What is the evidence that poor flexibility is a risk factor for sports injury?

Sports Medicine and Rehabilitation

Research into the area of physiological factors and their impact on injury incidence in sports has been considerable. It has become a priority to identify significant risk factors that are causing players and athletes to miss matches and sometimes a large slice of a season. With the increased numbers of games in a season and the increased standard of playing it is essential that an athlete is free from injury so that they can perform at their optimal level all of the time. Studies have been conducted so that the number of injuries can be analysed to show when they are happening, who too in particular and probably the most important why? The results can then be used to assess and ultimately prevent the risk factors and also measure the impact of an intervention programme that will reduce injury. This information can be of particular interest to sports companies who design protection such as shin pads and shoulder pads or to sports fitness instructors who can develop a regime that will rectify their problem.

What is flexibility?

Flexibility is one of these physiological factors and has been described as the ability of skeletal muscle and tendon to lengthen (Gleim and McHugh 1997). There are two types of flexibility that can be measured for analysis. Static flexibility is the range of movement available to a joint or series of joints and dynamic flexibility is the ease of movement within that range of movement. These biomechanical factors and their relation to sports injuries stimulated considerable interest and extensive research has been conducted to try and explain these statements and to generate data that will provide meaningful evidence [1, 2, 4, 5, 6, 8, 9, 10, 12, 13].

Evidence that poor flexibility is a risk factor.

A recent study examined the relationship between increasing hamstring flexibility and the result it had on lower extremity overuse injuries (Hartig and Henderson 1999). Its strength was that is was conducted on approximately 300 military basic trainees who were all performing their 13 week training course at the same time. This is a strength because the environment is easy to control because they are all living in the same place and although the test was not compulsory the likelihood is that they were made to see the test through by their superiors. Consequently the follow up will be extremely good because the dropout percentage will be low. It will also mean that all the subjects will be doing the same infantry basic training course at the same time and so exposure to other variables can be controlled. These factors combine to ensure that the quality of the study will be excellent in that area.

The design of the study looked at the effectiveness of implementing an intervention programme i.e. three hamstring stretching sessions that were incorporated into half of the military trainees existing fitness schedule. Throughout the 13-week training period any injury that the trainee occurred in the lower extremity was recorded by the troop medical clinic. This paper is also good because it does not give an absolute number of injuries but it states the incidence rate, which allows the data represented to be interpreted clearer i.e. the number of injuries compared to the number of training hours or trainees. Although the incidence is an important factor it is also important to find and record the severity of the injury. This is something that the researchers have failed to do and is a major weakness in this study. They have mentioned the types of injury that have occurred but have failed to show the number of each type in the total incidence rate. Therefore a sprained ankle could have been categorised the same as a torn ligament.

The results of this study showed that both the control group and the intervention group both increased their flexibility but with a larger increase in the intervention group. This difference was found to be significant. When considering the incidence of injuries it was established that the number of lower extremity overuse injuries was significantly lower in the intervention group compared with the control group (Hartig and Henderson 1999). Therefore this paper is excellent evidence that poor flexibility is an important risk factor in the development of a sports injury.

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Another study that supports these findings was conducted on female collegiate athletes and investigated into flexibility imbalances and their association with athletic injuries (Knapik et al., 1991). The testing took place on 138 female subjects who participated in eight weightbearing varsity sports during their athletic season. The fact that it was during the season is a strength because it means that factors such as motivation would not have effected the results i.e. testing at the end of season when motivation levels are low and participant is not fully committed. Selecting subjects from the same environment and subjects that were doing the same activities was highlighted in the previous study as a strength. Therefore because this study is looking at athletes from different sports it is difficult in comparing flexibility. Some athletes may compete more often than others and the physical demands placed on the individual will vary and so it is difficult to control their exposure of other factors. Another problem was that the study did not assess the effects of different playing positions and so these are weaknesses of the study. The design of the study allowed the researchers to collect information on flexibility imbalance preseason and then record all sports related injuries that occurred throughout the season. The results found that subjects that had a significant flexibility imbalance in the lower extremities, especially those with a greater range of motion in their left side, were almost twice as likely to experience injuries compared to subjects with a more balanced range of motion (Knapik et al., 1991). These findings further indicate that a major risk factor for sports injury is poor flexibility and, in concurrence with the previous study, that it is most apparent in the lower extremities.

