Gait & Posture 1993; 1: 85-91

Examination of the duration of gait initiation by use of an electrogoniometer

J P Gormleyl, D A Barr3, A J Bell’, J Ravey2, R A B Moltan

‘Department of Occupational Therapy and Physiotherapy, 2 Department of Psychology, University of Ulster, Jordanstown. 3 Department of Orthopaedic Surgery, Queens University, Belfast, Northern Ireland

Summary

This paper describes a series of experiments that examine the duration of the initiation of gait, In all, 30 subjects participated in the experiments. Information about the gait pattern of each subject was obtained by the use of electrogoniometers attached across both knee joints. These provided a direct measure of knee angle versus time, from which a value of stride rate was mathematically derived. This measurement of stride rate was used to identify the step at which gait initiation was completed as defined by the attainment of a constant value for stride rate. This was found to vary amongst subjects with a mode value of five steps and a range between two and 11 steps.

Key words: Gait mitiatton, stride rate, steady state

introduction

The first step of walking to a constant state of motion would appear to involve a simple transition; however, it is a complex interaction of ‘neural and muscular activity”. Normal level walking has been examined extensively in the literature, reflecting its importance, but the initiation of gait has received scant attention. This does not reflect its value, as it is believed to provide important information in the identification and diagno- sis of disabilities in the locomotor system’. This study is concerned with the duration of gait initiation, i.e. the number of steps necessary to reach a steady state of walking. The identification of when initiation ceases and steady state walking begins is essential to both researchers and clinicians engaged in the assessment of gait.

Review of previous work

The earliest work on gait initiation was carried out by Carlsoo’ who examined the distribution and size of foot

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Dr D.A. Barr. Department of Orthopaedic Surgery. Musgrave Park Hospital. Stockmans Lane. Belfast. N. Ireland.

(‘ 1993 Butterworth-Heinemann Ltd 0966.-6362 ;93 ‘0200X547

pressures and the muscle activity of certain leg muscles. The forces acting through both limbs and the electro- myographic activity of muscles involved were examined, but perhaps the most interesting feature of this study was a lateral shift of the centre of pressure towards the swing limb prior to the initiation of the first step. The next work by Hermann et aL4 examined the neurophysiological processes involved in gait initiation and concluded that it was controlled at a ‘supraspinal’ level and was therefore not simply a reflex action. The first study to examine the relationship between gait initiation and normal walking was carried out by Cook and Cozzen9. Their results strongly suggested that constant walking velocity was related to the velocity during initiation. This study was carried a stage further by Mann et al.’ in an attempt to describe in more detail how gait was initiated. They concluded that the velocity of the centre of gravity reached a steady level between the second and third steps. Another study examining the duration of gait initiation was carried out by Breniere and Doh, who investigated the velocity of the centre of gravity during the first two steps. Their conclusions contradicted Mann et al.‘, asserting that steady-state walking commenced at the end of the first step. To date two further studies have been carried out on gait initiation. A study was carried out by Nissan and Whittle? to establish a database of kinetic and kinematic data for future reference. The results provided valuable information about the magni- tude of various joint angles and forces that was absent from the previous studies. Brunt et al.’ examined various

86 Gait & Posture 1993; 1: No 2

characteristics of gait initiation at various speeds of walking. In these two studies gait initiation was assumed to have been completed at the end of the first step as only the first step of walking was examined.

The problem with previous work examining the duration of the initiation of gait is that the subjects were only examined for a maximum of three steps. In the case of Mann et al.’ tine-film analysis was used to determine the speed of walking during the first three steps. Breniere and Do6, using a single force plate, used a process of integration to determine instantaneous velocity of the centre of gravity at the end of the first step, which was then compared with mean velocity during the second step. Each of these workers identified the step at which a steady state of walking was reached as the step at which initiation was completed. The duration of gait initiation may therefore be most appropriately defined as the number of steps or strides necessary to reach a steady state of walking, using the convention that a step is between consecutive footfalls and a stride is between consecutive footfalls of the same foota.

