ELSEVIER 0268-0033(95)00068-2 Clinical Eiomerhanics Vol. 11. No. 5, pp. ?S3-25Y, 1996 Copynght 0 1996 Published by Elsevicr Science Limited. All rights rcscrved Printed in Great Britain 0268-0033196 $15 .OO + 0.00 Relationship between vertical ground reaction force and speed during walking, slow jogging, and running T S Keller’, A M Weisberger*, J L Ray3, S S Hasan4, R G Shiavi4, D M Spengler* ‘Department of Mechanical Engineering, University of Vermont, ‘Department of Orthopaedics and Rehabilitation, 3Department of Mechanical Engineering, and 4Department of Biomedical Engineering, Vanderbilt University, USA Abstract Objective. To obtain descriptive information between vertical ground reaction force (GRF)-time histories and gait speed, running style, and gender. Design. GRF-time history measurements were obtained from male and female subjects during walking, slow jogging, jogging and running on an indoor platform. Background. Previous studies have established GRF descriptor variables for male subjects running at speeds from 3 to 6 m s-‘, but very little descriptive data exists for slower or faster running, nor have previous studies reported GRF descriptors separatelyforfemale subjects. Methods. GRF-time histories were recorded for 13 male and 10 female recreational athletes during walking and slow jogging at speeds between 1.5 and 3.0 m s-l, and running at speeds between 3.5 and 6.0 m s -‘. Vertical GRF-time data for trials with speeds within 0.2 m s-’ of the prescribed speed were analysed to determine thrust maximum GRF (F,) and loading rate (G,). Results. In both male and female subjects, F, increased linearly during walking and running from 1.2 BW to approximately 2.5 BW at 6.0 m s-‘, remaining constant during forward lean sprinting at higher speeds. F, was linearly correlated to G,, the latter ranging from 8 to 30 BW s-’ over this speed range. Slow jogging was associated with a > 50% higher F, and G, in comparison to walking or fast running. Conclusions. Similar GRF descriptor data and velocity relationships were obtained for male and female subjects. Impact forces were greatest when the subjects adopted a higher, less fixed centre of gravity during slow jogging. Relevance These results suggest that vertical GRF norms can be established for male and female subjects alike, and that slow or fast running with a lower, fixed centre of gravity decreases impact forces. Copyright @ 1996 Published by Elsevier Science Ltd. Key words: Gait, ground reaction force, thrust maximum, speed, running, biomechanics C/in. Biomech. Vol. 11, No. 5, 253-259, 1996 Introduction The popularity of recreational running has increased dramatically over the past few years, as has the incidence of overuse or repetitive loading injuries. Clinical evidence suggests that workout intensity plays a major role in the development of overuse injuries. In a study *conducted by James and associates’ in- volving 180 patients. 65% of the chronic injuries Received: 25 January 1995; Accepted: 13 October 1995 Correspondence and reprint requests to: Tony S Keller PhD, University of Vermont, Department of Mechanical Engineering, 119 Votey Building, Burlington, VT 05405-0156, USA occurred among dedicated distance runners logging high mileage on a daily basis. Two-thirds of the chronic injuries were attributed to high mileage, workout intensity, running up hills and on hard surfaces, and/or rapid change in training routine. Other researchers have postulated that impact forces associated with repeated loading are responsible for certain types of overuse injuries of the musculoskeletal system’.“. The notion that there may be a positive relationship between impact force and overuse injuries during running, together with the need to assess athlete performance, has prompted numerous experimental studies of ground reaction force (GRF)-time histories during the past 20 years4. Ground reaction force-time
