Validity of vertical jump measurement devices

This article was downloaded by: [Loughborough University] On: 16 February 2012, At: 02:58 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Sports Sciences Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rjsp20

Validity of vertical jump measurement devices a

a

Matthew Buckthorpe , John Morris & Jonathan P. Folland a

a

School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK

Available online: 23 Nov 2011

To cite this article: Matthew Buckthorpe, John Morris & Jonathan P. Folland (2012): Validity of vertical jump measurement devices, Journal of Sports Sciences, 30:1, 63-69 To link to this article: http://dx.doi.org/10.1080/02640414.2011.624539

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Journal of Sports Sciences, January 2012; 30(1): 63–69

Validity of vertical jump measurement devices

MATTHEW BUCKTHORPE, JOHN MORRIS, & JONATHAN P. FOLLAND School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK

Downloaded by [Loughborough University] at 02:58 16 February 2012

(Accepted 14 September 2011)

Abstract Vertical jump height is thought to provide a valuable index of muscular power, which is an important factor in sports performance and for assessing the mobility and functional capacity of injured or aged individuals. The purpose of the present study was to investigate the criterion validity of four popular devices for measuring vertical jump height. A belt mat, contact mat, portable force plate, and Vertec were compared to a criterion device, a laboratory force plate. Forty participants performed three maximal countermovement jumps on each device in a counterbalanced order, using block randomization. The criterion device presented the highest mean value (50.3 cm). The portable force plate and belt mat devices recorded similar jump height values to the criterion device (within 1 cm). The contact mat and Vertec devices recorded significantly lower values than the criterion device (P 5 0.001). The mean difference + limits of agreement were: belt mat 70.1 + 5.5 cm, contact mat 711.7 + 6.4 cm, portable force plate 70.8 + 3.9 cm, and Vertec 72.4 + 6.6 cm. In conclusion, the portable force plate and belt mat devices provided valid measures of vertical jump height, whereas the Vertec and contact mat devices did not.

Keywords: Field tests, force plate, validity, vertical jump height

Introduction Assessment of the vertical jump height an individual can attain is often undertaken in various sport and exercise settings. The propulsive action of the lower limbs during a vertical jump is thought to provide a valuable index of the muscular power of the legs in sedentary individuals, elite athletes, and some patient groups (Bach, Jones, Sweet, & Hager, 1994; Bosco & Komi, 1979, 1980; Markovic, Dizdar, Jukic, & Cardinale, 2004). Given that muscular power is an important determinant of performance in many individual and team sports (Newton & Kraemer, 1994), and a critical factor in the mobility and the functional capacity of injured or aged individuals, valid measures of vertical jump performance are desirable. Recording the vertical ground reaction force with a laboratory force plate during take-off using an appropriate sampling frequency and double integration of the recorded signal is considered the ‘‘gold standard’’ reference method for the measurement of vertical jump height (Hatze, 1998; Vanrenterghem, De Clercq, & Van Cleven, 2001). As laboratory force plates are typically housed in a concrete floor to minimize extraneous vibration, this method of

measuring vertical jump height is inaccessible for most practitioners working within many sport and exercise settings. As a result, numerous alternative devices for assessing vertical jump height in the nonlaboratory setting have been used, including: the traditional sergeant jump and its derivations, which examine the jumper’s ability to jump and reach; contact mats that calculate jump height from flight time; belt mat systems that use a tape measure to determine changes in the height of the jumper’s waist; and portable force plates that typically utilize automated software to calculate jump height from the vertical ground reaction forces. Given the ubiquity of vertical jump assessment, it is important to understand the validity of the various measurement devices and associated methods. Validity is generally referred to as the ability of a measurement tool to reflect what it is designed to measure (Atkinson & Nevill, 1998). Although there are different types of validity (logical, content, criterion, and construct), in the presence of a gold standard measure it is particularly useful to establish criterion validity, which assesses the extent to which scores on a test are related to some recognized standard (Thomas, Nelson, & Silverman, 2005).

