SYMMETRY AND VARIABILITY OF VERTICAL GROUND REACTION FORCE AND CENTER OF PRESSURE IN ABLE-BODIED GAIT Yu Wag ad Kazuhiko Wataabe Graduate School of Educatio, Hiroshima Uiversity, Hiroshima, Japa, wagyu@hiroshima-u.ac.jp INTRODUCTION Gait is a basic requiremet for daily activity ad is kow as oe of the most uiversal ad complex of all huma activities (Sadeghi et al, 2000). The aim of gait is to support the body agaist gravity while geeratig movemets to propel the body forward. For producig the smooth rhythmical motio of the able-bodied gait, it requires precise coordiatio betwee the tasks of propulsio ad balace (Witer & Crago, 2000). It is foud that, i able-bodied gait, oe lower limb is maily resposible for support ad body weight trasfer while the cotralateral limb cotributes more to propulsio (Hirasawa, 98; Sadeghi et al, 997, 2000). Sadeghi et al. (997) reported that the limb havig the fuctio of propulsio was characterized as predomiatly muscle power geeratio while the limb havig the supportig ad cotrollig fuctio was characterized as predomiatly power absorptio behaviour. To date, symmetry ad variability i able-bodied gait have received little attetio despite the fact that it offers potetial isight ito euromuscular cotrol mechaisms. Able-bodied gait ivolves the movemets of a large umber of segmets ad therefore perfect symmetry is ot possible due to variable factors. It is just like what Berstei suggested that variability i movemet is a prerequisite for skilled motor performace (Berstei, 967). As the first ad oly poit of direct cotact betwee the body ad the exteral eviromet, foot plays a importat role o maitaiig balace ad adjustig limb trajectories durig gait. Pressure measuremet system allows the dyamic aalysis of COP o the platar surface of the foot durig mid-gait, which is thought more represetative of the ormal walkig stride. Ceter of pressure (COP) is the cetroid of the vertical force distributio o the platar surface of the foot (Cavaagh, 978). I gait aalysis, the path of the istataeous COP durig stace is cosidered as the direct reflectio of the subject's balace ad patter of progressio (Perry, 992). Therefore, the purpose of this study was to idetify whether or ot the limb domiace affects the symmetry ad variability i the ceter of pressure (COP) ad vertical groud reactio force (vgrf) durig the stace phase of able-bodied gait. METHODS Subjects 5 subjects, female ad 4 male, voluteered to participate i this study (mea (±SD) age: 25.5 (4.5) year, height:.62 (0.07) m, weight: 56. (9.7) kg). All subjects reported that they were free of ay kow eurological dysfuctio or disease that might impair their performace i this test. 2 subjects were right foot domiat ad 3 of them were left foot domiat. The tests used to determie the domiat foot iclude kickig a ball, steppig up o a chair ad leadig off i the log jumpig (Nachsho et al, 983). Procedures The Parotec system (Paromed GmbH, Neubeuer, Germay) was used to collect platar foot pressure data with a samplig frequecy of 00 Hz. The isoles are approximately three millimetres thick ad each has 24 sesors areas. The isoles were placed betwee two cotto socks wor by the subject o each foot. To attach the isole to the foot, a small piece of two-sided tape was placed at the heel area. The outer socks were the wrapped with thi elastic tape. Subjects were asked to walk wearig isoles at their most comfortable speed o a walkway about 0 meters log ad meter wide. The subjects were allowed several practice trials to become accustomed to the istrumetatio ad eviromet. For data collectio, three trials were performed for each subject. I each trial, four steps of each foot durig mid-gait were aalysed. Data aalysis Pressure data were coverted to ASC format ad aalysed with custom-writte