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TECHNICAL NOTES JOURNAL OF APPLIED BIOMECHANICS, 1993,9, 149-159 Q 1993 by Human Kinetics Publishers, Inc. A Panning Videographic Technique to Obtain Selected Kinematic Characteristics of the Strides in Sprint Hurdling John W. Chow This paper describes a panning videographic technique for measuring stride lengths and horizontal velocities of strides over an entire hurdle race. The technique requires that tapes of alternate black and white sections be placed at the inside border of the inside lane and the outside border of the outside lane of a track as spatial reference and that a vertical reference be videotaped when it is erected at different locations within the track. The stride lengths and horizontal velocities obtained with the panning technique were compared with the corresponding values that were obtained with conventional stationary camera techniques. The results indicate that if three panning cameras are used to cover the entire hurdle race, the average absolute errors in stride length and horizontal velocity are 0.07 m and 0.15 mls, respectively. Such errors are considered acceptable for some applications. Certain properties of the panning technique are discussed. A single-camera panning videographic technique for obtaining the stride lengths of a runner during a 100-m race was proposed by Chow (1987) several years ago. The technique requires that paired background markers be placed at 10-m intervals on opposite sides of the eight-lane track. The known locations of the markers are used as spatial reference for determining the stride lengths. The technique was later adapted by Hay and Koh (1988) to obtain the stride lengths during the approach run of the long and triple jumps. Instead of using background markers, Hay and Koh (1988) painted parallel dashed lines (6 cm wide) on each side of the runway. These lines were placed at 1.80-m intervals, and the known locations of these lines relative to the takeoff board aided in determining the horizontal distance from the toe of the support foot to the front edge of the takeoff board for each support phase of the approach. The technique has been further developed so that the horizontal velocity of the center of gravity (CG) during the flight phase of a stride can be determined. John W. Chow is with the Department of Exercise Science, El01 Field House, University of Iowa, Iowa City, IA 52242.

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TECHNICAL NOTES

JOURNAL OF APPLIED BIOMECHANICS, 1993,9, 149-159 Q 1993 by Human Kinetics Publishers, Inc.

A Panning Videographic Technique to Obtain Selected Kinematic Characteristics of the Strides

in Sprint Hurdling

John W. Chow

This paper describes a panning videographic technique for measuring stride lengths and horizontal velocities of strides over an entire hurdle race. The technique requires that tapes of alternate black and white sections be placed at the inside border of the inside lane and the outside border of the outside lane of a track as spatial reference and that a vertical reference be videotaped when it is erected at different locations within the track. The stride lengths and horizontal velocities obtained with the panning technique were compared with the corresponding values that were obtained with conventional stationary camera techniques. The results indicate that if three panning cameras are used to cover the entire hurdle race, the average absolute errors in stride length and horizontal velocity are 0.07 m and 0.15 mls, respectively. Such errors are considered acceptable for some applications. Certain properties of the panning technique are discussed.

A single-camera panning videographic technique for obtaining the stride lengths of a runner during a 100-m race was proposed by Chow (1987) several years ago. The technique requires that paired background markers be placed at 10-m intervals on opposite sides of the eight-lane track. The known locations of the markers are used as spatial reference for determining the stride lengths.

The technique was later adapted by Hay and Koh (1988) to obtain the stride lengths during the approach run of the long and triple jumps. Instead of using background markers, Hay and Koh (1988) painted parallel dashed lines (6 cm wide) on each side of the runway. These lines were placed at 1.80-m intervals, and the known locations of these lines relative to the takeoff board aided in determining the horizontal distance from the toe of the support foot to the front edge of the takeoff board for each support phase of the approach.

The technique has been further developed so that the horizontal velocity of the center of gravity (CG) during the flight phase of a stride can be determined.

John W. Chow is with the Department of Exercise Science, El01 Field House, University of Iowa, Iowa City, IA 52242.

The purpose of this study was to examine the validity of the refined technique in measuring selected kinematic characteristics of the strides in sprint hurdling. The sprint hurdle was selected in this validation study because most previous studies in hurdling have focused on the hurdle stride itself and it appears that no attempt has been made to obtain more complete kinematic data for the entire hurdles race. The present technique will be a valuable tool for future such attempts.

