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Reliability and validity of field-based measures of leg stiffness and reactive strength index in youths

Rhodri S. Lloyd a; Jon L. Oliver a; Michael G. Hughes a; Craig A. Williams b a Cardiff School of Sport, University of Wales Institute Cardiff, Cardiff b Children's Health and Exercise Research Centre, University of Exeter, Exeter, UK First published on: 04 December 2009

To cite this Article Lloyd, Rhodri S., Oliver, Jon L., Hughes, Michael G. and Williams, Craig A.(2009) 'Reliability and

validity of field-based measures of leg stiffness and reactive strength index in youths', Journal of Sports Sciences, 27: 14, 1565 — 1573, First published on: 04 December 2009 (iFirst) To link to this Article: DOI: 10.1080/02640410903311572 URL: http://dx.doi.org/10.1080/02640410903311572

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Journal of Sports Sciences, December 2009; 27(14): 1565–1573

Reliability and validity of field-based measures of leg stiffness and reactive strength index in youths

RHODRI S. LLOYD1, JON L. OLIVER1, MICHAEL G. HUGHES1, & CRAIG A. WILLIAMS2 1

Cardiff School of Sport, University of Wales Institute Cardiff, Cardiff and 2Children’s Health and Exercise Research Centre, University of Exeter, Exeter, UK

Downloaded By: [University of Wales] At: 11:00 16 December 2009

(Accepted 4 September 2009)

Abstract The aim of the study was to assess the reliability of a mobile contact mat in measuring a range of stretch–shortening cycle parameters in young adolescents. Additionally, vertical leg stiffness using contact mat data was validated against a criterion method using force–time data. The reliability study involved 18 youths completing a habituation and three separate test sessions, while 20 youths completed a single test session for the validity study. Participants completed three trials of a squat jump, countermovement jump, and maximal hopping test and a single trial of repeated sub-maximal hopping at 2.0 Hz and 2.5 Hz. All tests were performed on the contact mat. Reliability statistics included repeated-measures analysis of variance, intraclass correlation coefficient, and coefficient of variation (CV), while the correlation coefficient (r) and typical error of estimate (TEE) were reported for the validity study. Squat jump height was the most reliable measure (CV ¼ 8.64%), while leg stiffness during sub-maximal hopping, and reactive strength index produced moderate reliability (CV ¼ 10.17–13.93% and 13.98% respectively). Measures of leg stiffness obtained from contact mat data during sub-maximal hopping were in agreement with the criterion measure (r ¼ 0.92–0.95; TEE ¼ 6.5–7.5%), but not during maximal hopping (r ¼ 0.59; TEE ¼ 41.9%). The contact mat was deemed a valid tool for measuring stretch–shortening cycle ability in sub-maximal but not maximal hopping. Although reliability of performance was generally moderate, the tests offer a replicable assessment method for use with paediatric populations.

Keywords: Stretch–shortening cycle, hopping, spring-mass model

Introduction The stretch–shortening cycle, which is characterized by an initial eccentric action and a subsequent concentric contraction, forms the basis of human locomotion (Komi, 2000; Nicol, Avela, & Komi, 2006). Its relationships with performance enhancement, injury prevention, and fatigue mechanisms have previously been examined (Bishop, Fiolkowski, Conrad, Brunt, & Horodyski, 2006; Kubo et al., 2007; Padua et al., 2006). These studies typically used a laboratory-based measurement device to calculate stretch–shortening cycle function, devices that are both expensive and time-consuming. However, alternative field methods of measuring stretch– shortening cycle function are available. These use simple contact mats and assess performance in a variety of jump tests, including countermovement jumps (Markovic, Dizdar, Jukic, & Cardinale, 2004), rebound jumps (Cormack, Newton, McGuigan, &

Doyle, 2008), and hopping tasks (Dalleau, Belli, Viale, Lacour, & Bourdin, 2004). Previous studies have highlighted the effectiveness of contact mats in measuring jump performance owing to their strong reliability, mobility, and ease of administration (Dalleau et al., 2004; Isaacs, 1998; Markovic et al., 2004). Researchers have previously measured squat jump and countermovement jump heights independently, and established the performance difference between the two jumps resulting from the pre-stretch component of the countermovement jump (Harrison, Keane, & Coglan, 2004; Markovic et al., 2004). Although the reliability of squat and countermovement jumps has been reported in the literature (Markovic et al., 2004), the focus has been on adult populations and currently there are no reports on the reliability in youths of the calculated pre-stretch component. It has also been argued that the absence of any pre-loading means that squat and

Correspondence: R. S. Lloyd, Cardiff School of Sport, University of Wales Institute Cardiff, Cyncoed Campus, Cardiff CF23 6XD, UK. E-mail: [email protected] ISSN 0264-0414 print/ISSN 1466-447X online Ó 2009 Taylor & Francis DOI: 10.1080/02640410903311572


