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Measurement of momentary velocity in a study of human gait
J. Biomcc honey.
I 972. Vol. 5. pp. 273-276.
Pergamon Press.
Printed in Great Bntain
MEASUREMENT OF MOMENTARY VELOCITY IN A STUDY OF HUMAN GAIT* N. H. MOLEN and W. BOON Institute of Biomechanics and Rehabilitation. Free University. Amsterdam. The Netherlands Abstract-This study describes a method of measurement in the field of biomechanical research, which determines the walking-speed of the subject in a forward movement. The measuring is based on the procedure of a subject stretching a magnetic tape, while it runs through a playback head. Pulses have been fed into the tape in advance so that the signal frequency is proportional to the momentary walking-speed of the subject in the course of movement. In a frequency/ d.c. converter, the signal frequency is converted into a proportional d.c.-voltage. The frequency response of the output signal is being analysed and compared to the frequency response obtainable in a speed measuring system in which a tachogenetator is applied.
INTRODUCTION
field of research on locomotion, one of the quantities of importance in the analysis of human walking, is the velocity in the centre of gravity of the body in a forward course of movement (Drillis, 1958). The determination of the average walkingspeed can be reduced to either a timemeasurement, while the subject covers a known distance, or a measurement of the distance while the subject moves in a given space of time. The accuracy with which the average speed can be measured depends on the exactness of the measurements of distance and time. However, the determination of the momentary velocity of the centre of gravity of the body requires the measuring of the distance covered in a time AI, in which At approximates to zero. In publications that bear reference to the investigation of walking patterns of man, a method has been described for the determination of the momentary velocity in which a tachogenerator is being used, i.e. by means of a rope the subject/patient drives the tachogenerator. The terminal voltage or the signal frequency is, thus, a measure for the momentary velocity (Drillis, 1958). In view of the low cut-off frequency we were in doubt as to IN THE
*Received
16 March
B.M. Vol. 3 No. 3-C
1971.
whether or not the above mentioned measuring method would render all information about the momentary velocity: therefore, we opted for another method which we developed as follows. DEMXIPTION
OF THE METHOD
The development was based on the fact that the signal-frequency, reproduced from a magnetic tape, is proportional to the speed of the tape. Figure 1 represents an outline of the set-up for measurements. On the level of the body’s centre of gravity, the subject/patient is connected with the beginning and the end of a magnetic tape which has been stretched through guides lengthwise over the walkway. Beforehand, the tape is tightehed with a weight and a vibration damper situated in a loop of the tape, as shown in Fig. 2. A double-track record/reproduce head serves as a transducer, in such a way that it scans that part of the tape that is under pulling stress. In consequence of the varying sizes of the is completely subjects, the transducer adjustable. In advance, a signal has been fed to the magnetic tape with a hxed frequency at unvarying speed of the tape, with a magnetic tape recorder plus a function generator.
N. H. MOLEN
274
and W. BOON
Maqnet~c
Fig. 1 S&em&c I
drawing of the measunment
It is now quite clear that while the subject is walking, an alternating current will be generated in the play-back head whose frequency is proportional to the speed of the tape running along the head. This frequency is also proportional to the momentary speed of the centre of gravity of the body in the line of movement. In order to enable the measurement of the average speed of the subject/patient as well, two pulse-codes are fed into the magnetic tape; the distance between those two pulses on the tape is equal to the stretch on which the average speed is to be measured. While the subject is walking, the code pulses appear on the recording instrument after a distance in accordance with the time in which the stretch is covered. The average waking speed can now be calculated from the distance and the time basis of the recording instrument. Having passed the amplifier, the nature of the frequency-modulated signal is such that it lends itself admirably to the purpose of magnetic tape recording. This offers the possibility of recording and reproducing the speed signal at discretion. The signal frequency at the highest walkingspeed has to be adapted to the range of frequency of the magnetic tape recorder, e.g. 50- 14MO c/s. Assimilation of the measure signal
In a block diagram (Fig. 31, the assimilation of the measuring signal is demonstrated. The voltage, generated in the play-back head while
lope
8
of momentary v&city.
L-J ? ? cd v
Amplifier 20ch-20kch
Squarer
Oif. network
Fig. 3. Block diagram of the assimilation of the measuring signal.
the tape runs along the transducer, is raised in an amplifier with a frequency range of 20 c/s-20 kc/s, and then converted into a square wave with a stabilised amplitude and a constant rise time.
Fig. 2. Set-up of the record/reproduce head with tapeguides and vibration dam per. (Fat :ing p. 274)
:ight adju ;ter.
MOMENTARY
VELOCITY
By means of a rate action network and a diode gate the ensuing positive pulses are presented to a low-pass active filter. In this way we have constructed a frequency/d.c. converter with a linearity deviation of less than 1 per cent. The gain is O-5 V/kc/s. The cut-off frequency of the low-pass filter at the end of the circuit is determined by the frequency range of the speed signal that is to be expected and is fixed on fo = 20 c/s. DISCUSSION
In order to analyse the frequency response of the speed signal, three low-pass filters with fo of 10, 20 and 100 c/s, were built into the circuit. Next, the momentary velocity of a healthy subject (V = 1 m/set) was recorded simultaneously by way of the three separate filters. The results are shown in Fig. 4. If we compare the signal recorded with a 20 c/s filter. to the signal obtained with a 100 c/s filter, we will note that both curves are identical in shape; however, upon the signal filtered through 100 c/s. a vibration with a slight amplitude and a frequency of about 30 c/s is superimposed.
