David Davis

02-17-1997, 10:07 PM

Dear All,

I recently wrote the following question:

"I hope that someone may be able to help / put me straight. I am currently

working on gait analysis during load carraige. This involves subjects of

differing sizes , with differing loads free speed walking over a force plate

and recording the results. Now in order to accurately compare the results I

at first thought that expressing the forces as a percentage of body weight /

total weight was best. But it occurred to me that with free speed walking,

intrasubject variation of velocities as well as intersubject variation of

velocities occurred and (this is the bit that concerns me) since velocity is

a factor of acceleration and acceleration is a factor of force, are not the

ground reaction forces affected by velocity changes. Or do we safely assume

that during terminal swing the only acceleration acting is gravity??

I hope someone can explain a method of normalizing for this or at least tell

me of an experimental protocol that avoids this or tell me I'm wrong!!!! , I

will post a summary of replies."

I'd like to thank all those who replied, the information I have recieved has

been of great use.

Here follows is a summary of the replies I received. At the end of the

document are the references that people supplied/lead me to, for those

interested.

"G.GIAKAS"

Walking velocity affects GRF and in the Anterior-Posterior direction. This

is why we ask the subjects to start and finish at least 7-10m before and

after the force plate, so they maintain their normal speed. We assume

constant velocity, so zero walking acceleration. We normalise our results

with body weight and total stance phase. There is always intra- and inter-

subject variability in GRF (time but also in frequency domain analysis). For

the frequency domain variability check the December 1996 issue of Spine the

work by Giakas and Baltzopoulos.

"Craig Nevin"

The variation in velocity of a subject has vertical and horizontal

components. The vertical component, being subject to gravity, is

independant of mass. The horizontal fluctuation is dealt with by assuming

that the body is moving at a constant veleocity, any acceleration of the

body during one part of the cycle is recovered during another part of the

gait cycle (equal and opposite reactions to maintain the costant speed).

Since the gait cycle is invariably studied at a constant velocity, the

movement is without beginning or end, which is perpetual motion.

Jesus Dapena

On can divide the forces by the body weight. When you divide by body weight,

in essence you are computing accelerations (force/mass) --or more

precisely, values that are directly proportional to accelerations, since

you are dividing by weight and not by mass. You are calculating how much

each horizontal and vertical force contributes by itself to the

acceleration of the whole system (if you are dividing by total system

weight or mass).

Irene S McClay

Most all of the literature reports GRF in bodyweights. However, you are

correct in assuming that gait velocity will influence GRF (Doris Miller has

published in this area). There are two possible ways to deal with this.

Either you control the speed or you can treat velocity as a covariate as you

analyze the data.

"D. Gordon E. Robertson"

In a previous study we compared the subjects against their own unloaded

walking patterns. For example, the mediolateral ground reaction forces were

averaged for unloaded walking and a mean and a standard deviation curve were

produced. The same was done for a loaded condition. We would say the the

loaded condition was significantly different from the unloaded condition

whenever it fell outside the mean +/- a confidence interval of the unloaded

condition. The confidence interval was 1.96*s.d. which represents the 95%

confidence level.

Scott & Joanne Aldworth

At the BloorView Medical Centre in Toronto, Ontario, Canada whilst

performing a gait analysis investigation between normals, above and below

knee amputees in children based on age differences. We normalized to the

weight of the subject and it seems that we only used gravtity at the point

of heel strike.

Sylvain Grenier

We normalize work to velocity * mass, in "free speed" walking trials over a

force platform.

"Vladimir M. Zatsiorsky"

Try the Local Proportional Scaling method (see the Motor Control, v. 1, #1,

1997).

"Henk C. Schamhardt"

It may be better to collect baseline data of each subject, and compare

loaded data with those. What I mean is a certain kind of paired comparison.

Dividing GRF by body mass (whether or not including additional mass)

neglects the influence of speed on GRF. Since a tendency towards lower

speeds at increasing loads may be expected, this might seriously interfere

with your data.

The influence of walking speed on GRF has to be dealt with separately (one

should avoid trying to get subjects to walk at a forced speed) for example

by application of appropriate statistical techniques such as ANOVA.

"Ruben Lafuente Jorge"

Velocity is a main factor affecting gait curve morphology, but not gait

performance. Gait performance is the same irrespective of speedSome other

factors affecting gait performance and also curve morphology are pathology

(of course), type of shoe, sex and age. There is at present no scheme for

speed normalization. Maybe you could think about the inverse pendulum model

and try to normalise the centrifugal force or consider velocity an explicit

factor which must be taken into consideration for determining the reference

curves. That is, you calculate separately average curves for slow speeds,

for medium speeds and for fast speeds. This way, you can get rid of a part

of the variability.