Method

Data Collection

The data collection session was conducted at the synthetic-surface track of the University of Iowa. A male former college hurdler volunteered to be the subject. He signed an informed consent document in accordance with the University of Iowa policy for the protection of the rights of research participants.

Setup. Two Panasonic S-VHS video cameras (Model AG-450) were used to record the subject's performance. A panning camera was placed on a tripod at an approximate horizontal distance of 16 m from the outside lane of the track and 6.5 m above ground level (Figure 1). The legs of the tripod were adjusted such that the camera was allowed to rotate about an axis approximately 75" to horizontal. When the optical axis of the camera was perpendicular to the lanes, the center of the optical field coincided with the ninth hurdle of a 100-rn hurdle race; the width of the optical field was about 9 m for the inside lane and 6 m for the outside lane at this instant. When the camera was aimed at the sixth hurdle in either the inside or outside lane, about 16 m of that lane could be observed in the optical view.

A stationary camera was placed in the infield of the track about 23 m away from the inside lane (Figure 1). The optical axis of this camera was perpendicular to the lanes, parallel to and about 1 m above the ground. The width of the optical view was about 7 m and 9 m for the inside and outside lanes, respectively. The location of this camera varied with trials and depended on the hurdle of interest.

So the error in spatial measurement made by the stationary camera could be estimated, a 2-m reference pole with major divisions at 0.5-m intervals was positioned in various orientations and locations throughout the optical view in the vertical planes of the second and seventh lanes and was videotaped. The picture (or frame) in which the reference pole was located in the middle of the optical view was used as linear scale for estimating the lengths of the pole in other locations.

Two nylon tapes (5 cm wide and 50 m long) of alternate black and white pattern were placed on the inside border of the inside lane and the outside border of the outside lane (Figure 1). Because one end of each tape was placed at the finishing line, the tapes appeared as two parallel dashed lines and the length of each black or white section was 0.61 m.

Three hurdles (height 0.84 m) were used throughout the data collection session. They were placed at consecutive hurdle locations (Figure 1). The station- ary camera was positioned such that it aimed at the second hurdle in each trial.

Trials. A total of 10 trials were conducted. The hurdle setup was different in each trial. The second hurdle was placed in either the second or the seventh

Panning Videographic Technique

i: 'I

i: i: hurdles

j F INISH I

hurd(e f o r 100-m hurdle race

I I

STATIONARY CAMERA

Figure 1 - Overhead view of the experimental setup. The panning and stationary cameras are 6.5 m and 1 m above ground level, respectively.

lane and at one of the 6th to 10th hurdle locations of the 100-m hurdle race (2 lanes x 5 hurdle locations = 10 trials).

In each trial, the subject started from a stationary position about 8 m from the first hurdle. The panning camera followed the subject's action over the three hurdles (only two hurdles in the last two trials when the second hurdle was at the 10th hurdle location). The strides prior to (preparatory stride), over (hurdle stride), and after (landing stride) the second hurdle were recorded by the stationary camera.

Before each trial, the reference pole was erected next to the second hurdle

and was recorded by both cameras. Two spirit levels were attached to the pole to ensure vertical alignment. The pole acted as a linear scale for the stationary camera and provided a vertical reference for the panning camera.

Data Reduction

A Peak Motion Measurement System (Peak Performance Technologies, Inc., USA) was used to obtain coordinate data from the video recording. The supporting hardware of the system includes a 13-in. (340-mm) Sony Trinitron video monitor (Model PVM-1341), a Panasonic VCR (Model AG-7300), and an AST Premium/ 286 computer. The digitizing units across the monitor screen ranged from 7 to 503 in steps of 0.5.

For each stride, takeoff was defined at the instant at which the hurdler first broke contact with the ground, and touchdown was defined as the instant at which he regained contact. The video pictures in which the hurdler was first seen to be off the ground and back in contact with it were thus taken to represent the takeoff and touchdown, respectively. The x- and y-coordinates of 2 1 points defining a 14-segment model of the human body were recorded for each picture analyzed. The CG locations of the hurdler were computed by use of the inertial data of Clauser, McConville, and Young (1969) and the basic segmental procedure described in Hay (1985).

For the purpose of this study, the stride length was determined by the locations of the toes of the takeoff and touchdown feet of a stride. The horizontal velocity of a stride was computed as the horizontal distance traveled by the CG of the hurdler during the flight divided by the corresponding flight time.

