Lack of hamstring muscles extensibility conditions a decrease of pelvic mobility (Kendall et al., 2005). This invariably leads to biomechanical changes in the pressure distribution of the spine and consequent spinal disorders (da Silva Dias and Gomez-Conesa, 2008). Therefore, poor hamstring extensibility has been associated with thoracic hyperkyphosis (Fisk et al., 1984), spondylolysis (Standaert and Herring, 2000), disc herniation (Harvey and Tanner, 1991), changes in lumbopelvic rhythm (Esola et al., 1996; Lopez-Minarro and Alacid, 2009) and low back pain (Biering-Sorensen, 1984; Mierau et al., 1989). Additionally, individuals with shortened hamstring muscles present gait limitations, increased risk of falls, and susceptibility to musculoskeletal injuries (Erkula et al., 2002; Jones et al., 1998).
Nowadays different kinds of tests are used to assess hamstring extensibility. Flexibility is typically characterized by the maximum range of motion in a joint or series of joints (McHugh et al., 1998). Thus, angular tests that specifically measure hip flexion with the knee extended (straight leg raise test), or the range of knee extension with the hip flexed to 90 degrees (knee extension or popliteal angle test), have been widely considered the criterion measures of hamstring extensibility (e.g., Ayala et al., 2011; Hartman and Looney, 2003; Lopez-Minarro and Rodriguez-Garcia, 2010c). Nevertheless, due to the necessity of sophisticated instruments, qualified technicians, and time constraints, the use of these angular tests seem to be limited in several settings such as in a school context or large scale studies (Castro-Pinero et al., 2009b).
Unlike the angular tests, lineal tests have a simple procedure, are easy to administer, require-minimal skills training for their application, and the equipment necessary to perform them is very affordable (Castro-Pinero et al., 2009b; Lopez Minarro et al., 2008c). Sit-and-reach (SR) tests in which a fingertips-to-tangent feet distance is measured are probably the most widely used lineal measures of flexibility (Holt et al., 1999; Castro-Pinero et al., 2009a). However, as the SR is a test which involves the movement of the whole body, it has been suggested that the position of the fingertips does not give valid information about hamstring extensibility (Hoeger et al., 1990). The main factors that seem to affect the validity of SR tests to estimate hamstring extensibility are the differences in length proportion between the upper and lower limbs (Hoeger et al., 1990), the position of the head (Smith and Miller, 1985) and the position of the ankles (Kawano et al., 2010; Liemohn et al., 1997). In addition, recent studies have also found that the levels of hamstring extensibility influence the criterion-related validity of SR tests (Lopez-Minarro et al., 2011; Lopez-Minarro and Rodriguez-Garcia, 2010c).
The choice of a flexibility test must be based on its functionality and validity (Lopez-Minarro, 2010). Although the angular tests have the advantage of being the criterion measure to assess flexibility, due to several practical reasons they have the disadvantage of having a limited use in several settings (Castro-Pinero et al., 2009b). In these settings, as the SR tests have the advantage of allowing for an evaluation in a short amount of time with minimal skills and instruments, potentially they could be a useful alternative to estimate flexibility. Nevertheless, as in the application of any fitness field test, the SR tests' results are a simple estimation and, therefore, the evaluators must be aware of validity coefficients in order to interpret the scores of these tests correctly. Unfortunately, the studies examining criterion-related validity of SR tests for estimating hamstring and lumbar extensibility have shown inconclusive results (Baltaci et al., 2003; Hui and Yuen, 2000; Hui et al., 1999; Jones et al., 1998).
Each primary study that is published about criterion-related validity of the SR tests only constitutes as a single piece of a constantly growing body of evidence (Cooper et al., 2009). For example, in some studies the correlation coefficient is statistically significant, while in others a statistically significant association is not found. In some cases the strength of the association is quite high, while low in others. To make sense of the often conflicting results found in the scientific literature, researchers have to conduct meta-analyses (Cooper et al., 2009; Hunter and Schmidt, 2004; Lipsey and Wilson, 2001). Hence, the meta-analyses remain a useful tool for the evaluation of evidence (Flather et al., 1997), forming a critical process for theory development in science (Hunter and Schmidt, 2004).