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`254 Ctin. Biomech. Vol. 11, No. 5, 1996 histories provide dcscriptil e information concerning the magnitude. direction and point of application of the impact fori~:: In general the vertical component of the GRF domimites the impact force--time histor) in comparison t/1 the other two components (backward-. forward. me&al-lateral). and hence is the easiest to quantify. The vertical GKI’ also shows the least vari- abitit! between and within subjects’ i. Studies have indicated that the descriptive data characterizing the verlicitl GRE (loading rate. impact peak. relative minimum. :hl u\t maximum. decay rate) are dependent up0n numcroiis external factors such its suhjcct body masi. loading rate. running speed. running stvle, area tri !hc foot sround contact. :IS well as the mechanical lm~p~rtic’s of’ the fool. shoe. ;Ind surface involved’~” “. <ii thy nl:\!?v esterna! tactors which intluencc the GRF. gait ++~ed has been the central focus of man) invcstigiltiony ’ - !(I i ’ .I’hcs1: studies have consistentI! rtcttcd thilt. rhr: nla,gnitutie c lf the vertical CiRE‘ increasec; witli rni,rc;t3iti!; speed %>?‘cr thy range 111‘ spends ~~xrtmined. i>:lr h;~vc genr~rall\ been limited to a narro\\ tati~c 01 \\,;iliiing 01 runnirtg speeds ;md:‘or 3 small 1rumt~r <,I x?ibjcctb. In :]I! c>ffort TV) establish reference stanthrrdu l’<)r C;RF dat,r as il function of running speed. Munr~) CI ~1.. .:oliccted GRF data from 20 adult males a1 \ptc’(fs rzmging from i-5 m 5 ’ tit the SQWdS ~x;m~~nc~l. th majorirt of the subjects in this stud! \+tcrc rC;ir--t(>~ !I 5trikcrs LL~OSC GKF-time history wan, ~h;ir;+~t~~r~~c:c! t)\ ;I,] iriiti;jl sharp peak (impact Inax- mum) i’olic~t:~l hi ;I sccc,nd pc;ik at mid-stance (thrust m;t*imilJn! i hc( nored that rhr impact maximum irrcrc.;\\etl i aira gut i .i-Iold) in it linear manner from j CL %~lvwcigh! (l3W) t(i 2.S HW over this range 01 >pcctl~ - ir,irc;i5C:s iI, the thrust maximum and the ;~~,c~-:Q~c \trr?ir~ri C;KF cxcrtctl throughout the stance phahz howc.\ J:- ~c’ce i+s remarkable ( I, I -fold and 1 “.11,ldi ;tbL’i h ..I ?hcxe running speeds. and exhibited ;I ??lOrL’ 11011. iir!t.;tr relationuhlp kvith i;peed (increasing less wit!? ;noi-c;rslll,z3 running speed). Nipg et al: found SilTiililj ti’?,ittii 1~11 14 rllalc\ running at speeds ranging r”riK: .; f. iJ’l Whili~ these results establish useful \tnndnr&. !%:I. (; KI-‘de>cripi;pr \,:lriables for nialc runners. the; do noi l>r(!l J&Z descriptive data for women. nor do thcW rcsuii~ ;lr pre\%~u\ IitCraturc consider running at ifXW.l$ IC\S t!-c,ii\ : 111 \ ’ or grcatcl- that) f, 111 x ‘, ‘Z%C, i>f>jt’cti\‘* of thE> <~uJy W;IS LO re-examine the relationship between 111~ vertical CiKF and speed tzncompassmg ;i wide r;\nge of physiological running speeds. In particular WC‘ wished to answer the following qut,stions: (1 ) docc the vertical GKI~ increase in a lineal manner at running speeds greater than S m s ‘?. (7) does gender intluencc the vertical GKF‘.‘. and (3) what ei’f~ct does sIo\~ .joggmg as opposed to fast walking have on the vcrtlcal GKF’.’ Age iyears) 25.2 (SD 4.31 28.4 (SD 5.4) Height (cm1 178.4 (SD 7.0) 168.3 (SD 7.0) Mass (kg} 75.6 (SD 12.0) 57.6 (SD 5.81 Methods Twenty-three subjects (13 males and 10 females) were used in this study (Table 1). All subjects were recre- ational athletes who participated on a regular basis in a variety of activities including: basketball. squash, cyc- ling, soccer. racquetball, distance running. volleyball, tennis, weight lifting, triathlons. and other sports. All were within the range of normal weight for their height. The majority of subjects surveyed indicated that they run at average speeds of X-c) minutes/mile for females and 7- S minutes/mile for males. ‘I’hese speeds correspond approximately to 3.0-3.3 m s ’ and 3.4--3.X m s-’ for females and males respectively. During the tests the male and female subjects wore Nike Aircraft running shoes with identical soles and cushioning. Subjects were asked to walk. jog, and run over a 11-m running platform. ‘This arrangement provided 6 m for the subjects to accelerate :md decelerate. Football dummie\ were placed at the end of the runway as ;I buffer during deceleration and subjects were en- couraged