September 2013

Core stability and lower extremity injury risk

9core-iStock23285105v1A growing body of scientific evidence suggests that clinical tests of core strength and stability can help predict risk of lower extremity injury in athletes, which in turn supports the need for a multisegmental approach to the development of injury prevention programs.

By Valerie Snider, MS, ATC, and Gary Wilkerson, EdD, ATC

Lower extremity injuries account for more than 50% of all injuries sustained during participation in 15 different collegiate sports.1 Multi­segmental coordination of core and lower extremity muscle activity provides dynamic stability for the kinetic chain that extends from the lumbar spine to the foot and is widely believed to be enhanced by an adequate level of core stability.2 The multifidus and transverse abdominus muscles are activated before any extremity movements occur,3 suggesting that dysfunction in these muscles may disrupt an optimal neuromuscular activation sequence between the core and the lower extremities.

Rapid fatigue of the core musculature, which may be associated with a history of low back dysfunction, may severely limit an athlete’s ability to counteract external forces effectively. Poor endurance and delayed activation of the gluteus maximius and gluteus medius have been documented in patients with low back pain, ankle dysfunction, or a history of knee surgery.4  Furthermore, an individual’s capacity to tolerate the loads imposed by sport-related activities can be enhanced through training-induced improvements in neuromuscular function (i.e., strength, endurance, antagonist coactivation, postural balance, agility, and reactive responses to external stimuli).5

Rapid fatigue of the core musculature appears to be an important indicator of poor core stability, which can be assessed through administration of a timed “hold” test of the ability to maintain a static postural position.

Injury risk screening procedures

Preparticipation assessment of injury risk is increasingly used to identify individuals who would benefit from targeted training interventions for prevention of sprains and strains. A growing body of research evidence supports a variety of injury risk assessment methods, giving clinicians numerous options.

The choice of an assessment method should be based on evidence of its predictive power for a given type of injury in a specific population, which can be represented by either a relative risk (RR) or an odds ratio (OR) value. The RR value compares the injury incidence over a specified time period (e.g., a sport season) for a group of athletes who possessed a given risk factor to the injury incidence for a group of athletes who did not possess the risk factor. Thus, the relative probability of injury (RR) is based on the proportion of injury cases in high-risk versus low-risk groups. An RR value of 1 would indicate no difference in injury incidence for the two groups. A value of 1.1 can be interpreted as a relatively small, but potentially meaningful, difference, whereas a value of 3 or greater represents a substantial difference.

Original and modified versions of wall-sit hold test: A) bilateral support of body weight, B) unilateral support of body weight with nonsupporting extremity in figure-4 position, and C) unilateral support of body weight with non- supporting extremity foot lift.

Original and modified versions of wall-sit hold test: A) bilateral support of body weight, B) unilateral support of body weight with nonsupporting extremity in figure-4 position, and C) unilateral support of body weight with non- supporting extremity foot lift.

The odds for injury occurrence within a group is a number derived from the ratio of probability for injury (p) to the probability for injury avoidance (1 – p); the OR for a high-risk group relative to a low-risk group is based on the ratio of the odds for each group. An OR value of 2 or higher is considered an indication of a meaningful difference between groups, and a value of 8 or higher is considered substantial.

Injury risk factors

Volume of exposure to game conditions is the dominant risk factor for any type of sport-related injury. Agel et al6 documented an injury incidence rate for 15 different National Collegiate Athletic Association sports that was 3.5 times greater for games than for practice sessions. For football, the incidence rate was reported to be 9.5 times greater for games. Although game exposure is not generally considered a modifiable risk factor, its dominant influence must be included in any analysis of injury risk to avoid a confounding effect.

Another consistently strong injury predictor is a history of previous injury. Dvorak et al7 reported an OR of 2.8 for injury occurrence among soccer players who had sustained more than six previous injuries. The results of a longitudinal study reported by van Mechelen et al8 documented an OR of 9.4 for occurrence of any type of injury among physically active adults who had a history of a previous injury. Thus, incomplete postinjury recovery of functional capabilities appears to be an extremely important injury risk factor that needs to be quantified. Numerous surveys have been developed to quantify patient perceptions of pain and joint function for documentation of treatment outcomes, which can also be used to assess preparticipation injury risk status.

Susceptibility to lower extremity injury appears to be increased by low back dysfunction, and lower extremity dysfunction appears to increase susceptibility to low back injury.4 A history of low back injury has been associated with a six-fold increase in the probability of sustaining another low back injury during participation in college sports.9

The Oswestry Disability Index (ODI) has been shown to be a useful tool for quantification of the effects of low back dysfunction. Wilkerson et al10 found that an ODI score of 6 or higher (on a 0-100 scale of disability) was associated with an OR of 2.64 for prediction of core and lower extremity sprains and strains among college football players, which suggests that even a mild degree of low back dysfunction has an effect on lower extremity injury susceptibility. A possible explanation for this association is the reduction of multifidus cross-sectional area that results from a low back injury, which has been found to persist beyond resolution of acute symptoms among elite athletes who are engaged in physically demanding activities.11,12

Anthropometric measures that have been associated with elevated injury risk include body mass index, estimated mass moment of inertia (MMOI), foot width index, and Q angle. Milgrom et al13 reported that body mass (kg) multiplied by squared height (m2) provides a good estimate of MMOI, which they found to be associated with risk of ankle injury. A large proportion of body mass that is located at a great distance above the axes of rotation of weight-
bearing joints creates a large external moment, which must be counteracted by generation of an internal moment (i.e., muscle tension) to maintain dynamic joint stability. An unpublished analysis of data collected at our institution strongly suggests that optimal endurance of the posterior core musculature (i.e., back extensors, hip extensors, and hamstrings) can compensate to some extent for elevated ankle sprain risk imposed by a high MMOI.