Stationary Camera. For each trial, the instants of takeoff and touchdown of the three recorded strides were analyzed. For the estimation of error in spatial measurement, five points on the reference pole (two endpoints and three major divisions) were digitized for those pictures (11 for the second lane and 10 for the seventh lane) showing the pole at different locations and orientations. With the established scale values, four length measurements--0.5 m, 1 m, 1.5 m, and 2 m-were computed for each pole location.

Panning Camera. The stride lengths and horizontal velocities of the three strides recorded by the stationary camera were also computed from the instants of takeoff and touchdown recorded by the panning camera.

Consider the views of a hurdler at the instants of takeoff and touchdown of a hurdle stride (Figure 2). Points A, B, C, and D are the junctions of adjacent blacklwhite sections in the reference tapes. In the overhead view, these four points form a rectangle. Points E and F are the locations of the toes of the supporting feet at the instants of takeoff and touchdown, respectively. For compu- tation of horizontal displacement parameters, Lines GH and IJ are constructed so that they pass through Points E and F and are parallel to Lines AB and CD, respectively. KL is a line bisecting the region GHU, and the hurdler's CG is assumed to travel on the vertical plane above KL during the flight phase.

Points M and N are the CG locations of the hurdler at the instants of takeoff and touchdown, respectively. With the reference pole that was videotaped before the trial as a vertical reference, the projection ~ i n e s MO and NP are constructed. Because the locations of the Points A, B, C , and D are known, the distances PO, EH, and FJ can be obtained by simple proportion. For example, the distance OP is computed by the following:

Panning Videographic Technique

Figure 2 - Computation of stride length and horizontal velocity of a stride in a hurdle race.

AB (actual) x OP (digitizing units) OP = KL (digitizing units)

The stride length is obtained by subtraction of the distance FJ from EH. The horizontal velocity of a stride is the distance OP divided by the time elapsed between takeoff and touchdown.

Analysis of Data

The stride lengths and horizontal velocities obtained with the panning camera were compared with the corresponding values obtained with the stationary camera. The absolute difference between the paired values was treated as the absolute error (AE). Average AE and standard deviations (SD) were computed for each measurement. In addition, percent error, defined as the ratio of the AE to the value of the parameter measured by the stationary camera, was also computed for each stride.

Results and Discussion

The average absolute errors in spatial measurements made by the stationary camera were found to be 0.01 m, 0.02 m, 0.03 m, and 0.04 m for length measures of 0.5 m, 1 m, 1.5 m, and 2 m, respectively, for the second lane (N = 11). The corresponding values for the seventh lane (N = 10) were 0.02 m, 0.02 m, 0.03 m, and 0.03 m. An examination of the sign of the errors revealed that the errors were random in nature. When these error values were combined with the flight distances and times of the strides measured from the stationary camera data, and when the possible errors in locating the CG locations with the segmen- tation method were neglected, the error in speed measurement made by the stationary camera was estimated to be about 0.1 m/s for the hurdle stride and 0.15 m/s for the other two strides.

The stride lengths and horizontal velocities obtained with both cameras and the absolute errors for all trials are presented in Table 1. Some data are not available because the takeoff feet of the preparatory strides were out of the optical field of the stationary camera in those trials conducted in the second lane. The average AEs in stride length and horizontal velocity for each trial are graphically presented in Figures 3 (second-lane trials) and 4 (seventh-lane trials).

The average AEs in stride length and horizontal velocity measurements are summarized in Table 2. For all trials, the average AEs for the stride length and velocity measurements (N = 25) are 0.10 m (SD = M.ll m) and 0.22 mls (SD = M.23 mls), respectively. However, if the data of the sixth hurdle are excluded (N = 20), the average AEs are reduced to 0.07 m (SD = M.06 m) and 0.16 m/s (SD = M . l l m/s). An examination of the sign of the errors revealed that the errors were random in nature.

For the trials conducted in the second lane, the average AEs for the stride length and horizontal velocity (N = 8) are 0.06 m (SD = H.05 m) and 0.15 mls (SD = k0.11 m/s), respectively, if the data of the sixth hurdle are excluded. The corresponding average AEs for the trials conducted in the seventh lane (N = 12) are 0.07 m (SD = M.05 m) and 0.15 mls (SD = M.10 mls). It is apparent that there is not much difference between the results of the second and seventh lanes.

