of the displacement scalars of the COP between adjacent time steps (0.01 s). This value was divided by the measurement time (30 s) to calculate average sway speed. Standard deviation was calculated as the root mean squares of the distance between the center of the COP trajectory (average position during the data collection period) and the instantaneous position of the COP. As described previously, eight trials were performed by each participant under each condition. The first measurement of each condition served as practice, though the participants were not aware of this. Therefore, each condition contained seven repeated trials for analysis. The mean values of individual variables were calculated for each participant in each condition. These mean values were used for statistical analysis that evaluated the effects of balance disk and finger touch. The significance of the effects of balance disk and finger touch was tested using a 2 2 (2 standing conditions 2 hand positions) analysis of variance with a randomized block factorial design (R, The R Foundation for Statistical Computing, Vienna, Austria). The level of significance criterion was set at p <.05. Results In the first part of the experiment, the participants stood directly on the force platform. The changes in the mean value of the average sway speed (31.46 mm/s and mm/s), standard deviation (4.79 mm and 4.81 mm), maximal mediolateral range (11.51 mm and mm), and maximal anteroposterior range (20.64 mm and mm) between the no-touch and the finger-touch conditions were + 0.1%, + 0.4%, + 5.5%, and 4.6%, respectively (Figure 3, Figure 4, panels a through d). In the second part of the experiment, the participants stood on the balance disk. The changes in the mean value of the average sway speed (83.34 mm/s and mm/s), standard deviation (28.43 mm and mm), maximal mediolateral range ( mm and mm), and maximal anteroposterior range ( mm and mm) between the no-touch and the finger touch conditions were 7.1%, 6.3%, 9.2%, and 3.8%, respectively (Figure 3, Figure 4, panels a through d). For the average sway speed and standard deviation (Figure 4, panels a and b), the factors of standing condition (A), hand position (B), and the interaction between them (C), exhibited statistically significant effects (Table 1). This statistical analysis indicates that (1) standing on the balance disk increased postural sway, (2) light touch on the upper legs reduced postural sway, and (3) the effect of finger touch

6 Finger Touch on Leg Reduces Postural Sway 353 Figure 3 Typical trajectories of the COP. FP: standing directly on the force platform, BD: standing on the balance disk. a b c d Figure 4 Postural sway measures: (a) average sway speed; (b) standard deviation; (c) maximal mediolateral range; (d) maximal anteroposterior range. FP: standing directly on the force platform. BD: standing on the balance disk. Average and standard deviation are shown graphically.

7 354 Nagano et al. on postural sway was greater when standing on the balance disk. For the maximal mediolateral and anteroposterior ranges (Figure 4, panels c and d), only factor (A) was found to be statistically significant (Table 1). This statistical result suggests that standing on the balance disk increased postural sway, whereas the effect of finger touch was not large enough to exhibit statistical significance. Table 1 Statistical Analysis Results (Two by Two Analysis of Variance with a Randomized Block Factorial Design) FP/BD No/Finger touch Interaction Average sway speed p < p < p < Standard deviation p < p < p < Maximal mediolateral p < n.s. n.s. range Maximal anteroposterior range p < n.s. n.s. Note. The level of significance was set at p <.05. FP = standing directly on the force platform; BD = standing on the balance disk. Discussion The purpose of this study was to test whether a light finger touch on the legs reduces postural sway. As many previous studies have reported that postural sway decreases with finger contact with an external support object, we hypothesized that postural sway would decrease with a finger touch to the upper legs. The most interesting finding of this study was that postural sway (the average sway speed and standard deviation) significantly decreased with finger touch while standing on the balance disk (Table 1; Figure 4, panels a and b). This result not only supported our hypothesis, but also was consistent with the results of preceding studies (Jeka et al., 1996; Jeka & Lackner, 1994, 1995). The effect of finger touch on the maximal mediolateral range and maximal anteroposterior range were not statistically significant, although these variables tended to decrease with finger touch. In the past, researchers have reported that average sway speed is the most reliable (reproducible) measure of postural sway (Geurts, Nienhuis, & Mulder, 1993; Lafond, Corriveau, Hebert, & Prince, 2004). While it may be necessary to collect