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countermovement jumps do not provide an adequate representation of the stretch–shortening cycle (Komi & Gollhoffer, 1997), with repeated hopping providing a more accurate model. Measures of stretch–shortening cycle function during rebound jumping include leg stiffness and the reactive strength index. The reactive strength index is used to examine the interaction of jump height and ground contact time, thus reflecting the rebound capabilities of the performer. Specifically, the measure is derived from dividing the height jumped (mm) by the time spent in contact with the ground (ms) developing those forces to perform the jump (Flanagan & Comyns, 2008; McClymont, 2005), and provides an insight into the strain experienced by the musculotendinous unit during activities requiring the utilization of the stretch– shortening cycle (McClymont, 2005). Despite being recommended as a useful assessment tool (Logan, Fornasiero, Abernethy, & Lynch, 2000) and being used in previous research (Flanagan, Ebben, & Jensen, 2008; McClymont, 2005; McClymont & Hore, 2004), there appears to be very limited information on the reliability of the measurement, particularly in the paediatric literature. While Flanagan et al. (2008) reported high reliability (a 4 0.95) for reactive strength index measures, interpretation of their results could be misleading, since they collected data from a single test session. Such an approach fails to accommodate for the random variation commonly seen in repeated-measures studies where multiple tests are implemented over time. Leg stiffness has been shown to be well related to hopping (Hobara, Kimura, Omuro, Gomi, Muraoka, Iso, & Kanosue, 2008), maximum speed (Chelly & Denis, 2001), and running economy (Kerdok, Biewener, McMahon, Weyand, & Herr, 2002; Kram, 2000). Leg stiffness has traditionally been measured in the laboratory utilizing the properties of the spring-mass model, measured via a force plate (Granata, Padua, & Wilson, 2002). Recently, a fieldbased measurement has been proposed that enables the quantification of leg stiffness from ground contact times, flight times, and body mass (Dalleau et al., 2004). While strong correlations for contact mats have previously been reported when validating leg stiffness against force–time data during submaximal (r ¼ 0.94) and maximal (r ¼ 0.98) hopping in adults (Dalleau et al., 2004), data on youths has yet to be published. Although the validity of fieldbased estimations of leg stiffness have been reported for sub-maximal and maximal hopping with a small sample size (n ¼ 8) of adults (Dalleau et al., 2004), no such data exist for youths. There is currently a lack of information on the reliability and validity of field-based measures of leg

stiffness and reactive strength index in youths. The current study was devised in two parts. In the first, we used a contact mat to measure the reliability of various stretch–shortening cycle measures [countermovement jump height, squat jump height, prestretch augmentation, and reactive strength index and leg stiffness during maximal hopping (5max) and sub-maximal hopping (2.0 Hz and 2.5 Hz)] in paediatric populations. In the second, we wished to validate leg stiffness measures using contact mat data against those acquired from ground-fixed force plate measurements.

Methods Participants Eighteen male participants (age 13.5 + 0.5 years; height 1.67 + 0.09 m; body mass 57.2 + 14.5 kg; mean + s) volunteered to participate in the reliability study. None of the participants reported any injury at the time of testing, and all were involved in regular physical education lessons. An additional validity study was undertaken on an independent sample of both male (n ¼ 12; age 16.5 + 0.5 years; height 1.75 + 0.06 m; body mass 68.1 + 9.5 kg) and female (n ¼ 8; age 16.5 + 0.5 years; height 1.65 + 0.06 m; body mass 59.8 + 1.9 kg) participants. For each study, parental consent and participant assent were obtained, and both studies were granted ethical approval by the University Research Ethics Committee. Procedures The reliability study was performed over three separate test sessions with a minimum of 48 h between sessions. Testing was completed at the same time on each test day, at the same indoor venue, and by the same tester. Participants were asked to wear the same clothing and footwear and to avoid drinking, eating or participating in exercise activities in the hour before testing. Participants completed a familiarization session in which they were provided with the opportunity to practise each of the jump protocols. Additional practice of the tests was provided before each test session. The tests used within the study were in the following order: squat jump, countermovement jump, maximal hopping (5max), and sub-maximal hopping at 2.0 Hz and 2.5 Hz. For all jump protocols, participants were instructed to: keep hands on the hips at all times, thus avoiding upper-body interference (Lees, Vanrenterghem, & Clercq, 2004); jump and land on the same spot; land with legs fully extended (i.e. triple extension at the acetabulofemoral joint, femorotibial

Reliability and validity of stretch–shortening cycle parameters joint, and the talocrural joint); and to look forward at a fixed position to aid balance maintenance. Adherence to these instructions was carefully monitored by the tester during each test session. For both the reliability and validity test sessions, the participants were instructed to complete a generic warm-up consisting of 3 min of continuous sub-maximal running and practice attempts of each test.