Matlab (Versio 6., The MathWorks Ic. USA) programs. Stace phase time (time from iitial cotact to toe off) ad swig phase time (time from toe off to iitial cotact) were measured. Stace phase was sub-classified ito four phases as suggested by Perry (992): loadig respose, mid-stace, termial stace ad pre-swig phase. Each phase was defied i terms of a progressio of foot cotact patters (Ta et al, 999). The loadig respose phase was defied as the period from the iitial cotact util shortly before the foot became flat, mid-stace phase as the period immediately followig util the heel was lifted, termial stace phase from immediately after mid-stace util the iitial cotact of the cotra-lateral side, ad pre-swig phase from this util the toe is lifted. The istataeous positio of COP was calculated as a weighted average of the measured pressure values for each sesor at each sample iterval usig Eqs. () ad (2) (Keeth et al, 2000): X cop = PiAiXi PiAi ()
Y cop = PiAiYi PiAi where (X COP, Y COP ) is the istataeous positio of the COP, P i is the pressure measured by sesor i; A i is the area of sesor i; (X i, Y i ) is the positio of the ceter of sesor i; ad is the umber of sesors. The coordiate at the iitial cotact, mea coordiate, atero-posterior ad medio-lateral displacemet of coordiate, displacemet velocity of COP trajectory durig stace phase were measured by istataeous positios of COP (Figure ). To evaluate the COP path, COP data of each step were ormalized to 00 poits so that comparisos could be made i X, Y axle betwee steps. The coefficiet of variatio (CV) of each poit was the calculated ad averaged over the total stace time of each step for each subject. (2) Figure 2: Five distict regios were defied o the foot, based o the specific isole sesors show. (%) 80 Medial forefoot Arch Medial heel Lateral forefoot Lateral heel X Toe off Velocity of pre-swig Cotralateral Iitial cotact Velocity of termial stace Heel lift 0 Medial lateral displacemet % Body weight 60 40 20 00 80 60 First peak force Secod peak force Velocity of mid-stace 40 20 Mid-peak of low force Foot flat Velocity of Loadig respose Iitial cotact Aterior posterior displacemet 0 0 00 % Stace phase a. Forces i the whole foot (%) Y Figure : Illustratios of ceter of pressure variables durig stace phase. For aalysig vertical groud reactio force (vgrf) distributio, the sole was divided ito five regios: medial heel, lateral heel, arch, medial forefoot ad lateral forefoot (Figure 2). The force of the whole foot (Figure 3a)ad the forces distributed i five areas (Figure 3b)were expressed as a percetage of dyamic body weight (mea support forces measured o-lie durig sigle support itervals). The absolute symmetry idex (ASI) was used for calculatig the asymmetry betwee the feet (Herzog et al, 989): (D N) ASI = 00 (3) 0.5 (D + N) where D ad N are the values of the gait variable measured for the domiat ad o-domiat foot, respectively. Whe ASI=0, the gait is symmetrical. A acceptable degree of symmetry for this type of parameter was cosidered to have a ASI of less tha 0% (Giakas et al. 997; White et al. 999). % Body weight (%) 80 60 40 20 00 80 60 40 20 Forces i the lateral heel Forces i the medial heel Forces i the arch Forces i the lateral forefoot Forces i the medial forefoot 0 0 00(%) % Stace phase b. Forces of the five regios i the foot Figure 3: Illustratios of vertical groud reactio force variables used i this study. Coefficiet of variatio (CV) was used to determie the variability of the results, which was calculated as the mea stadard deviatio divided by the rage of the mea data value, multiplied by 00. stadard deviatio CV = 00 (4) mea A value for the CV of less tha 2.5% is usually cosidered a acceptable level of variability for experimetal data of this type (White et al. 999).
