BIOMECHANICAL ANALYSIS OF PHYSICAL ACTIVITY
Student Project - Data Reduction, Analysis, and Reporting:
Stroboscopic Photography of Human Gait
Dr. Eugene W. Brown
Purposes:
This student project has several purposes. They include:
1.
learning about stroboscopic photography as an experimental technique
a.
subject praparation
b.
setting preapration
c.
data collection
d.
film and film processing
2.
learning about the analysis of two dimensional photographic images
3.
reviewing how to define biomechanical events and intervals of physical
activities
4.
developing standards for determining absolute and relative angles
5.
reviewing techniques of temporal analysis of human movement (absolute and
relative values)
6.
applying results of temporal and kinematic analysis to expected outcomes
7.
meaningfully representing data in various formats (tables, spreadsheats, and
graphs)
8.
developing an understanding of experimental methods in biomechanics,
9.
calculating kinematic variables: angular position, velocity, and acceleration;
and linear displacement and velocity
10. understanding the relationship of experimental error to measurements
recorded,
11. preparing subjects for participation in research experiments,
12. setting up experimental procedures in biomechanics,
13. drawing meaningful relationships between temporal and kinematic data
14. learning how to report the results of laboratory experiments.
15.
List of Equipment and Supplies:
1.
2 or more large sheets of graph paper
2.
2 stroboscopic photography slides
3.
calculator
4.
dark room with smooth wall surface
5.
fine-tip colored marking pens
6.
level
7.
masking tape
8.
meter stick
9.
plumb bob
10. protractor
11. ruler
12. slide projector
13. spreadsheet computer software program with graphics capabilities
Definition of Terms:
1.
absolute angle - angular position; orientation in a global or laboratory
reference system
2.
activity plane – primary plane of movement; usually two dimensional plane
that best represents movement being studied (e.g., sagittal plane for standard
gait patterns, frontal plane for jumping jack exercises)
3.
amplitude – difference between minimum and maximum angle
4.
angular acceleration ()– change in angular velocity/change in time; slope of
the tangent to the angular velocity curve; units of deg.  s-2 or rad.  s-2
5.
angular position – absolute angle; orientation in a global or laboratory
reference system
6.
angular velocity () – change in angular position or relative angle/change in
time; units of deg.  sec-1 or rad.  s-1
7.
data sample rate – see flash rate (Note that the data sample rate should be at
least two times the expected frequency of the signal.)
8.
events – definable actions that occur in a brief moment of time (e.g., heel
contact with the ground, toe off the ground)
9.
fiducial – two or more marks placed in the field of view of a video or motion
picture camera (usually at the outer edges of the field of view) to be used to
align sequential images to a laboratory coordinate system
10. flash rate – the number of short duration high intensity light flashes from a
strobe light per minute (e.g., 900 flashes/minute, 1200 flashes/minute)
11. functional range of movement – minimum to maximum angle of a joint in the
performance of a physical activity
12. instantaneous center of rotation – joint center of rotation at a given point in
time
23. intervals – phases; definable periods of time that start and end with definable
events (e.g., support phase defined from foot contact with the ground to foot
off the ground)
14. joint axis of rotation – center of rotation of two segments composing a joint
determined by their relative motion
15. metronome – device that provides a repetitive auditory and/or electronic
signals equally spaced in time (Note that the frequency of most metronomes
can be adjusted.)
16. optic axis of lens – line through the center of the lens of the 35 mm camera
that was perpendicular to the activity plane or line through the center of the
lens of the slide projector that was perpendicular to the graph paper screen
surface
17. perspective error – error which occurs when parts of a body or implement lie
outside the principle photographic plane; image of segment closer to the
camera appears larger and segment farther away appears smaller
18. plumb bob – weighted line that hangs vertical and is used for spatial
orientation
19. range of movement (ROM) – minimum to maximum angle that can be
achieved at a joint
20. reference measure – an object of known length (e.g., meter stick) that is
21.
22.
23.
24.
25.
26.
27.
28.