Part I: Reliability study protocols The squat jump was performed starting from an initial semi-squat position (908 knee flexion), determined from visual inspection. Once achieved, participants held this position for 2 s before jumping vertically for maximum height on the command of the tester. In accordance with previous research, each participant was visually observed during the squat jump to ensure that no countermovement was implemented (Bobbert, Gerritsen, Litjens, & Van Soest, 1996). The countermovement jump involved the participants lowering themselves from an initial standing position to a selfselected squat position, followed immediately by a vertical jump. Participants were encouraged to perform the eccentric phase of the jump as quickly as possible, with the depth of the countermovement phase being self-selected by the participant to maximize jump height (Cormack et al., 2008). The maximal hopping test involved participants performing five repeated maximal vertical hops on the contact mat. Participants were instructed to maximize jump height and minimize ground contact time (Dalleau et al., 2004). The first jump in each trial served as a countermovement jump and consequently was discounted for analysis. The remaining four hops were averaged for analysis of leg stiffness and the reactive strength index. Sub-maximal two-legged hopping was performed at frequencies of 2.0 Hz and 2.5 Hz. Participants were asked to hop two-legged on top of the contact mat for 20 consecutive hops at each frequency. Hopping frequency was maintained using a quartz metronome (SQ-44, Seiko, UK). Using the digital metronome instead of allowing participants to selfselect their preferred frequency enabled greater consistency of movement coordination in the lower extremities. The frequencies of 2.0 Hz and 2.5 Hz were within the boundaries previously defined as allowing for the broadest possible range of hopping frequencies (Hobara et al., 2008). Frequencies below 1.5 Hz have led to an inability to maintain true spring-mass model behaviour (Farley, Blickhan, Saito, & Taylor, 1991), whereas frequencies above 3.0 Hz have caused the unsuccessful maintenance of desired hopping pace (Hobara et al., 2008). Because those studies used adult populations, the choices of 2.0 Hz and 2.5 Hz were deemed more attainable and

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sustainable for younger participants. Also, the hopping frequency of 2.5 Hz was selected in addition to 2.0 Hz to represent an intensity that would enable the examination of the effect of increased hopping frequency on leg stiffness. Within each test session, participants were given three trials of the squat jump, countermovement jump, and maximal hopping test, in an order that provided the participants with a gradual increase in neuromuscular stress. The best trial of each test was subsequently used for further analysis. For the maximal hopping test (5max), the selection criterion was based on the best mean jump height scores after discounting the initial countermovement jump. For the sub-maximal hopping tests, each participant was given one trial at each frequency. Ten consecutive ‘‘acceptable hops’’ were used for analysis, with the selection criterion based on ten consecutive hops where the participants’ hopping frequency was closest to the designated metronome rate. All jumps were performed on a mobile contact mat (Smartjump, Fusion Sport, Australia), and data instantaneously collected via a hand-held PDA (iPAQ, Hewlett Packard, USA). Part II: Validity study protocol Participants were given one trial of the maximal hopping test and sub-maximal hopping at 2.0 and 2.5 Hz. All tests were performed as in the reliability study. Participants performed the tests on the mobile contact mat, positioned directly over a 900 6 600 mm ground-fixed force plate (type 9287BA, Kistler Instrumente AG, Winterthur, Switzerland) fitted with an integrated charge amplifier. Vertical force output data were automatically captured on a PC, at a sampling rate of 1000 Hz, and saved in the Bioware1 v. 3.2.6 software package. Data was then exported to Microsoft Excel1 for subsequent calculations of peak ground reaction force and centre of mass displacement. Contact mat variables Data obtained from the contact mat enabled the calculation of the following variables: . .

Flight time: The amount of time (s) between leaving and returning to the mat. Jump height: Calculated from the flight-time method as described by Flanagan and Comyns (2008): Jump height ¼ (gravity*(flight time)2)/8.

.

Contact time: The amount of time (in seconds) the participant was in contact with the ground.

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Reactive strength index: The ratio between jump height and contact time (mm/ms) (McClymont & Hore, 2004), calculated during the maximal hopping test. Vertical leg stiffness (kN m71): Calculated using the equation and methods as proposed by Dalleau et al. (2004).

Force plate variables For data obtained from the force plate, the following variables were determined: .


.

.

. .

Jump height (cm): Calculated via flight time (Flanagan & Comyns, 2008). Flight time and ground contact time (both in seconds): Calculated from force plate data by determining the point at which a significant change in force data values (an increase of more than 10 N) occurred for take-off and touchdown. Vertical leg stiffness: The ratio (kN m71) between the peak ground reaction force and displacement of centre of mass values (McMahon & Cheng, 1990). Peak maximal ground reaction force (N): Calculated from force–time data. Peak centre of mass displacement (m): Calculated from the double integration of vertical acceleration with respect to time (Farley & Morgenroth, 1999)

Statistical analysis Reliability study. Means and standard deviations were calculated for each trial of the jump protocols. A repeated-measures analysis of variance (ANOVA) was used to test for possible systematic bias between trials. Where required, Tukey’s HSD test was used to highlight significant pair-wise differences. Mauchly’s test for sphericity was used to ensure non-violation of the respective assumptions and, where violated, a Greenhouse-Geiser adjustment was implemented. Single and average intraclass correlation coefficients (ICC) were determined to assess the trial-to-trial reliability of jump and hopping test data. Mean coefficients of variation (CV) were calculated using the anti-logged root mean square error, obtained from the two-way ANOVAs on log-transformed data (Hopkins, 2000b), with log-transformation reducing the effects of any non-uniformity of error (Hopkins, 2000c). Ninety-five percent confidence intervals (95% CI) were reported for the coefficient of variation. Statistical significance for all tests was set at P 5 0.05. Descriptive statistics, coefficients of variation, and confidence intervals were computed