V I m/set _--__----_-
!‘u<,
f,=lOHz
I
IN HUMAN
GAIT
275
After a few alterations in the parameters of the measurement system, we arrived at the conclusion that the vibration of 30 c/s must be mechanical by nature and that it originated somewhere in the system and could not possibly be part of the speed signal Comparison of the signals recorded with the 10 and 20 c/s filters, indeed bears evidence to the fact that the higher frequencies are being cut off in an intolerable way. This leads to the conclusion that application of the 20 c/s filter would not be detrimental to the speed signal. In the measurement system we used, we therefore applied a 20 c/s low-pass filter. Furthermore, we have tested a tachogenerator for the purpose of momentary speed calculation. The results we obtained demonstrated that a cut-off frequency of 15 c/s could hardly be reached. We, therefore, are of the opinion that in the case at issue, a tachogenerator might well be applied in so far as the analysis of normal locomotion patterns is concerned. When making researches into pathological patterns of walking, however, we are inclined to give priority to the method of measurement with a magnetic tape as described above.
,YAy‘
v.LL
l&
I
i
Fig. 4. Effect of filtering on a momentary veloctty signal, by means of a low-pass active filter with a mt-off frequency of 10.20 and I00 c/s.
N. Ii. MOLEN and W. BOON
276 SPECIFICATIONS
1. Frequency to d.c. converter Input frequency range : 20-20,000 c/s, output voltage : o-lOV, : 0.5 V/kc/s, gain output frequency response: O-20 c/s, linearity greater than : 99 per cent, 2. Set-up for measurements Length of walkway : 12m, velocity range depending on the prerecorded : O-l mlsec, O-2 mlsec,
acceleration range
:
prerecorded pulses on magnetic tape magnetic tape 3. Tachogenerator Hewlett Packard 40-40,OOOrevlmin.
type
O-20 m/set*, O-40 m/set*,
20,000 pulses/m, 10,000 pulses/m, : 4 in. standard thickness 52 p :
508 A,
60 c/rev.
REFERENCES Drillis, R. ( 1958) Objective recording and biomcchanics of Pathological Gait. Ann. New York Acnd. Sci. 74, 86-109.
I 972. Vol. 5. pp. 273-276.
Pergamon Press.
Printed in Great Bntain
MEASUREMENT OF MOMENTARY VELOCITY IN A STUDY OF HUMAN GAIT* N. H. MOLEN and W. BOON Institute of Biomechanics and Rehabilitation. Free University. Amsterdam. The Netherlands Abstract-This study describes a method of measurement in the field of biomechanical research, which determines the walking-speed of the subject in a forward movement. The measuring is based on the procedure of a subject stretching a magnetic tape, while it runs through a playback head. Pulses have been fed into the tape in advance so that the signal frequency is proportional to the momentary walking-speed of the subject in the course of movement. In a frequency/ d.c. converter, the signal frequency is converted into a proportional d.c.-voltage. The frequency response of the output signal is being analysed and compared to the frequency response obtainable in a speed measuring system in which a tachogenetator is applied.
INTRODUCTION
field of research on locomotion, one of the quantities of importance in the analysis of human walking, is the velocity in the centre of gravity of the body in a forward course of movement (Drillis, 1958). The determination of the average walkingspeed can be reduced to either a timemeasurement, while the subject covers a known distance, or a measurement of the distance while the subject moves in a given space of time. The accuracy with which the average speed can be measured depends on the exactness of the measurements of distance and time. However, the determination of the momentary velocity of the centre of gravity of the body requires the measuring of the distance covered in a time AI, in which At approximates to zero. In publications that bear reference to the investigation of walking patterns of man, a method has been described for the determination of the momentary velocity in which a tachogenerator is being used, i.e. by means of a rope the subject/patient drives the tachogenerator. The terminal voltage or the signal frequency is, thus, a measure for the momentary velocity (Drillis, 1958). In view of the low cut-off frequency we were in doubt as to IN THE
*Received
16 March
B.M. Vol. 3 No. 3-C
1971.
whether or not the above mentioned measuring method would render all information about the momentary velocity: therefore, we opted for another method which we developed as follows. DEMXIPTION
OF THE METHOD
The development was based on the fact that the signal-frequency, reproduced from a magnetic tape, is proportional to the speed of the tape. Figure 1 represents an outline of the set-up for measurements. On the level of the body’s centre of gravity, the subject/patient is connected with the beginning and the end of a magnetic tape which has been stretched through guides lengthwise over the walkway. Beforehand, the tape is tightehed with a weight and a vibration damper situated in a loop of the tape, as shown in Fig. 2. A double-track record/reproduce head serves as a transducer, in such a way that it scans that part of the tape that is under pulling stress. In consequence of the varying sizes of the is completely subjects, the transducer adjustable. In advance, a signal has been fed to the magnetic tape with a hxed frequency at unvarying speed of the tape, with a magnetic tape recorder plus a function generator.