29.
placed in a plane that is perpendicular to the optic axis of the lens of a camera
that is used to assist in determining distance measurements in the same plane
relative angle – orientation of one segment with respect to another
relative joint angle – the angle formed by lines representing two segments
with respect to each other
standard walking frequency – 120 steps per minute; rate often used in
experimental analyses of walking gait patterns in subjects free of any
handicapping conditions
step – in a gait pattern, from heel strike of one foot to the next heel strike of
the opposite foot
stride – in a gait pattern, from heel strike of one foot to the next heel strike of
the same foot
subject identification number – alpha and/or numeric value used to
differentiate and identify subjects; code placed in the field of view of a camera
that is used to distinguish each subject
temporal analysis – report and comparison of when (time) defined events
occur and the interval of time between defined events
a.
absolute temporal analysis – uses actual times and time intervals
b.
relative temporal analysis – divides time values by some standard (e.g.,
total time for the completion of a cycle) and reports them as a decimal
value (Note that this is a normalizing process that permits the
comparison of time intervals that compose performances of different
total times.)
trial number – alpha and/or numeric value used to differentiate among trials of
a subject; code placed in the field of view of a camera that is used to
distinguish individual performances of each subject
walkway – narrow mat used to assist subjects in maintaining their movement
pattern in a constricted plane; also used to minimize perspective error in two
dimensional photography of primarily planar movements
Premise:
The Shifty Shoe Company has been doing stroboscopic studies of the gait patterns
of subjects wearing its new experimental model shoes to get a “leg up” on its
competition. Recently, it has run into some difficulty. Its chief research biomechanist
has defected to a competing company.
The Shifty Shoe Company managed to retain the stroboscopic photography
negatives of the gait patterns of subjects wearing its new experimental models and has the
details of the Experimental Methods used to produce these negatives.
You have been hired to complete one comparison study. Your responsibility is to
produce information requested by the Shifty Shoe Company and to respond to questions
about the performance of gait patterns under the conditions represented in the two
negatives that you have received.
Experimental Methods:
Subject Preparation
One adult female subject (55.79 kg, 170 cm, 26 years of age) was used for all gait
conditions studied. In some of the experiments, the pace of the metronome was adjusted
(slow, fast) to achieve two separate gait speeds. The setting of the metronome is not
currently known. Prior to data collection, she received practice walking, to the regular
beat of a metronome, in each of the experimental shoes. For data collection, she was
dressed in black leotards. Prior to donning the leotards, a system of tacks projecting
through moleskin patches was used to mark joint centers of the right ankle, knee, hip,
shoulder, elbow, and wrist. The leotards were stretched over the joint markers that were
adhered to the subject’s skin via the moleskin patches. 3M Company reflective tape was
adhered to the exterior of the leotard to designate the joint centers that were established
by the tacks. Large shapes (e.g., triangles, arrows. diamonds,) were cut out of the
reflective tape and the points of these shapes were positioned at the point of the center of
the tack. In addition, narrow lines were cut out of the reflective tape and adhered to the
exterior of the leotard to connect the joint center markers.
Setting Preparation
A thin black vinyl walkway mat was placed on the wood floor surface of the gait
laboratory. This mat was narrow and guided the subject in a narrow activity plane in an
attempt to reduce perspective error associated with two dimensional photography. The
background was a matte black color so that it would reflect a minimum amount of light
back to the 35mm camera. The camera was positioned 6.1 meters from the right side of
the subject. At this distance, the use of a 55mm focal length lens resulted in the
maximization of subject’s size, for the completion of two steps, in the field of view. The
camera’s optic axis was aligned perpendicular to the plane of movement formed by the
right side of the subject. The optic axis of the lens of the camera was at a height of 1
meter off the ground. The camera was leveled in this position. A plumb line, and subject
identification and trial numbers were hung in the field of view of the camera. A General
Radio strobolume set at the high intensity output was position just outside the field of
view of the camera at 5.64 meters from the right side of the subject. The frequency of the
strobolume was established by the strobotac.