through Microsoft Excel1 2007, while all repeatedmeasures ANOVAs and intraclass correlation coefficients were processed through SPSS1 (v. 12, Chicago, IL). Validity study. Two-tailed paired t-tests were performed to determine the existence of statistically significant differences in leg stiffness measures between the two test devices for both sub-maximal hopping frequencies and the maximal hopping test. Mean differences and standard deviations were reported for both sub-maximal and maximal conditions. Pearson’s correlation coefficients (r) were calculated to assess the relationship between the two test methods. Additionally, the typical error of estimate (TEE, expressed as a percentage) was used to determine the distribution of the residuals from the line of best fit (Hopkins, 2000a). Statistical significance was set at P 5 0.05 for all tests.

Results Reliability Means + standard deviations (s) and coefficients of variation for leg stiffness and ground contact times are displayed in Table I, while all other stretch– shortening cycle measures are presented in Table II. Significant within-participant differences were observed for the reactive strength index (decrease between trials 1 and 2: P ¼ 0.035), but differences in leg stiffness were non-significant (P 4 0.05). Significant decrements in squat jump height (between trials 1 and 2: P ¼ 0.048; between trials 1 and 3: P ¼ 0.013) and an increase in pre-stretch augmentation (between trials 1 and 3: P ¼ 0.001) were evident. Mean countermovement jump height scores were not significantly different (P 4 0.05) and remained sufficiently consistent during the study to be deemed a reliable measure. Table III displays intraclass correlation coefficients for leg stiffness during both sub-maximal and maximal hopping, reactive strength indexes, and squat and countermovement jump heights. Acceptable single intraclass correlation coefficients (r ¼ 0.7–0.9) were reported for all variables with the exception of countermovement jump height and pre-stretch augmentation, while high average intraclass correlation coefficients (r 0.9) were reported for all measures excluding pre-stretch augmentation. Despite the mean bias in squat jump height and pre-stretch augmentation (510%), moderate coefficients of variation (410%) were reported for all other variables. Mean contact times during sub-maximal hopping were deemed sufficiently reliable (2.0Hz:CV ¼ 7.84%; 2.5Hz:CV ¼ 7.48%),

Reliability and validity of stretch–shortening cycle parameters

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Table I. Means and coefficients of variation for sub-maximal and maximal leg stiffness, and contact times across all three test sessions. Mean results

Mean coefficients of variation (%)

Performance variable

Test session 1

Test session 2

Test session 3

Test sessions 1 and 3

Test sessions 2 and 3

Kleg 2.0 Hz (kN m71) Kleg 2.5 Hz (kN m71) Kleg 5max (kN m71) CT 2.0 Hz (s) CT 2.5 Hz (s) CT 5max (s)

18.80 + 6.07 26.05 + 5.55 17.19 + 5.91 0.219 + 0.03 0.185 + 0.02 0.199 + 0.03

18.29 + 5.41 26.62 + 5.95 17.12 + 5.91 0.219 + 0.02 0.183 + 0.02 0.210 + 0.05

19.52 + 5.59 25.55 + 6.05 18.90 + 7.26 0.213 + 0.02 0.189 + 0.02 0.201 + 0.03

13.93 10.17 21.37 7.84 7.48 36.41

11.0 1 9.48 19.14 6.34 7.47 32.05

(11.44–17.82) (8.39–12.93) (17.55–27.35) (6.44–10.03) (6.17– 9.51) (27.32–54.58)

(8.42–15.9) (7.29–13.55) (14.65–27.64) (4.85–9.16) (5.75–10.68) (24.05–48.05)

Note: Kleg 2.0 Hz ¼ leg stiffness during sub-maximal hopping at 2.0 Hz; Kleg 2.5 Hz ¼ leg stiffness during sub-maximal hopping at 2.5 Hz; Kleg 5max ¼ leg stiffness during maximal hopping; CT 2.0 Hz ¼ contact time during sub-maximal hopping at 2.0 Hz; CT 2.5 Hz ¼ contact time during sub-maximal hopping at 2.5 Hz; CT 5max ¼ contact time during maximal hopping.

Table II. Means and coefficients of variation for all other performance variables across all three test sessions.


Mean results

Mean coefficients of variation (%)

Performance variable

Test session 1

Test session 2

Test session 1

Squat jump height (cm) CM jump height (cm) Pre-stretch augmentation (%) Maximal hopping height (cm) Reactive strength index (mm/ms)

30.00 + 5.23 30.72 + 5.06 1.35 + 5.80 24.12 + 3.90 1.27 + 0.29

28.23 + 5.57* 29.23 + 6.01 1.90 + 11.40 23.56 + 4.49 1.17 + 0.34*

27.82 + 5.59* 29.46 + 5.17 5.64 + 9.29 22.61 + 3.38 1.21 + 0.31*

Test session 2 8.64 12.88 9.43 14.19 13.98

(6.99–11.32) (10.29–17.22) (7.54–12.6) (10.86–20.49) (11.31–18.32)

Test session 1 7.47 14.48 10.81 15.63 14.24

(5.61–11.2) (10.7–22.41) (7.99–16.73) (10.92–27.43) (10.69–21.35)

Note: CM ¼ countermovement. *Significant difference (P 5 0.05) compared with test session 1.