Statistical aalysis All parameters were averaged over 2 steps of each foot for each subject. The absolute symmetry idex ad coefficiet of variatio were calculated for each parameter. All statistics were performed usig SPSS statistical software (.0J for Widows). Paired t-test was used to determie the sigificat differeces betwee the domiat ad o-domiat foot. The level of sigificace was set at 5%. RESULTS Temporal parameters Gait cycle, stace phase ad swig phase durig gait are show i Table. No statistically sigificat differeces were foud betwee the domiat ad o-domiat foot. The CV ad ASI of these temporal parameters were very low. Table : Duratios of gait cycle, stace ad swig phase of the domiat ad o-domiat foot Domiat foot No-domiat ASI Mea (SD) CV Mea (SD) CV Mea Gait cycle.02 (0.04).6.02 (0.04).5 0.6 Stace phase 0.58 (0.03).6 0.59 (0.03).5.4 Swig phase 0.44 (0.02) 2.5 0.43 (0.02) 2.2 2.0 There were o sigificat differeces i each of sub-phase duratios (Table 2). The CVs of these parameters were high both i the domiat ad o-domiat foot. The CVs of o-domiat foot show greater values tha that of the domiat foot. However, they did ot exceed acceptable level of variability (2.5%). The ASI of each sub-phase s duratio decreased from the loadig respose phase to the pre-swig phase. The high value of ASI i the loadig respose phase suggests the poor symmetry durig this period of gait. Table 3 shows that velocities of the COP durig the loadig respose phase, mid-stace phase, termial stace phase ad pre-swig phase were 42.9±3.0, 40.0±6.5, 4.±4., 22.9±0.6 cm/sec i the domiat foot ad 4.2±4.8, 40.3±5.7, 2.8±4.0, 2.2±.8 cm/sec i the o-domiat foot, respectively. I the termial stace phase, COP velocities were sigificatly differet betwee the domiat foot ad o-domiat foot. The domiat foot had a greater COP velocity. I each of sub-phases, the CVs of the o-domiat foot showed greater values tha that of the domiat foot. Amogst ASI values i each sub-phase, oly the ASI of the mid-stace phase is lower tha 0%. This suggests that the mid-stace phase is the most symmetrical phase tha the others durig the stace phase. Spatial parameters Spatial parameters of COP path are show i Table 4. (X, Y) coordiates of COP i iitial cotact i the domiat ad o-domiat foot were (42.6±5.3mm, 200.7±.3mm) ad (4.7±3.8mm, 200.3±.9mm), respectively. There is o sigificat differece i both feet ad the locatio of the iitial cotact correspoded to the ceter of the heel. Table 2 Time ad percetage of each sub-phase durig stace phase Domiat foot o-domiat foot ASI Mea (SD) CV (%) Mea (SD) CV (%) Mea (%) Loadig respose (s) 0.08 (0.02) 9.8 0.08 (0.02). 4.0 Loadig respose % 7.69 (.96) 0.2 7.85 (2.38).0 4. Mid-stace (s) 0.24 (0.04) 8.6 0.23 (0.05) 0.9 2.4 Mid-stace % 23.39 (3.76) 8.0 23.06 (4.28) 0.6 2.3 Termial stace (s) 0.9 (0.04) 9.7 0.20 (0.04).2.7 Termial stace % 8.50 (4.07) 0.0 9.23 (4.4).3.9 Pre-swig (s) 0.08 (0.0) 6.8 0.08 (0.02) 7.9.0 Pre-swig % 7.83 (.23) 6.9 8.02 (.56) 8.0.2 (% Gait cycle) Table 3 Velocity of ceter of pressure (COP) i each of sub-phase durig gait Domiat foot No-domiat foot ASI (cm/sec) Mea (SD) CV (%) Mea (SD) CV (%) Mea (%) Velocity of iitial cotact 42.9 (3.0) 2.2 4.2 (4.8) 2.6 8.3 Velocity of mid-stace 40.0 (6.5) 9.3 40.3 (5.7) 9.8 7.6 Velocity of termial stace 4. (4.)* 2.5 2.8 (4.0)* 6.6 5.6 Velocity of pre-swig 22.9 (0.6) 3.7 2.2 (.8) 8. 38.8 (*: P<0.05)