to USC them at higher speeds. A A-channel force pl;itform (,Model OR&3, Advanced Mechanical Technology. Inc.. Newton. MA). with a natural frequency 01’ 401) Hz. was located flush in the centrc of the platform and was directly connected to a IWP I 1123 computer. A I:-bit A0 converter was used to sample the GKF --time history data at 3% samples per second. l‘his SilJllpling frequency was based upon Nigg’s’” reconimendution that the appropriate frequency for data acquisition should be at least five times the maximum frequency content of the analysed signal. During running up to 6 m s ’ the frequency content of GRI; -time histories is not more than 3) lIzI-‘. The force platform surface measured 50X mm x 357 mm and was outlined with a bright yellow. ‘r-mm wide I;I~c’. Subjects were allowed as many practice trials a\ needed to achieve acceptable foot contact (within the bounds of the force platform minus 25 mm on each side) and were given ;i rest of at least I min between speed trials. A line of 20 1X1) lights. spaced 0.5 m apart and set to blink in sequence at the desired specda. \tas placed OJI the margin of’ the running stage in order to guide the subject. Subjects were required to contact the force platform using the same foot (right or I&t), because the data acquisition program required consistent foot usage throughout. Data from both right and left foot strikers were combined into distinct speed categories according to the method of Munro and associates’. An KGB video camera (Hitachi, model KP-C’IOSA. Hitachi Denshi Ltd. Japan) and a h-head videocassette recorder (I litachi. model VT-33OA, Hitachi I.td. Japan) were used to film and record (at 30 frames per second) the foot-strike pattern and contact angle during each trial. The camera was located adjacent to the force platform facing either the medial or lateral aspect of the foot, depending upon whether a left or right foot strike occurred. respectively. A minimum of four walking speeds (1.5. 2.0. 3.5. 3.0 m s .‘) and four running speeds (3.5. 4.0. 5.0,
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`Keller et al: Ground reaction force and gait speed 255 6.0 m s-‘) were measured for males. Female subjects were measured at the same walking and running speed intervals up to 4.0 m s-l, after which the speed inter- vals increased at 0.5 m SC’ intervals up to the subjects’ maximum speed. In order to obtain GRF-time histories at the subjects’ maximum speed, male and female subjects were encouraged to run as fast as possible above 6.0 m s-’ and 5.0 m SC’ respectively. In a subset of 12 subjects (6 males, 6 females), the subjects were asked to slow jog at speeds of 1.5, 2.0, 2.5, and 3.0 m s-‘. Slow jogging was distinguished from walk- ing by the absence of a double support phase. Walking, slow jogging, and running speeds were measured by two photoelectric cells located 1 m from the centre of the force platform, and mounted so that the photo- electric cells were triggered by the subjects’ waist. Up to 10 trials at each speed were recorded, and only trials in which there was good foot contact within the perimeter of the tape, a steady stride, and speeds within ?Z 0.2 m SC’ of the prescribed speed were analysed. The six-channel GRF-time history data was later processed on a PDP 11173 computer using data analysis methods described previously”. The main variables reported in this paper are the vertical thrust maximum force (F,), vertical thrust maximum loading rate (G,), and speed (v). In order to precisely determine the magnitude and time duration of the vertical thrust maximum force, a four-point interleave filter (3 db cut- off = 15 Hz) was used to smooth the 256 samples/ second GRF-time history data. This smoothing process produced data records containing 65 samples/s (256/4 + 1). Thrust maximum loading rates were calcu- lated by dividing F, by the time interval between initial foot contact and the occurrence of the vertical thrust maximum force. In accordance with Munro et al.