Data Collection
Data was collected in a darkened room. The perimeters of the camera’s field of
view were marked on the walkway. The only light sources in the room during data
collection were the General Radio strobotac and strobolume. These strobe lights were
turned on prior to the subject’s initiation of her walking pattern. The metronome was
subsequently turned on and the subject imitated its frequency with her step rate.
Immediately after the subject entered the field of view, the shutter of the camera was
opened and left open until the subject exited the field of view.
Film and Film Processing
High speed 400 ASA black and white print film was used for the stroboscopic
photography pictures. The film was push-processed with Acufine developer to create an
ASA of 1000. Based on the General Radio instruction manual the guide number was
determined to be 200. This resulted in a calculated f-stop of 11 for the camera. After the
film was processed, the individual negatives were put in slide mounting jackets.
Stroboscopic Photography Negative Records of Walking Patterns:
Slide
Numbers
and/or
Subject
and Trial
Numbers
Corresponding
Descriptions of
the 2
Comparison
Slide Negatives
1/13
slow, bare
feet/slow, high
heels
fast, bare
feet/fast, high
heels
fast, bare
feet/fast, high
heels
slow
sneakers/slow,
hiking boots
slow,
sneakers/slow,
hiking boots
fast,
sneakers/fast,
hiking boots
fast,
sneakers/fast,
hiking boots
slow, clogs/fast,
clogs
3/15
4/16
9/17
10/18
11/19
12/20
5/7
Stroboscopic
Photography
Flash
Rate
(f./min.)
1200
1200
900
1200
900
1200
900
1200
6/8
slow, clogs/fast,
clogs
900
21/23
slow,
orthopedic/fast,
orthopedic
slow,
orthopedic/fast,
orthopedic
1200
22/24
900
Purpose of the Individual Comparison
Experiment
to determine the influence of high heel shoes
on the kinematics of slow gait patterns in
comparison to unshod slow gait
to determine the influence of high heel shoes
on the kinematics of fast gait patterns in
comparison to unshod fast gait
to determine the influence of high heel shoes
on the kinematics of fast gait patterns in
comparison to unshod fast gait
to determine the kinematic differences in slow
gait patterns between subjects wearing
sneakers or hiking boots
to determine the kinematic differences in slow
gait patterns between subjects wearing
sneakers or hiking boots
to determine the kinematic differences in fast
gait patterns between subjects wearing
sneakers or hiking boots
to determine the kinematic differences in fast
gait patterns between subjects wearing
sneakers or hiking boots
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
clogs
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
clogs
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
orthopedic shoes
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
orthopedic shoes
Stroboscopic Photography Negative Records of Walking Patterns: (continued)
25/27
28/30
02/05
03/06
04/07
05/09
06/08
slow, cowboy
boots/fast,
cowboy boots
slow, cowboy
boots/fast,
cowboy boots
standard, bare
feet/standard
high heels
standard, bare
feet/standard
high heels
standard, bare
feet/standard
high heels
standard high
heels/ one high
heel on left and
one bare foot on
right side
standard high
heels/one high
heel on left and
one bare foot on
right side
1200
900
900
900
900
900
900
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
cowboy boots
to determine the kinematic differences between
slow and fast gait patterns for subjects wearing
orthopedic shoes
to determine the influence of high heel shoes
on the kinematics of standard gait patterns in
comparison to unshod slow gait
to determine the influence of high heel shoes
on the kinematics of slow gait patterns in
comparison to unshod slow gait
to determine the influence of high heel shoes
on the kinematics of slow gait patterns in
comparison to unshod slow gait
to determine the kinematic differences in
standard gait patterns between subjects
wearing high heels, and one high heel and one
unshod
to determine the kinematic differences in
standard gait patterns between subjects
wearing high heels, and one high heel and one
unshod
Procedures for Obtaining Data from Negative of Each Slide:
1.
Use masking tape to adhere a large sheet of graph paper on a smooth wall surface.