Table III. Intraclass correlation coefficients (95% CI) for all performance variables across all three test sessions. Intraclass correlation coefficient Variable Kleg 2.0 Hz (kN m71) Kleg 2.5 Hz (kN m71) Kleg 5max (kN m71) Squat jump height (cm) Countermovement jump height (cm) Pre-stretch augmentation (%) Maximal hopping height (cm) Reactive strength index (mm/ms)

Single measures

Average measures

0.83 (0.68–0.92)

0.94 (0.87–0.97)

0.83 (0.68–0.92)

0.93 (0.86–0.97)

0.74 (0.54–0.87)

0.89 (0.78–0.95)

0.82 (0.66–0.92)

0.93 (0.85–0.97)

0.62 (0.34–0.83)

0.83 (0.60–0.94)

0.03 (70.22–0.38)

0.07 (71.14–0.65)

0.75 (0.52–0.89)

0.90 (0.77–0.96)

0.75 (0.54–0.89)

0.90 (0.78–0.96)

Note: Kleg 2.0 Hz ¼ leg stiffness during sub-maximal hopping at 2.0 Hz; Kleg 2.5 Hz ¼ leg stiffness during sub-maximal hopping at 2.5 Hz; Kleg 5max ¼ leg stiffness during maximal hopping.

while contact times during the maximal hopping test proved unreliable (CV ¼ 36.41%). No noticeable improvement in 95% confidence intervals was evident in the reliability scores when discounting the initial trial.

Validity Results for both mean leg stiffness and ground contact times during both sub-maximal and maximal hopping conditions are presented in Tables IV and V respectively for both force plate and contact mat data. The contact mat consistently overestimated leg stiffness during both maximal and sub-maximal hopping. The mean difference between the two measuring devices was not significant during sub-maximal hopping at 2.0 Hz, whereas significance was observed at a frequency of 2.5 Hz (P ¼ 0.001) and during maximal hopping (P ¼ 0.01). Strong correlations (r 4 0.9; Hopkins, 2000c) and acceptable typical errors of the estimate (2.0 Hz: TEE ¼ 6.5%; 2.5 Hz: TEE ¼ 7.6%) were observed between the contact mat and force plate during both sub-maximal hopping frequencies (Figure 1). Leg stiffness measures obtained during maximal hopping produced a weaker correlation (r ¼ 0.59) and revealed a high typical error of the estimate (41.9%), suggesting that the maximal hopping test was not a valid means to quantify leg stiffness. Discussion The results of this study demonstrate that a range of stretch–shortening cycle measures obtained from

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R. S. Lloyd et al. Table IV. Validity statistics for vertical leg stiffness during sub-maximal and maximal hopping. Mean sub-maximal leg stiffness (kN m71 + s)

Variable 2.0 Hz 2.5 Hz 5max

Practical

Criterion

23.8 + 6.3 23.7 + 4.9 29.8 + 5.7 27.9 + 5.1 22.3 + 5.2 18.8 + 7.6

Mean difference (kN m71 + s) Pearson correlation (95% CI) Typical error of estimate (95% CI) 0.09 + 2.25 1.87 + 2.01* 3.49 + 5.14*

0.95 (0.88–0.98) 0.92 (0.80–0.97) 0.59 (0.21–0.82)

6.5% (4.8–9.7) 7.5% (5.6–11.3) 41.9% (30.3–67.8)

Note: 5max ¼ maximal hopping. *Significant difference at P 5 0.05.

Table V. Validity statistics for ground contact times during sub-maximal and maximal hopping.


Mean ground contact time (seconds + s) Variable

Practical

Criterion

Mean difference (kN m71 + s)

Pearson correlation (95% CI)

Typical error of estimate (95% CI)

2.0 Hz 2.5 Hz 5max

0.23 + 0.03 0.19 + 0.02 0.23 + 0.05

0.20 + 0.03 0.22 + 0.03 0.23 + 0.03

0.03 + 0.00* 0.04 + 0.01* 0.00 + 0.02

0.98 (0.95–0.99) 0.94 (0.85–0.98) 0.86 (0.68–0.94)

2.7% (2.0–4.0) 3.8% (2.8–5.6) 11.2% (8.4–17.0)

Note: 5max ¼ maximal hopping. *Significant difference at P 5 0.05.

