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DYNAMIC HAMSTRING FLEXIBILITY
Some studies have suggested that static flexibility tests of the hamstring muscle group are not limited by neural or biomechanical characteristics of muscle (Osternig et al., 1990; Halbertsma et al., 1996; McHugh et al., 1998) and that the actual length of the hamstring muscle, determined by the number of sarcomeres in series, may be a factor (Gajdosik, 1995). Various models have been developed to investigate the contribution of biomechanical constraints to flexibility. An early attempt at measuring active dynamic flexibility utilised a damped oscillation technique, which involved gentle perturbation of a loaded musculotendinous system, during which the free response of the system was recorded and a second order linear equation applied to the resulting damped oscillation to calculate the stiffness of the system (Bach et al., 1983; Shorten, 1987).
This technique has possible errors in that the downward force applied by the experimenter is generally described as a ‘downward gentle push’ in the region of 100-200N magnitude (Ditroilo et al., 2011) and often estimated rather than being objectively measured. Differences in perturbation amplitude can influence stiffness assessment, particularly in terms of reflex contribution (Kearney et al., 1999; Hunter & Spriggs, 2000). There is evidence that the damping response is somewhat curvilinear as opposed to completely linear (Watsford et al., 2010). While Wilson et al. (1994) described reliability values for performance measures determined through the use of intraclass correlations (ICCs), they did not report any values for stiffness measures and two earlier studies failed to make any attempts at confirming reliability of the system (Wilson et al., 1991; Wilson et al., 1992). Other studies have reported ICCs of r = 0.87 to 0.94 (p < 0.01) but did not state which particular ICC was used (Walshe et al., 1996; Murphy et al., 2003). The choice of ICC test is not inherently obvious and different tests can yield different results, which should be considered when interpreting this value. Additionally, convenience samples of participants were used in all studies and as these can exhibit high inter-subject variability, artificially high reliability values can be obtained (Atkinson & Nevill, 1998).
In an attempt to improve objectivity and to allow further understanding of dynamic flexibility, Magnusson et al. (1995) developed a model which used an isokinetic dynamometer to examine the stiffness and energy in a dynamic phase, and viscoelastic response in a static phase of a standardised stretch manoeuvre of the human hamstring muscle group. Due to the insignificant differences between trials as measured by paired t-tests and having Pearson product moment correlation coefficients of r = 0.91 to 0.99 (p < 0.05), the method was considered highly reliable (Magnusson et al., 1995). The use of PPM correlation coefficients in reliability studies is however inappropriate (Chen & Barnhart, 2008) as they are bivariate statistics and reliability measures are univariate in nature (Weir, 2005). Despite the challenges associated with the objective measurement of flexibility and the equivocal nature of the research considering the importance of flexibility for injury prevention, a variety of strategies aimed at improving flexibility and performance are utilised.
STRETCHING TECHNIQUES
Static stretching is generally the most widely used and recommended stretching technique because the exercises are relatively easy to perform and have little associated risk of injury (Kolber & Zepeda, 2004). These involve joints being placed in the outer limits of their ROM and then subjected to an elongation torque or force, which is maintained for a period ranging from a few seconds to minutes (Thacker et al., 2004). The slow build-up of tension and the absence of pain involved with static stretching are believed to minimise stretch reflex responses thus inducing muscular relaxation and allowing further stretching (Guissard & Duchateau, 2006).
Ballistic stretching is a dynamic and fast movement in which a bouncing type of stretch torque is applied into the extreme ROM limits of the joints concerned (Covert et al., 2010). This technique was thought a useful method of developing dynamic flexibility even though research often reports ballistic stretching to be less effective at improving flexibility than other types of stretching (Sady et al., 1982; Wallin et al., 1985; Bandy et al., 1998; Bacurau et al., 2009; Covert et al., 2010). The inhibitory effect of the stretch reflex is thought to be one reason for the lower effectiveness of ballistic stretching for improving static flexibility (Guissard & Duchateau, 2006). Additionally, because higher forces are involved in ballistic stretching compared to other stretching methods, it is associated with increased potential of injury to the musculotendinous unit and thus has traditionally been avoided (Hartig & Henderson, 1999; Hedrick, 2000). While it is possible that the musculotendinous units of untrained and sedentary individuals may not be able to withstand this vigorous type of stretching without sustaining muscle damage, sports people often put their joints through large ranges while exerting considerable forces. It is therefore possible that ballistic-type stretching may be appropriate in these groups of people (Shrier, 2005). More recently, ballistic stretching procedures have been adapted to become more controlled, through range movements rather than end of range techniques, and these types of stretches have been termed dynamic stretching (Behm & Chaouachi, 2011).