Correspondence: M. Buckthorpe, School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough LE11 3TU, UK. E-mail: [email protected] ISSN 0264-0414 print/ISSN 1466-447X online Ó 2012 Taylor & Francis http://dx.doi.org/10.1080/02640414.2011.624539


64

M. Buckthorpe et al.

Research investigating the validity of devices used to measure vertical jump height is limited, but even where research has been undertaken the findings of the various studies, when compared, seem equivocal. Leard and colleagues (2007) compared Vertec (a derivation of the jump and reach method) and contact mat devices against the change in centre of gravity assessed with a three-dimensional (3D) motion analysis system, the latter acting as their criterion device. They found that the Vertec device significantly under-reported jump height, but that jump height measured using a contact mat did not differ significantly from the criterion device. This latter finding was perhaps surprising as contact mats utilize the flight time method for calculating jump height, and this method, even when used with laboratory force plate recordings, has been found to under-report vertical jump height when compared with either the double integration of force recordings during take-off (Moir, 2008) or 3D motion analysis of the centre of mass (Aragoń-Vargas, 2000). The discrepancies (Moir, 2008: –9.4 cm; Aragoń-Vargas, 2000: –11.8 cm) are thought to be due to the fact that the flight time algorithms underpinning any contact mat device fail to account for displacement of the centre of mass prior to take-off. To our knowledge, the validity of portable force plate or belt mat devices has not been compared with that of any other device or criterion method. Therefore, further research is needed to clarify, and establish in some cases, the validity of different devices, and provide some comparative information on the values they produce. The aim of this study was to assess the criterion validity of four popular devices (contact mat, jump and reach, belt mat, and portable force plate) used in a variety of sport and exercise settings to measure the vertical jump height an individual can attain, versus the measurements made using a ‘‘gold standard’’ device, a laboratory force plate.

Methods Participants Forty young healthy adults (31 males, 9 females) provided written informed consent before participating in this study, which had local ethics committee approval (mean + s: age 24.4 + 3.6 years; stature 1.75 + 0.08 m; body mass 72.0 + 10.7 kg). To provide a range of jump heights, the participants ranged from sedentary individuals to national standard athletes. Experimental protocol Four methods of assessing vertical jump height were compared with a criterion method (a laboratory force

plate). As it was not possible to assess the same jump simultaneously with all five devices, three maximum jumps were performed with each measurement system in a counterbalanced order, using block randomization. Three sets of five jumps were performed, with one jump of each set randomly assigned to each measurement device. This ensured that all 120 possible combinations were performed once, eliminating any potential order effect. The participants were given 60 s rest between jumps. The average of the three jump trials with each measurement device was used for statistical analysis. To quantify any reduction in performance over the course of the protocol, in addition to the validity assessment, participants performed three additional jumps with the criterion measurement device before and after the 15 block randomized jumps; therefore a total of 21 maximum jumps were completed by each participant. The participants’ body mass, stature, and reach height were measured using the laboratory force plate, Holtain stadiometer (Holtain, Ltd., Crymych, UK), and the Vertec (Sports Imports, Hilliard, OH, USA) apparatus, respectively. Following a 5 min warm-up on a cycle ergometer at a light intensity (*60 W), participants were familiarized with the protocol for each jump and performed three submaximum practice jumps before data acquisition. Before each jump the participants were instructed to ‘‘stand tall’’ on the respective measurement device for 3 s, which involved standing perfectly still in an upright manner, with their shoulders back and arms by their sides to standardize the starting position. At the end of this period, participants were reminded to jump maximally, before being instructed to ‘‘jump!’’ They then performed a maximum countermovement jump (CMJ) with arm swing. If a participant jumped forward out of the landing area, the jump was repeated. Measurement devices Laboratory force plate. A laboratory force plate (Type 9281B Kistler, Instrumente AG, Winterthur, Switzerland) served as the criterion device. Force data were recorded with Bioware software (Kistler, Instrumente AG, Wintherthur, Switzerland) at a frequency of 2000 Hz for 13 s. The vertical force data were exported to additional software (Spike 2, Cambridge Electronic Design, Cambridge, UK) for analysis of vertical jump height. Acceleration was derived from the vertical ground reaction force and body mass was measured using Newton’s second law of motion (force ¼ mass 6 acceleration), adjusting for the gravitational constant (g ¼ 9.81 m s–2). Body mass was measured for each individual jump over a 2 s period of stationary activity before

