There is a growing belief amongst the medical fraternity that prevention is better than cure. This view is progressively working along the chain to include administrators in sport and business who are adopting the principle that productivity is greater if resources are invested in programmes to limit the likelihood of injury and therefore poor performance or even no performance (Alexander, 2004). Considerable work has been performed on investigating ergonomics and lumbo-pelvic dysfunction (see Chapter IX) however contrastingly little research has been performed on the effect of core stability function and peripheral injury rates (Lycholat, 2004). This chapter will review the evidence behind core stability and peripheral injuries.
On considering simple biomechanical principles of stress and strain and the effect this has on human tissue it is not difficult to accept that if a tissue is progressively loaded the cumulative effect will result in an alteration of the initial state (Best and Garrett, 1993; Booth and Gould, 1975; Curwin and Stanish, 1984). This may have a positive or negative effect on the individual. In the case of osteoporosis progressive loading can have a very positive effect by increasing bone mineral density (Rutherford, 2002). In the case of many soft tissue structures the effect of malalignment of a limb beyond a certain threshold appears to have a potentially undesirable effect on tissue (Bartlett, 1999; Nigg and Herzog, 1994). Depending on the location it could result in tissue hypertrophy or the reverse such as necrotic breakdown due to wringing out the vascular supply as in Achilles tendonosis (Clement et al., 1984; Nigg, 1985).
On further investigation into conditions that appear to potentiate peripheral injury one must consider the direction of biomechanical forces that could result in such an injury and determine if the individual was capable of resisting these forces under normal conditions or did an apparent acceptable force result in an injury? One of the most difficult determinates appears to be ascertaining the degree of variation from the norm in terms of limb alignment that could place an individual in a precipitous position for manifesting an injury. In association with this an understanding of the threshold at which undesirable tissue changes occur and ultimately when symptoms are experienced by the individual. Foundation studies have investigated the tensile properties of soft tissue structures such as ligaments and tendons (Butler et al., 1978) however they do not provide adequate data that would provide a knowledge base for determining hazardous forces in activities of daily living where the preload is significantly less than the point of tissue failure.
Human tissue provides a rather complex paradigm to consider as it is non-homogenous (material behaves in different ways depending on location e.g. lateral and medial cortex of bone) and anisotrophic (properties of material are dependent on direction they are loaded) (Bartlett, 1999). The other factor to consider is the viscoelastic properties of biological tissue that demonstrates a ‘creep’ effect where they continue to deform over time under a constant applied load (Bartlett, 1999).
When considering the biomechanical chain of the lower limb it is clear that forces acting on the knee for example can come from three basic directions: superiorly, inferiorly or be a translatory force. More than likely all three are acting concurrently. In the case of the knee the angulation of the patellofemoral joint can be influenced by the strength of the hip abductors from above and the degree of pronation of the forefoot from below (McConnell, 2002). Weak hip abductors could be linked in with muscle inhibition due to pathology at the sacroiliac joint (Liebenson, 2004a; Richardson et al., 2002; Vleeming et al., 1990a and 1990b) that is related to a decrease in the supportive function of the thoracolumbar fascia (Vleeming et al., 1995) that in turn is resulting from inhibition of the multifidus muscle Hides et al., 1994 and 1996) at a specific spinal level. At this stage it can now be seen how muscles involved in providing a core stabilising function of the lumbo-pelvic region could influence the patellofemoral joint. In contrast the symptoms at the knee may also be as a result of forefoot pronation (Clement et al., 1984). This generates a clinical decision that warrants careful consideration of what variables the clinician must influence and when. This review found no direct evidence to provide answers to such specific questions however a limited number of intervention studies did provide some guidance for future research.
Leetun et al. (2004) considered the structural differences between sexes to determine if the apparent gender bias for women in relation to lower limb injuries had any relationship to core stability measures. In recent times it has been established that women tend to be at a greater risk of certain lower limb injuries such as rupture of the anterior cruciate ligament, iliotibial band friction syndrome and femoral, pubic, tibial and metatarsal stress fractures (Sallis et al., 2001; Taunton et al., 2002). An important criticism of this study was the interchangeable use of the terms core strength and core stability. Leetun et al. (2004) described a battery of tests that were devised to test the strength of muscles that contributed to anterior, posterior and lateralcore stability. Isometric strength of hip abductors and hip external rotators was performed using a hand held dynamometer. A modified Biering-Sorensen test was used to test the muscle capacity of the posterior core. Lateral core muscle capacity was measured using the side bridge. Anterior core capacity was measured using the straight leg lowering test for the first year and the flexor endurance test for the second year which tests the athlete’s ability to sustain a 60° angle of the trunk from the horizontal. Apart from the hip measurements which were measures of force the other tests measured endurance capacity. While the endurance tests may have some validity when considering the criteria for objectively measuring stabilising muscles the stated goal in the methods section was to measure the strength of these muscles. This highlights another study that uses the terms stability and strength indiscriminately. The described tests are not measures of strength but measures of endurance capacity.