Table 4 Spatial parameters of ceter of pressure (COP) of the domiat ad o-domiat foot Domiat foot No-domiat foot ASI (mm) Mea (SD) C V Mea (SD) C V Mea (%) Iitial X-coordiate 42.6 (5.3).4 4.7 (3.8).9 9.0 Iitial Y-coordiate 200.7 (.3) 0.3 200.3 (.9) 0.3 2. Mea X-coordiate 40.8 (4.6) 5.0* 40.9 (3.5) 7.* 7.5 Mea Y-coordiate 06.3 (9.5) 3.3 05.7 (9.9) 3.3 2.8 ML displacemet of COP 6.2 (2.3) 46.5* 6.3 (3.0) 67.3* 45.5 AP displacemet of COP 60.9 (3.7).4 57.6 (6.5).4 6.6 Stadard deviatio i X-coordiate 2.4 (0.8)* 6.0* 3.2 (.7)* 7.9* 36.5 Stadard deviatio i Y-coordiate 4.5 (.7) 4.7 4.4 (.5) 4.7 3.5 (*: P<0.05) Table 5 Vertical groud reactio forces o the whole foot ad o each of five distict regios durig gait Domiat foot No-domiat foot ASI Mea (SD) C V (%) Mea (SD) C V (%) Mea (%) Vertical groud reactio forces o the whole foot: First peak force 5.6 (4.) 3.7 5.4 (3.9) 3.7 9.5 Secod peak force 2.2 (0.7) 3.0 2. (8.9) 4.4 6.8 Mid-peak low force 78.6 (0.) 3.5 76.2 (7.8) 3.0 5.7 Vertical groud reactio forces o each of 5 regios: Peak force of lateral heel 40.5 (3.3) 7.2 42.6 (.3) 6.8 30.2 Peak force of medial heel 37.5 (4.) 8.3 36.6 (.8) 6.9 38.8 Peak force of arch 6.0 (5.6) 9.8 5.4 (5.9) 23.6 37.6 Peak force of lateral forefoot 56.4 (4.) 5.2* 54.6 (.3) 2.9* 2.5 Peak force of medial forefoot 67.6 (6.5) 3.0 70.0 (7.8) 5.0 8.6 (% Body weight) (*: P<0.05) The mea X, Y coordiates were calculated by averagig all coordiates durig the stace phase ad correspoded to the ceter of platar surface. There is o sigificat differece i mea X, Y coordiates betwee the domiat foot ad o-domiat oe. However, the CVs of the o-domiat foot are sigificatly greater tha that of the domiat foot. Durig the stace phase, atero-posterior (AP) ad medio-lateral (ML) displacemets of COP were 60.9±3.7, 6.2±2.3mm i the domiat foot ad 57.6±6.5, 6.3±3.0mm i the o-domiat foot, respectively. No statistically sigificat differeces were foud betwee the domiat ad o-domiat foot. The CVs of ML displacemet i the o-domiat foot were greater tha that of the domiat foot. The ASI of ML displacemet was greater tha 0%. I the X-coordiate of COP path, o-domiat foot showed greater variability. Both the mea stadard deviatio ad its CV i o-domiat foot were sigificatly greater tha that of the domiat foot. However, there is o sigificat differece o these parameters i the Y-coordiate of COP path. Vertical groud reactio force Vertical groud reactio forces i the whole foot ad i each of five distict regios durig gait are show i Table 5. The st peak force, 2d peak force ad mid-peak of the low force were 5.6 ±4., 2.2 ±0.7, 78.6 ±0.% of the body weight i the domiat foot ad 5.4±3.9, 2.±8.9, 76.2±7.8% of the body weight i the o-domiat foot, respectively. No statistically sigificat differeces were foud betwee the domiat ad o-domiat foot. The CVs of these parameters showed o differet betwee the feet. I additio, the ASI of them are lower tha 0%. These results are cosistet with the previous study (Giakas & Baltzopoulos, 997). There are o differeces i peak forces of each five distict regios betwee the domiat ad o-domiat foot. However, the peak forces i lateral forefoot showed greater CV i the o-domiat foot. The CVs of peak forces i the arch, lateral forefoot ad medial forefoot showed greater values tha acceptable level of variability (2.5%). The ASI of these parameters showed greater values tha 0% (Giakas & Baltzopoulos, 997). DISCUSSION Gait is a activity i which the movig body is supported successively by oe leg ad the other. Durig gait, the lower limbs support the body agaist gravity while geeratig movemets to propel the body forward. I recet studies, it
