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
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
21.relative angle – orientation of one segment with respect to another
22.relative joint angle – the angle formed by lines representing two segments
with respect to each other
23.standard walking frequency – 120 steps per minute; rate often used in
experimental analyses of walking gait patterns in subjects free of any
handicapping conditions
24.step – in a gait pattern, from heel strike of one foot to the next heel strike of
the opposite foot
25.stride – in a gait pattern, from heel strike of one foot to the next heel strike of
the same foot
26.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
27.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.)
28.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
29.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 / Strobo-scopic Photo-graphy Flash Rate (f./min.) / Purpose of the Individual Comparison Experiment1/13 / slow, bare feet/slow, high heels / 1200 / to determine the influence of high heel shoes on the kinematics of slow gait patterns in comparison to unshod slow gait
3/15 / fast, bare feet/fast, high heels / 1200 / to determine the influence of high heel shoes on the kinematics of fast gait patterns in comparison to unshod fast gait
4/16 / fast, bare feet/fast, high heels / 900 / to determine the influence of high heel shoes on the kinematics of fast gait patterns in comparison to unshod fast gait
9/17 / slow sneakers/slow, hiking boots / 1200 / to determine the kinematic differences in slow gait patterns between subjects wearing sneakers or hiking boots
10/18 / slow, sneakers/slow, hiking boots / 900 / to determine the kinematic differences in slow gait patterns between subjects wearing sneakers or hiking boots
11/19 / fast, sneakers/fast, hiking boots / 1200 / to determine the kinematic differences in fast gait patterns between subjects wearing sneakers or hiking boots
12/20 / fast, sneakers/fast, hiking boots / 900 / to determine the kinematic differences in fast gait patterns between subjects wearing sneakers or hiking boots
5/7 / slow, clogs/fast, clogs / 1200 / to determine the kinematic differences between slow and fast gait patterns for subjects wearing clogs
6/8 / slow, clogs/fast, clogs / 900 / to determine the kinematic differences between slow and fast gait patterns for subjects wearing clogs
21/23 / slow, orthopedic/fast, orthopedic / 1200 / to determine the kinematic differences between slow and fast gait patterns for subjects wearing orthopedic shoes
22/24 / slow, orthopedic/fast, orthopedic / 900 / 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 / slow, cowboy boots/fast, cowboy boots / 1200 / to determine the kinematic differences between slow and fast gait patterns for subjects wearing cowboy boots28/30 / slow, cowboy boots/fast, cowboy boots / 900 / to determine the kinematic differences between slow and fast gait patterns for subjects wearing orthopedic shoes
02/05 / standard, bare feet/standard high heels / 900 / to determine the influence of high heel shoes on the kinematics of standard gait patterns in comparison to unshod slow gait
03/06 / standard, bare feet/standard high heels / 900 / to determine the influence of high heel shoes on the kinematics of slow gait patterns in comparison to unshod slow gait
04/07 / standard, bare feet/standard high heels / 900 / to determine the influence of high heel shoes on the kinematics of slow gait patterns in comparison to unshod slow gait
05/09 / standard high heels/ one high heel on left and one bare foot on right side / 900 / to determine the kinematic differences in standard gait patterns between subjects wearing high heels, and one high heel and one unshod
06/08 / standard high heels/one high heel on left and one bare foot on right side / 900 / 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