The Leetun et al. (2004) study reported that male athletes generally demonstrated greater core stability measures than female athletes. Hip abduction, external rotation and side bridging measures demonstrated significant differences between male and female athletes. Male athletes also performed slightly better during the straight leg lowering test and the flexor endurance test. They found that athletes who experienced injuries during the season generally demonstrated lower core stability measures most notably hip abduction and external rotation. Hip external rotation strength was found to be the single most effective predictor of injury during the season. It was suggested that the hip and trunk weakness demonstrated by the female athletes predisposed them to a greater incidence of lower limb injuries due to their inability to stabilise the hip and trunk to the large external forces associated with athletics. A limitation of this study was the lack of dynamic assessment of function. It is possible that more tests exist that could identify proneness to injury. Leetun et al. (2004) concluded that proximal stabilisation was important in preventing injuries in the lower extremities.
In a study by Steele and White (1986) they investigated injury prediction in female gymnasts. They found they were able to predict whether a gymnast was of a high or low risk of injury by considering 5 variables (height, weight, age, mesomorphy and lumbar curvature in standing). Interestingly they did not find any correlation with injury and extreme flexibility or hypermobility. They concluded that injury proneness in young female gymnasts was related to their anthropometric characteristics. Unfortunately their statistical analysis did not extend to determining whether specific injuries could be related to a specific variable. The angle of lumbar curvature has been linked to low back pain (Micheli, 1979), however it would have been interesting to see if there was any correlation with peripheral injury states and lumbar curvature. Further statistical analysis on the data from this study would have been beneficial.
Hennessy and Watson (1993) performed a study on flexibility and posture in relation to hamstring injury. Interestingly no relationship was found between posture and hamstring flexibility or between hamstring flexibility in injured and noninjured subjects. However, there was a significant difference in lumbar lordosis between injured and noninjured subjects. Subjects with hamstring strains had a larger variation in the lumbar lordotic curve and tended to be considered to have poorer posture. The authors hypothesised that overactivity of the iliopsoas muscle due to straight leg kicking, straight leg raising or abdominal sit-up exercises with straight knees could contribute to the lordosis. They recommended that regular assessment of posture in sports people and the prescription of corrective exercises would improve this finding.
Comerford and Mottram (2000) suggested that anatomical characteristics such as muscle imbalance about a joint, poor strength or decreased range of motion provided a greater risk of injury than postural disturbances. The impact of posture on dynamic exercise performance and injury occurrence warrants further investigation.
Alexander’s (2004) paper considered the influence of strength and conditioning on sportspeople. He outlined that the two main aims of such programmes were to improve performance and to prevent injury. In his review he considered how strength and conditioning training had improved most performance parameters such as speed, strength, endurance, vertical jump height, VO2 max, anaerobic thresholds and immunological activity. His concern was the fact that as these parameters were improving injury rates appeared to be increasing. This view was supported by a number of studies (de Loes, 1990 and Michaud et al., 2001). From his experience with professional rugby teams and Olympic triathletes he observed common muscle imbalances that he felt may lead to injury. Based on his review of studies on muscle fibre types, anti-gravity research and his own practical trial he recommended the addition of slow or static low intensity sensori-motor exercises to an athletes training programme. These types of exercises have been previously described in detail by Richardson et al. (1999). Alexander (2004) proposed that traditional strength and conditioning programmes bias the development of type II fibres which have a lesser endurance capacity and therefore don’t have the same capacity to stabilise and protect the musculoskeletal system. As the local stabilising muscles are essentially type I fibres any loss in the number of type I fibres could impair an athletes’ ability to maintain dynamic joint stability during sport. Abernathy et al. (1990) in their review of muscle function reported that strength training increases the size and proportion of type II fibres and reduces type I fibres. Further suggesting that if the slow twitch fibres of the stabilising muscles aren’t trained in a manner akin to their intended function then they will lose the capacity to provide joint stability. Alexander (2004) also suggested that proprioception could be impaired by the selective recruitment of type II fibres therefore providing less time for feedback of the afferent system to the central nervous system.
Hewett et al. (2001) described the concept of Dynamic Neuromuscular Analysis (DNA) training with reference especially to injury prevention. They described three components of DNA training as being dynamic sport-specific movement skills, neuromuscular patterning based on the identification of underlying neuromuscular imbalances, and constant biomechanical analysis by the instructor and feedback to the athlete both during and after training . Their specific interest was investigating the effect of DNA training on preventing knee injuries. They described the neuromuscular component of DNA training as being a balance between challenging the proprioceptive abilities of the athlete and exposing the athlete to movement patterns that generate dynamic control of peripheral joints. They purported that this type of proprioceptive stress may aid in the development of spinal reflexes that more quickly and effectively stabilise the joint than the voluntary muscular movements that require an afferent-efferent pathway along with cerebral input. This concept is similar to that of Comerford et al. (2000) who purported the benefits of dynamic stability and muscle balance. It would appear that if the spinal reflexes are operating efficiently and muscular activation patterns are in synch then any system of exercise that enhances this concept would be of benefit both to the core and peripheral structures. As techniques to objectively measure neural components progress these theories may be confirmed with evidence.
Table 11 summarises the intervention studies considered in this guideline development. The evidence table associated with the guideline development for Key Question 4 can be seen in Table 17, Appendix F.
|Table 11. Intervention studies for Question 4: Is there any correlation between core stability and peripheral injury rates?