Acommonly used form of strength and power assessment is the measurement of vertical jump performance. The vertical jump has many derivations that enable information to be gathered about various neuromuscular and performance qualities of an individual. For example, the squat jump has been used to evaluate the concentric strength of the leg extensors (1, 12, 14). Because of the inclusion of an eccentric component to the jumping motion, the countermovement jump has been used to measure the reactive strength of the lower body (1, 12, 14, 15). Because of the long duration (>250 milliseconds) of the eccentric-concentric contraction (stretch-shorten cycle, or SSC), this jump is also thought to be a measure of slow SSC ability (7, 14). Drop jumps, on the other hand, are thought to be measures of fast SSC performance and give a measure of stretch-load tolerance when used across different heights (1, 3, 7, 14–16).
Many different protocols and devices have been used to assess jump performance. These include the use of Vertecs or yardsticks, contact mats, optical encoders, rotary encoders, accelerometers, and force platforms. According to publications over the past 100 years, force platforms would appear to be one of the most commonly used measuring devices in biomechanics and are hence regarded as the “gold standard” in force measurement (4). However, such assessment must take place in the laboratory and not in the field because force platforms are very sensitive to extraneous vibrations and therefore must be mounted as specified by the manufacturer's instructions to preserve the integrity of the signal. Furthermore, the cost of these devices and accompanying electronics places them beyond the budget of many laboratories. Consequently, the development of portable, cost-effective equipment that allows kinematic and kinetic information to be gathered similar to the force platform would have obvious advantages in a field-testing situation. The purpose of this study, therefore, was to determine whether the movement characteristics of 3 different jumping activities (squat jump, countermovement jump, and drop jump) as measured by a linear position transducer were similar to the information gathered simultaneously on a force platform. A secondary purpose was to determine the reliability of the linear position transducer. If proven valid and reliable, the linear position transducer will offer a portable, cost-effective technique for the assessment of force and power.
Experimental Approach to the Problem
The reliability and validity of a linear position transducer to measure jump performance was determined by comparing the mean force, peak force, and time-to-peak force measurements with data obtained simultaneously with a force platform.
The subjects were 25 male volunteers who all had training histories in a variety of individual or team sports. Their mean (±SD) age, mass, and height were 23.4 ± 4.6 years, 76.8 ± 10.6 kg, and 176 ± 8.6 cm. The Human Subject Ethics Committee of the Auckland University of Technology approved all the procedures undertaken, and all subjects signed an informed consent before their participation in the research.
A linear position transducer (P-80A, Unimeasure, Corvallis, OR—average sensitivity 0.499 mV·V-1·mm-1, linearity 0.05% full scale) was countersunk into a custommade base. The wire from the transducer was attached to a waist harness on the subject and measured vertical displacement relative to the base with an accuracy of 0.1 cm. The transducer and base were placed upon the force platform (see Figure 1). This arrangement allowed simultaneous data collection from the linear position transducer and the force platform. The linear position transducer was calibrated to a known distance before testing.
A force platform (AMTI Force Plate and Amplifier, Advanced Technology Inc., Watertown, MA) was used for the jump assessment. The force plate was calibrated to a known mass before testing.
Each subject performed a warm-up that included cycling and static stretching of the legs. Subjects were instructed regarding the performance of each jump and were able to familiarize themselves before the jump assessment. Three types of jump were performed: a squat jump, a countermovement jump, and a drop jump. The subjects kept their hands on their hips for all jumps. Before the jump assessment, a linear position transducer was attached by harness to each subject. For the squat and countermovement jumps, the subjects began on top of the custom-made base. The squat jump (concentric only) involved flexing the knee joint to an angle of approximately 90°, maintaining that position for 4 seconds, and thereafter extending the knee joint (1, 11, 14). Countermovement jumps were performed under the same conditions as the squat jump but involved flexion of the knee joint (eccentric contraction) followed immediately by extension of the legs. For the drop jump assessment, subjects began on top of a wooden box 30 cm above the force platform. The drop jump involved stepping off the box from an erect standing position, and upon ground contact the subjects attempted to minimize contact time and jump for maximum height (1, 11, 14). Drop jump heights of 30–40 cm have been recommended previously and are thought to be safe heights for such assessments (14). Three trials for each jump were collected, and a rest period of 1 minute was implemented between trials. Trials 2 and 3 of each jump assessment were used for analysis.
The variables of interest in this study were calculated from the mass-displacement characteristics of the linear position transducer and the force data from the force platform.