‘, F, and G, were normalized to the subject’s bodyweight (BW). Vertical impact peak forces were not determined from the GRF-time histories, since these short duration peaks were attenuated by of the smoothing scheme used to process the data. The smoothing scheme, however, produced only a small reduction (about 2-5%) in the thrust force values at the highest speeds. Foot-contact patterns for each trial of each subject were quantified by examining digitized images obtained from the videotape recordings. Originally we had intended to determine both the foot-strike index (rear- foot, mid-foot, fore-foot)-i and the contact angle from the video recordings of the foot-strike patterns. Both are important parameters which are required for dynamic analysis and modelling of rigid body motion of the lower extremities. Subsequent analysis of digitized images of each foot strike, however, indicated that while this procedure was adequate for determining the foot-strike index, we could not obtain accurate contact angle measurements above 3 m s-’ with the frame rate used (30 Hz). This paper, therefore, presents only the former. It should be noted that one can perform centre of pressure measurements to determine contact patterns2,‘, but such measurements cannot be used to compute contact angle measurements. Percent of Total ReDetitions 100 80 60 40 20 0 1 1.5 2 2.5 3 3.5 4 5 6 6.5 7 Speed (meters/second) Forefoot E2 Midfoot 0 Reatfoot Figure 1. Foot strike indices (rear-foot, mid-foot, fore-foot) versus speed for all subjects. indices are depicted in terms of the percentage of total walking and running repetitions (n = 8791. Foot strike patterns change from predominantly rear-foot to predominantly mid-foot at 6 m 5-l. Means and standard deviations (SD) of the descriptive variables were obtained at each of the fixed walking, slow jogging, and running speeds. Linear regression models were also applied to the force-velocity data, and R2 values and levels of significance were calculated for the regression equations. An analysis of covariance (ANCOVA, equality of slopes) was also performed to determine if the regression models were significantly different for male and female subjects. GRF descriptor variable differences between men and women, at differ- ent speeds, and between slow jogging and walking were assessed using a one-way analysis of variance (ANOVA). Results Analysis of the foot strike indices indicated that the majority of subjects were rear-foot strikers at speeds less than 5 m SC’ (Figure 1). At speeds above 3 m SC’ there was an increasing frequency of mid-foot and fore- foot strikes. Eighty-six percent of the subjects were mid-foot or fore-foot strikers at 6.0 m SC’. Eight females achieved speeds of 5 m s-’ and two completed five trials at 6 m s- ‘. All males achieved speeds of 6 m s-l and four completed four or more trials at 7 m s-‘. One male subject completed three trials at a speed of 8 m SC’ using a rear-foot strike pattern. Many subjects increased their stride length and assumed a more crouched, forward leaning posture during their high-speed running trials. The vertical GRF-time histories exhibited a double peak during walking and running below speeds of 2.5- 3.0 m SC’ (Figure 2). At these speeds the thrust maxi- mum force was generally the first peak recorded and occurred between 15 and 25% of the total stance time. At higher running speeds, the GRF-time histories consisted of a single peak (thrust maximum) located at about 40-50% of the total stance time. The mean values for F, ranged from 1.15 BW at 1.5 m SC’ to 2.54 BW at 4.5 m s-’ for females, and from 1.23 BW at 1.5 m SC’ to 2.46 BW at 5 m s-l for males (Table 2). The average loading rate increased from 7.77 to 30.0 BW SC’ and 8.20 to 29.1 BW SC’ in the speed range 1.5-6.0 m SC’ for the female and male subjects respectively.
