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A Comparison of 2 Optical Timing Systems Designed to Measure Flight Time and Contact Time During Jumping and Hopping

Bosquet, Laurent1,2; Berryman, Nicolas1; Dupuy, Olivier2

Journal of Strength and Conditioning Research: December 2009 - Volume 23 - Issue 9 - p 2660-2665
doi: 10.1519/JSC.0b013e3181b1f4ff
Original Research
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Bosquet, L, Berryman, N, and Dupuy, O. A comparison of 2 optical timing systems designed to measure flight time and contact time during jumping and hopping. J Strength Cond Res 23(9): 2660-2665, 2009-This study was designed to investigate the interchangeability of 2 commercial optical timing systems for measuring flight time and contact time during jumping and hopping. Seventy-three physical education students (33 men and 40 women) participated in this study. They were instructed to perform 3 jump protocols (squat jump, countermovement jump, and countermovement jump free arms) and a hopping test (10 seconds with straight legs at a frequency of 2 Hz). Flight time and contact time were measured with 2 optical timing systems (Optojump, Microgate, Italia and IR-mat, Ergotest, Sweden), consisting of 2 bars placed opposite to each other. Both systems trigger a timer with a precision of 1 millisecond each time the infrared light is interrupted by the feet. Jump height was given by the systems, whereas leg stiffness was computed from contact time and flight time. Flight time was higher when measured with the IR-mat (bias ± 95% LOA [limits of agreement] = 5 ± 14 ms, p < 0.001). This difference was trivial (effect size <0.2) and clinically meaningless. The high correlation between sets of data (r = 0.99) together with narrow 95% LOA (3%) support the interchangeability of both systems to measure flight time. Similar results were found with contact time (bias ± 95% LOA = 8 ± 23 ms, p < 0.001, effect size <0.2 and r = 0.99), with the exception that it was the Optojump that provided the higher values. These trivial but significant differences between both systems had minor impact on jumping height (bias ± 95% LOA = 0.6 ± 1.7 cm, p < 0.001), effect size <0.2 and r = 0.99), and stiffness (bias ± 95% LOA = 0.8 ± 1.4 N·m−1·kg−1, p < 0.001, effect size <0.2 and r = 0.98). We concluded that both systems can be used interchangeably.

1Department of Kinesiology, University of Montreal, Montreal, Quebec, Canada; and 2Faculty of Sport Sciences, University of Poitiers, Poitiers, France

The research was conducted in the Laboratory of Exercise Physiology, Department of Kinesiology, University of Montreal, Montreal (Quebec, Canada).

Address correspondence to Laurent Bosquet, laurent.bosquet@umontreal.ca.

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Introduction

The ability to produce high amounts of force over a short period is a major determinant of performance in many sports (9,10,19-21). As such, the development of power is given a high priority in conditioning programs (12). Optimization of this development requires not only a sound understanding of the mechanisms underlying muscular power and a repertoire of strategies to enhance these underlying factors, but also valid and reliable tests and measures to assess this component of physical fitness.

Vertical jump height is commonly used by health and conditioning professionals for this purpose (6,13). Several protocols have been developed-including squat jump, countermovement jump, drop jump, or hopping with or without arm swing-that allow the specific assessment of concentric and plyometric power of leg extensors and jumping skill or leg stiffness.

The most widespread method for measuring vertical jump height is the jump and reach test, which requires only a height gauge fixed on a wall (18). Although very accessible, the accuracy of this method is questionable (15). Alternative methods using data obtained from force platforms (8) or 3-dimensional analysis systems (1) are also available. These measures can be considered as gold standards, but it has to be recognized that their cost and difficulty to use often are dissuasive for conditioning professionals.

Bosco et al. (4) were probably among the first to use basic kinematic equations to calculate jump height from flight time. Their original device to measure flight time was a contact mat that allowed a timer (± 0.001 seconds) to start each time the feet of the subject released or touched the platform. Considering the accessibility and accuracy of this method, the principle has been developed by others. Infrared mats have now replaced contact mats to measure flight time. There are actually 2 devices available commercially: the Optojump (Microgate, Italia) and the IR-Mat (Ergotest, Sweden). Although they have been built on the same principle (i.e., a ± 0.001-second timer that is triggered each time the infrared signal is interrupted), it is legitimate to question the interchangeability of their measures.

The aim of this study was to determine whether data obtained during vertical jump tests (flight time and height) or during hopping (contact time, flight time, and stiffness) were the same when measured by the Optojump (Microgate, Italia) or the IR-mat (Ergotest, Sweden).