Legend

stride Length

a Horizontal Velodty

< - 6th 7t h 8th 9th 10th Hurdle Location

Figure 3 - Average absolute errors in stride length and horizontal velocity for the second-lane trials.

Panning Videographic Technique

Legend

Stride Length

Horizontal Velocity

Hurdle Location

Figure 4 - Average absolute errors in stride length and horizontal velocity for the seventh-lane trials.

When the data of the sixth hurdle are excluded, the average percent errors in stride length are 5.24% (SD = +3.79%, N = 4), 2.33% (SD = +1.23%, N = 8). and 2.59% (SD = +0.80%, N = 8) for the preparatory, hurdle, and landing strides, respectively. The corresponding average percent errors for the horizontal velocity are 2.97% (SD = +1.28%), 1.48% (SD = +1.34%), and 2.42% (SD = f 1.59%). The average percent error in stride length for the preparatory stride is considerably larger than the other two strides. This is not the case when the average AEs for the three strides are compared (Table 2). It is apparent that the large average percent error of 5.24% is partly due to the short stride length of the preparatory stride.

Several reasons account for the larger AEs found in the trials of the sixth hurdle. The recorded image became smaller and out of focus when the hurdler and reference tapes were farther away from the panning camera. When the reference pole was positioned next to the ninth hurdles in the second and seventh lanes, the lengths of the pole as observed by the panning camera were 113.6 and 157.7 digitizing units (DUs), respectively. The length of the pole decreased progressively as the optical view moved away from the ninth hurdles. When the panning camera was aimed at the sixth hurdles in the second and seventh lanes, the lengths of the pole were 82.2 and 95.3 DUs, respectively. The small and

156 Chow

Table 1

Stride Lengths and Horizontal Velocities of Preparatory, Hurdle, and Landing Strides Obtained With Stationary and Panning Cameras

Preparatory Hurdle Landing

Stride Hor. Stride Hor. Stride Hor. length velocity length velocity length velocity

Hurdle Lane (m) (mls) (m) @IS) (m) (m/s)

S - 3.49 6.73 1.44 6.50 6 2 P 1.74 6.70 2.94 5.83 1.33 6.45

AE - - 0.55 0.90 0.1 1 0.05

S 2.01 8.1 1 3.65 7.15 1.55 7.23 6 7 P 1.90 7.28 3.85 7.57 1.33 6.73

AE 0.11 0.83 0.20 0.42 0.22 0.50

S - 3.52 6.78 1.45 6.88 7 2 P 1.70 6.92 3.49 6.72 1.40 6.98

AE - - 0.03 0.06 0.05 0.10

S 1.69 7.10 3.37 6.57 1.45 6.33 7 7 P 1.67 7.39 3.45 6.56 1.47 6.29

AE 0.02 0.29 0.08 0.01 0.02 0.04 S - - 3.55 6.82 1.44 6.98

8 2 P 1.80 7.95 3.43 6.54 1.47 6.67 AE - - 0.12 0.28 0.03 0.31

S 1.75 7.71 3.46 6.94 1.47 6.70 8 7 P 1.70 7.44 3.55 6.89 1.42 6.95

AE 0.05 0.27 0.09 0.05 0.05 0.25

S - - 3.43 6.90 1.44 7.03 9 2 P 1.78 7.20 3.45 6.87 1.49 6.77

AE - - 0.02 0.03 0.05 0.26

S 1.80 7.32 3.40 6.81 1.44 6.75 9 7 P 1.65 6.98 3.33 6.75 1.47 6.68

AE 0.15 0.23 0.08 0.06 0.03 0.07

S - - 3.43 6.51 1.41 6.52 10 2 P 1.78 7.28 3.28 6.37 1.45 6.47

AE - - 0.15 0.14 0.04 0.05

S 1.86 7.02 3.25 6.37 1.47 6.36 10 7 P 1.70 6.94 3.18 6.21 1.50 6.59

AE 0.16 0.08 0.07 0.16 0.03 0.23

Note. S = stationary camera. P = panning camera. AE = absolute error = d m .