The displacement and force signals were sampled simultaneously (1,000 Hz) and transmitted to a computer (MacIntosh G4 Computer, Cupertino, CA) via a 12-bit A/D converter card (Instrunet, GW Instruments, Boston, MA) for the force platform and the linear transducer. The signals were digitally filtered with a low-pass Hamming filter with a cutoff frequency of 10 Hz and a zero phase lag. An analysis program (Superscope Version 3, GW Instruments) was used to calculate the mean force, peak force, and time-to-peak force for both devices.
Statistical analysis was performed by SPSS for windows Version 10.0 (Chicago, IL). Data are presented as mean values and SDs. Paired Student's t-tests were carried out between variables (mean force, peak force, and time-to-peak force) to determine whether differences existed within trials and between means. The Pearson product moment correlation coefficient (r) was used to examine the validity of the mean force, peak force, and time-to-peak force as measured by the linear position transducer and force platform. Reliability was tested according to procedures outlined by Atkinson and Nevill (2). The “relative” reliability is the degree to which individuals maintain their position in a sample with repeated measures and is best measured by a correlation coefficient. The relative reliability of the linear position transducer was tested by assessing the intraclass correlation coefficient (ICC) between the mean force, peak force, and time-to-peak force of the 2 trials measured with this device. These ICCs were compared with the same variables as calculated from the force platform data. The ICCs were calculated with a 2-way mixed effect (people effect random, measure effect fixed)-consistency definition model, and the single-measure ICC is reported. “Absolute” reliability refers to the degree which repeated measures vary for individuals and can be described by the coefficient of variation (CV). The CV was calculated (CV = SD/Mean × 100) for each single case, and then the mean CV was determined for the sample (6). Statistical significance was determined by a probability level of p ≤ 0.05.
The mean values and SDs for mean force, peak force, and time-to-peak force (n = 25) and the Pearson product moment correlation coefficient for each of the jumps as assessed by the linear position transducer and force platform can be observed in Table 1. Pearson correlation coefficients across the 3 jumps for the mean force (r = 0.95–0.96), peak force (r = 0.86–0.93), and time-to-peak force (r = 0.92–0.99) were high. The only significant difference in means between the linear position transducer and the force platform data was the peak force of the drop jump (t = -3.702, p = 0.001).
The trial-to-trial reliability of mean force, peak force, and time-to-peak force for the 2 devices across the various jump techniques can be observed in Table 1. The trial-totrial reliability of the jumps measured by the linear position transducer gave an ICC of 0.92–0.97 for mean force, 0.97–0.98 for peak force, and 0.72–0.96 for time-to-peak force. Similar ICCs of 0.92–0.97 for mean force, 0.86–0.97 for peak force, and 0.77–0.92 for time-to-peak force were also observed for the force platform (see Table 2). The CVs were 2.1–4.5% for mean force, 2.5–8.4% for peak force, and 4.1–11.8% for time-to-peak force. The ICCs and CVs were similar to those calculated for the force platform. Once more, similar CVs can be observed for the force platform (see Table 2).
To determine the measurement error associated with calculating the force variables with the linear position transducer, relative (ICC) and absolute (CV) reliability measures were calculated. Such an approach is common in the literature (8, 10, 13). Although there are no preset standards for acceptable reliability measures, it has been suggested that ICC values above 0.75 may be considered reliable and this index should be at least 0.90 for most clinical applications (9). As can be observed in Table 2, the ICCs for the force measures assessed by the linear position transducer across the 3 jumps meet these requirements, with the exception of time-to-peak force for the drop jump condition (ICC = 0.72). However, a similar correlation coefficient for the force platform (ICC = 0.77) suggests that the reliability of this measure is questionable.
Some scientists have arbitrarily chosen an analytical goal of the CV being 10% or below, but the merits of this value are the source of conjecture (2). Nonetheless, only 2 measures from the linear position transducer were found to lie outside these suggested limits, both of which were associated with time-to-peak force (CV = 10.8–11.7%). For the majority of the variables measured from the linear transducer data, the CVs were less than 5%. Similar CVs were found with the force platform. The CVs and ICCs for time-to-peak force, in particular for the drop jump, suggest that this measure should be interpreted with caution.
Both the ICC and CV were calculated from trials 2 and 3 of each jump condition in an effort to minimize measurement error, especially systematic bias. Trial 1 of each jump assessment provided familiarization in addition to that provided in the warm-up. Paired sample Student's t-tests found no statistical difference between the 2 trials for any of the variables measured, indicating that factors such as learning effects, motivation, protocol inconsistencies, and others were not influencing the assessment. The high ICCs, low CVs, absence of any statistical difference among trials, and similar values noted with the force platform indicated that the linear position transducer data were reliable.