data from a greater number of participants to obtain statistically significant results for the maximal mediolateral and anteroposterior ranges, the tendency found in this study suggests that these variables decrease with finger touch on the balance disk. The interaction between the factor of standing condition (A) and the factor of hand position (B) was significant for average sway speed and standard deviation (Table 1). Because the postural sway associated with the task of upright standing on a stable surface with no finger touch was very small to begin with (Figure 4, panels a through d), it is likely that even with the additional cutaneous afferent feedback obtained through touching one s own legs, postural sway can not be further reduced. In other words, having the sensation of finger touch in addition to proprioceptive and vestibular feedback did not help the participants decrease postural sway when standing directly on the force platform. In numerous preceding studies, reductions in postural sway have been reported when spatial feedback was

8 Finger Touch on Leg Reduces Postural Sway 355 obtained from fingers by touching external support objects (Clapp & Wing, 1999; Krishnamoorthy et al., 2002; Riley et al., 1999; Tremblay, Mireault, Dessureault, Manning, & Sveistrup, 2004). The reason why such contact is more informative seems to be that touching external support objects provides information regarding the alignment between body segments and the environment, information that cannot be obtained from proprioceptors alone. The sensation obtained by touching one s own legs was useful to reduce postural sway on the balance disk, which seems to be related to the difficulty of the task. In the past, many researchers who studied the effects of finger touch on an external object asked participants to stand with their feet placed in tandem or in a similar position, whereby the feet are aligned with the toes of one foot touching the heel of the other (Jeka, Oie, & Kiemel, 2000; Lackner et al., 2001; Oie, Kiemel, & Jeka, 2002; Rabin et al., 1999). It seems that those researchers chose unstable positions to increase the difficulty of the task, such that the sensory information from the fingers became relatively more important. We used the balance disk to achieve the same objective in this study. The average sway speed increased by ~150% on the balance disk compared to standing directly on the force platform (Figure 4, panel a). The basic idea is identical to that which requires participants to stand on foam rubber (Rogers et al., 2001) or on one leg (Holden et al., 1994). In previous studies, researchers reported that a light touch on an external support decreased postural sway by ~60% (Holden et al., 1994; Jeka, 1997). In this study, the decrease in postural sway was up to ~7%. This difference in the magnitude most likely comes from the fact the fingers touched the participants own legs in this study, instead of an external support object. In this study, the finger touch detected only the relative movement of the body segments. It is reasonable to propose that gaining mechanical information from the contact between the fingers and an external support object ( absolute information) is more helpful than this relative information. The novel finding of the current study is that postural sway can be reduced by touching one s own body when external support is not available. In a preceding study, Reginella and coworkers tested the effects of finger contact with an Earth-fixed surface and with a body-referenced surface (i.e., a surface that swayed in synchrony with the body sway) (Reginella, Redfern, & Furman, 1999). They found that finger contact with an Earth-fixed surface decreases postural sway, whereas finger contact with a body-referenced surface does not. The findings of the current study indicate that touching one s own legs is of greater benefit than touching a body-referenced surface to reduce postural sway, but of less benefit than touching a fixed object. In the no-touch condition, we asked the participants to close their hands in loose fists. The aim was to minimize the contact of the fingers and palms with the body. At the same time, it was necessary to minimize the change of the location of the body s center of mass between the no-touch condition and the finger-touch condition. Therefore, we asked the participants to keep their arms straight and cross them in front of their body (Figure 1). de Leva (1996) reported that the average mass of an arm (shoulder to hand) relative to the whole body was 4.94%. A slight change in the location of the arm segments that have only a small fraction of total body mass does not greatly change the location of the body s center of mass. Specifically, the vertical position of the body s center of mass shifted by less than 8 mm as the result of this manipulation, which is less than 1% of the original height.