N. H. MOLEN
274
and W. BOON
Maqnet~c
Fig. 1 S&em&c I
drawing of the measunment
It is now quite clear that while the subject is walking, an alternating current will be generated in the play-back head whose frequency is proportional to the speed of the tape running along the head. This frequency is also proportional to the momentary speed of the centre of gravity of the body in the line of movement. In order to enable the measurement of the average speed of the subject/patient as well, two pulse-codes are fed into the magnetic tape; the distance between those two pulses on the tape is equal to the stretch on which the average speed is to be measured. While the subject is walking, the code pulses appear on the recording instrument after a distance in accordance with the time in which the stretch is covered. The average waking speed can now be calculated from the distance and the time basis of the recording instrument. Having passed the amplifier, the nature of the frequency-modulated signal is such that it lends itself admirably to the purpose of magnetic tape recording. This offers the possibility of recording and reproducing the speed signal at discretion. The signal frequency at the highest walkingspeed has to be adapted to the range of frequency of the magnetic tape recorder, e.g. 50- 14MO c/s. Assimilation of the measure signal
In a block diagram (Fig. 31, the assimilation of the measuring signal is demonstrated. The voltage, generated in the play-back head while
lope
8
of momentary v&city.
L-J ? ? cd v
Amplifier 20ch-20kch
Squarer
Oif. network
Fig. 3. Block diagram of the assimilation of the measuring signal.
the tape runs along the transducer, is raised in an amplifier with a frequency range of 20 c/s-20 kc/s, and then converted into a square wave with a stabilised amplitude and a constant rise time.
Fig. 2. Set-up of the record/reproduce head with tapeguides and vibration dam per. (Fat :ing p. 274)
:ight adju ;ter.
MOMENTARY
VELOCITY
By means of a rate action network and a diode gate the ensuing positive pulses are presented to a low-pass active filter. In this way we have constructed a frequency/d.c. converter with a linearity deviation of less than 1 per cent. The gain is O-5 V/kc/s. The cut-off frequency of the low-pass filter at the end of the circuit is determined by the frequency range of the speed signal that is to be expected and is fixed on fo = 20 c/s. DISCUSSION
In order to analyse the frequency response of the speed signal, three low-pass filters with fo of 10, 20 and 100 c/s, were built into the circuit. Next, the momentary velocity of a healthy subject (V = 1 m/set) was recorded simultaneously by way of the three separate filters. The results are shown in Fig. 4. If we compare the signal recorded with a 20 c/s filter. to the signal obtained with a 100 c/s filter, we will note that both curves are identical in shape; however, upon the signal filtered through 100 c/s. a vibration with a slight amplitude and a frequency of about 30 c/s is superimposed.
V I m/set _--__----_-
!‘u<,
f,=lOHz
I
IN HUMAN
GAIT
275
After a few alterations in the parameters of the measurement system, we arrived at the conclusion that the vibration of 30 c/s must be mechanical by nature and that it originated somewhere in the system and could not possibly be part of the speed signal Comparison of the signals recorded with the 10 and 20 c/s filters, indeed bears evidence to the fact that the higher frequencies are being cut off in an intolerable way. This leads to the conclusion that application of the 20 c/s filter would not be detrimental to the speed signal. In the measurement system we used, we therefore applied a 20 c/s low-pass filter. Furthermore, we have tested a tachogenerator for the purpose of momentary speed calculation. The results we obtained demonstrated that a cut-off frequency of 15 c/s could hardly be reached. We, therefore, are of the opinion that in the case at issue, a tachogenerator might well be applied in so far as the analysis of normal locomotion patterns is concerned. When making researches into pathological patterns of walking, however, we are inclined to give priority to the method of measurement with a magnetic tape as described above.
,YAy‘
v.LL
l&
I
i
Fig. 4. Effect of filtering on a momentary veloctty signal, by means of a low-pass active filter with a mt-off frequency of 10.20 and I00 c/s.
N. Ii. MOLEN and W. BOON
276 SPECIFICATIONS
1. Frequency to d.c. converter Input frequency range : 20-20,000 c/s, output voltage : o-lOV, : 0.5 V/kc/s, gain output frequency response: O-20 c/s, linearity greater than : 99 per cent, 2. Set-up for measurements Length of walkway : 12m, velocity range depending on the prerecorded : O-l mlsec, O-2 mlsec,
acceleration range
:
prerecorded pulses on magnetic tape magnetic tape 3. Tachogenerator Hewlett Packard 40-40,OOOrevlmin.
type
O-20 m/set*, O-40 m/set*,
20,000 pulses/m, 10,000 pulses/m, : 4 in. standard thickness 52 p :
508 A,
60 c/rev.
REFERENCES Drillis, R. ( 1958) Objective recording and biomcchanics of Pathological Gait. Ann. New York Acnd. Sci. 74, 86-109.