Attainment of steady state is characterized by mean values for all variables of gait. There are, however, several variables (e.g. stride rate and knee angular velo- city) that are not constant after three steps of walking. An investigation was therefore planned and executed to evaluate these other gait variables up to the step at which their value becomes constant.

To examine three or more steps using conventional kinetic equipment would be difficult, as an investigation of the movement of the centre of mass over such a long distance would require several force plates. Furthermore a study of body segment kinematics using three-dimen- sional photogrammetry would restrict the walking dis- tance to a few metres. For these reasons an alternative system was chosen to facilitate continuous monitoring of joint angles over a long walkway. This simplified approach greatly restricted the set of gait variables that could be quantified from the data, but it had the advan- tage that it could be used to examine many more than three steps of continuous walking. The system used was a twin-axis goniometric system (Penny and Giles Gonio- metrics).

Experiment 1

Aim

This first experiment was a pilot study to provide base- line information on a number of gait variables, i.e. stride rate and knee angular velocities in flexion and extension.

Method

Eight normal female subjects participated in this study. Their mean age was 19.2 (SD 1.4) years. The knee joints were monitored simultaneously for each subject and the duration of the walk was set at 10 steps. Each subject wore shorts and low-heeled footwear. After calibration

"0 1 2 3 4 5 6 7

Time (s)

5o t ‘Rl BR2 BR3 ‘R4 BR5

a 40

2 30

F 20 a

10

0 0 1 2 3 4 5 6 7

Time (s)

Figure 1. Typical angle/time graph obtained from electro- goniometer. Upper trace, left leg. Lower trace, right leg.

according to manufacturer’s instructions the gonio- meters were attached to the lateral aspect of each knee by means of adhesive tape while the subject was in the supine posture. The sampling frequency of the system was set at 200 samples per second. Each subject was given a trial walk with the goniometers attached in order to aquaint her with the procedure and to ensure that there was no problem with the fixation of the gonio- meter. At the beginning of each test the subjects were requested to stand with both arms by their sides and feet together. The subjects were requested to start walking in their own time. Each subject was free to initiate walking with whichever leg she wished. Only one episode of 10 steps was recorded per subject.

Results

A typical angle/time graph obtained from the gonio- meter is shown in Figure 1. The top trace is the angular displacement of the left leg and the bottom trace is the angular displacement of the right leg. This subject initiated walking with her left leg as the first angular motion can be seen to appear on the upper trace. With reference to the angle/time graph from one gait cycle illustrated in Figure 2, from time A to time B the knee is flexing toward the maximum value of knee flexion occur- ring at time B. The knee then begins to extend reaching a minimum value of knee flexion at time C. This process is then repeated with the knee beginning to flex again. The gradient of the chord between time A and B is a measure of the mean angular velocity during the period of increasing flexion angle. Likewise the gradient between time B and C is a measure of the mean angular velocity during the period of increasing extension angle. To com- pensate for the non-linearity of the curve at times A, B, and C threshold values at 10% and 90% were imposed. Times D and E (Figure 2) are the 10% and 90% instants

Gormley er al.: Durarion of gair iniriarton 87

Angle at E(f&) - angle at o(@,)

Time at E(T,) - time at o(T,)

50

40

30

6

er n

:

20

10

0 t I

2.5 3.0 3.5

Time (s)

Figure 2. Angle/time graph from one gait cycle with points of interest marked.

for knee flexion respectively. Likewise F and G are the

90% and 10% instants for knee extension.

A computer program was designed to identify these

points of interest. By moving a cursor along the trace, the

A and B instants were marked, the program then auto-

matically pinpointed times D and E. The two angles and

the two corresponding times for these points were then

automatically recorded in a text file on the computer

disk. A similar procedure was used to find and record the times and angles for instants F and G. The times and

angles for instants B and C. i.e. maximum knee flexion and maximum knee extension. were also recorded. As

the study period was five steps on each leg 30 time values

and 30 angle values were recorded for each leg. These

points were then transferred to a ‘Lotus’ spreadsheet and graphs of the following variables were examined.