Unfortunately, to our knowledge there are not any meta-analyses addressing the criterion-related validity of SR tests. Beyond the simple but important function of describing and summarizing the scientific findings of a research area, the main contribution of a meta-analysis is to estimate as accurately as possible the population parameters (Hunter and Schmidt, 2004). Therefore, the results of a meta-analysis let us generalize the research findings, as well as test hypotheses that may have never been tested in primary studies. Finally, the meta-analyses permit us to examine today's lack of knowledge in a specific area and to guide scientists in future research (Cooper et al., 2009).
Consequently, the main purpose of the present meta-analysis was to examine the scientific literature on criterion-related validity of SR tests for estimating hamstring and lumbar extensibility in apparently healthy individuals. More specifically, the objectives of this study were: (a) to describe and summarize the up-to-date scientific findings of criterion-related validity of SR tests for estimating hamstring and lumbar extensibility; (b) to estimate and compare the overall population mean of the criterion-related validity coefficients of each SR test for estimating hamstring and lumbar extensibility; and (c) to examine the influence of some study features (sex of the participants, age of participants, and level of hamstring extensibility) in criterion-related validity coefficients of SR tests.
The following seven electronic databases were searched from their inception through December 2012: SportDiscus, Scopus, Medline, Pubmed, Web of Science, ERIC, and Dissertations & Theses Database. The search terms used were based on two concepts. Concept one included terms for the SR test (sit and reach) and concept two included terms for validity (validity, related, relationship, correlation, comparison, hip, hamstring, flexibility, ROM, range of motion, range of movement, straight leg raise, knee extension, popliteal angle, lumbar, back, Macrae and Wright, Macrae & Wright, Schober, radiography, goniometer, and inclinometer). The terms of the same concept were combined together with the Boolean operator "OR" and then the two concepts were combined using the Boolean operator "AND" (Benito Peinado et al., 2007). The keywords that consisted of more than one word were enclosed in quotes. In addition, the reference lists of all included papers were manually searched.
The selection criteria to identify studies that examined the criterion-related validity of SR tests for estimating hamstring and/or lumbar extensibility were: (a) studies with apparently healthy participants who did not present any injury, physical and/or mental disabilities; (b) studies with SR tests that yielded the values of the maximum reach of the fingertips; and (c) studies in which hamstring and/or lumbar extensibility criterion measurements used are widely accepted in the scientific literature (e.g., straight leg raise or knee extension tests for hamstring extensibility and Macrae & Wright or inclinometer methods for lumbar extensibility). In addition to papers, master/doctoral dissertations and conference proceedings were also accepted. No language or publication date restrictions were imposed.
For this meta-analysis, data were collected from studies that reported relationships between SR tests and hamstring and/or lumbar extensibility criterion measures with apparently healthy participants of any age. From each selected study the following data were coded: Study identity number, sample size (n), sex of participants (1 = males, 2 = females), age of participants (1 = children,
In addition, although various protocols for evaluating quality of single studies have been described, there is no widespread agreement on the validity of this type of evaluation approach. Thus, rejecting certain single studies and accepting others for inclusion in a meta-analysis on the basis of a quality score remains a controversial procedure (Flather et al., 1997). Hence, according to Flather et al. (1997), our approach has been to ensure that the design has not been flawed (e.g., conducted by scientifically evidenced criterion measures), and that there has been a complete reporting of relevant outcomes. For a study to be included in this meta-analysis, sample size, SR test protocol, hamstring and/or lumbar criterion measures and Pearson's r were considered to be critical. In the event that the authors mixed subgroups of a study feature (e.g., males mixed with females), failed to identify a study feature (e.g., criterion measure or reliability scores) or were ambiguous (e.g., hamstring extensibility scores around 80[degrees] shown graphically) the data was omitted. When in the same study data for males and females were expressed both separately and together, only the separate data were coded. When in the same study data were expressed for both legs separately or for two different days from the same sample (i.e., such as in Mier, 2011), the average value of the coefficients was coded.
Finally, in the event that included studies used multiple validity coefficients for hamstring and/or...