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Methods

Experimental Approach to the Problem

This study was designed to investigate the interchangeability of 2 commercial optical systems for measuring flight time, contact time, or both during jumping and hopping. To address this issue, participants were asked to perform a series of jumping and hopping tests on a jumping surface that was delimited by the 2 devices mounted perpendicular to each other, so that performance could be recorded simultaneously by both systems.

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Subjects

Seventy-three physical education students (33 men and 40 women) with no history of lower-extremity surgery participated in this study. Their mean (SD) age, stature, and body mass were 21 (1) years, 169 (8) cm, and 66 (13) kg, respectively. The protocol was reviewed and approved by the Research Ethics Board in Health Sciences of the University of Montreal, Canada.

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Procedures

Testing took place over a 2-day period with half the group tested each day. All participants performed a standardized warm-up by skipping intermittently for 5 minutes and getting accustomed to the different tests for an additional 5 minutes. A passive recovery of 5 minutes was allowed between the end of the warm-up and the beginning of the tests. The order of tests was (a) squat jump, (b) countermovement jump, (c) countermovement jump free arms, and (d) hopping test. All tests were carried out by the same evaluator.

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Jumping Tests

Whatever the jumping test, participants were instructed to jump vertically for maximal height and to land in the same position and at the same place from takeoff to avoid lateral or horizontal displacement (22). To emphasize the use of leg extensors, participants were asked to maintain their torso in an upright position (3). They performed 2 trials per jumping test, separated by 30 seconds of passive recovery.

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Squat Jump

The hands are placed on the hips throughout the entire jump. When cued, the participant moves from the starting position into a semi-squat position and must stay motionless for 2 seconds before jumping.

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Countermovement Jump

The hands are placed on the hips throughout the entire jump. When cued, the participant makes a countermovement before jumping. No specific instruction was given regarding the depth or speed of the countermovement.

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Countermovement Jump Free Arms

When cued, the participant makes a countermovement before jumping with arms swinging back during the eccentric phase and forward during the concentric phase of the countermovement. No specific instruction was given regarding the depth of speed of the countermovement or the arm swing.

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Hopping Test

The hands are placed on the hips throughout the entire hopping test. When cued, the participant hops in place for 10 seconds with the legs as straight as possible. A frequency of 2 Hz was imposed with an auditory signal. Participants were asked to land in the same position and at the same place from takeoff to avoid lateral or horizontal displacement (22).

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Measurement Equipment

The 2 devices used in this study consist of 2 bars placed opposite to each other and connected directly to a PC via the serial port (Optojump, Microgate, Italia) or to a data collection unit that connects the PC via the USB port (IR-mat and Musclelab, Ergotest, Norway). Both systems transmit an infrared light 1 to 2 mm above the floor. When the light is interrupted by the feet, the units trigger a timer with a precision of 1 ms, which allows the measurement of flight time and contact time.

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Stiffness Calculation

Stiffness was calculated according to the method of Dalleau et al. (7). Briefly, ground reaction force is modeled as a sine wave, as it is expected from oscillation of pure spring mass. Assuming that the area under the curve is equal to the impulse of the ground reaction force, the vertical stiffness is calculated as follows:

where K is the stiffness (N·m−1), M the body mass, Tc is the ground contact time, and Tf is the flight time.

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Statistical Analyses

Standard statistical methods were used for the calculation of means and standard deviations. Normal Gaussian distribution of the data was verified by the Shapiro-Wilk test and homogeneity of variance was verified by the Levenne test. A paired t-test was used to compare the measures of both devices. The magnitude of the difference was assessed by the effect size (ES). Because there is no control group per se, pooled standard deviation was used to compute this statistic. The scale proposed by Cohen (1988) was used for interpretation. The magnitude of the difference was considered as trivial (ES <0.2), small (0.2 ≤ ES < 0.5), moderate (0.5 ≤ ES < 0.8) and large (ES ≥ 0.8). Pearson product moment correlation and Bland and Altman plots (2) were used to evaluate the association and the level of agreement between the measures of both devices. Statistical significance was set at p < 0.05.

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Results

Jumping Tests

Mean flight time and jumping height are reported in Table 1. We found a difference between both systems (p < 0.001), with the IR-mat providing measures that were higher than the Optojump, whatever the test. This difference could be considered as trivial because the effect size ranged from 0.05 to 0.08 and the mean bias represented ∼1% of the mean performance for flight time and ∼2% for jumping height. The association between measures and the bias and 95% limits of agreement (LOA) are presented in Table 1 and Figure 1. Pearson product moment correlation was very high (r = 0.99). On 95 occasions among 100 new tests, the difference between Optojump and IR-mat was −5 ± 14 ms for flight time and −0.6 ± 1.7 cm for jumping height. The LOA represented 3 and 6% of the mean result, respectively.