I. Angular velocity in extension. This was calculated by

Angle at (0,) - angle at P’(&-) Time at G(T,) - time at F(T,)

2. Angular velocity in flexion. This was calculated by

3. Stride rate

The rate at which gait cycles occur may be calculated

from the time between one maximum knee flexion and

the next corresponding maximum knee flexion on the

same leg. Mathematically this rate may be calculated

using any point on one cycle and its corresponding point

on the next cycle. For example, with reference to Figures

1 and 2 the cycle period could equally well be measured

from the first point B to the next corresponding point B.

Due to equipment constraints, one particular point in the

gait cycle. i.e. heelstrike. has traditionally been used to

quantify cycle rate and this measurement is then referred

to as stride rate. As the point of heelstrike is difficult to

identify on the traces of joint angle. the more easily

identifiable point of maximum knee flexion was chosen

in these studies to quantify cycle rate. Assuming that

maximum knee flexion consistently divides successive

gait cycles by the same proportion it will make no mathe-

matical difference which point on the cycle is chosen to

determine cycle rate. so the term stride rate was adopted

herein as having more meaningful significance.

In a similar argument the step rate, which traditionally

was measured from the time between one heelstrike and

the next heelstrike on the opposite leg. may alternatively

be measured as the time between one maximum knee

flexion and the next maximum knee flexion on the

opposite leg. Each consecutive point of maximum knee

flexion on alternate legs may therefore be used to dis-

tinguish the individual steps that contribute to the gait

pattern. For example. with reference to Figure I, BLI

occurs in step 1. BR2 in step 2, BLZ occurs in step 3, etc.

It is possible to associate with each point of maximum

knee flexion, a value for stride rate as determined from

the time interval to the next point of maximum knee

flexion on the same leg. Graphical representation of

these variables for one subject is shown in Figure 3.

When viewed collectively for all eight subjects these

flexion and extension angular velocity graphs do not

reveal any pattern. Figure 3c of stride rate shows an

increase in rate up to the end of step 5 and then a

decrease to step 8. Results for other subjects all demon-

strated an increase in rate in the first few steps. The step

number at which the rate appeared to level off varied

between step 3 and step 5.

These results also demonstrated that with some other

subjects there was a decrease in stride rate after the initial

increase. This was surprising in that a levelling off had

been expected but not a decrease. It was possible that a reduction in the rate of walking after four or five steps

was a natural occurrence. but a probable explanation

was that these subjects in the study anticipated the end of

the walk and began to slow down their pace prematurely

in preparation for stopping. The duration of the walk

used, i.e. 10 steps, was therefore deemed to be of insuf- ficient length to determine when a steady state of walking

88 Gait & Posture 1993; 1: No 2

-0.04 Average speed of knee extension

-0.161 I 1 2 3 4 5 6 7 8 9 10

Step number

0.12- Average speed of knee flexion

O.ll-

0.061 I 1 2 3 4 5 6 7 8 9 10

Step number

0.93 Stride rate

0.90 -

0.87-

1 2 3 4 5 6 7 8 9 10

Step number

Figure 3. Average speed of knee extension, knee flexion and stride rate for one subject.

had been achieved. As a result it was decided to repeat the experiment and to require each subject to walk for a longer distance.

Experiment 2

Aim

The aim of this experiment was to establish at what step a steady state of walking was achieved.

Method

Eight subjects, three male and five female, participated in this experiment. The mean ages were 18.6 (SD 0.57) years for the males and 21.4 (SD 0.54) years for the females. None of these subjects had participated in the previous experiment. The same experimental procedure with regard to attachment of the goniometer, clothing, and footwear of the subjects and the sampling frequency of the goniometer applied in this experiment. However, the

subjects were required to walk for a longer distance of approximately 20 metres and they each repeated the experiment four times in all.

