Curling is a winter sport that is played on ice by two teams of four players who aim to deliver stones into a target area of four concentric circles by sliding them individually over an approximately 28-m long sheet of ice. Although two teammates are able to sweep the ice to guide the delivered stone towards the target, the performance of the shooter delivering the stone primarily determines the final placement of the stone and therefore team success (Kivi and Auld, 2012). The delivery involves the shooter pushing off the hack (i.e., the starting blocks), then gliding on the ice in a deep lunge position over 10 m and finally releasing the stone smoothly (Berry et al., 2013; Kivi and Auld, 2012).
A precise stone delivery involves a low, extended lunge position close to the ice in order to better visualize the target. Thus, to efficiently execute stone delivery, good balance, core strength, stability and flexibility have been considered to be important qualities in curling (Behm, 2007; Kivi and Auld, 2012). These physical attributes provide curlers with a stable delivery position, which optimize both speed and control of the stone (Shank and Lajoie, 2013). An additional unique challenge in curling is the need to perform the delivery and sweeping actions on slippery ice, so well-developed stability and balance are of further importance to exert substantial sweeping pressures and prevent the players from falling (Behm, 2007; Weinberg and Gould, 2014).
These skill- and task-dependent challenges are affected by both static and dynamic balance. Static balance, defined as the ability to sustain a stable static position, is more important in the delivery position, whereas the ability to transition from a static to dynamic or dynamic to static position (i.e., dynamic balance) is more important during sweeping (Clark et al., 2012; DiStefano et al., 2009). In both cases, to establish and maintain good body balance it is essential for a curler to keep the vertical projection of the body's center of mass inside the perimeter of the base of support (DiStefano et al., 2009; Heyward, 2010). In order to maintain a stable delivery position and good postural control over the body, a player must also possess optimal joint range of motion, flexibility and core strength as important determinants of balance (Bressel et al., 2007; Palmieri et al., 2002; Thorpe and Ebersole, 2008). While recognized as an essential quality for successful curling performance (Behm, 2007; Shank and Lajoie, 2013), no testing protocols appear to have been developed to test curling-specific balance in the delivery position.
To be established in practice, any new sport-specific test should firstly be investigated for its validity, reliability and usefulness (Boddington et al., 2019; Pojskic et al., 2018a; Pojskic et al., 2018b; Sekulic et al., 2017). The validity concept refers to what extent the test measures what it is intended to measure. In particular, face validity may refer to the extent to which a test logically measures an ability that it is intended to measure (Gratton and Jones, 2010). For instance, the current study aimed to establish face validity by simulating the curling delivery position. On the other hand, content validity might be evidenced through exact statistical analysis by identifying the relationship between balance performances in the delivery and standing single-leg positions. This is based on the concept that balance is task-specific rather than a general ability (Bachman, 1961; Tsigilis et al., 2001), so the lack of relationship would indicate that the tests measure idependent facets of balance. Construct validity, as another important type of validity, is often evidenced by comparison of the measurement scores between different expertise and playing levels (e.g., elite vs. sub-elite), often referred to as discriminative validity (Butler et al., 2012; Pojskic et al., 2018a, Sekulic et al., 2017; Thorpe and Ebersole, 2008).
Measurement of reliability, by contrast, can be evidenced by multiple test-retest assessments of balance performance (Gratton and Jones, 2010; Hildebrandt et al., 2015; Hopkins, 2004). Specifically, absolute reliability may be examined by within-subject variation (i.e., the typical measurement error), whereas relative reliability is evidenced by consistency of the position of individuals in the group relative to others after test-retest trials (Weir, 2005). The greater the reliability, the more accurate the measurement may be and the easier it is to detect any changes in balance performance (Hopkins, 2004). The usefulness of a test reflects the ease of identifying a change in performance (Hopkins, 2004) and is established by comparing the typical error (TE) of the measurement and the smallest worthwhile change (SWC). Better measurement usefulness is evidenced by a higher SWC than TE (Hopkins, 2004).