Validity of vertical jump measurement devices


commencing the jump. The double integration method was performed on acceleration data to derive velocity and then displacement of the centre of mass in a manner by which each data point was calculated as the sum of the data points from the start of the data up until the current point. Jump height was defined as maximum vertical displacement of the centre of mass. To determine the role of rise height on any potential differences between the measurement devices, participants’ rise height prior to takeoff was calculated for the three jumps on the laboratory force plate. Rise height was determined as centre of mass displacement at the point of takeoff. Belt mat. The University of Toronto Belt Jump (Sports Books Publisher, Toronto, Ontario, Canada) consists of a rubber mat that the participant stands on, with a tape feeder attached to its centre. The tape measure was attached to a belt that fastened around the participant’s waist. The bottom of the tape measure passes through a feeder joined to the mat and the tape measure was set to 0 cm before the jump. The tape measure slides through the feeder until the participant reaches the apex of the jump. After the jump, height can be recorded from the length of the tape that has been pulled through the feeder. Contact mat. The contact mat (Eleiko Sport, Halmstad, Sweden) contains embedded micro-switches that detect compression. The mat is connected to a hand-held digital timer (+0.001 s) that activates when compression is removed as the participant takes off during the jump, and records until the moment the participant lands. The timer unit calculates jump height from recorded flight time with the equation: height of body centre of mass ¼ (t2 6 g)/8, where g ¼ 9.81 m s–2 and t ¼ flight time. Portable force plate. We used a portable force plate designed for field testing (Quattro Jump, Type 9290 AD, Kistler, Switzerland). This device records only the vertical ground reaction force at a sampling frequency of 500 Hz and jump height is automatically calculated by the Quattro jump software using double integration of the force signal using Simpson’s rule of integration. Vertec. The Vertec Vertical Jump Meter was used as a variant of the traditional sergeant jump. It comprises plastic swivel vanes arranged in half-inch increments attached to a telescopic metal pole that was adjusted for each participant’s reach height. The test requires participants to use their dominant hand to displace the highest possible plastic vane with an overhead

65

arm swinging motion at the apex of their jump. Jump height was determined as the number of vanes displaced above the metal pole and converted from inches to centimetres. All jumps were performed from a standardized position with the participant stood facing the vanes at a distance of 10 cm from the Vertec, with their dominant shoulder aligned with the end of the vanes. Data analysis and statistics The three jumps performed with each device were averaged to provide a representative value for each, before calculating group mean + standard deviation. A one-way analysis of variance (ANOVA) with repeated measures was conducted to determine any differences between methods. In the event of significant differences, Bonferroni post-hoc comparisons were made. The Bland and Altman 95% limits of agreement (LOA) were employed (Bland & Altman, 1986) to assess criterion validity. Before their calculation, the absolute differences between the criterion and comparison measurement devices were assessed for normality using the AndersonDarling normality test. In addition, a Pearson product–moment correlation coefficient was calculated using the absolute differences between the two measurement devices and their mean value, to assess for heteroscedasticity. In the presence of heteroscedasticity, the log-transformed values were computed and the ratio limits were reported (Nevill & Atkinson, 1997). A Pearson product–moment correlation coefficient was used to assess the relationship between each measurement device and the criterion method. To examine the mean error for each respective measurement device compared with the criterion device, the mean residuals were calculated as the mean absolute difference (i.e. irrespective of sign). The average of the pre- and post-validity assessment jumps with the laboratory force plate were compared with a paired t-test. An alpha of 5% was accepted as statistically significant. Results The criterion device, a laboratory force plate, recorded the highest jump height values (50.3 + 7.5 cm) (Table I). The portable force plate and belt mat devices recorded similar jump height values to the criterion (within 1 cm or 2%; P 4 0.05) (Figure 1), while the contact mat and Vertec devices recorded lower values than the criterion (contact mat: 11.6 cm or 23% lower; Vertec: 2.4 cm or 4.8% lower; P 50.01). While the belt mat had the lowest mean difference of all the devices (–0.1 + 5.5 cm), the portable force plate had the narrowest limits of agreement (–0.8 + 3.9 cm); the corresponding value

66

M. Buckthorpe et al. Table I. Comparison of four vertical jump measurement devices with the criterion laboratory force plate device.