Results

Graphs of the following variables for two subjects shown in Figure 4:

(I) angular velocity in flexion, (2) angular velocity in extension, (3) stride rate, for two subjects.

It was noticeable that the stride rate was similar in each case. However, no consistency was obvious in the graphs of angular velocity in flexion and angular velocity in extension. This lack of consistency was not confined to the two subjects illustrated but was observed in all eight subjects. The steady increase in stride rate for the first 5- 8 steps was clear in seven of the eight subjects. On further examination of stride rate it was noticeable that the rate started to decrease towards the end of the trial as with subject 1. This decrease in stride rate was found to occur in four other subjects. As a result it would be very diffi- cult to predict when a steady state of walking occurred. The likely explanation for this decrease was that the subjects were subconsciously slowing down as they reached the end of the room. However, repeating this experiment with the subjects performing a longer walk and concentrating on the variable which has been found to be most consistent (i.e. stride rate) should enable a determination of the step number at which a steady state of walking was reached.

Experiment 3

Aim

As in the previous experiment the aim of this experiment was to determine the number of steps required to achieve a steady gait.

Method

The procedure was much the same as in the previous experiment. Fourteen subjects (seven male and seven female) who had not previously participated were used. Their mean ages were 21.2 (SD 1.2) years for the females and 21.8 (SD 2.4) years for the males. The length of walk was increased to approximately 30 metres. Each subject walked this distance four times with a I-min pause between each trial. As noted above it was decided to derive only the stride rate in this experiment. Only the second, third, and fourth trials were analysed for each subject, treating the first attempt as a practice trial.

Results

The average stride rate versus step number for one sub- ject is illustrated in Figure 5 and was typical of all sub- jects. In this graph the results from each of the three trials

Gormley et al.: Duration of gait initiation 89

Subject 1 Subject 2

-0.04 * Average speed of knee extension

-0.04 + Average speed of knee extension

,-0.07- '; Ln -0.07- In o_-O.lO- 5 -O.lO- + z) $ m E-0.13- = * + + + + +

%

g-0.13- + + 0 + +O+0 0

0 v) =+0 L m -0.16-

+ 0 k -0.16- 0 m + =+a 7 0 0 0 C m-0.19- 0 0

D q &-0.19- Of +

: P

D : D- + + +

+

-0.22 -0.22 ” ” * ” ” j ” 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24

Step number Step number

0.22. Average speed of knee flexion D

0.16 D speed of knee flexion +

0 -

Average +

'; 0.19- 7 0.14- v) VI + +

n 0 0 _ 0.12- 0 m+ - 0.16- - + 0 D +m+n o

$ D D 0 + +

o- x 0.13- a 0 •I

+=+ + + + + I O.lO- D D ::

D O In h O.lO- co + + 0.08- + 7 += & F 0.07- + & 0.06-

< D 4 0.04.'- ’ ’ ’ s ’ ( ’ ’ 0.04-+, ' ' ' ' ' j '

0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24

0.94

7 VI 0.90 QI

.: 0.86 ;; - s

0.82

2 0.78 aI ‘D L 0.74 ;;

Stride rate +D o- 0 +m 0

+ Of -0 0 _+

c1 + +

0

0.94 Stride rate 0

'; +o+ + +

m 0.90- +0 0-O + D 0 0 D

aJ : 0.86- 0-0 +

;; -0.82- 3

o-

I * 0.78- 0 0 L 0.74- = lz

0.70 +' j ' ' ' ' ' ' ' 0.701-* ' ' ' ' ' j j ' 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24

Step number Step number

Step number Step number

1.02

1.00

0.98

0.96

z 0.94

3 0.92

J 0.90

I 0.88

a ‘D

0.86

2 0.84

vl 0.82

0.80

0.78

0.76

Figure 4. Variables obtained from two subjects during second experiment.