Panning Videographic Technique 157

Table 2

Average Stride Lengths, Average Horizontal Velocities, and Average Absolute Errors Over All Trials and Selected Trials

Preparatory Hurdle Landing

Stride Hor. Stride Hor. Stride Hor. length velocity length velocity length velocity (m) (m/s) (m) (m/s) (m) (m/s)

All trials S (N = 10)

P

All except S trials of sixth hurdle P (N = 8)

AE

Note. Standard deviations are in parentheses. aN=5.bN=4.

unclear image not only affected the validity of spatial measurements but also affected temporal measurements because the instants of takeoff and touchdown were more difficult to identify.

Figure 5 illustrates an important property of the proposed technique: The more the angle between the optical axis of the panning camera and the plane of action deviates from 90°, the smaller the image size and the larger the measure- ment error. AE and CF are the recorded images of an object at two different locations (i.e., AB and CD) along the plane of action. For the same magnitude of digitizing error (e), the projection of e onto CD (E2) is greater than the projection of e onto AE (El). To minimize this type of error, the camera should be placed as far away from the plane of action as possible and a telephoto lens should be used to increase the image size.

In videographic analysis, the instants of takeoff and touchdown are the first pictures of the athlete losing and gaining contact with the ground, respectively. For the present study, error may occur in the computation of horizontal velocity if the exact instants of takeoff and touchdown are not captured. For example, if the takeoff foot has left the ground, the Point E in Figure 2 cannot be accurately located. If the touchdown recorded has passed the initial contact, the deceleration of the CG during the early portion of the support phase will be included in the computation of the horizontal velocity during the flight phase. This problem can be solved by use of four pictures in the computation. The picture immediately prior to the takeoff (last picture in which the subject is on the ground) and the

AB, CD = OBJECT

AE, CF = RECORDED IMAGES

e = DIGITIZING ERROR

El, E 2 = MEASUREMENT ERRORS

AXIS DF RUTATION OF PANNING CAMERA

Figure 5 - Increase in measurement error when the angle between the optical axis of the panning camera and the plane of action deviates from 90".

picture of touchdown are used to provide the toe locations (i.e., Points E and F in Figure 2). The horizontal displacement and the time elapsed between the first and the last pictures of the flight phase are used for the computation of horizontal velocity. However, such computational procedure requires more pictures to be digitized, which means more time consumed in data reduction.

Summary and Conclusion

This paper describes a panning videographic technique for measuring stride lengths and horizontal velocities of strides over an entire hurdle race. When compared with stationary camera techniques, the present technique has as its major advantage the ability to cover larger portions of the race without reducing the image size. Furthermore, the oblique view (looking down at an angle) reduces the chance of hurdlers obscuring one another.

When the present technique is implemented in a competition situation, it is recommended that three panning cameras be used, each covering 3040 m of the race. When a standard videographic system is used for data collection and

Panning Videographic Technique 159

reduction, the estimated average errors in stride length and horizontal velocity are 0.07 m and 0.15 mls, respectively. Given the limited picture rate (or frame rate) and digitizing resolution for the system employed in this study, such errors are considered acceptable for certain applications such as technique analysis. However, data collection and reduction systems of higher resolution are desired when the present technique is used for research purposes. In addition to being used for sprint hurdling, the technique can also be used in the analyses of running strides along a straight line (e.g., sprint running or approach running in horizontal jumps and javelin throwing).

References

Chow, J.W. (1987). Maximum speed of female high school runners. International Journal of Sport Biomechanics, 3, 110-127.

Clauser, C.E., McConville, J.T., & Young, J.W. (1969). Weight, volume and center of mass of segments of the human body (AMRL Tech. Rep. 69-70). Dayton, OH: Wright-Patterson Air Force Base.

Hay, J.G. (1985). The biomechanics of sports techniques (pp. 132-139). Englewood Cliffs, NJ: Prentice Hall.

Hay, J.G., & Koh, T.J. (1988). Evaluating the approach in the horizontal jumps. Interna- tional Journal of Sport Biomechanics, 4, 372-392.

Acknowledgment

This study was funded in part by a student research grant provided by the University of Iowa Student Assembly (UISA). The author would like to acknowledge the two anonymous reviewers for their comments on a draft of this manuscript. Thanks are extended to John Gerot and Bing Yu for their technical assistance.