In addition to the research into flexibility and general overuse injuries, several studies have examined flexibility in relation to more specific injuries. Worrell (1994) highlighted the factors that were associated with hamstring injuries and identified that flexibility, or lack of, was an important aetiological factor in its incidence. The rational for this approach is strong as hamstring injuries are a significant occurrence in athletes who participate in running and sprinting activities and so identifying the risk factors is essential. The report showed that the injury occurs from a complex interaction of many factors but does continue to mention that an important factor was poor hamstring flexibility (Worrell 1994). The conclusions of this report were established in a recent study that examined hamstring injured athletes and compared them to a control group (Worrell et al., 1991, cited in Worrell 1994). The study showed conclusively that there was significantly less hamstring flexibility in the hamstring injured athletes than in the matched control group. Furthermore in the hamstring injured group, the injured hamstring muscle had significantly less flexibility than the non-injured hamstring.

Another study looked into this specific area of hamstring strains (Liemohn 1978) and the results reinforced the findings of Worrell (1994). The researchers were concerned about the high prevalence of hamstring injuries in track and field athletes and investigated into the factors that caused them. They concluded by stating that the study highlighted hip-joint flexibility as the most definite precipitator of hamstring injuries, with flexibility being lower in the injured groups compared to the non-injured groups. However a major downfall of the study is that it does not perform any statistical analysis and so it is unsure whether these relationships were significant.

Ekstrand and Gillquist (1983) performed a more sports specific test and assessed the etiological factors that influenced all types of soccer injuries. The paper is good because at the start of the study there is a clear definition of what they consider to be an injury i.e. any injury that has occurred during a scheduled game or practice and has caused a player to miss the next game or practice session (Ekstrand and Gillquist 1983). This ensures no ambiguity in the interpretation of results and increases the quality of the analysis. The results showed that amongst the 44 players who sustained muscle rupture or tendonitis, 34 were through muscle tightness, which was significant (p < 0.05). In general the soccer players were significantly less flexible than the reference group and this correlated to muscle tightness and therefore strains, predominately in the adductors.

These more specific studies are yet further evidence that confirms that poor flexibility is a risk factor for sports injury and in particular to hamstring injuries, which has been identified as one of the most common strains and pulls in sports performers

Ekstrand and Gillquist (1982) conducted a study on one hundred and eighty male soccer players to examine the relationship between muscle tightness and injuries. The design was again strong because the subjects were taken from the same division and so comparison of subjects could be controlled. If the players are taken from different divisions and leagues the results maybe inaccurate because the subjects are playing at different levels and are playing a different number of games in the season. The report collected information on past injuries and muscle tightness and then examined the range of movement in 5 actions of the lower extremities. These were also measured in non-soccer player so comparisons could be made. Their results gave evidence suggesting that poor flexibility was a risk factor as muscle tightness was higher in players that experienced strains. The investigation also found that the football players were in general less flexible than the non-players. But the study concluded by stating that the risk of poor flexibility was not significant. This highlights an uncertain area that suggests that poor flexibility is not a significant risk factor for sports injury, which has also been demonstrated in other research.

Evidence that poor flexibility is not a risk factor.

