Elsevier

Gait & Posture

Volume 46, May 2016, Pages 173-178
Gait & Posture

Reliability of knee biomechanics during a vertical drop jump in elite female athletes

https://doi.org/10.1016/j.gaitpost.2016.03.003Get rights and content

Highlights

  • Reliability was estimated from a large sample of elite female athletes (n = 41).

  • In general, the vertical drop jump demonstrated sufficient reliability to be used in both single-session and multiple-session studies.

  • The knee flexion angle can vary considerably within-session in some subjects.

  • A new approach for calculating medial knee displacement was found to be reliable.

Abstract

The purpose of the study was to assess the within-session and between-session reliability of knee kinematics and kinetics in a vertical drop jump task among elite female handball and football athletes. Specifically, we aimed to quantify the within-session waveform consistency and between-session consistency of the subject ranking for a variety of knee kinematics and kinetics.

Forty-one elite female handball and football (soccer) athletes were tested in two sessions. The reliability of three-dimensional knee biomechanical measurements was quantified by the intra-class correlation, Spearman's rank correlation, and typical error. All the selected discrete variables achieved excellent within-session reliability (ICC > 0.87). The typical error of valgus angles, internal rotation angles, and internal rotation moment was constant throughout the whole stance phase. For between-session reliability, the selected discrete variables achieved good to excellent reliability (ICC > 0.69), except peak internal rotation moment (ICC = 0.40). All between-session rank correlation coefficients ranged from 0.56 to 0.90. Most of the discrete variables achieved good to excellent reliability in both within-session and between-session analysis. Moreover, moderate to strong between-session consistency of subject rankings was found, implying that the measurements assessed during the vertical drop jump demonstrate sufficient reliability to be used in both single-session and multiple-session studies.

Introduction

Vertical drop jump tasks have been widely used in anterior cruciate ligament (ACL) injury-related research in the last decade. Three-dimensional knee kinematics and kinetics, quantified using marker-based motion analysis systems, have been used to identify potential risk factors for ACL injuries [1], [2]. Furthermore, knee kinematics and kinetics in vertical drop jumps are utilized for ACL injury risk assessment [2], [3] and evaluation of training interventions [4], [5].

Previous research has investigated both within-session and between-session reliability of various knee biomechanical variables in vertical drop jump tasks [6], [7]. Ford et al. [6] utilized the intra-class correlation coefficient (ICC) and typical error of various discrete biomechanical variables to quantify the between-session reliability. The majority of the knee kinematic and kinetic variables were shown to have fair to excellent reliability within- (ICC from 0.67 to 0.99) and between-sessions (ICC from 0.59 to 0.92) in young female high school athletes [6]. Malfait et al. [7] assessed the within-session reliability of the knee kinematics variables, and showed that the variability ranged from 1.1° to 3.8°.

The ICC is commonly used to describe reliability, however, there is considerable confusion concerning both the calculation and interpretation of the ICC [8]. The ICC will give high reliability when the subject range is large, even if trial-to-trial variability is large [8], [9]. Spearman's rank correlation will be unaffected by the range in the variable as it transforms the measurements to the ranking domain for the correlation calculation thus is less sensitive to between-subject variability. Spearman's rank correlation coefficient can theoretically provide additional information on reliability, in particular on subject rankings.

The coefficient of multiple correlations (CMC) has been used to assess the waveform reliability [6]. However, CMC coefficient measures are underestimating the reliability for small motions [10] and are generally insensitive to systematic error [11]. An alternative to the CMC, the waveform reliability can be quantified as the typical error of every time point. With this temporal presentation, the movement variability can be further described in a specific region such as initial contact or mid-stance. Using this approach, we could detect variation between sessions and attribute them to a specific phase of the movement. For example, a previous study found that most of the variability of the kinetics measurements were around impact (0–20% of contact phase) [7]. The current study would use this approach to present waveform reliability.

The low number of participants in the previous reliability studies is a major concern [12]. Methodology studies of reliability in sports medicine suggest that such studies should contain a minimum of 40 subjects [13]. The reliability of vertical drop jump tasks have, up until now, only been investigated in very limited populations, i.e. one study on 8 recreational athletes [7] and one on 11 high school athletes [6]. Likewise, the reliability of medial knee displacement was only reported from a study with five subjects [14].

Furthermore, previous studies have not investigated the reliability of vertical drop jump task in homogenous elite populations. Elite female handball and football cohorts are of particular interest, knowing that the risk of sustaining ACL injuries is higher, compared with other groups of athletes [15], [16].

The aim of the present study was to assess the within-session and between-session reliability of knee kinematics and kinetics in a vertical drop jump task among elite female handball and football athletes. Specifically, we aimed to quantify the within-session waveform error of measurements and between-session consistency of the subject measurements and rankings.

Section snippets

Subjects

Forty-one elite female handball and football (soccer) athletes (mean ± SD: 22 ± 4 years old, 168 ± 5 cm, 66 ± 8 kg) performed vertical drop jumps in our biomechanics laboratory. The Regional Ethics Committee approved the study and all subjects provided signed informed consent forms.

Sample size calculation

Sample size calculation was performed using the formula of Shoukri et al. [17]. The formula is specifically designed for reliability studies by setting the limit of the confidence interval width of the reliability coefficient.

Results

All the selected discrete variables achieved excellent within-session reliability with all ICC values greater than 0.87 (Table 1). The typical errors were generally small. The errors related to the valgus angles were <1.0°.

The within-session typical error of the knee valgus, internal rotation angle and internal rotation moment was relatively constant throughout the whole stance phase (Fig. 1, Fig. 2). We observed an increase in typical error during the mid-stance for the knee flexion angle,

Discussion

In the current study, we assessed the reliability of knee kinematics and kinetics in a vertical drop jump task in elite female handball and football athletes. Most of the discrete knee biomechanical variables achieved good to excellent reliability in both within-session and between-session analyses. Moreover, we found a strong between-session consistency of subject measurements and rankings, implying that the estimates could reliably reproduce the testing results in both single-session and

Conclusion

Our results suggest that biomechanical variables of the vertical drop jump task are reliable and can be used for research purposes. Additionally, the strong and positive between-session consistency of subject measurements and rankings imply that the ranking of athletes based on knee biomechanics in the vertical drop jump task can be reproduced reliably, which is critical for injury risk screening purposes. The vertical drop jump task can reliably measure knee valgus angles and internal rotation

Author's contribution

All authors have made substantial contributions to all of the following: (1) the conception and design of the study, or acquisition of data, or analysis and interpretation of data, (2) drafting the article or revising it critically for important intellectual content, (3) final approval of the version to be submitted.

Conflict of interest

The authors declare that there are no conflicts of interest.

Acknowledgements

The Oslo Sports Trauma Research Center has been established at the Norwegian School of Sport Sciences through generous grants from the Royal Norwegian Ministry of Culture, the South-Eastern Norway Regional Health Authority, the International Olympic Committee, the Norwegian Olympic Committee & Confederation of Sport and Norsk Tipping AS. The authors acknowledge Mr. Oliver Faul for this assistance in data collection and technical support.

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