Figure 1. Relationship for combined stiffness measures (kN m71) at both sub-maximal hopping frequencies: contact mat vs. force plate.

simple vertical jump tests exhibit modest reliability (CV 4 10%). Squat jump height and pre-stretch augmentation were the only two variables possessing coefficients of variation below the 10% cut-off used to determine test reliability (Cormack et al., 2008; Cronin, Hing, & McNair, 2004; Hunter, Marshall, & McNair, 2004). Owing to the lack of previously published data on the measurement error associated

with leg stiffness and the reactive strength index, the moderate reliability values reported for both (CV 5 10%), while suggesting the existence of some measurement variability, are deemed important findings. Acceptable reliability was evident for the rebound jump heights during maximal hopping, suggesting that reduced reliability in the reactive strength index


Reliability and validity of stretch–shortening cycle parameters values were attributable to variations in ground contact times. This is reflected by a coefficient of variation of 36.41% across the three trials for contact times during the maximal hopping test. This suggests that the participants displayed an inability to control loading forces during interaction with the ground. Mean leg stiffness during sub-maximal hopping frequencies and maximal hopping revealed coefficients of variation above the 10% cut-off. Maximal hopping proved to be the least reliable condition for measures of leg stiffness (21.37%), while the faster of the two sub-maximal hopping frequencies proved to possess the highest reliability (10.17%). This suggests that the reliability of leg stiffness measures improved concomitantly with an increase in hopping frequency. It could be deduced that at the highest hopping frequency, the participants were acting in more of a ‘‘spring-mass’’ manner, thus reducing any possible impact peak in the ground reaction force and producing a force–time curve more representative of the sine-wave form that the calculation-based method requires (Dalleau et al., 2004). This suggestion is supported by Morin and colleagues (Morin, Dalleau, Kyro¨laïnen, Jeannin, & Belli, 2005), who stated that the accuracy of their basic sine-wave postulate for leg stiffness increased at the highest running velocities during treadmill running. While moderate reliability values were reported for the reactive strength index and leg stiffness during sub-maximal hopping, these coefficients of variation are lower than those previously reported for alternative vertical jump protocols (CV 4 15%), which were deemed to be excellent measures for quantifying vertical jump height (Aragoń-Vargas, 2000). Additionally, and somewhat conversely, the intraclass correlation coefficients suggested that all stretch– shortening cycle measures, with the exception of prestretch augmentation, had moderate single intraclass reliability (ICC 0.60–0.80), and high to very high average intraclass reliability (ICC 0.80–0.94). Both single and average intraclass reliability for the reactive strength index were lower in the current study than the correlations reported by Flanagan et al. (2008) (ICC ¼ 0.97–0.99) for the same measure. Owing to the difference in mean age between the sample of the current study and that of Flanagan et al. (2008), we suggest that age-related differences in motor control strategies may have existed during the vertical jump or rebounding protocols. This explanation has previously been suggested by Harrison and Gaffney (2001) to explain differences in pre-stretch augmentation between 6- and 23-year-old males. The moderate single intraclass results highlight the need for multiple trials of the test protocols to examine the stretch–shortening cycle ability of paediatric populations. Also, because the tests are relatively short, multiple trials should be performed to reduce the

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noise of the tests, thus improving reliability (Pyne, 2004) without impinging on the time required to collect the data. Strong correlations (r 4 0.9), low typical errors of the estimate (TEE 5 8%), and low mean bias values for sub-maximal hopping at both 2.0 and 2.5 Hz indicated a strong relationship between the vertical leg stiffness measures obtained from both the force plate and the contact mat. The correlations were in agreement with those reported previously for adult populations during sub-maximal hopping on a force plate and contact mat (r ¼ 0.94; Dalleau et al., 2004). These results suggest the contact mat is a valid tool to measure leg stiffness in populations of youths during sub-maximal hopping conditions. The values obtained for mean difference suggest that at both hopping frequencies, vertical leg stiffness was slightly overestimated by the contact mat. These overestimations were non-significant at a hopping frequency of 2.0 Hz but significant at 2.5 Hz (P 5 0.05). However, it is worth noting that typical errors of the estimate during both hopping conditions remained low, a measure not previously reported in the literature examining leg stiffness using contact mat data (Dalleau et al., 2004; Morin et al., 2005). It should also be noted that the measures of vertical leg stiffness by Morin et al. (2005) were obtained from uni- as opposed to bi-lateral hopping. Since the typical error of the estimate was not reported in previous studies (Dalleau et al., 2004; Morin et al., 2005), comparisons between results are difficult, and caution must be adhered to when directly comparing stiffness results between studies. Maximal hopping produced a greater mean bias value (720.8 + 12.8%) and much weaker correlation coefficient (r ¼ 0.60) than sub-maximal hopping. These values were in contrast to the correlation previously reported for maximal hopping (r ¼ 0.98; Dalleau et al., 2004). It is suggested that this is due to maximal hopping producing movement patterns less indicative of the spring-mass model (i.e. greater impact peaks during ground contact time), compared to those attributed to sub-maximal hopping. The mean difference between the two measures revealed a significant overestimation (P 5 0.05) and it is therefore recommended that the contact mat is unsuitable for measuring leg stiffness during maximal hopping for paediatric populations using the present procedures. Arguably, the participants may have been less able to accommodate for the additional loading effect placed on them during maximal hopping, and consequently exhibited a decreased resemblance to the movement patterns associated with the typical spring-mass model (McMahon & Cheng, 1990). Both maximal and sub-maximal hopping are whole-body, multi-joint activities that require high