The accuracy of a new assessment tool is usually studied by comparing the new device with that of another (5, 8, 13). This study determined the validity of a linear position transducer to measure jump performance by comparing its mean force, peak force, and time-to-peak force measurements with data obtained simultaneously with a force platform. It is thought that if a high (r > 0.80) and statistically significant correlation coefficient is obtained between the 2 devices, the equipment is deemed to be sufficiently valid (2). Pearson correlation coefficients greater than 0.80 were found for all the force measurements across the 3 jump types. The results suggest that the linear position transducer measurements were valid.
Kinematic and kinetic measures that have been profiled on this device include duration of contraction, mean and peak velocity, peak acceleration, mean and peak force, mean and peak power, instantaneous power, force at 30 and 100 milliseconds, total impulse, and total work done. The only measure that was found to have unacceptable reliability was force at 30 and 100 milliseconds, and we found this to be the case with force platform measurements, too.
The high Pearson correlation coefficients, high ICCs, low CVs, absence of any statistical difference among trials, and similar values noted with the force platform indicate that the linear position transducer is reliable and valid. Furthermore, it offers information (e.g., force, impulse, power) that is often limited to laboratory-type assessment and hence can give much better prognostic and diagnostic information in a field setting than can many of the other devices described previously. The system has the added advantage of adapting to any weightlifting apparatus. Hence, it may also be used to assess and monitor changes in strength and power performance. The results, therefore, suggest that the linear position transducer offers a cost-effective, versatile, and valid means for the measurement of force.
1. Arteaga, R., C. Dorado, J. Chavarren, and J.A.L. Calbert. Reliability of jumping performance in active men and women under different stretch loading conditions. J. Sports Med. Phys. Fitness.
2. Atkinson, G., and A.M. Nevill. Statistical methods for assessing measurement error (reliability) in variables relevant to sports medicine. Sports Med.
3. Hatze, H. Validity
and reliability of methods for testing vertical jumping performance. J. Appl. Biomech.
4. Nigg, B.M., and W. Herzog. Biomechanics of the Musculo-Skeletal System.
Chichester, England: John Wiley & Sons Ltd., 1994.
5. Rahmani, A., G. Dalleau, F. Viale, C.A. Hautier, and J.R. Lacour. Validity
and reliability of a kinematic device for mesauring the force developed during squatting. J. Appl. Biomech.
6. Sale, D. Testing strength and power. In: Physiological Testing of the High Performance Athlete.
J.D. Mac Dougall, H.A. Wenger, and H.J. Green, eds. Champaign, IL: Human Kinetics, 1991. pp. 21-106.
7. Schmidtbleicher, D. Training for power events. In: Strength and Power in Sport.
P.V. Komi, ed. Boston: Blackwell Scientific Publications, 1992. pp. 381-395.
8. Thompson, C.J., and M.G. Bemben. Reliability and comparability of the accelerometer as a measure of muscular power. Med. Sci. Sports Exerc.
9. Walmsley, R., and T. Amell. The application and interpretation of intraclass correlations in the assessment
of reliability in isokinetic dynamometry. Isokinetic Exerc. Sci.
10. Walshe, A., G. Wilson, and A.J. Murphy. The validity
and reliability of a test of lower body musculotendinous stiffness. Eur. J. Appl. Physiol.
11. Wilson, G.J., A.D. Lyttle, K.J. Ostrowski, and A.J. Murphy. Assessing dynamic performance: A comparison of rate of force development tests. J. Strength Cond. Res.
12. Wilson, G.J., A.J. Murphy, and A. Giorgi. Weight and plyometric training: Effects on eccentric and concentric force production. Can. J. Appl. Physiol.
13. Wilson, G.J., A.D. Walshe, and M.R. Fisher. The development of an isokinetic squat device: Reliability and relationship to functional performance. J. Appl. Physiol.
14. Young, W. Laboratory strength assessment
of athletes. New Studies in Athletics.
15. Young, W. A simple method for evaluating the strength qualities of the leg extensor muscles and jumping abilities. Strength Cond. Coach.
16. Young, W.B., J.F. Pryor, and G.J. Wilson. Effect of instructions on characteristics of countermovement and drop jump performance. J. Strength Cond. Res.