The graph paper should be oriented so that its lines are horizontal and perpendicular
to the environment. There should be no air pockets between the graph paper and
wall surface and the graph paper should be free of any wrinkles.
2. Level a slide projector and orient it with the optic axis of its lens perpendicular to
the surface of the graph paper. Adjust the distance of the slide projector from the
graph paper and the height of the slide projector from the floor so that a projected
slide is centered within the frame of the graph paper and the image size is as large
as possible.
3. Check the level and orientation of the slide projector and slide by projecting one of
the negatives. The plumb bob included within the image should be aligned with
one of the vertical lines on the graph paper.
4. Once alignment of the graph paper and slide projector is achieved, do not move
either until data collection is completed.
5. Attempt to identify the first heel strike right. Use the pattern of the ankle marker. It
should be the first reflective marker of the ankle that stops its vertical displacement.
Put a colored mark at this point.
6. Repeat 5. for the second heel strike right. Use the same colored marker as in 5. for
the ankle associated with the second heel strike.
7. Determine a color scheme for the ankle, knee, hip, shoulder, elbow, and wrist. This
scheme should have each joint with its own color.
8. Mark all joints with their assigned color marker, beginning two images before the
first heel strike and ending two images after the second heel strike.
9. Use a straight edge to draw a line connecting the ankle to the knee to the hip and the
shoulder to the elbow to the wrist for first heel strike and also for second heel strike.
These lines should be distinct from subsequent lines making the same connections.
10. Proceed to connect the remaining corresponding joint markers in a manner similar
to what was used in 9..
11. Count the number of images from first heel strike to the second heel strike of the
same foot (i.e., stride consisting of two steps).
12. Determine the time, in seconds, from first heel strike to second heel strike.
Time = (number of images counted –1) /(flash rate)
For example: (13 images, 900 flashes/minute)
(13-1)/(900/60 seconds) = 0.8 seconds per stride
13. Determine stride length by using a ratio of the image of the projected distance of the
ankle from first heel strike to the second heel strike of the same foot and the
projected distance of the 1 meter reference measure.
1 meter
= unknown stride length
projected meter length
projected stride length
14. Calculate velocity of gait from calculated length of stride and time period for stride.
15. Determine the absolute angle of the shank, thigh, arm, and forearm and relative
angle for the knee and elbow for all images, beginning two images prior to the first
heel strike right and ending two images after the second heel strike right. This
could be done on the basis of the Cartesian coordinates of the joint centers or via
the use of a protractor. See the following figure for standards on absolute and
relative angles.
Key:
a – absolute angle of
the arm
b – relative angle of
the elbow joint
c – absolute angle of
the forearm
d – absolute angle of
the thigh
e – relative angle of
the knee
f – absolute angle of
the shank
a
b
c
e
d
direction of gait
f
Figure 1. Standard for determining absolute and relative angles.
16. Repeat proceeding process for subsequent slides.
Results:
The results are the responses to the statements that follow. They are to be written in
a scientific format. You should develop figures, graphs, and spreadsheet tables and refer
to these in your write-up to make the results easy to read. Your format should differ from
the normal scientific format in that you must show your work (i.e., how you calculated
your results). If there are several iterations of the same calculation process, you only
need to show the first to demonstrate your understanding.
Experiment – ______________________________________________________
________________________________________________________________________
1.
Temporal Analysis
a.
On the basis of the experimental methods used to collect data in this
stroboscopic photography experiment precisely define events in the
space provided in Table 1.
b.
For all events of the stride right under the two experimental conditions,
display the absolute and relative temporal values in the tables provided.
c.
List the order of events for the experimental conditions. Is this order as
expected? Are there changes in order among experimental conditions?
Explain.
d.
Determine and compare the absolute times for all defined events of the
stride right under the two experimental conditions. Are these results as
expected? Explain.
e.
Determine and compare the relative intervals for all defined events of
the stride right under the two experimental conditions. Are these results
as expected? Explain.
f.
What relationships existed between the absolute and relative temporal
values for the two experimental conditions? Explain.
g.
What problems existed in data collection to cause errors in the temporal
data? Explain.