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levels of motor coordination. It is reasonable to suggest that an immature neurophysiological status may lead to greater variability in the functioning of the pre-motor cortex to accurately maintain postural control and accommodate for rapid corrections during the movement by innervating the appropriate motor units. Consequently, discrepancies would exist in ground contact times and jump heights, both of which are integral components of the reactive strength index. Since the participants were of an adolescent age, it could be argued that the development of motor coordination was diverse due to variations in biological maturity (Jones, Hitchen, & Stratton, 2000). Consequently, their inability to consistently coordinate movement during the ground contact phase led to greater variation in deformation of the ‘‘spring’’, and therefore greater variation in leg stiffness measures. This notion is reinforced by Laffaye and colleagues (Laffaye, Bardy, & Durey, 2005), who indicated the existence of variable motor control in participants of differing jumping expertise as a possible cause for subsequent variations in vertical and horizontal jumping profiles. Further analysis suggested participants of different sporting profiles and different jump training experience possessed incompatible motor control signatures. The findings of Laffaye et al. (2005) support the existence of activity-specific motor control adaptations, reflecting the potential effects of trainability but also growth and maturational factors on stretch–shortening cycle function, thus highlighting the need for future research. Owing to the mean age of both the reliability (13.5 + 0.5 years) and validity (16.5 + 0.5 years) cohorts, consideration must be given to age-related and maturity-related motor control discrepancies when applying the results of the current study to groups of different age ranges. Previous research has suggested that variability in the performance of stretch–shortening cycle tasks is greatest in younger participants and recedes as participants move towards adulthood (Gerodimos et al., 2008; Harrison & Gaffney, 2001). In conclusion, the current study has added to the sparse data base of reliability and validity issues pertaining to field-based measures of stretch–shortening cycle function, including squat jump height, pre-stretch augmentation, leg stiffness, and reactive strength index. Moderate reliability was reported for the use of the contact mat in measuring both leg stiffness and the reactive strength index. Owing to the moderate single intraclass correlation coefficient, the simplicity of the tests, and their relatively low physical demands, it is recommended that future research should obtain mean values for both measures from a greater number of trials within each test session to minimize the effects of

within-participant variation. The contact mat proved to be a valid measuring tool for leg stiffness during sub-maximal hopping but not maximal hopping. This supports its use within the field for quantifying leg stiffness measures in adolescent children. References Aragoń-Vargas, L. F. (2000). Evaluation of four vertical jump tests: Methodology, reliability, validity, and accuracy. Measurement in Physical Education and Exercise Science, 4, 215–228. Bishop, M., Fiolkowski, P., Conrad, B., Brunt, D., & Horodyski, M. (2006). Athletic footwear, leg stiffness, and running kinematics. Journal of Athletic Training, 41, 387–392. Bobbert, M. F., Gerritsen, K. G. M., Litjens, M. C. A., & Van Soest, A. J. (1996). Why is countermovement jump height greater than squat jump height? Medicine and Science in Sports and Exercise, 28, 1402–1412. Bret, C., Rahmani, A., Dufour, A.-B., Messonnier, L., & Lacour, J.-R. (2002). Leg strength and stiffness as ability factors in 100 m sprint running. Journal of Sports Medicine and Physical Fitness, 274, 274–281. Chelly, S. M., & Denis, C. (2001). Leg power and hopping stiffness: Relationship with sprint running performance. Medicine and Science in Sports and Exercise, 33, 326–333. Cormack, S. J., Newton, R. U., McGuigan, M. R., & Doyle, T. L. A. (2008). Reliability of measures obtained during single and repeated countermovement jumps. International Journal of Sports Physiology and Performance, 3, 131–144. Cronin, J. B., Hing, R. D., & McNair, P. J. (2004). Reliability and validity of a linear position transducer for measuring jump performance. Journal of Strength and Conditioning Research, 18, 590–593. Dalleau, G., Belli, A., Viale, F., Lacour, J.-R., & Bourdin, M. (2004). A simple method for field measurements of leg stiffness in hopping. International Journal of Sports Medicine, 25, 170–176. Farley, C. T., Blickhan, R., Saito, J., & Taylor, C. R. (1991). Hopping frequency in humans: A test of how springs set stride frequency in bouncing gaits. Journal of Applied Physiology, 7, 2127–2132. Farley, C. T., & Morgenroth, D. C. (1999). Leg stiffness primarily depends on ankle stiffness during human hopping. Journal of Biomechanics, 32, 267–273. Flanagan, E. P., & Comyns, T. M. (2008). The use of contact time and the reactive strength index to optimize fast stretch–shortening cycle training. Strength and Conditioning Journal, 30, 32–38. Flanagan, E. P., Ebben, W. P., & Jensen, R. L. (2008). Reliability of the reactive strength index and time to stabilization during depth jumps. Journal of Strength and Conditioning Research, 22, 1677–1682. Gerodimos, V., Zafeiridis, A., Perkos, S., Dipla, K., Manou, V., & Kellis, S. (2008). The contribution of stretch–shortening cycle and arm-swing to vertical jumping performance in children, adolescents, and adult basketball players. Pediatric Exercise Science, 20, 379–389. Granata, K. P., Padua, D. A., & Wilson, S. E. (2002). Gender differences in active musculoskeletal stiffness. Part II. Quantification of leg stiffness during functional hopping tasks. Journal of Electromyography and Kinesiology, 12, 127–135. Harrison, A. J., & Gaffney, S. (2001). Motor development and gender effects on stretch–shortening cycle performance. Journal of Science and Medicine in Sport, 4, 406–415. Harrison, A. J., Keane, S. P., & Coglan, J. (2004). Force–velocity relationship and stretch–shortening cycle function in sprint and endurance athletes. Journal of Strength and Conditioning Research, 18, 473–479.