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`256 C/in. Biomech. Vol. 11, No. 5, 1996 a 250 Time (% Stance) Time (% Stance) Figure 2. Anterior-posterior vertical GRF-time histories patterns as functions of running speed. Time histories shown were smoothed using a 4-point interleave filter and normalized as a percentage of the total stance time according to the method of Hasan et al. 14, Impact peaks were not present in the vertical GRF- time histories because of the smoothing procedure used to post-process the data. (a) Female subject (7) for speeds of 1.5-6.0 m S- ‘. Transition from double to single vertical force peak occurs at 2.5 m s ‘. (b) Male subject (9) for speeds of 1.5-7.0 m sm.‘. Transition from double to single veltical force peak occurs a? 3.0 m q ’ ‘1 1~ icrtlcai thrust maximum force increased in a lineal- manner with increasing speed up to abot~t 1 ,? !I-, c ’ for both males and females (,Figure 3). V’ari- :~l~ms in 2;, wt:re greatest in the speed transition region l>riM;<$<:n lvalking and running (e.g. 2.5.-3.0 m s ‘) at which point some subJccts walked and some jogged. ii; 3.5 nl :. the male and female subjects were run- ning at 53,11”:1 (SD 5.2) and 67.5(X, (SD 6.1) of their maximum speed respectively. Linear regression equa- rions and the coefficient of determination (R’) for F, (RN’) vc’rsux speed (walking and running gaits) in the range (11 I 5 ~TI 4- I c: i’ -C 3.5 m s -’ were: Males (n = 291) F, = 0.598 L’ + 0.249. R” = 0.65 (PCO.001) Females (n = 240) F, = 0.634 v + 0.159, R” = 0.66 (PCO.001) where n is the number of trials. Incremental changes in F, were statistically significant (ANOVA, P-cO.05) up to 3.5 m s-l for both male and female subjects. At speeds greater than about 3.5 m s-l there were no significant increases in F, for either group of subjects. In the male subjects there was a slight decrease in F, at the highest speeds. particularly for the subject who ran up to Table 2. Summary of vertical GRF variables (mean values) grouped by running speed and sex Females Thrust max. farce (F,, BWJ Loading rate fG, BW s ‘i ~___-- __-- Speed (lt 0.2 m s ‘) Males Thrust max. force Fz, SW! Loading rate (G, BW s-‘I 1.5 (r: 501 1.15 (0.10) 7.77 (1.781 1.5 (n = 65) 1.23” lO.10) 8.20 (1.84) 2.0 in 50; 1.36 (0.18) 11.5 (2.36) 2.0 (n = 641 1.42’ (0.14) 11 .o (2.29) 2.5 /n .= 49) ‘I .73 (0.43) 14.6 (3.71) 2.5 h = 65) 1.62 (0.24) 14.6 (2.46) 3.0 (r> = 50) 2.11 (0.46) 16.9 (3.97) 3.0 In = 61) 2.10 (0.50) 16.0 (3.30) 3.5 (a -= 41 J 2.36 (0.25) 19.1 (3.82) 3.5 (n = 37) 2.45 (0.28) 18.32 (3.36) 4.0 (n 46) 2.33 (0.32) 19.6 (4.65) 4.0 In = 58) 2.35 (0.48) 18.9 (4.85) 4.5 (n IO! 2.54 (0.27) 23.7 (4.91) 5.0 in = 38; 2.28 (0.32) 22.3 (4.61) 5.0 (n = 60) 2.46* (0.33) 22.8 (4.51) 5.5inx 101 2.13 (0.32) 22.5 (6.87) 6.C (P IO! 2 45 !0.13) 30.0 (2.63) 6.0 (n = 67) 2.38 (0.28) 29.1 (15.2) 6.58 (n ; 26) 2.34 (0.23) 37.8 (29.3) 7.0% (n = 17) 2.29 (0.19) 36.5 (22.5) 8.0% (n = 3) 1.89 (0.49) 58.5 (37.6) so m parentheses. n - number of trials f Significant difference ~ANOVA, P<O.O5) compared to females. *iAopruximate running speed across force platform since subjects were accelerating between 4 and 6 mare speed measurement interval
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`Keller et al: Ground reaction force and gait speed 257 3- 3.57 2.8 - iz2., : g22.4 : E 3 2.2- E : .m 2 2- 2 HI- c ; 3 1.6- .u 5 > 1.4- b -1; I I c JI I - 1 I 0 Females - walk, run 0 Males - walk, ran $3.0: IL” 0 g 0 2.5- 8 U E i 2.0- 3 2 1.5- t; - 0 2 f 1.0- - / 0 Male 0 Female 1.2- 0.01 I I 81, i I I I I I I II ,I -0 1 I 0 10 20 30 40 Thrust Maximum Loading Rate (BWs”) Figure 3. Comparison of male (open squares) and female (open circles) vertical thrust maximum force versus speed (1.5-6.0 m ss’). Mean and Figure 4. Vertical thrust maximum force (F,) versus thrust maximum standarddeviationsareshown.Bestfitlineforcombinedmaleandfemale loading rate (G,) for male (open squares) and female (open circles) during subjects is also shown for speeds up to 3.5 m s-’ (see text for linear walking and running. Both male and female subjects exhibited a similar regression equation). Differences between male and female subjects positive linear relationship between F, and G,. Best fit line for combined were significant (ANOVA, P<O.O5) at speeds of 1.5,2.0, and 5.0 m s- ‘. male and female subjects is shown in the range 2.9 < G, < 26 BW s-’ (see text for linear regression). 