Mean + s (cm) Range (cm) Mean difference (+LOA) Correlation (diff. vs. mean) Mean residual (cm) Pearson’s r Bonferroni P

Laboratory force plate

Belt mat

Contact mat

Portable force plate

Vertec

50.3 + 7.5 36.5–66.7

50.2 + 7.8 34.0–66.7 –0.1 + 5.5 0.111 2.2 0.93 1

38.6 + 6.5 27.0–53.4 –11.7 + 6.4 0.979* 11.6 0.90 50.001

49.5 + 7.2 37.0–65.3 –0.8 + 3.9 70.176 1.7 0.97 0.072

47.9 + 7.9 32.9–64.2 –2.4 + 6.6 0.118 3.4 0.91 50.001


*Significant correlation (P 5 0.05). Bonferroni P ¼ one-way ANOVA with Bonferroni post-hoc calculation of P-value.

Figure 1. Vertical jump height measured with four field testing devices (A, portable force plate; B, belt mat; C, Vertec; D, contact mat) versus jump height measured with a laboratory force plate as the criterion. Individual data points (n ¼ 40) and the line of identity are shown.

for the Vertec device was –2.4 + 6.6 cm (Table I). Although the limits of agreement for the contact mat are presented in Table I, there was a strong significant correlation between the absolute differences and the mean jump height (r ¼ 0.979). The

log-transformed data still presented a significant relationship (r ¼ 0.409), but much less strongly than before. The new mean (bias) ratio and agreement ratio for the contact mat were 0.76 6/7 1.15. For comparison, the mean (bias) ratio and agreement


Validity of vertical jump measurement devices ratio for the belt mat, portable force plate, and Vertec were 0.98 6 /7 1.12, 0.97 6 /7 1.06, 0.93 6 /7 1.15, respectively (Table II). The absolute differences between the criterion device and the other measurement devices were normally distributed (all P 4 0.05). When the jump heights recorded with the four devices were plotted against the criterion measure (Figure 1), all the points for the contact mat were below the line of identity (Figure 1D), and for the Vertec 75% of the points were below the line of identity (Figure 1C). Qualitatively the portable force plate values appeared more tightly clustered to the line of identity than for the other devices (Figure 1A), and this device had the lowest mean residual value (1.7 cm). All methods significantly correlated to the criterion method (P 50.001). The t-test results showed a significant decline in jump height from 51.1 + 7.5 cm to 49.6 + 7.1 cm over the course of the protocol (P 50.001). The rise height of the centre of mass before takeoff was on average 14.6 + 2.2 cm. When individual rise heights were used to correct the jump height recorded with the contact mat device, corrected jump height was on average 53.3 + 6.8 cm, which was still significantly different from the criterion (P 50.001). However, the corrected values were more strongly correlated to the criterion method (r ¼ 0.963), with a mean difference + LOA of 3.0 + 4.1 cm. Discussion The aim of the present study was to assess the criterion validity of four devices commonly used to assess vertical jump height. When a variety of similar devices can be used to make a measurement, it is important to know the validity of a particular device so that reference values can be examined, meaningful comparisons of the results from different studies can be made, and the true capability of an individual’s performance evaluated. The results from this study suggest that the portable force plate and belt mat devices produced similar vertical jump height measurements to the criterion method, but that the

67

contact mat and Vertec devices significantly underreported vertical jump performance. The portable force plate and belt mat devices were found to be valid measures of vertical jump performance, producing, on average, measurements within 1 cm or 2% of those using the criterion device. The portable force plate determines jump height based on centre of mass displacement due to force application, in the same way as the criterion method; therefore, it was to be expected that it would demonstrate good agreement and hence validity. The fact that the belt mat also proved to be a valid device was perhaps more surprising, although it has been argued previously that a belt mat is better than other traditional vertical jump devices (jump and reach and contact mat; Klavora, 2000). While the position of the waist would be expected to provide a reasonable representation of the position of the centre of mass during a jump, it was not clear if the tape measure and feeder mechanism could accurately record the maximum change in displacement during the jump. The results from this study indicate that a belt mat device can provide a valid measure of vertical jump height. That the Vertec under-reported vertical jump height is in line with previous research (Leard et al., 2007). The differences evident when the Vertec and criterion devices were compared probably derive from the skill requirement of the Vertec method. Vertec jump height performance is determined by the ability of the jumper to extend at the shoulder and reach the highest possible vane at the apex of the jump. Therefore, poor coordination or timing may result in underestimations of vertical jump height. The results of the present study suggest that the Vertec does not provide a valid representation of vertical jump height, although it is possible that the results would be different in a sample of skilled jumpers, who possess superior timing and coordination. Vertical jump height, measured using the contact mat device, was found to be 11.7 cm lower on average than the measurement recorded using the criterion force plate. Consequently, the contact mat would not be regarded as a valid device for