Clinical tests of functional capabilities

The functional movement screen (FMS) is probably the most widely used method for assessment of multisegmental musculoskeletal dysfunction. The FMS involves assignment of a 0 to 3 rating for each seven fundamental movement patterns believed to provide the foundation for more complex athletic movements of both the upper and lower extremities (i.e., maximum score = 21). Kiesel et al14 reported an OR of 11.7 for FMS prediction of “serious” injuries among professional football players that were classified on the basis of a score of 14 or lower (of a possible 21 points), but much smaller values for predictive power have been reported by other researchers.15-17

The Y-balance test is a simplified three-component version of the original eight-component Star Excursion Balance Test (SEBT), both of which involve simultaneous maintenance of unilateral postural balance and achievement of maximum reach distance with the foot of the nonstance lower extremity. A composite performance value is calculated to represent average reach distance as a percentage of leg length for different reach directions (leg length equal to the distance from the anterior superior iliac spine to the distal tip of either the fibular malleolus18 or the tibial malleolus19,20).

For prediction of lower extremity injuries in male and female high school basketball players, Plisky et al18 reported an adjusted OR of 3 for risk categorization on the basis of a three-direction composite reach distance (i.e., anterior, posteromedial, and posterolateral directions) of 94% of leg length or less. Sato et al19 reported a lower extremity injury OR of 5.3 for three-direction composite reach distance of 69.5% of leg length or less in male and female high school basketball players. For prediction of lower extremity injuries in college football players, Ford et al20 reported an OR of 5.4 for anterior reach distance of 68% of leg length or less. Thus, the anterior reach component of the Y-balance test and the SEBT, which requires both unilateral postural stability and eccentric quadriceps control of the weight-bearing extremity, appears to be the best predictor of lower extremity injury risk.

The wall-sit hold test that was first reported by Wilkerson et al10 has been repeatedly modified in an effort to enhance its sensitivity for detection of injury risk and to make its administration more efficient (Figure 1). The original bilateral wall-sit hold test average performance for college football players was 79 ± 34 seconds;10 a hold time of 60 seconds or less was associated with an OR of 2.1 (Figure 1A).

Subsequent test modification from bilateral to unilateral support of body weight reduced average performance to 61 ± 27 seconds, and a hold time of 45 seconds or less was associated with an OR of 2.1 (Figure 1B). Further modification of the unilateral test procedure reduced average performance to 28 ± 14 seconds, while preserving an OR of 2.0 for a hold time of 30 seconds or less (Figure 1C).

The wall-sit hold test appears to effectively identify athletes who experience rapid fatigue in muscles that are important for maintenance of optimal alignment of the lumbar spine, pelvis, hip, knee, and ankle. The prolonged muscle activation required to maintain the unilateral wall-sit hold position may relate to the ability to avoid excessive hip adduction and knee valgus during dynamic activities, which are clearly related to knee injury risk.21 Because the test imposes a demand on the quadriceps to prevent knee flexion and a simultaneous demand on the hamstrings to prevent hip flexion, poor test performance might indicate a diminished ability to dynamically stabilize the knee joint through cocontraction of the antagonistic muscle groups.

Strategies for reducing injury risk

The findings of numerous studies have demonstrated the importance of core stability, proprioception, postural balance, and neuromuscular control for reduction of lower extremity injury risk. Most injury prevention studies have addressed risk for a specific type of injury, but the apparent association between core stability and various types of lower extremity injury suggests a need for a multisegmental approach to development of injury prevention programs.4,22 Wilkerson et al10 have demonstrated the combination of a high level of exposure to game conditions, poor core muscle endurance, and a relatively low level of low back dysfunction may substantially elevate risk for any core or lower extremity sprain or strain during participation in football. Although exposure to game conditions was the strongest predictor, both the other predictors relate to the core stability concept.

A recent literature review by Reed at al23 supported the importance of a strong and stable core for optimal athletic performance, but only 13 of the 24 reviewed studies involved participants who were competitive athletes, and the best results were reported for athletes who performed sport-specific exercises. Further research is needed to identify the specific core training techniques that are most effective for reduction of injury risk. As more research evidence becomes available, clinicians will probably be able to perform increasingly accurate assessments for identification of individual athletes who possess elevated injury risk, and injury risk reduction strategies will probably be targeted to address the individual athlete’s needs in a more sophisticated manner than the traditional one-size-fits-all approach to injury prevention.

Valerie Snider, MS, ATC, is an athletic trainer at Tennessee Ortho­paedic Clinics in Knoxville, TN, and Gary Wilkerson, EdD, ATC, is a tenured professor in the Graduate Athletic Training Program at the University of Tennessee at Chattanooga.

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