8 m s-l (F, = 1.89, SD 0.49 m s-‘. Changes in G, were also linear with regards to speed throughout the range of walking and running speeds examined. However, the relationship between F, and G, was most linear only up to about 26 BW s-l, after which F, remained relatively constant (Figure 4). The following linear regression equation and coefficient of determination (R*) was obtained for F, (BW) versus G, during walking and running: (Table 3). Differences in F, and G, for slow jogging versus .walking were statistically significant (ANOVA, P<O.OOl) at speeds ranging from 1.5-2.5 m s-r in female subjects and 1.5-3.0 m s-l in male subjects. Females exhibited a smaller difference in forces between slow jogging and walking than males. Both groups indicated that walking was preferable to slow jogging or ‘slogging’. Males (n = 436) F, = O.O89G, + 0.520, R* = 0.79 (P<O.OOl) Discussion Females (n = 356) F, = O.O90G, + 0.482, R2 = 0.77 (P<O.OOl) where G, < 26 BW SK’. An ANOVA indicated that the difference in F, (BW) between male and female subjects was significant for the following gait speeds: 1.5, 2.0 and 5.0 m s-l, but these differences were small (<8%). There were no significant differences in G, (BW s-l) between the male and female subjects at any of the speeds examined. An ANCOVA indicated that there were no significant differences between the force-velocity and loading rate-velocity linear regression equations (equality of slopes) obtained for the male and female groups. Consequently the data for male and female subjects was combined, yielding the following linear regression relationships: Males + females (n = 531) F, = 0.614 v + 0.208, R* = 0.65 (P<O.OOl) In this study, GRF-time histories and foot-strike indices were analysed for 23 young male and female recreational athletes during walking, slow jogging, and running on a force platform. Normative data for vertical GRF descriptor variables (thrust maximum, average loading rate) were presented and relationships between the GRF descriptors and speed were studied. The notion that altered running gait (slow jogging versus walking) may influence the GRF-time histories was also examined. In order to establish normative GRF data, a relatively large number of subjects wearing shoes with identical soles and cushioning was studied. Over 1100 GRF-time histories and foot contact patterns were collected and analysed for walking, slow jogging, and running at speeds ranging from 1.5 m s-l -8.0 m s-l. Despite limitations in the runway length, most of the male and two of the female recreational athletes examined in this study were able to achieve constant running speeds up to 6 m s-‘. Males + females (n = 753) F, = O.O89G, + 0.503, R* = 0.78 (P<O.OOl) where 1.5 < v < 3.5 m s-’ and 2.9 < G, < 26 BW s-l. Thrust maximum forces and loading rates were as much as 62 and 65% greater, respectively, during slow jogging than during walking at the same speed The magnitudes of the vertical thrust maximum forces obtained in this study for walking (less than about 2.5-3.0 m s-‘) and running (greater than 2.5- 3.0 m s-l) compare favourably with previously pub- lished results*,3,S,9,“,‘2~IS-*O . Results from these studies are summarized graphically in Figure 5 for comparison to the present study. Examination of
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`258 C/in. Biomech. Vol. 11, No. 5, 1996 Table 3. Retative difference 1%) in vertical GRF descriptors between slow jogging and walking gaits Females Males ..___ . . ---_- __-._____-__ Speed F, I;. FJ G, tl?.2ms ’ _ _- ________ -.. 15 49.0* 45.1 62.2X 65.4* 2.0 39.4” 13.5* 49.6* 36.0” 2.5 18.7* 0.3 42.5* 11.6’ 3.a 5.6 1.3 l&6* 15.4x *SwmiRcanr differrncr !nNovn , PcO.001) between slow jog and walk. i-igurc 5 indicates that walking is associated with vertical thrust maximum forces between 1.1 and 1 .s 13w. The range of thrust maximum forces measured during the fast running speeds most commonly reported in the literature (4-4.5 m s ‘) are about double that of walking gait values. which is also consistent with the present study. I)nly two prior studies have performed comprc- hens& GRF--time history studies over a range of run- ning speeds’,i. For speeds of 3-S m se-‘, Munro and associat& reported vertical thrust maximum force values ranging from 2.51 BW (SKI 0.21) to 2.83 BW (SP 0 I?