Table II. Comparison of four vertical jump measurement devices with the criterion laboratory force plate device using log transformed data for assessing the limits of agreement. Laboratory force plate

Belt mat

Contact mat

Portable force plate

Vertec

3.92 + 0.15

3.90 + 0.16 –0.02 + 0.06 0.98 (1.12) 0.409*

3.64 + 0.16 –0.28 + 0.07 0.76 (1.15) 0.308

3.89 + 0.15 –0.03 + 0.03 0.97 (1.06) 0.019

3.85 + 0.17 –0.07 + 0.07 0.93 (1.15) 0.465*

Mean + s Difference (+s) Ratio limits Correlation (diff. vs. mean) *Significant correlation (P 50.05).


68

M. Buckthorpe et al.

measuring vertical jump height. The lower heights seen using the contact mat are similar to those recorded by Aragoń-Vargas (2000), who reported a mean difference of –11.8 cm between a flight time based device and a criterion device. Other studies indicate that contact mat devices generate lower vertical jump heights when compared with belt mat devices (Markovic et al., 2004; Slinde, Suber, Suber, Edweń, & Svantesson, 2008), which further corroborates our findings. The likely reason for these discrepancies is that the methodology underpinning contact mat devices is based on measurement of flight time, and the timer starts when the participant leaves the ground, thus it fails to capture the initial rise of the centre of mass before take-off. Rise height before take-off was on average 14.6 cm and more than offset the observed difference between the contact mat and criterion devices (11.7 cm). Furthermore, a strong relationship (r ¼ 0.78, P 50.001) was found between individual rise height values and the discrepancy between the contact mat and criterion device. Therefore, the inability of the contact mat to account for rise height does appear to explain a large portion of the discrepancy evident in the jump heights recorded by the contact mat and the criterion laboratory force plate in the present study. When the contact mat was corrected for each participant’s rise height, there was an improvement in its relationship to the criterion method (from r ¼ 0.90 to r ¼ 0.96). However, despite this, the corrected contact mat vertical jump heights were still different from those of the criterion device (þ3.0 cm; P 50.05). A major assumption of the flight time method is that take-off and landing positions are identical, and therefore the duration of the ascending and the descending phases of the flight time are the same. Previous research has identified that the descending phase is longer than the ascending phase, as participants land in a slightly crouched manner (Aragoń-Vargas, 2000; Kibele, 1998). It has been calculated that this difference in landing position during a countermovement jump with hands on hips may equate to an overestimation of vertical jump height of 2.3 cm (Kibele, 1998) or 2.8 cm (Enoksen, Tonnessen, & Shalfawi, 2009), which is very similar to the 3 cm value noted above, and appears to explain the discrepancy between our corrected contact mat vertical jump heights and those measured using the criterion device. There were some possible sources of error in this study that should be noted. For example, it was not possible to measure vertical height during a jump using all five methods at the same time, and a decline in jump height over the course of the protocol was found, which was likely a function of fatigue or a decline in motivation. It is acknowledged that these sources of error could potentially have influenced the

results presented. However, the counterbalanced block randomization design would be expected to minimize the impact of these errors on the study’s results for any particular measurement method. When considering the individual variability between devices assessed using the residuals and/or Bland and Altman analysis, it is important to acknowledge that measured jump height may differ due to the inherent variability in vertical jumping. It should also be noted that we did not address reliability. All the devices assessed in this study give a measure of vertical jump height, and providing a participant jumps with the same device in a consistent manner, and the device has acceptable reliability, it could still be used to assess the impact of an intervention or to monitor training. Although the current paper deals with the measurement of vertical jump height by a variety of devices, it should be acknowledged that there are additional advantages of measuring jump height using the force plate method (either portable or inbuilt). Force plates enable the assessment of force and power production throughout both the eccentric and concentric phases of the movement (e.g. Cormie, McGuigan, & Newton, 2010) rather than providing just a single measure of jump height, which allows for more detailed analysis of an individual’s training or rehabilitation requirements. In conclusion, the portable force plate and belt mat devices provided valid measures of vertical jump height when compared with a criterion device, a laboratory force plate, whereas a Vertec device and a contact mat device did not. Consequently, when testing in a field-based setting, the results of the present study would suggest that a portable force plate or a belt mat are the most valid devices for the assessment of vertical jump height.