Proprioceptive Neuromuscular Facilitation (PNF) is a collection of techniques for facilitating muscle contraction, strengthening and increasing flexibility. These techniques were originally formulated and developed as a physical therapy procedure for the rehabilitation of stroke patients (Knott & Voss, 1957; Kabat, 1958) and PNF stretching procedures were subsequently developed on the basis of several important neurophysiological mechanisms (Chalmers, 2004). Contract-relax stretching involves an initial maximal isometric contraction of the muscle to be stretched (the antagonist) followed by the relaxation and passive stretch of the muscle to the limit of its ROM (Sharman et al., 2006). The isometric contraction of the muscle to be stretched is followed by relaxation, thought to stem from autogenic inhibition by the Golgi tendon organs (Ferber et al., 2002; Chalmers, 2004; Guissard & Duchateau, 2006). This suppresses the excitability of muscle spindles and thus allows the muscle to relax and stretch further (Bonnar et al., 2004). Consequently, the intention of PNF stretching is to reduce reflex activity, thus diminishing resistance and thereby improving joint ROM (Etnyre & Abraham, 1986). Paradoxically, it has been found that PNF stretching techniques that were most effective in increasing static flexibility also produced the highest levels of EMG activity in the stretched muscles, indicating that full muscular relaxation is not necessary for elongation to take place (Sharman et al., 2006; Mitchell et al., 2009).
CHAPTER 1
INTRODUCTION
1. INTRODUCTION
1.1 FLEXIBILITY
1.2 INJURY RISK IN SPORT
1.3 HAMSTRING STRAIN MECHANISMS
1.4 STRETCHING
1.5 RATIONALE
1.6 STATEMENT OF THE PROBLEM
1.7 ORGANISATION OF THE THESIS
1.8 AIMS OF THE STUDY
CHAPTER 2
LITERATURE REVIEW
2. LITERATURE REVIEW
2.1 MEASUREMENT OF STATIC FLEXIBILITY
2.2 STATIC FLEXIBILITY MEASURES
2.3 MUSCLE AND JOINT STRUCTURE AND FLEXIBILTY
2.4 DYNAMIC HAMSTRING FLEXIBILITY
2.5 STRETCHING TECHNIQUES
2.6 STRETCHING AND FLEXIBILITY
2.7 STRETCHING AND PERFORMANCE
2.8 STRETCHING AND JOINT POSITION SENSE
2.9 STRETCHING AND INJURY PREVENTION
2.10 FATIGUE AND INJURY SUSCEPTIBILITY
2.11 SUMMARY
CHAPTER 3
INTRA AND INTERDAY RELIABILITY OF HAMSTRING FLEXIBILITY MEASURES
3.1 INTRODUCTION
3.2 PILOT STUDIES
3.3 METHODS
3.4 RESULTS
3.5 DISCUSSION
3.6 CONCLUSIONS
CHAPTER 4
COMPARISON BETWEEN DIFFERENT METHODS OF MEASURING STATIC AND DYNAMIC HAMSTRING FLEXIBILITY
4.1 INTRODUCTION
4.2 METHODS
4.3 RESULTS
4.4 DISCUSSION
4.5 CONCLUSIONS
CHAPTER 5
IMPACT OF FATIGUE AND POST-EXERCISE STATIC HAMSTRING STRETCHING ON MEASURES OF STATIC AND DYNAMIC HAMSTRING FLEXIBILITY AND PERFORMANCE
5.1 INTRODUCTION
5.2 METHODS
5.3 RESULTS
5.4 DISCUSSION
5.5 CONCLUSIONS
CHAPTER 6
REFERENCES