0.74' I" 0 10 20 30 40

Step number

Figure 5. Stride rate versus step number. Trial 1; Cl, trial 2; C, trial 3. +

were illustrated by symbols while the continuous line through the points indicates the mean value.

A steady increase in rate up to about the fifth or sixth

step is evident and after this point the rate appeared to

level off. The rate from this point on is not constant but

shows slight variations from one cycle to another. This

posed a problem, therefore, in determining the exact

value of steady state. To overcome this a mean value of

steady state was calculated using steps 16-25 for each

trial. This mean rate was used as the steady state for that

trial.

Before confidence could be placed in a steady-state

value the experimental error had to be taken into account. There were two potential sources of experimen-

tal error. One involved the measurement of joint angle,

the other involved the signal analysis in selection of the

instants A, B, and C on the waveform for each step.

The goniometer was attached carefully to the subject’s

knee to minimize any error in placement. The stride rate

could be calculated accurately even if the alignment of

the goniometer was incorrect by five or ten degrees

because stride rate was derived from the time at which

maximum knee flexion occurred and not the magnitude of the angle of maximum knee flexion. The cursor used

to locate the point of maximum knee flexion could be

90 Gair & Posture 1993; 1: No 2

Table 1. Stride rates calculated at each recorded step along the walkway for one subject

10

Trial number

Step number One Two Three

1 2 3 4 5 6

: 9

IO 11 12 13 14 15

Values of steady state

Values of steady state minus 2.5% error

0.735 0.806 0.775 0.793 0.862 0.826 0.819 0.877 0.847 0.840 0.884 0.862 0.840 0.917 0.892 0.862 0.917 0.925 0.900 0.892 0.925 0.900 0.934 0.900 0.900 0.934 0.917 0.900 0.934 0.925 0.925 0.917 0.909 0.917 0.917 0.900 0.892 0.952 0.909 0.900 0.934 0.943 0.900 0.925 0.943

0.92 0.933 0.944

0.897 0.909 0.9204

- Number of steps necessary to attain a value of steady state

moved in intervals of IO ms; therefore the time recorded at any instant on the graph had a potential error of f 10 ms. As stride rate was calculated by the subtraction of two time values an error of f 20 ms was possible in the values obtained. As this period usually varied between 800 and 1200 ms the percentage errors varied between 2.50 and I .66%. Therefore a 2.5% error was taken as the error in this experiment.

Discussion

To establish the minimum reliable value of steady- state stride rate, 2.5% error was subtracted from the 42 mean rates calculated previously. These values were then used as thresholds to identify when a steady state of walking commenced for each trial. The procedure used to identify steady state is described with the aid of Table I.

This Table presents the stride rates calculated at each of the first fifteen steps taken by one subject in three separate trials. Appended to the Table are the mean values of steady state calculated for each of the three trials. together with the corresponding threshold value calculated by subtracting the 2.5% error from the mean. It is clear from column I of this Table that six steps were necessary in trial I for the stride rate to attain the thres- hold value of 0.897. Similarly in trial 2 a total of four steps was necessary for the stride rate to attain a thres- hold value of 0.909. and in trial 3 a total of five steps was necessary for the stride rate to attain a threshold value of 0.9204. This analysis was repeated for each of the 14 subjects and the distribution of results is shown in Figure

6.

The first and second experiments provided insight into the measurement of stride rate and also the difficulty experienced in producing a sufficiently long period of steady-state walking. The angular velocity measurements did not prove successful in the earlier experiments, wher- eas stride rate did appear to offer promise even in the pilot study. The reason for this is thought to lie in the identification of the points used to derive these three variables. Maximum knee flexion was an easily dis- tinguishable point, but the IO and 90% points used to compute the angular velocities were not always clearly defined. Perhaps with further refinement of the method these variables could prove to be as useful as stride rate.

