Introduction and Justification of the Study essay

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Muscular stretching prior to exercise historically has been recommended by physical educators, individuals involved in athletics, and a variety of healthcare practitioners as an integral component of a preparatory program to prevent musculoskeletal injury (Best, 1995; Garrett, 1990; Gleim & McHugh, 1997; Hartig & Henderson, 1999; Safran, Seaber, & Garrett, 1989; Smith, 1994) and improve muscular performance (Anderson & Burke, 1991; Baechle & Earle, 2000; Shellock & Prentice, 1985).  This educational message often has been dispersed without appropriate scientific support (Shrier, 1999).  In fact, critical evaluation of clinical and basic science evidence demonstrates conflicting findings related to the value of stretching prior to exercise (Shrier, 1999).  Consequently, there currently is insufficient evidence to suggest that acute stretching prior to exercise is beneficial for the prevention of musculoskeletal injuries.   

An acute bout of pre-exercise stretching appears to be detrimental for muscle strength and power performance measures for certain athletic populations (Young & Behm, 2002) as a result of alterations in neurological properties (Cramer et al., 2004) and viscoelastic properties (Avela, Finni, Liikavainio, Niemela, & Komi, 2004) of the stretched muscle.  Therefore, it may not be appropriate to alter the fine dynamic interactions of neural, architectural, and electrophysiological properties present in muscle for force production immediately prior to participation in exercise (Fowles, Sale, & MacDougall, 2000).  Nonetheless, many athletes continue to perform pre-exercise stretching (Young & Behm, 2003) and continue to believe it is important to engage in this practice (Nyland, Kocabey, & Caborn, 2004).  In light of this information, one might conclude that there is a psychosocial component to these perceptions of the benefit of pre-exercise stretching. 

One psychological phenomenon that has been extensively researched across a variety of performance domains and may provide insight into the current perceptions of the value of stretching is the self-perception of efficacy, also known as self-efficacy (Bandura & Locke, 2003; Feltz, 1982; Feltz & Weiss, 1982; Moritz, Feltz, Fahrbach, & Mack, 2000).  Stronger self-efficacy has been shown to enhance performances in a variety of physical activities including bowling (Boyce & Bingham, 1997), diving (Feltz, Landers, & Raeder, 1979), gymnastics (McAuley, 1985), bench pressing (Wise & Trunnell, 2001), and muscle endurance (Feltz & Reissinger, 1990; George, Feltz, & Chase, 1992; Gould & Weiss, 1981).  It plays a major role in human performance, and teachers, physical educators, and coaches can influence individuals’ perceptions of efficacy (Chase, 1998; Feltz & Doyle, 1981; Gould, Hodge, Peterson, & Giannini, 1989; Harrison, Fellingham, Buck, & Pellett, 1995). 

Traditional teachings and beliefs of many coaches, teachers, physical educators, and healthcare providers regarding the impact of stretching on human performance may have influenced certain athletes’ perceptions of the value of pre-exercise stretching (Nyland et al., 2004).  This perception of that value may, in turn, affect specific exercise self-efficacy, which could potentially influence motor performance.  Perhaps, athletes fear their confidence level or physical performance will suffer without engaging in pre-exercise stretching because it has been a part of their educational background and exercise routine for a long time.  Therefore, evaluating the relationship between an individual’s perception of the value of stretching and their motor performance following a stretching sequence warrants investigation.  Likewise, it stands to reason that the potentially harmful effects of stretching prior to exercise may be present regardless of his or her perception of the value of pre-exercise stretching; therefore, appropriate education regarding pre-exercise stretching and injury prevention becomes increasingly important. 

Stretching and Control    

Greater numbers of individuals are participating in athletics or other recreational physical activities because of social changes and a growing knowledge of the value of healthy lifestyles (Thacker, Gilchrist, Stroup, & Kimsey, 2004).  Thus, education about injury prevention becomes more important.  One specific injury which has recently received increased educational and public awareness efforts regarding injury mechanisms and preventive measures is the injury of the anterior cruciate ligament (ACL), an important stabilizing knee ligament. 

Several risk factors for ACL injuries have been identified and understanding these factors could offer a basis for the development of prevention strategies in high-risk populations.  These factors can fall into the following categories:  environmental, anatomical, hormonal, biomechanical, and neuromuscular (Griffin et al., 2000).  Biomechanical and neuromuscular risk factors have received a large focus in the recent literature.  It has been well established that impaired passive joint restraints and/or active muscular control systems that biomechanically influence the dynamic stability of the knee may increase one’s risk for non-contact ACL injuries (Dugan, 2005; Griffin et al., 2000; Lephart, Abt, & Ferris, 2002).  These non-contact episodes typically take place during deceleration, lateral pivoting, or landing tasks and are often associated with high external knee joint loads (Boden, Dean, Feagin, & Garrett, 2000). 