9 356 Nagano et al. The participants kept their eyes closed during the measurement. This is because vision has a great stabilizing influence on postural sway (Fox, 1990; Isotalo, Kapoula, Feret, Gauchon, Zamfirescu, & Gagey, 2004; Rougier, 2003). In the past, researchers reported that vision stabilized postural sway even when muscle fatigue (Vuillerme, Nougier, & Prieur, 2001) or prolonged static stretch (Nagano et al., 2006) impaired postural control. Therefore, we did not test conditions in which vision is active, consistent with decisions made by researchers in previous studies. On the balance disk, the motion of the participants did not stabilize with the eyes closed. Nevertheless, none of the 10 participants fell off the balance disk during the course of this experiment. When the change in the magnitude of postural sway associated repeated measurements was evaluated, it was found that postural sway did not increase with repetition. As postural sway tends to increase with muscle fatigue (Lepers, Bigard, Diard, Gouteyron, & Guezennec, 1997), we believe the participants experienced negligible fatigue in the experiment. In this study, the experimenters watched the participants and confirmed that they did not provide mechanical support with their fingers in the finger-touch condition. We believe this method is reliable, as applied force causes a visible deformation of the hand and fingers, compared with only light contact. In preceding studies, researchers have measured the force that is developed between the fingers and an external support (Holden et al., 1994; Lackner et al., 2001; Rabin et al., 1999) by using force sensors. That approach was not used in this study because inserting sensors between the fingers and the leg surface could have changed the sensation of the fingers. However, it would be interesting to address this issue in future studies, by examining the effect of pressure on postural sway. This study showed that postural sway decreases by ~7% with a light finger touch on the upper legs. The change in the postural touching sway was smaller than that reported in previous studies (~60%), which evaluated the effects of an external support. Therefore, it might be of more benefit to touch an external support when one is available. However, this study has shown for the first time that self-contact can be used as a strategy to reduce postural sway in the absence of external supports. If this change is also relevant while walking, lightly touching the legs may be shown to decrease the incidence of falls. The extent to which postural stability may be affected by self-contact, and the mechanisms responsible for such changes, are issues of great interest and worthy of further attention. Acknowledgments Akinori Nagano would like to thank the Special Postdoctoral Program of RIKEN. Shinsuke Yoshioka would like to thank the Junior Research Associate Program of RIKEN. All the authors would like to thank Dr. Ryutaro Himeno of RIKEN for his support. References Clapp, S., & Wing, A.M. (1999). Light touch contribution to balance in normal bipedal stance. Experimental Brain Research, 125, de Leva, P. (1996). Adjustments to Zatsiorsky-Seluyanov s segment inertia parameters. Journal of Biomechanics, 29,

10 Finger Touch on Leg Reduces Postural Sway 357 Fernie, G.R., Gryfe, C.I., Holliday, P.J., & Llewellyn, A. (1982). The relationship of postural sway in standing to the incidence of falls in geriatric subjects. Age and Ageing, 11, Fox, C.R. (1990). Some visual influences on human postural equilibrium: binocular versus monocular fixation. Perception and Psychophysics, 47, Geurts, A.C., Nienhuis, B., & Mulder, T.W. (1993). Intrasubject variability of selected forceplatform parameters in the quantification of postural control. Archives of Physical Medicine and Rehabilitation, 74, Holden, M., Ventura, J., & Lackner, J.R. (1994). Stabilization of posture by precision contact of the index finger. Journal of Vestibular Research, 4, Isotalo, E., Kapoula, Z., Feret, P.H., Gauchon, K., Zamfirescu, F., & Gagey, P.M. (2004). Monocular versus binocular vision in postural control. Auris, Nasus, Larynx, 31, Jeka, J., Oie, K.S., & Kiemel, T. (2000). Multisensory information for human postural