As previously mentioned hamstring injuries have been quite a common research area and therefore Hennessy and Watson (1993) also investigated into flexibility assessment in relation to hamstring injuries. The investigation took place on subjects who were divided into to two groups i.e. non-injured group and injured group. The subjects in the injured group had a history of hamstring strain within the previous twelve months. The findings of the study seem to contradict the previous study (Worrell et al., 1991) because the results indicated that there was no significant difference in flexibility between the two groups and more importantly no significant result was found between the injured leg and the non-injured leg. This may be explained by the fact that posture was also examined as a risk factor. The researchers examined ten postural components and so the study was quite extensive. For the reason the sample of the study was relatively small (n = 34) compared to other studies. This gave an increase in the quality of the results but Gleim and McHugh (1997) stated that such a relationship between a significant change in flexibility and a significant reduction in injury could only be adequately examined if the study is conducted on a large sample. It is also apparent from the study that the subjects were taken from various sports i.e. rugby, hurling and Gaelic football and as mentioned this can be a weakness as exposure cannot be controlled and range of movement will be different between sports.

This is not the only study to challenge the fact that flexibility is not a significant risk factor for sports injury. A long-term study that took place over four years examined the physical characteristics that were risk factors for sports injuries (Twellaar et al., 1997). The study assessed sixteen flexibility indices of 136 physical education students prior to their course starting. The subjects were of similar age and physical condition and were exposed to the same accident hazards and physical exertions. As emphasised earlier on a controlled environment is important and therefore this study design is fundamentally ideal. Despite this the results showed that the influence of flexibility had no effect on the risk to sustain a sports injury. This could be explained by the major weakness of the study, which is the large number of dropouts. As with most longitudinal studies the number of people who fail to complete the study is high with 79 out of the 215 tested failing to finish the testing, which is almost 40 %. But apart from this it is a strong study and therefore is good evidence that poor flexibility is not a significant risk factor is the development of sports injuries and consequently conflicts with other research.

Gymnastics is a sport, which requires a high degree of flexibility and a large range of movement. For that reason a study conducted by Kirby et al., (1981) examined the flexibility in female gymnasts and the data generated an interesting result. Sixty competitive gymnasts were compared to 35 age-matched controls and the investigation showed that they were more flexible in most areas. But the interesting finding was that the gymnasts with low back pain had greater toe-touching ability than those without symptoms. This suggests that a high level of flexibility may be just as significant a risk factor as poor flexibility. This relationship is known as the U shaped relationship (Fig. 1) and it is suggested that ligament laxity is a reason for injuries to people with high flexibility.

Fig.1 U-shaped relationship of flexibility and sports injury.

This association was also found in a subsequent study conducted by Jones et al., (1993). The study was carried out on 303 army infantry trainees and examined the impact of past physical activity, current physical fitness, and Army physical training on the incidence of injuries. The results showed that both high and low flexibility were risk factors with the trainees more than two times more likely to sustain injury than the average group. Statistical analysis of the results demonstrated a significant U-shaped relationship with flexibility and injury incidence.

Conclusion.

From the conflicting evidence examined it is apparent that the area of flexibility and sports injury is very unclear with several quite strong contradictions in the research done. Several studies have found that poor flexibility is a significant risk factor (1, 2, 4, 9, 10, 13,) whilst others have disagreed (5, 12). Some studies have even highlighted that a high level of flexibility may be a significant risk factor (6, 7). This may be down to design limitations such as the lack of subjects needed to give adequate results (Gleim and McHugh 1997) and that most of the studies were retrospective. The fact that they were retrospective means that when selecting the subjects there may be a possible bias. Another problem was that some of the studies failed to have a clear definition of injury and so analysis of the subject was inaccurate. But the major contributing factor towards these divergent results that has been highlighted in the studies is the difficulty in defining a normal range. The effective assessment of flexibility there must be a normal range in which the measurements can be compared. A normal range must be defined but this is difficult because of the number of different sports that the subjects are participating in. Each individual sport will place varying demands on the participant and the difference in attainable range of movement can be quite considerate. Therefore flexibility patterns, which represent risk factors for one particular sport, may not give a fair representation of another sport (Gleim and McHugh 1997). This can also be said for difference in playing position or differences between divisions. The role of flexibility and its range depends on a number of factors such as the type and force of muscular contractions involved in their specific movement. For example a normal range of movement for a gymnast will differ considerably from the range of movement of other sports. Gleim and McHugh (1997) stated that female gymnasts manifest such dramatic range of movement that a non-gymnast would almost certainly be injured attaining the same range of movement. This difficulty has lead to limitations and weaknesses in many studies that have examined flexibility as highlighted in this assignment. This is because no clear relationship has been found between injury and flexibility that can be applied to all sports and all levels of play and consequently comparison has been difficult.