Table 1

Table 1

Figure 1

Figure 1

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Hopping Tests

Mean flight time, contact time, and stiffness are reported in Table 2. We found a difference between systems (p < 0.001), with the IR-mat providing higher measures for flight time and stiffness and lower measures for contact time. Once again, these differences could be considered as trivial because the effect size ranged from 0.12 to 0.17 and the mean bias represented ∼3% of the mean performance for flight time and contact time and ∼4% for leg stiffness. The association between measures and the bias and 95% LOA are presented in Table 2 and Figure 2. Pearson product moment correlation ranged from 0.96 to 0.98. On 95 occasions among 100 new tests, the difference between Optojump and IR-mat was −7 ± 22 ms for flight time, 8 ± 23 ms for contact time, and −0.8 ± 1.4 N·m−1·kg−1 for stiffness. The LOA represented 9, 9, and 4% of the mean result, respectively.

Table 2

Table 2

Figure 2

Figure 2

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Discussion

This study was designed to investigate the interchangeability of 2 commercial optical systems for measuring flight time, contact time, or both during jumping and hopping. Interchangeability, which refers to the level of agreement between 2 methods of measuring the same thing, was assessed with the procedure of Bland and Altman (2).

Flight time was systematically higher when measured with the IR-mat (Ergotest, Sweden), either during jumping or hopping. This difference was trivial and clinically meaningless. The high correlation between sets of data was expected because flight time was measured simultaneously by both devices. However, added to the narrow 95% LOAs, it supports the possibility of using the Optojump and the IR-mat interchangeably. The same conclusion applied to contact time, with the exception that it was the Optojump that provided the higher values. Flight time and contact time are interesting because they allow the calculation of vertical jump height and leg stiffness. At this stage, it is important to determine whether the trivial but significant bias we observed between both devices in the measurement of these variables had repercussions on the calculation of vertical jump height and leg stiffness. The bias and 95% LOAs were slightly higher for jumping height when compared with flight time (2 ± 6% vs. 1 ± 3% of the mean measure, respectively) but still acceptable in a clinical perspective. The same conclusion applied to leg stiffness because the bias and 95% LOAs were 4 ± 4% of the mean measure. Both sets of data were highly correlated (0.98 < r < 0.99).

We conclude that optical systems available commercially agree sufficiently to be used interchangeably. Isaacs et al. (14) and Leard et al. (15) provided data supporting the higher accuracy of timing systems vs. jump and reach devices such as the Vertec (Sports Imports, Columbus, Ohio, USA). This does not mean, however, that technical errors of measurement are not possible. Timing systems are not sensitive to several factors known to affect the measure of jump and reach devices, such as the accurate determination of the starting position height, the shoulder range of motion, or the correct timing and coordination of the arm swing to ensure that the measurement is recorded at the maximal height of the jump (15). Nevertheless, they are sensitive to all strategies that might increase flight time and thus artificially inflate jump height, such as bending the legs before contact with the floor or landing flat-footed rather than landing toes first as most people do naturally (5,14). It is therefore important to standardize protocols and to address these issues when giving the instructions to the subjects.

When the tests are correctly administered, timing systems such as the Optojump or the IR-mat allow an accurate determination of leg power or stiffness. However, for selection or talent identification, coaches might be interested in assessing the functional jumping height (i.e., the maximal height reached by the hand or the head) independently of leg power or jump technique. Lees et al. (16) have shown that an arm swing increases both takeoff velocity and the height of the body's center of mass at takeoff. The consequence is an increase in jumping height, which is typically 10% higher than the height reached during a countermovement jump without arm swing (17). Flight path of the body's center of mass cannot be altered after takeoff. However, altering the body position in the air by tilting the body and dropping the nonreaching arm from its original overhead takeoff position should allow reaching a higher height (5). This technique is possible with jump and reach devices such as the Vertec, but it is much more difficult when timing systems are used to measure performance. This is probably the reason why subjects obtained a better performance with the Vertec in the study by Burr et al. (5). In this context, the use of an overhead goal, which is known to affect both leg biomechanics and performance (11), should be considered when using a timing system.

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Practical Implications

Jumping and hopping tests are commonly used by health and conditioning professionals to measure leg power, leg stiffness, or jumping skill. On the field, performance can be measured with jump and reach devices or timing systems. The higher accuracy of timing systems has already been shown. Our study provides data supporting the interchangeability of optical timing systems available commercially to measure flight time and contact time and calculate jumping height or leg stiffness. This allows comparisons between field or laboratory measures or norms using 1 of these systems.

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Acknowledgments

No sources of funding were used to assist in the preparation of this manuscript. The authors have no conflicts of interest that are directly relevant to the content of this article.

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Keywords:

limits of agreement; vertical jump height; leg stiffness; timing system

© 2009 National Strength and Conditioning Association