3. Biomechonics Vol. 25, No. 10, pp. 1233-1236, Printed in Great Britain

1992.

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TECHNICAL

GROUND

REACTION

$5.00+ .xJ

1992 Pergamon Press Lid

NOTE

FORCES DURING HUMAN GAIT

TERMINATION

OF

R.J. JAEGER*~$ and P.VANITCHATCHAVAN~§~ t Pritzker Institute of Medical Engineering, 3 Section of Orthopaedics and Rehabilitation Medicine, Pritzker School of Medicine, The University of Chicago, Chicago, IL 60637, U.S.A.; QDepartment of Computer Science, Illinois Institute of Technology,Chicago, IL 60616, U.S.A. and IDepartment of Computer Science, Silpakorn University, Nakorn Pathoom, Thailand Abstract-This is the first published report of the ground reaction forces during gait termination. Two mechanisms appear to be used to stop walking: increased braking forces and decreased push-off force. There appears to be a short interval of time during the gait cycle in which a decision to take an additional step is to be made.

INTRODUCIION Rhythmic movements could theoretically be controlled either on a continuous volitional basis, or be initiated voluntarily and then controlled on a more subconscious level. Many models have been proposed for the neural circuitry that generates rhythmic movements and much of this work has centered on gait (Grillner, 1981).However, very little work has been done to address the question of how rhythmic movements are terminated, particularly when constraints are imposed with respect to terminal position, e.g. both feet on floor in a specified position. Human walking is one example of a rhythmic movement that must be terminated with accurate control to minimize the likelihood of a poorly controlled stop and possible fall. The biomechanical and electromyographic events during steady-state gait have been well described, and the gait cycle diagram (in several variations) has now become standard in both scientific and clinical applications (Murray et al., 1964, Winter, 1983; Inman et al., 1981). Gait initiation (Mann et al., 1979; Carlsoo, 1966; Herman et al., 1973; Breniere et al., 1987) and backward walking (Nissan et al., 1990, Winter et aI., 1989; Smith et al., 1988; Buford et al., 1990; Buford and Smith, 1990) have also been studied. This study was concerned with the termination of human gait, defined as transition from steady-state walking to a quiet standing posture; this has not been extensively described. We hypothesized that there is a short interval of time during the gait cycle in which the decision to make an additional step must be made. We also hypothesized that the ground reaction forces should give some insight as to the mechanisms by which gait is terminated. Knowledge of how gait is terminated may have applications in better understanding the operation of hypothesized central pattern generators and afferent feedback in rhythmic movements (Grillner, 1981; Brooks, 1986), as well as the ability of volitional intent to interact with an ongoing motor process not under continuous voluntary control. In addition to these scientific applications, this work may also be of value in exploring the etiology and mechanisms of certain types of

Received in finoi fonn 12 February 1992. *Author to whom correspondence should be. addressed at: R. J. Jaeger, Pritzker Institute of Medical Engineering, Illinois Institute of Technology, El-125, Chicago, IL 60616, U.S.A. Tel: (312)-567-5324.

falls in the elderly (Wolfson et al., 1985), gait training in rehabilitation medicine, and programming the movements of legged robots (McGhee and Iswandhi, 1979; Vukobratovic et al., 1990). METHODS Human subjects (characteristics summarized in Table 1) performed walking trials along a 8.5 m walkway. In most of the trials, the subjects walked the entire length of the walkway to provide normative data of the steady-state gait. Trials in which no stop command was given were termed as steady-state trials. In randomly selected trials, a stop command (auditory tone) was given at some time during the trial. In trials in which a stop command occurred, it was given at different delay times after a randomly selected left- (or right-) heel contact. Two sets of experiments were performed. In the first set, subjects were asked to stop after the command with feet symmetrically placed. In the second set of experiments, subjects were asked to stop as soon as possible after the tone, irrespective of the position of their feet (Fig. 1). An ultrasonic ranging device monitored the approximate position of the subject’s center of mass during walking. An average gait velocity prior to termination was computed from this signal. Any data indicating the anticipation of a stop command by departures of gait velocity from the normal walking trials were discarded. Foot switches were placed on the heel and on the first metatarsal. The entire foot-floor contact pattern is digitized and collected into a computer for subsequent analysis. Timing values are also computed from these d&a. A biomechanics platform (AMTI) measured the ground reaction forces; attention in this paper is restricted to the force parallel to the direction of progression (F,, anteroposterior shear force) and the force perpendicular to the direction of progression (F,, vertical force). In a set of experiments on each subject, the subject was instructed to walk 30 times; six were normal ‘steady-state’ trials without any stop command, the remaining 24 were ‘stop trials’. These stop trials are equally divided into two groups of 12 each. In the first group, the stop command was given in such a way that the ground reaction forces were measured at location 1 (stance phase prior to termination). In the second group, the ground reaction forces were measured at the terminal position at location 2 and 3, shown in Fig. IC. All the 30 trials were chosen in random order. The stop command was given as soon as the second computer detected the subject’s heel contact, either left- or right-heel contact depending on the