The distribution of results found in the third experi- ment was quite surprising. The number of steps necess- ary to reach a steady state far exceeded any previously quoted values. The concentration of most of the results between steps two and seven would suggest that gait initiation was completed between the end of the third and the eighth step. The trials in which gait initiation was not completed until either step nine or step I I were

randomly shared amongst eight of the 14 subjects and did not correlate with the trial number within the experi- ment. Because the experimental error was recognized and taken into account the results may be relied upon and it was concluded that sometimes it may take up to ten or 12 steps to reach a steady state of walking.

It is clear from this distribution that most subjects These results differed from the results obtained by reach a steady state of walking between steps two and both Mann et al.’ and Breniere et al.h. The main kinetic seven. However it is noticeable that there were a number events are most probably concentrated in the first step as of trials where up to nine and Ii steps were necessary in the body is changing from a state of rest to a state of order to reach a steady state. Using the measurement of motion’. However, the rate of motion of the body can

123456789 10 11 12

Number of steps

Figure 6. Distribution of results for the 14 subjects showing the number of steps necessary to reach steady state.

stride rate and a walking distance of 30 metres the aim of this experiment was achieved.

increase until about ten or 12 steps into a walk. The full

significance of this will only become clear when an exam-

ination of the difference in tracings prior to reaching

steady state is conducted; this was beyond the scope of this present study. However. these results are supported

by Zahedi et al.“. who. in an attempt to determine the

length of walkway necessary to obtain repeatable results

from a force plate. found that step number seven pro-

duced the most repeatable results using a walkway I5 m

long. This is the field where these results may have the

most importance. The acquisition of either kinetic or

kinematic data during the first seven to ten steps of a

walk may therefore provide a distorted view of gait.

Conclusion

The experiments described in this paper provide a differ-

ent approach to assessing the duration of gait initiation

from those previously carried out. The methodology

used was both novel and unique. The results suggest that

the majority of people reach a steady state of walking

between the start of the third step and the end of the

eighth. The number of steps required to reach a steady

rate of walking would appear not to be a constant value

and it varied both among and within subjects. This work

may have important implications in the clinical field of

gait analysis. particularly when specifying the length of

Gormiey et al.: Duration of ga/t inirlarion 91

walkway available for a patient to attain a steady state of

walking.

References

I.

1 _.

3 _

4.

5.

6.

Mann RA. Hagy JL. White V. Liddell D. The initiation of gait. J Bo~c, Jai/t/ Swg 1979: 6 I A: 132-9. Nissan M. Whittle M. Initiation of gait in normal subjects: a preliminary stud.?: J Biorlx4 f%p 1990: 12: 165-7 I Carlsoo S. The inmation of walkinp. .1r,rtr .4rlnr 1966: 65: l-9 Herman R. Cook T. Cozzens B. Freedman W. Control of postural reactions in man: the’initiation of gait. In: Klop- stes PE. Wilson PD. rd. Corll,.o/ of Po.strrw tmtl Locom-

riorz. (Advances in Behavioural Biology). New York: Ple- num Press. 1973: 7: 363-88 Cook T. Cozzens B. Human solutions for locomotion: the initiation of gait. In: Herman RM. Grillner S. Stein PSG. Stuart DG. ed. !Vc~ral Co~rtr.o/ (!f Louwwrio~~. New York: Plenum Press. 1976: 65-76 Breniere Y. Do M C. When and how does steady gait move induced from upright posture begin? J Biorwch 1986: 19: 103540 Brunt D. Lafferty MJ. McKeon A et al. Invariant charac- teristics of palt Initiation. .-IuI J f/11..\ .Llcll Rchohil 1991; 70: 206-I 7 Booker P. Messenger N. The measurement of gait. Ch

Relrahil 1988: 2: 89-97 Zahedi MS. Spence WD. Solomonidis SE. Paul JP. Repea- tibility of kinetic and kinematic measurements in gait stud- ies of lower limb amputees. Plo.cth @~/IO //II 1987; I I: 55-64

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