is foud that oe lower limb is maily resposible for support ad body weight trasfer while the cotralateral limb cotributes more to propulsio (Hirasawa, 98; Sadeghi et al, 997, 2000). Usig the traditioal methods, for example, force plate, the subject would be coscious of measuremet eviromets. It is difficult to record the atural gait patter for aalysig fuctioal gait asymmetry. However, the platar pressure measurig system (Parotec system) has a advatage that ca record the pressure distributios at the foot-groud iterface up to 0 steps durig midgait. Thus, i this study, Parotec system was used for idetifyig whether or ot the limb domiace affects the symmetry ad variability i the ceter of pressure (COP) ad vertical groud reactio force (vgrf) durig the stace phase of able-bodied gait. Temporal parameters The duratios of gait cycle, stace ad swig phase are cosistet with the previous research documetig the able-bodied gait. It suggests that the subjects utilized i this study ca be cosidered typical i their gait. I temporal parameters, there is o sigificat differece i the duratios of each sub-phase betwee the domiat ad o-domiat foot. Durig ormal barefoot walkig, the loadig experieced by the foot progresses from the heel to the metatarsal heads to the hallux. The heel is the first part of the foot to cotact the groud ad loadig is oly through the subcalcaeal tissue for supportig of the upper body ad shock absorptio. The ASI of the loadig respose duratio shows greater value tha the others durig the stace phase ad suggests its poor symmetry. I the COP velocity, the highest values were foud i the loadig respose phase ad this meas a rapid forward trasfer of force. After iitial cotact, the COP velocity decreased durig the mid-stace phase ad the ASI is less tha 0%. The COP is located i the mid-foot for up to 23% of the gait cycle ad it is the logest duratio i 4 sub-phase durig stace. Because of this phase is characterized by keepig the body s balace, people has eough time tha the others to carefully cotrol the COP path. Thus, for this reaso, the ASI is kept lower tha 0%. Sadeghi et al. (997) reported that the domiat limb which had a propulsio fuctio was characterized by a strog hip power tha that of the o-domiat limb durig gait. The characteristic of COP velocity cocerig the domiat ad o-domiat foot i this study is cosistet with the results of Sadeghi s research. There was sigificat differece betwee the domiat foot ad o-domiat foot. Velocity of the domiat foot was faster tha that of the o-domiat foot. Durig the termial stace phase, people geerate forward velocity by positioig the body s ceter of gravity forward to COP (Perry, 992). The o-domiat foot is cosidered as to cotrol for the body balace durig the stace phase for its lower velocity. Additioally, i the CVs of COP velocity, the values of o-domiat foot were higher tha the domiat foot s. It is suggested that o-domiat foot has the fuctio o shock-absorbig ad cotrollig the velocity of COP. Spatial parameters I gait aalysis, the path of the istataeous COP durig stace is cosidered as the direct reflectio of the subject's balace ad patter of progressio (Perry, 992). I this study, the iitial cotact positios of the heel are ot differet betwee the domiat foot ad o-domiat foot. Also, the mea (X,Y) coordiates i the stace phase are the same. However, i the o-domiat foot, the CV of medio-lateral coordiate showed higher value. This result suggests that although the foot cotact positios are the same for two feet, the o-domiat foot has more variability of platar pressure distributio i the medio-lateral directio. The medio-lateral displacemet of COP was defied as the distace travelled by COP betwee the iitial cotact to the ext iitial cotact of the cotralateral foot i the medio-lateral coordiate. While the iitial cotact, the locatio of body s ceter of mass reaches to the highest positio durig gait cycle. The higher CV value of this parameter i o-domiat foot implys it is resposible for the