************************************************** **************************

*********

References:

Giakas G., Baltzopoulos ?., December 1996, ...frequency domain analysis...,

Spine (incomplete ref.)

???, 1997, Local Proportional Scaling Method, Motor Control, Vol.1,No.1,

pp.?? (incomplete ref.)

Tatara T., Iida T., Walking patterns of normal human gaits with loads on a

level, Biomechanics X-A, pp.397-401, Jonsson B.(ed.), Human Kinetics:

Champaign Il,1987

Millar J.F., Stamford B.A.,1987, Intensity and energy cost of weighted

walking vs running for men and women, Journal of applied physiology, vol.64,

pp.1497-1501

Martin P.E., Heise G.D., Morgan D.W., 1993, Interrelationships between

mechanical power, energy transfers, and walking and running economy.

Medicine and science in sports and exercise, Vol.25, pp.508-515

Webb P et al., 1988, The work of walking: a calorimetric study, Medicine

and science in sports and exercise, Vol.20, pp.331-337

Nagle F.J., Webb, Wanta., 1990, ???, Medicine and science in sports and

exercise, Vol.22, pp.540-544

Sun M., Hill J.O. , 1993, A method for measuring mechanical work and

efficiency during human activities. Journal of biomechanics, Vol.26, pp.229-

241

Crowe A et al., 1996, The influence of walking speed on parameters of gait

symmetry determined from ground reaction force, Human movement science,

15(3) pp. 347-367

DeVita, P., April 1994, The selection of a standard convention for analyzing

gait data based on the analysis of relevant biomechanical factors, Journal

of biomechanics, Volume 27, Number 4, pp.501-

Fioretti S, 1996, Signal processing in movement analysis, Human movement

science, 15(3) pp. 389-410

Vaughan CL, 1996, Are joint torques the holy-grail of human gait analysis,

Humna movement science, 15(3) pp.423-443

----------------------------------------------------------------------------

-----------

Department of Human Sciences

Loughborough University

Leics.

LE11 3TU

UK

Tel: 01509 223086

Fax: 01509 223941

Email: D.Davis@lboro.ac.uk

I recently wrote the following question:

"I hope that someone may be able to help / put me straight. I am currently

working on gait analysis during load carraige. This involves subjects of

differing sizes , with differing loads free speed walking over a force plate

and recording the results. Now in order to accurately compare the results I

at first thought that expressing the forces as a percentage of body weight /

total weight was best. But it occurred to me that with free speed walking,

intrasubject variation of velocities as well as intersubject variation of

velocities occurred and (this is the bit that concerns me) since velocity is

a factor of acceleration and acceleration is a factor of force, are not the

ground reaction forces affected by velocity changes. Or do we safely assume

that during terminal swing the only acceleration acting is gravity??

I hope someone can explain a method of normalizing for this or at least tell

me of an experimental protocol that avoids this or tell me I'm wrong!!!! , I

will post a summary of replies."

I'd like to thank all those who replied, the information I have recieved has

been of great use.

Here follows is a summary of the replies I received. At the end of the

document are the references that people supplied/lead me to, for those

interested.

"G.GIAKAS"

Walking velocity affects GRF and in the Anterior-Posterior direction. This

is why we ask the subjects to start and finish at least 7-10m before and

after the force plate, so they maintain their normal speed. We assume

constant velocity, so zero walking acceleration. We normalise our results

with body weight and total stance phase. There is always intra- and inter-

subject variability in GRF (time but also in frequency domain analysis). For

the frequency domain variability check the December 1996 issue of Spine the

work by Giakas and Baltzopoulos.

"Craig Nevin"

The variation in velocity of a subject has vertical and horizontal

components. The vertical component, being subject to gravity, is

independant of mass. The horizontal fluctuation is dealt with by assuming

that the body is moving at a constant veleocity, any acceleration of the

body during one part of the cycle is recovered during another part of the

gait cycle (equal and opposite reactions to maintain the costant speed).

Since the gait cycle is invariably studied at a constant velocity, the

movement is without beginning or end, which is perpetual motion.

Jesus Dapena

On can divide the forces by the body weight. When you divide by body weight,

in essence you are computing accelerations (force/mass) --or more

precisely, values that are directly proportional to accelerations, since

you are dividing by weight and not by mass. You are calculating how much

each horizontal and vertical force contributes by itself to the

acceleration of the whole system (if you are dividing by total system

weight or mass).