One such condition where these biomechanical and neuromuscular factors have been examined in the recent literature is landing from a jump.  In fact, some authors have reported that 58% to 61% of non-contact injuries occur during landing from a jump (Gerberich, Luhmann, Finke, Priest, & Beard, 1987; Gray et al., 1985).  In a retrospective analysis, it was established that the body positioning associated with most non-contact ACL injuries consisted of the following:  external rotation of the tibia, the knee was close to full extension, the foot was planted, and a deceleration occurred which resulted in valgus collapse at the knee (Boden et al., 2000).

Many biomechanical and neuromuscular deficits have been identified in the literature and are thought to contribute to a higher incidence of ACL injuries (Lephart, Abt, & Ferris, 2002).  Some of these deficits include:  decreased knee flexion angles, greater knee valgus angles, increased quadriceps muscle activation, and decreased hamstring muscle activation found during the stance phases of running and cutting maneuvers (Malinzak, Colby, Kirkendall, Yu, & Garrett, 2001).  Malinzak et al. suggested that these body positions and muscle actions during high risk activities result in increased strain on the ACL.  These findings have been supported by several studies investigating kinematic and kinetic variables in the sagittal plane (Chappell, Yu, Kirkendall, & Garrett, 2002; Decker, Torry, Wyland, Sterett, & Steadman, 2003; Hass et al., 2005; Huston, Vibert, Ashton-Miller, & Wojtys, 2001; Lephart, Ferris, Riemann, Myers, & Fu, 2002; McLean, Lipfert, & Van Den Bogert, 2004; Salci, Kentel, Heycan, Akin & Korkusuz, 2004) and frontal plane (Chappell et al., 2002; Ford, Myer, & Hewett, 2003; Ford, Myer, Toms, & Hewett, 2005; Hass et al., 2005; Hewett, Myer, & Ford, 2004; Hewett  et al., 2005; McLean, Lipfert, & Van Den Bogert, 2004) during landing and other high risk activities. 

Of special interest in this investigation is the interaction between muscular activation and dynamic knee valgus positioning with a landing sequence that follows acute static stretching.  Coordinated muscular activation surrounding the knee and hip during landing is important for controlling the dynamic knee valgus positioning and moments which have been shown to be predictive of ACL injury (Hewett et al., 2005). 

Acute static stretching has the potential to alter neuromuscular (Cramer et al., 2004) and biomechanical (Avela et al., 2004) properties of muscle; therefore, the influence of stretching on neuromuscular and biomechanical measures surrounding the knee during a landing sequence warrants investigation.  Exploring this interaction will provide valuable insights into the role of acute stretching in biomechanical behaviors at the knee and, combined with survey measures will provide an understanding of the psychosocial interactions between educational messages, stretching perceptions, and stretching practices.  

Purpose Statement                   

There were several objectives in conducting this investigation.  First, this study examined the influence of acute static stretching on selected biomechanical measures of neuromuscular control that affect the knee and hip during landing.  More specifically, electromyography (EMG), kinematic, and ground reaction force (GRF) measurements of the dominant lower extremity were obtained during the initial landing and ground contact (GC) phases of a vertical drop jump test.  Second, this study evaluated the relationship between subjects’ historical educational background and current perceptions regarding the value and appropriate practice of pre-exercise stretching.  Specifically, subjects completed a survey instrument using a standard visual analogue scale (VAS).  Third, using the same survey instrument, this investigation evaluated the relationship between subjects’ perceptions of the influence of the warm-up intervention on their current performance outcomes and the actual change in performance outcomes (vertical jump height).  Fourth, using the same survey instrument, this investigation examined the difference in subjects’ perceptions of the influence of the warm-up intervention on their current performance outcomes between those that stretched (experimental group) and those that did not stretch (control group).  Finally, this study evaluated the influence of current written educational messages on subjects’ perceptions of the value of pre-exercise stretching and their future level of participation in pre-exercise stretching.  Specifically, subjects were presented with either a positive or a negative written educational message about the value of pre-exercise static stretching on the current measurement outcomes.   After reading this scenario, the subjects were asked to complete the same survey questions as above dealing with their current perceptions of the value of pre-exercise stretching.

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