load redistributio. I additio, the higher ASI value i medio-lateral coordiate showed its poor symmetry. The results of this study are i accordace with the previous studies (Giakas et al., 997; White et al., 999). I able-bodied gait, the gait parameters i the medio-lateral directio are characterized by substatial asymmetries ad probably due to the high variability i this directio. Durig gait, variability i movemet is a prerequisite for skilled motor performace (Berstei, 967). I the path of COP, the mea stadard deviatio i X-coordiate ad its CV showed higher values i o-domiat foot. It implys there is more variability i the medio-lateral coordiate of COP. Iformatio provided from the muscles, tedos, as well as the cutaeous ad subcutaeous structures of the ski help to cotrol balace durig gait i the o-domiat foot. Matsusaka et al. (995) reported that medio-lateral balace was mostly cotrolled by the o-domiat limb. I ormal walkig, the domiat limb produces 56-6% of total positive work (Devita et al., 99). From the characteristics of COP path, it seems that the domiat limb is to propel the body segmets forward while the o-domiat limb is to cotrol forward progressio durig able-bodied gait. Vertical groud reactio force Vertical groud reactio force is cosidered as a iformative parameter sice it ca be able to provide isight ito the effect of the movemet. Hamill et al. 984 used force plate to measure vertical groud reactio force ad foud o sigificat differeces betwee the limbs i vertical characteristics of the groud reactio forces durig walkig. I the preset study, there is also o differece i all three peak values of vgrf. From ASI of the three peak values of vgrf, the gait seems symmetry. However, there are sigificat differeces o the CVs of vertical groud reactio forces o each of 5 distict regios. I the o-domiat foot, the CV of lateral forefoot was greater tha that of the domiat foot. Additioally, force o the medial forefoot i the o-domiat foot showed greater value tha that of the domiat foot, though it is ot
statistically sigificat. Cosiderig with the spatial parameters, the o-domiat foot has greater variability i the medio-lateral directio, especially i the timig of push-off. This also suggests that the o-domiat limb has a role of cotrollig forward progressio durig able-bodied gait. CONCLUSION From these results, atural fuctioal gait differeces were foud betwee domiat ad o-domiat foot. It was suggested that the domiat limb is to propel the body segmets forward while the o-domiat limb is to cotrol forward progressio durig able-bodied gait. REFERENCES Berstei NA (967). The Co-ordiatio ad Regulatio of Movemet. (st ed), Oxford: Pergamo Press. Cavaagh P.R. (978). J. Biomech., 487 49. Devita P. et al (99). J. Biomech. 24(2): 9 29. Giakas G, Baltzopoulos V (997). Gait & Posture 5:89 97. Hamill J. et al (984). Res. Q. 55: 288 293. Ha, T.R. et al (999). Gait & Posture, 0: 248-254 Herzog W. et al (989). Med. Sci. Sports Exer. 2():0-4. Hirasawa Y (98). Saiesu 6:32-44. Hirokawa S (989). J. Biomech. Eg. : 449-456. Keeth JC. et al (2000). Gait & Posture 2: 28-33. Matsusaka N. et al (985). I: D.A. Witer, R. Norma, R. Wells, PR., Hayes, K. Patla, A. (eds), Biomechaics IX-A, Champaig, IL: Huma Kietics. Nachsho I. Et al (983).. It. J. Neurosci. 8(-2): -9 Perry J (992). Gait Aalysis: Normal ad Pathological Fuctio, Slack Ic, Thorofare. Sadeghi H. et al (997). Hum. Mov. Sci. 6:243-58. Sadeghi H. et al (2000). Gait & Posture 2: 34-45. Tai RH. et al (999). Gait & Posture 0: 248 254. White R. et al (999). Cli. Biomech. 4(3): 85-92. Witer JM, Crago PE (2000). Biomechaics ad Neural Cotrol of Posture ad Movemet. New York, NY: Spriger