Irene S McClay

Most all of the literature reports GRF in bodyweights. However, you are

correct in assuming that gait velocity will influence GRF (Doris Miller has

published in this area). There are two possible ways to deal with this.

Either you control the speed or you can treat velocity as a covariate as you

analyze the data.

"D. Gordon E. Robertson"

In a previous study we compared the subjects against their own unloaded

walking patterns. For example, the mediolateral ground reaction forces were

averaged for unloaded walking and a mean and a standard deviation curve were

produced. The same was done for a loaded condition. We would say the the

loaded condition was significantly different from the unloaded condition

whenever it fell outside the mean +/- a confidence interval of the unloaded

condition. The confidence interval was 1.96*s.d. which represents the 95%

confidence level.

Scott & Joanne Aldworth

At the BloorView Medical Centre in Toronto, Ontario, Canada whilst

performing a gait analysis investigation between normals, above and below

knee amputees in children based on age differences. We normalized to the

weight of the subject and it seems that we only used gravtity at the point

of heel strike.

Sylvain Grenier

We normalize work to velocity * mass, in "free speed" walking trials over a

force platform.

"Vladimir M. Zatsiorsky"

Try the Local Proportional Scaling method (see the Motor Control, v. 1, #1,

1997).

"Henk C. Schamhardt"

It may be better to collect baseline data of each subject, and compare

loaded data with those. What I mean is a certain kind of paired comparison.

Dividing GRF by body mass (whether or not including additional mass)

neglects the influence of speed on GRF. Since a tendency towards lower

speeds at increasing loads may be expected, this might seriously interfere

with your data.

The influence of walking speed on GRF has to be dealt with separately (one

should avoid trying to get subjects to walk at a forced speed) for example

by application of appropriate statistical techniques such as ANOVA.

"Ruben Lafuente Jorge"

Velocity is a main factor affecting gait curve morphology, but not gait

performance. Gait performance is the same irrespective of speedSome other

factors affecting gait performance and also curve morphology are pathology

(of course), type of shoe, sex and age. There is at present no scheme for

speed normalization. Maybe you could think about the inverse pendulum model

and try to normalise the centrifugal force or consider velocity an explicit

factor which must be taken into consideration for determining the reference

curves. That is, you calculate separately average curves for slow speeds,

for medium speeds and for fast speeds. This way, you can get rid of a part

of the variability.

************************************************** **************************

*********

References:

Giakas G., Baltzopoulos ?., December 1996, ...frequency domain analysis...,

Spine (incomplete ref.)

???, 1997, Local Proportional Scaling Method, Motor Control, Vol.1,No.1,

pp.?? (incomplete ref.)

Tatara T., Iida T., Walking patterns of normal human gaits with loads on a

level, Biomechanics X-A, pp.397-401, Jonsson B.(ed.), Human Kinetics:

Champaign Il,1987

Millar J.F., Stamford B.A.,1987, Intensity and energy cost of weighted

walking vs running for men and women, Journal of applied physiology, vol.64,

pp.1497-1501

Martin P.E., Heise G.D., Morgan D.W., 1993, Interrelationships between

mechanical power, energy transfers, and walking and running economy.

Medicine and science in sports and exercise, Vol.25, pp.508-515

Webb P et al., 1988, The work of walking: a calorimetric study, Medicine

and science in sports and exercise, Vol.20, pp.331-337

Nagle F.J., Webb, Wanta., 1990, ???, Medicine and science in sports and

exercise, Vol.22, pp.540-544

Sun M., Hill J.O. , 1993, A method for measuring mechanical work and

efficiency during human activities. Journal of biomechanics, Vol.26, pp.229-

241

Crowe A et al., 1996, The influence of walking speed on parameters of gait

symmetry determined from ground reaction force, Human movement science,

15(3) pp. 347-367

DeVita, P., April 1994, The selection of a standard convention for analyzing

gait data based on the analysis of relevant biomechanical factors, Journal

of biomechanics, Volume 27, Number 4, pp.501-

Fioretti S, 1996, Signal processing in movement analysis, Human movement

science, 15(3) pp. 389-410

Vaughan CL, 1996, Are joint torques the holy-grail of human gait analysis,

Humna movement science, 15(3) pp.423-443

----------------------------------------------------------------------------

-----------

Department of Human Sciences

Loughborough University

Leics.

LE11 3TU

UK

Tel: 01509 223086

Fax: 01509 223941

Email: D.Davis@lboro.ac.uk