However the majority of the evidence has shown poor flexibility to be a risk factor in the development of sports injuries and therefore confirms the general consensus in this area. Twellaar et al., (1997) stated that an increased density and decreased lengthening of collagen fibres in connective tissue of ligaments, tendons, muscles and joint capsules might be associated with poor flexibility. Consequently an increase in flexibility will result in an increase in the range of movement around a joint or joints with less stiffness. Maffulli and Baxter-Jones (1995) acknowledged that flexibility was especially important when considering young athletes and their vulnerability to sustaining sports injuries. The incidence of sports injury in young athletes has been quite high not only because there is a highly number of participants but also because they show a biomechanical imbalance. Initially during the growth spurt of the young athlete the long bones grow, which then stimulates the muscles to elongate and grow. During this time period were the bones are growing first the athlete may experience tightness around muscles, especially were those which cross two joints (Krivickas 1997).

To summarise it must be stated that increased flexibility can be important for sports that require a large range of motion. Whereas in sports which require a relatively small range of movement a decrease in flexibility may increase the effectiveness of movement and therefore enhance performance. Thus highlighting the problems in assessing whether flexibility is a risk factor for sports injury.

References:

Ekstrand, J. and Gillquist, J. (1982) The frequency of muscle tightness and injuries in soccer players. The American Journal of Sports Medicine, 10(2), 75- 78.

Ekstrand, J. and Gillquist, J. (1983) The Avoidability of Soccer Injuries. International journal of Sports Medicine, 4, 124- 128.

Gleim, G.W. and McHugh, M.P. (1997) Flexibility and Its Effects on Sports Injury and Performance. Sports Medicine, 24(5), 289-299.

Hartig, D.E. and Henderson, J.M. (1999) Increasing Hamstring Flexibility Decreases Lower Extremity Overuse Injuries in Military Basic Trainees. The American Journal of Sports Medicine, 27(2), 173- 176.

Hennessy, L. and Waston, A.W.S. (1993) Flexibility and posture assessment n relation to hamstring injury. British journal of Sports Medicine, 27(4), 243- 246.

Jones, B.H., Cowan, D.N., Tomlinson, J.P., Robinson, J.R., Polly, D.W. and Frykman, P.N. (1993) Epidemiology of injuries associated with physical training among young men in the army. Medicine and Science in Sports and Exercise, 25(2), 197- 203.

Kirby, R.L., Simms, F.C., Symington, V.J. and Garner, J.B. (1981) Flexibility and musculoskeletal symptomatology in female gymnasts and age-matched controls. The American Journal of Sports Medicine, 9(3), 160- 165.

Krivickas, L.S. (1997) Anatomical Factors Associated with Overuse Sports Injuries. Sports Medicine, 24(2), 132- 146.

Knapik, J.J., Bauman, C.L., Jones, B.H., Harris, J.M. and Vaughan, L. (1991) Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes. The American Journal of Sports Medicine, 19(1), 76- 80.

Liemohn, W. (1978) Factors related to hamstring strains. Journal of Sports Medicine, 18, 71- 76.

Maffulli, N. and Baxter-Jones, A.D.G. (1995) Common Skeletal Injuries in Young Athletes. Sports Medicine, 19(2), 137- 149.

Twellaar, M., Verstappen, F.T.J., Huson, A. and van Mechelen, W. (1997) Physical characteristics as risk Factors for sports Injuries: A Four Year Prospective study. International journal of Sports Medicine, 18(1), 66-71.