References Aragoń-Vargas, L. (2000). Evaluation of four vertical jump tests: Methodology, reliability, validity, and accuracy. Measurement in Physical Education and Exercise Science, 4, 215–228. Atkinson, G., & Nevill, A. M. (1998). Statistical methods for assessing measurement error (reliability) in variables relevant to sports medicine. Sports Medicine, 26, 217–238. Bach, B. R., Jones, G. T., Sweet, F. A., & Hager, C. A. (1994). Arthroscopy-assisted anterior cruciate ligament reconstruction using patellar tendon substitution: Two- to four-year follow up results. American Journal of Sports Medicine, 22, 758–767. Bland, J., & Altman, D. G. (1986). Statistical methods for assessing agreement between two methods of clinical measurement. Lancet, 1, 307–310. Bosco, C., & Komi, P. V. (1979). Potentiation of the mechanical behaviour of human skeletal muscle through prestretching. Acta Physiologica Scandinavica, 106, 467–472. Bosco, C., & Komi, P. V. (1980). Influence of aging on the mechanical behaviour of leg extensor muscles. European Journal of Applied Physiology, 45, 209–219.


Validity of vertical jump measurement devices Cormie, P., McGuigan, M. R., & Newton, R. U. (2010). Changes in eccentric phase contribute to improved stretch–shortening cycle performance after training. Medicine and Science in Sports and Exercise, 42, 1731–1744. Enoksen, E., Tonnessen, E., & Shalfawi, S. (2009). Validity and reliability of the Newtest Powertimer 300-series1 testing system. Journal of Sports Sciences, 27, 77–84. Hatze, H. (1998). Validity and reliability of methods for testing vertical jump performance. Journal of Applied Biomechanics, 14, 127–140. Kibele, A. (1998). Possibilities and limitations in the biomechanical analysis of countermovement jumps: A methodological study. Journal of Applied Biomechanics, 14, 105–117. Klavora, P. (2000). Vertical-jump tests: A critical review. Strength and Conditioning Journal, 22 (5), 70–75. Leard, J. S., Cirillo, M. A., Katsnelson, E., Kimiatek, D. A., Miller, T. W., Trebincevic, K. et al. (2007). Validity of two alternative systems for measuring vertical jump height. Journal of Strength and Conditioning Research, 21, 1296–1299. Markovic, G., Dizdar, D., Jukic, I., & Cardinale, M. (2004). Reliability and factoral validity of squat and countermovement jump tests. Journal of Strength and Conditioning Research, 18, 551–555.

69

Moir, G. L. (2008). Three different methods of calculating vertical jump height from force platform data in men and women. Measurement in Physical Education and Exercise Science, 12, 207– 218. Nevill, A. M., & Atkinson, G. (1997). Assessing agreement between measurements recorded on a ratio scale in sports medicine and sport science. British Journal of Sports Medicine, 31, 314–318. Newton, R. U., & Kraemer, W. J. (1994). Developing explosive muscular power: Implications for mixed methods training strategy. Strength and Conditioning Journal, 16, 20–31. Slinde, F., Suber, C., Suber, L., Edweń, C. E., & Svantesson, U. (2008). Test–retest reliability of three different countermovement jumping tests. Journal of Strength and Conditioning Research, 22, 640–644. Thomas, J. R., Nelson, J. K., & Silverman, S. J. (2005). Research methods in physical activity. Champaign, IL: Human Kinetics. Vanrenterghem, J., De Clercq, D., & Van Cleven, P. (2001). Necessary precautions in measuring correct vertical jumping height by means of force plate measurements. Ergonomics, 44, 814–818.