Reliability and validity of stretch–shortening cycle parameters Hobara, H., Kimura, K., Omuro, K., Gomi, K., Muraoka, T., Iso, S. et al. (2008). Determinants of difference in leg stiffness between endurance- and power-trained athletes. Journal of Biomechanics, 41, 506–514. Hopkins, W. G. (2000a). Measures of validity. In A new view of statistics. Internet Society for Sport Science Retrieved from http://www.sportsci.org/resource/stats/index.html. Hopkins, W. G. (2000b). Reliability from consecutive pairs of trials (Excel spreadsheet). In A new view of statistics. Internet Society for Sport Science Retrieved from http://www.sportsci.org/resource/stats/index.html. Hopkins, W. G. (2000c). Measures of reliability in sports medicine and science. Sports Medicine, 30, 1–15. Hunter, J. P., Marshall, R. N., & McNair, P. (2004). Reliability of biomechanical variables of sprint running. Medicine and Science in Sports and Exercise, 36, 850–861. Isaacs, L. D. (1998). Comparison of the Vertec and Just Jump systems for measuring height of vertical jump by young children. Perceptual and Motor Skills, 86, 659–663. Jones, M. A., Hitchen, P. J., & Stratton, G. (2000). The importance of considering biological maturity when assessing physical fitness measures in girls and boys aged 10 to 16 years. Annals of Human Biology, 27, 57–65. Kerdok, A. E., Biewener, A. A., McMahon, T. A., Weyand, P. G., & Herr, H. M. (2002). Energetics and mechanics of human running on surfaces of different stiffnesses. Journal of Applied Physiology, 92, 469–478. Komi, P. V. (2000). Stretch–shortening cycle: A powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33, 1197–1206. Komi, P. V., & Gollhoffer, A. (1997). Stretch reflex can have an important role in force enhancement during SSC-exercise. Journal of Applied Biomechanics, 33, 1197–1206. Kram, R. (2000). Muscular force or work: What determines the metabolic energy cost of running? Exercise and Sport Science Reviews, 28, 138–143. Kubo, K., Morimoto, M., Komuro, T., Tsunoda, N., Kanehisa, H., & Fukunaga, T. (2007). Influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performance using single joint. European Journal of Applied Physiology, 99, 235–243.

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Laffaye, G., Bardy, B. G., & Durey, A. (2005). Leg stiffness and expertise in men jumping. Medicine and Science in Sports and Exercise, 37, 536–543. Lees, A., Vanrenterghem, J., & Clercq, D. D. (2004). Understanding how an arm swing enhances performance in the vertical jump. Journal of Biomechanics, 37, 1929–1940. Logan, P., Fornasiero, D., Abernerthy, P., & Lynch, K. (2000). Protocols for the assessment of isoinertial strength. In C. J. Gore (Ed.), Physiological tests for elite athletes (pp. 200–221). Champaign, IL: Human Kinetics. Markovic, G., Dizdar, D., Jukic, I., & Cardinale, M. (2004). Reliability and factorial validity of squat and countermovement jump tests. Journal of Strength and Conditioning Research, 18, 551–555. McClymont, D. (2005). Use of the reactive strength index (RSI) as an indicator of plyometric training conditions. In T. Reilly, J. Cabri, & D. Arau´jo (Eds.), Science and football V: Proceedings of the 5th World Congress on Science and Football, Part VI – Football training (pp. 408–416). London: Routledge. McClymont, D., & Hore, A. (2004). Use of the reactive strength index (RSI) as an indicator of plyometric training conditions. Journal of Sports Sciences, 22, 495–496. McMahon, T. A., & Cheng, G. C. (1990). The mechanics of running: How does stiffness couple with speed? Journal of Biomechanics, 23, 65–78. Morin, J. B., Dalleau, G., Kyro¨laïnen, H., Jeannin, T., & Belli, A. (2005). A simple method for measuring stiffness during running. Journal of Applied Biomechanics, 21, 167–180. Nicol, C., Avela, J., & Komi, P. V. (2006). The stretch–shortening cycle: A model to study naturally occurring neuromuscular fatigue. Sports Medicine, 36, 977–999. Padua, D. A., Arnold, B. L., Perrin, D. H., Gansneders, B. M., Carcia, C. R., & Granata, K. P. (2006). Fatigue, vertical leg stiffness, and stiffness control strategies in males and females. Journal of Athletic Training, 41, 294–304. Pyne, D. B. (2004). Commentary on how to interpret changes in an athletic performance test. Retrieved August 8, 2008 from http:// www.sportsci.org/2004/index.html).