Table 1. Temporal Results – Absolute Time (1st heel strike = 0.0 seconds)
Absolute Time
*defined events:
Experimental condition 1
time (sec.)
Experimental condition 2
time (sec.)
*
1 = first heel strike right
1
2
3
4
5
6
2 = maximum knee flexion right during support right
3 = maximum elbow flexion right
4 = maximum knee flexion right during non-support right
5 = maximum forward swing of thigh right during non-support right
6 = second heel strike right
**defined intervals:
a
b
c
d
Experimental condition 1
time (sec.)
Experimental condition 2
time (sec.)
** a = absorption phase (event 1 to 2)
b = maximum knee flexion from support to non-support right (event 2 to 4)
c = maximum knee flexion right during support to maximum elbow flexion (2 to 3)
d = stride (cycle) time (event 1 to 6)
Table 2. Temporal Results – Relative Values
Relative Time
*defined events:
Experimental condition 1
Proport. of stride from start
Experimental condition 2
Proport. of stride from start
*
See Table 1
**defined intervals:
Experimental condition 1
Proportion of right step
Experimental condition 2
Proportion of right step
** See Table 1
2.
1
2
a
3
b
4
5
c
6
d
Kinematic Analysis
a.
Calculate stride length and stride velocity for the two experimental
conditions. Are these results as expected? Explain.
b.
Develop a spreadsheet containing all defined absolute and relative
angles (beginning two images prior to the first heel strike right and
ending two images following the second heel strike right). In a separate
column of this spreadsheet indicate the occurrence of all defined events
(see Table 1).
c.
Complete Table 3 for all defined events and intervals (see Table 1) for
all absolute and relative angles (see Figure 1) for the two experimental
conditions.
d.
On a single sheet of graph paper, for the two experimental conditions,
plot the absolute angles of the forearm and arm and the relative angles
for the elbow from the spreadsheet developed for 2.a.. On the plot, show
vertical lines representing all defined events.
e.
On a single sheet of graph paper, for the two experimental conditions,
plot the absolute angles of the shank and thigh and the relative angles
for the knee from the spreadsheet developed for 2.a.. On the plot, show
vertical lines representing all defined events.
f.
Compare the absolute and relative angles for all defined events and
intervals for the two experimental conditions. Are these results as
expected? Explain.
g.
What problems existed in data collection to cause errors in the kinematic
data? Explain.
h.
Expand the spreadsheet in 2.a. to include average angular velocities
determined from all absolute and relative angles.
i.
On a single sheet of graph paper, plot the average angular velocities
from 2.h. for the arm, forearm and elbow for the two experimental
conditions. On the plot, show vertical lines representing all defined
j.
k.
l.
m.
n.
o.
p.
q.
events. Remember that these are average velocities and should be
represented in the midpoint of each of two successive image times.
Is the plot for 2.i. as expected? Explain.
On a single sheet of graph paper, plot the average angular velocities
from 2.h. for the thigh, shank, and knee for the two experimental
conditions. On the plot, show vertical lines representing all defined
events. Remember that these are average velocities and should be
represented in the midpoint of each of two successive image times.
Is the plot for 2.k. as expected? Explain.
Expand the spreadsheet in 2.a. to include average angular accelerations
determined from all calculated average angular velocities.
On a single sheet of graph paper, plot the average angular accelerations
from 2.m. for the arm, forearm and elbow for the two experimental
conditions. On the plot, show vertical lines representing all defined
events. Remember that these are average accelerations and should be
represented in the midpoint of each of two successive times for the
average angular velocities.
Is the plot for 2.m. as expected? Explain.
On a single sheet of graph paper, plot the average angular accelerations
from 2.m. for the thigh, shank, and knee for the two experimental
conditions. On the plot, show vertical lines representing all defined
events. Remember that these are average accelerations and should be
represented in the midpoint of each of two successive times for the
average angular velocities.
Is the plot for 2.p. as expected? Explain.