‘\. The majority of the subjects examined by Munro and associates were rear-foot strikers. Nigg ct at.” reported that vertical thrust maximum force values ranging from 1.86 kN (SU 0.17) to 2.26 kN (XI 0.42) or approximately 2.6-3.2 BW (estimated using mean BW reported) are produced during running at 1% h m 3. ’ Nigg and associates reported that the foot contact pattern changed from a rear-foot strike to a mid-foot strike pattern at the highest speeds. Their data also suggest that the vertical thrust maximum forces wcrc more variable at the highest speed, which was speculated to be the result of’ variations in foot contact patterns. WC observed a similar change in foot strike pattern. but a more consistent force variability with increasing running speed. Both of the aforementioned studies’.’ indicated that the increase in the vertical thrust maximum force during running was linear with increasing speed in the range 3-6 m s ‘. We also noted a Linear increase in the vertical thrust maximum force with increasing speed, but the relationship between the vertical thrust maximum force and speed in our study was linear only up to 4.0 m se ’ or about SO-60% of maximum running speed of the subjects. Above this speed the vertical thrust maximum force remained relatively constant. Noteworthy. therefore. was the finding that the vertical thrust maximum forces during running at S--h m s i were longer in magnitude (about lo-20%) than that of Nigg et al.’ and Munro et al.i An explana- tion for this finding may be the fact that the subjects in the present study adopted a forward leaning running posture at higher speeds. particularly during their tastcst running trials. A forward leaning running style lowers the centre of gravity of the subjects. and reduces the downward velocity of the head, arms and trunk at touchdown, thereby reducing the magnitude of the CRF in comparison to running with more upright postures. Bobbert et al.” noted that a ‘groucho’ style running (running while keeping the centre of gravity low) at 3.6-4.2 m s-l produced GRF forces which were about 25% lower than normal heel-toe running at the same speeds. Our findings associated with forward leaning during running are consistent with this. An interesting finding in this study was that slow jogging or ‘slogging’ produced forces that were significantly greater than those during walking. Differences between walking and ‘slogging’ were greatest in the male subjects, who exhibited vertical thrust maximum forces at 2.5-3.0 m s-l which were comparable to F, values during running at speeds between 3 and 8 m s I. The greater ‘slogging’ versus walking differences for the male subjects may be due to the fact that most females are already jogging at a speed of 3.0 m s ‘. whereas the majority of the male subjects chose to walk at 3.0 m s -I. Walking versus running preferences reflect differences in stature among the subjects; males tended to be taller and had a larger stride length. One explanation for the observed increased vertical thrust maximum forces associated with ‘slogging’ is the fact that both the male and female subjects adopted a higher centre of gravity in order to slow jog, which increases the downward velocity of the head, arms and trunk, and therefore increases the magnitude of the GRF. Although ‘slogging’ at walking speeds was a less natural gait, none of the subjects had any problems with this style of running. ‘Slogging’, however, produced a more bouncy and jarring style of running. Given the choice between ‘slogging’ and walking at the lowest speeds, therefore, all subjects 350, 2 $ 300 - 1 d -I A b 150 2 5 2 100 ” Symbols _ Literature - - Present Study 50(,~,,,1,,,,,,,,,,,,,,,,,,,,,, 0 1 2 3 4 5 6 7 Speed Ims-‘) Figure 5. Comparison of vertical thrust maximum force (F&speed data from the literature with the results of this study (linear regression line for male t female subjects). In contrast to that observed in previous studies (symbols in plot), F, values obtained during running did not increase significantly above speeds of 3.5 m s ‘. This was hypothesized to be the result of the lower centre of gravity associated with the fotward leaning running posture adopted by most subjects. Symbol key: A --- Alexander and Jayes” (7 male subjects, age range 7 23-58 years); - Munro et al.’ (20 male subjects, mean age = 25.4 years); A 7 Nigg et al.’ (14 male subjects, mean mass = 73 kg, F, data normalized with respect to mean mass of subjects); * = Bates et al. ll; c = Cavanagh and Lafortune3 (10 male and 7 female subjects, mean age 24 years); + 7 Frederick et al.” (6 male and 3 female subjects); :> = Dickinson et al.16 (6 male subjects, mean age 7 26.3 years); H = Hamill et al.“; c: - Roy”.
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`indicated that they would prefer to walk. Based upon the fact that ‘slogging’ produced a significant increase in vertical GRFs, this type of running style should be avoided, since increased forces are highly undesirable from the point of view of injury prevention. In this regard, a running style with a lower centre of gravity would appear to be optimal for minimizing GRFs. To our knowledge this study represents the first comprehensive study in which vertical GRF descriptors are reported separately for male and female subjects. Both the men and women subjects examined in this study were young recreational athletes, active in a number of different sports. While statistically signifi- cant differences in F, values were noted between males and females at some walking and running speeds, these differences were generally unremarkable. The small differences noted may reflect the fact that the female subjects changed from walking to jogging at a lower speed, and also had a lower maximum running speed. More noteworthy was our finding that similar F,- velocity and F,-G, relationships were obtained for both sexes. Our results suggest that normative vertical GRF descriptor data and velocity relationships can be applied to both male and female subjects alike. Additional work is needed to ascertain whether or not these observations apply to other GRF descriptors such as vertical impact force and impact loading rate. Conclusion Knowledge of the relationship between gait speed and vertical ground reaction force is important for develop- ing models of musculoskeletal adaptation to altered activity, improvement of our understanding of the aetiology and treatment of injuries associated with running, and assessment of running performance. The results of this study of 13 men and 10 women recre- ational athletes indicated that the vertical ground reaction forces increased linearly with gait speed up to about 60% of the subjects’ maximum speed. At higher speeds, vertical forces remained constant at approxi- mately 2.5 times body weight. This finding is different from that reported previously for running at speeds of 3-6 m SC’, and is hypothesized to be the result of a lower centre of gravity associated with the forward leaning running style adopted by the subjects in order to achieve fast running speeds on a relatively short runway. Male and female subjects had similar values for the vertical GRF descriptors at all the speeds examined, and linear regression relationships between vertical thrust maximum force and velocity were similar for both groups of subjects. These results suggest, therefore, that vertical GRF norms can be established for males and females alike. Moreover, slow jogging or ‘slogging’, characterized by a higher centre of gravity and more bouncy running style, produced vertical forces as much as 1.6 times greater than normal walking at the same speed or running at higher speeds. Running style, therefore, appears to be a particularly important determinant of vertical GRF-time history descriptor variables. Keller et al: Ground reaction force and gait speed 259 Acknowledgements The authors would like to thank James P Bohan Jr, Melvyn A Harrington Jr and Mike Stanfill for their technical assistance during th