The primary aim of the present study was to develop a new curling-specific balance test and to investigate its reliability, validity and usefulness characteristics. The secondary aim was to examine the differences between elite and sub-elite curlers for core strength and flexibility, which have previously been identified as important qualities in curling and determinants of balance (Behm, 2007; Bressel et al., 2007; Overmoyer and Reiser, 2015; Palmieri et al., 2002; Thorpe and Ebersole, 2008). It was hypothesized that the new test would be a valid, reliable and useful testing tool in assessing sport-specific balance in curlers (Hildebrandt et al., 2015). Moreover, it was expected that elite curlers would display superior standing single-leg balance, flexibility and core strength than their less-skilled counterparts (Behm, 2007; Butler et al., 2012; Paillard et al., 2011).
Both within- and between-subject experimental designs were used to determine the reliability, validity and usefulness of a newly-constructed curling-specific balance test (CSBT). The study was performed in a laboratory during the curling season and consisted of several phases. In the first phase, to establish validity, the CSBT was hypothetically created in consultation with several exercise scientists and national- and international-level curling coaches and players. All experts agreed on two important curling-specific tasks; the delivery and sweeping. They identified whole-body stability and balance over the front foot in the delivery position (i.e., during a deep lunge) as critical qualities for delivery performance. In the second phase the CSBT was designed and pilot testing was conducted with four curlers. In the third phase, 20 curlers (10 elite and 10 sub-elite) were recruited and familiarized with all physical tests before experimental measurements of curling-specific and single-leg balance were made, as well as tests of hamstrings and lower-back flexibility and core strength. In the fourth and final phase a range of analyses were made: reliability, through the test-retest measurements of the CSBT; usefulness, by comparing the SWC and TE of measurement; construct validity, through comparison of the elite and sub-elite curling groups in CSBT performance; content validity, through identification of the relationship between the CSBT and single-leg balance test (SLBT). Sample size was estimated a priori using SLBT score means and standard deviations from previous studies (Hildebrandt et al., 2015; Wojtyczek et al., 2014). Using G-Power software (version 126.96.36.199; Heinrich Heine University Dusseldorf, Dusseldorf, Germany), it was estimated that nine subjects (df = 8) would provide an appropriate sample size for paired-samples differences (P [less than or equal to] 0.05, power = 0.90).
Twenty curling players (13 females aged 19.0 [+ or -] 3.1 years; 7 males aged 19.6 [+ or -] 2.3 years) from five Swedish super-league curling clubs voluntarily participated in the study (Table 1). All participants had at least five years of competitive playing experience at a national level and were required to be actively competing at the time of testing to be included in the study. Participants were divided into two groups according to their playing level: an elite group, including players who had played at national senior level and in the Swedish super-league for at least two years, and a sub-elite group, including athletes who had played for at least five years but had not competed at national senior level (Table 1). The elite group included two leads, two second players, three third players and three fourth players. Seven out of the ten players were sweepers and three were skips. The sub-elite group included two leads, two second players, three third players and three fourth players. Six out of the ten players were sweepers and four were skips. All participants were healthy and free of any reported injuries, neuromuscular diseases, or visual or vestibular impairments for at least six months before testing. All participants delivered curling stones with their right-hand. Before commencing the study, participants were informed about the study design, protocols, benefits, potential risks and right to withdraw without explanation, then provided signed informed consent. Informed consent was also provided by a parent or guardian for participants under 18 years of age (n = 6). The study was performed in accordance with the Helsinki declaration and was approved by the local institutional ethical committee of Mid Sweden University (Number: MIUN 2017/211).
Participants attended one familiarization session and one testing session separated by 24 hours (Figure 1). They were asked to avoid high-intensity activity for at least 48 hours before testing. Participants were familiarized to the SLBT and the newly-designed CSBT, receiving detailed instructions on how to perform both tests. Special attention was paid to proper foot placement on the balance-testing platform, as well as proper body posture...