Table 3. Kinematic Results
Absolute Time
*defined events:
Experimental condition 1
Absolute and relative
angles (deg.):
a
b
c
d
e
f
Experimental condition 2
Absolute and relative
angles (deg.):
a
b
c
d
e
f
* See Table 1
**defined intervals:
Experimental condition 1
range (deg.)
Experimental condition 2
range (deg.)
** See Table 1
1
2
a
3
b
4
5
c
6
d
BIOMECHANICAL ANALYSIS OF PHYSICAL ACTIVITY
Student Project - Data Reduction, Analysis, and Reporting:
Stroboscopic Photography of Human Gait
Grade Report
Student: ______________________________________
Write-up Area/Comments
1. Temporal Analysis
Precisely define events in Table 1
Display absolute and relative temporal values for the two
experimental conditions in Tables 1 and 2
c.
List order of events for experimental conditions. Is order as
expected? Are there changes in order among experimental
conditions? Explain
d. Determine and compare the absolute times for all defined
events of the step right under the two experimental conditions.
Are these results as expected? Explain.
e.
Determine and compare the relative intervals for all defined
events of the step right under the two experimental
conditions. Are these results as expected? Explain.
f.
What relationships existed between the absolute and relative
temporal values for the two experimental conditions?
Explain
g.
What problems existed in data collection to cause errors in
the temporal data? Explain.
a.
b.
2. Kinematic Analysis
a.
Calculate step length and step velocity for the two
experimental conditions. Are these results as expected?
Explain.
b. Develop a spreadsheet containing all defined absolute and
relative angles (beginning two images prior to the first heel strike
right and ending two images following the second heel strike
right). In a separate column of this spreadsheet indicate the
occurrence of all defined events (see Table 1).
c.
Complete Table 3 for all defined events and intervals (see
Table 1) for all absolute and relative angles (see Figure 1)
for the two experimental conditions.
d. On a single sheet of graph paper, for the two experimental
conditions, plot the absolute angles of the forearm and arm
and the relative angles for the elbow from the spreadsheet
developed for 2.a.. On the plot, show vertical lines
representing all defined events.
Points
Received
Points
Possible
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
On a single sheet of graph paper, for the two experimental
conditions, plot the absolute angles of the shank and thigh
and the relative angles for the knee from the spreadsheet
developed for 2.a.. On the plot, show vertical lines
representing all defined events.
Compare the absolute and relative angles for all defined
events and intervals for the two experimental conditions.
Are these results as expected? Explain
What problems existed in data collection to cause errors in
the kinematic data? Explain.
Expand the spreadsheet in 2.a. to include average angular
velocities determined from all absolute and relative angles.
On a single sheet of graph paper, plot the average angular
velocities from 2.h. for the arm, forearm and elbow for the
two experimental conditions. On the plot, show vertical
lines representing all defined events. Remember that these
are average velocities and should be represented in the
midpoint of each of two successive image times.
Is the plot for 2.i. as expected? Explain.
On a single sheet of graph paper, plot the average angular
velocities from 2.h. for the thigh, shank, and knee for the
two experimental conditions. On the plot, show vertical
lines representing all defined events. Remember that these
are average velocities and should be represented in the
midpoint of each of two successive image times.
Is the plot for 2.k. as expected? Explain.
Expand the spreadsheet in 2.a. to include average angular
accelerations determined from all calculated average angular
velocities.
On a single sheet of graph paper, plot the average angular
accelerations from 2.m. for the arm, forearm and elbow for
the two experimental conditions. On the plot, show vertical
lines representing all defined events. Remember that these
are average accelerations and should be represented in the
midpoint of each of two successive times for the average
angular velocities.
Is the plot for 2.m. as expected? Explain.
On a single sheet of graph paper, plot the average angular
accelerations from 2.m. for the thigh, shank, and knee for
the two experimental conditions. On the plot, show vertical
lines representing all defined events. Remember that these
are average accelerations and should be represented in the
midpoint of each of two successive times for the average
angular velocities.
Is the plot for 2.p. as expected? Explain.