control: integrating touch and vision. Experimental Brain Research, 134, Jeka, J.J. (1997). Light touch contact as a balance aid. Physical Therapy, 77, Jeka, J.J., Easton, R.D., Bentzen, B.L., & Lackner, J.R. (1996). Haptic cues for orientation and postural control in sighted and blind individuals. Perception and Psychophysics, 58, Jeka, J.J., & Lackner, J.R. (1994). Fingertip contact influences human postural control. Experimental Brain Research, 100, Jeka, J.J., & Lackner, J.R. (1995). The role of haptic cues from rough and slippery surfaces in human postural control. Experimental Brain Research, 103, Krishnamoorthy, V., Slijper, H., & Latash, M.L. (2002). Effects of different types of light touch on postural sway. Experimental Brain Research, 147, Lackner, J.R., Rabin, E., & DiZio, P. (2001). Stabilization of posture by precision touch of the index finger with rigid and flexible filaments. Experimental Brain Research, 139, Lafond, D., Corriveau, H., Hebert, R., & Prince, F. (2004). Intrasession reliability of center of pressure measures of postural steadiness in healthy elderly people. Archives of Physical Medicine and Rehabilitation, 85, Lepers, R., Bigard, A.X., Diard, J.P., Gouteyron, J.F., & Guezennec, C.Y. (1997). Posture control after prolonged exercise. European Journal of Applied Physiology and Occupational Physiology, 76, Maki, B.E., Holliday, P.J., & Topper, A.K. (1991). Fear of falling and postural performance in the elderly. Journal of Gerontology, 46, M123-M131. Maki, B.E., Holliday, P.J., & Topper, A.K. (1994). A prospective study of postural balance and risk of falling in an ambulatory and independent elderly population. Journal of Gerontology, 49, M72-M84. Nagano, A., Yoshioka, S., Hay, D.C., Himeno, R., & Fukashiro, S. (2006). Influence of vision and static stretch of the calf muscles on postural sway during standing. Human Movement Science, 25, Oie, K.S., Kiemel, T., & Jeka, J.J. (2002). Multisensory fusion: simultaneous re-weighting of vision and touch for the control of human posture. Brain Research. Cognitive Brain Research, 14, Rabin, E., Bortolami, S.B., DiZio, P., & Lackner, J.R. (1999). Haptic stabilization of posture: changes in arm proprioception and cutaneous feedback for different arm orientations. Journal of Neurophysiology, 82, Reginella, R.L., Redfern, M.S., & Furman, J.M. (1999). Postural sway with earth-fixed and body-referenced finger contact in young and older adults. Journal of Vestibular Research, 9,

11 358 Nagano et al. Riley, M.A., Stoffregen, T.A., Grocki, M.J., & Turvey, M.T. (1999). Postural stabilization for the control of touching. Human Movement Science, 18, Rogers, M.W., Wardman, D.L., Lord, S.R., & Fitzpatrick, R.C. (2001). Passive tactile sensory input improves stability during standing. Experimental Brain Research, 136, Rougier, P. (2003). The influence of having the eyelids open or closed on undisturbed postural control. Neuroscience Research, 47, Tremblay, F., Mireault, A.C., Dessureault, L., Manning, H., & Sveistrup, H. (2004). Postural stabilization from fingertip contact: I. Variations in sway attenuation, perceived stability and contact forces with aging. Experimental Brain Research, 157, Vuillerme, N., Nougier, V., & Prieur, J.M. (2001). Can vision compensate for a lower limbs muscular fatigue for controlling posture in humans? Neuroscience Letters, 308, Weinstein, S. (1968). Intensive and extensive aspects of tactile sensitivity as a function of body part, sex, and laterality. In D. R. Kenshalo (Ed.), The skin senses. (pp ). Springfield, IL: Thomas. Appendix Calculation of the Position of Center of Pressure (COP) The following equations were used to determine the instantaneous position of the COP. First, moments applied on the force platform were calculated as: where M stands for moment, F stands for force, x, y, and z stand for the x, y, and z components, respectively (Figure 2). The numbers a and b are specific to this force platform (a = 120 mm, b = 200 mm). The subscript numbers correspond to individual sensors of the force platform. The x and y positions of the COP can be calculated as follows: where a z is the sum of a platform-specific number ( 54 mm) and the thickness of the balance disk ( 80 mm).