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Technical Note

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Table 1. Physical data from all the subjects participating in the ground reaction forces experiment

Number 1 2 3 4 5 6 7 8 9 10 Minimum Maximum Average SD.

Experiment TGFlO TGFll TGFl2 TGF13 TGF14 TGFl5 TGF16 TGF17 TGFl8 TGF19

Age (y)

Height (m)

Weight (kg)

22 25 42 30 24 24 25 23 35 37

1.70 1.80 1.80 1.65 1.75 1.80 1.88 1.80 1.65 1.70

65.9 75.3 80.1 68.5 84.3 75.7 101.6 69.4 82.2 66.0

22 42 28.7 7.0

1.65 1.88 1.75 0.1

65.9 101.6 76.9 10.9

I

x

x x

x

0

10

20

30

40

50

60

70

I

80

Delay Time (% aait cycle)

Fig. 2. Time to terminate gait, from one of the subjects, plotted vs delay of stop command from heel contact. First set of experiments: (a) stop with feet symmetrical; second set of experiments: (x) stop irrespective of foot positions. Solid line indicates ‘break’ for first data set; dashed line for second data set. The behavior of these times to terminate exhibits a cyclic pattern repeating with every heel contact.

Table 2. Summary of time ‘break’, which causes an abrupt change in time to terminate gait, for each subject. Also, time ‘offset’ between the two sets of experiment: stop with feet symmetrical and stop as fast as possible Subject

no.

Fig. 1. Terminal foot placement for two sets of experiments: (A) feet symmetrical; (B) irrespective of foot position (one possibility);(C) foot placement during gait termination showing two locations at which the ground reaction forces were measured. At position 1, the stance phase prior to termination was measured. At position 2 and 3, the final placement of feet were measured, both feet coming to rest on the same force plate.

location where ground reaction forces were to be measured. For any given trial, the subject had no prior knowledge as to whether a stop command would be given, or when it would be given during the trial. RESULTS

The time to terminate gait was defined as the interval between delivery of the stop command and both feet in terminal contact with the floor. This time was plotted vs delay of the stop command. Representative data for a single subject from both sets of experiments is shown in Fig. 2. Both sets of data (Fig. 2) show that the time to terminate gait decreases as the stop command is given with a greater delay from left-heel contact. On an average, for all subjects, at about 32% into the gait cycle for data set 1 and 18% into the gait cycle for data set 2, there is an abrupt increase in the time to terminate gait, as an additional step must be taken. After

Break point (% gait cycle) Set 1 Set 2

Offset (s)

1 2 3 4 5 6 7 8 9 10

24 33 32 40 40 40 30 40 30 15

5 25 7 15 22 24 15 15 40 -

0.401 0.531 0.490 0.191 0.275 0.394 0.663 0.663 0.956 -

Average S.D.

32.4 8.3

18.7 10.6

0.507 0.232

this ‘break’, the time to terminate continues to decrease until just after the next heel contact. The instruction to the subject regarding how to stop influences when the ‘break’ occurs. There is an almost constant vertical offset in time between the two sets of data on an average of 0.5 s (average of all the subjects), except in regions of the ‘break’. The data for all the subjects are summarized in Table 2. The ground reaction forces during steady-state gait have been well described. Figure 3 shows peak anteroposterior shear and vertical forces for the three cases (typical, single trial data from one subject and averaged template from the five steady-state trials). The data for the steady-state case are consistent with previous studies (Inman et al., 1981; Andriacchi et al., 1977). The most important observation from the termination data is the reduction in force during push-off (measure at 1) and the increase in braking force (measure at 2 and 3). Figures 4 and 5 illustrate the behavior of peak components of F,, and F, (normalized to body weight) for the data from all the subjects for the first part of stance

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Technical Note

Ground reaction forces during termination of human gait.

This is the first published report of the ground reaction forces during gait termination. Two mechanisms appear to be used to stop walking: increased ...
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