%ΔME (P = 0.29) or %ΔMA (P = 0.38) were not significantly different in male athletes who had occurrence of LBP in 1999–2000. These results were no different than that noted in 1998–1999: %ΔME (P = 0.51) and %ΔMA (P = 0.30), before incorporation of the core-strengthening program, were also found not to be predictive of LBP in males.
Core strengthening and LBP incidence.
LBP is a common problem in any sport that requires significant rotatory or twisting motions, repetitive flexion, and/or extension or where previous lower extremity injury may have altered the normal mechanics of the kinetic chain (4,6,16,19,27,31). Female athletes have been demonstrated to be more likely to suffer from its occurrence than males (7,10,21,26,29,30). According to NCAA Injury Surveillance Data 1997–1998, female athletes were almost twice as likely to sustain injury to their lower backs than males. In women’s basketball, the low back was the third most commonly injured body region after the ankle and knee. In women’s volleyball and soccer, low back injury was the most and second most common injury sustained during the spring season (29). According to NCAA Injury Surveillance Data 1998–1999, low back injury was the most common injury in women’s gymnastics during competition (30). In addition, it was the second and third most common injury in practice, in women’s gymnastics, field hockey, and volleyball, respectively (30). Nadler et al. previously demonstrated 8% more female athletes reporting LBP than male athletes (24). Gender difference in LBP occurrence may be a result of differences in gait, playing style, and pelvic anatomy, but at the present time, the exact reasons remain unknown (7,10,13,19).
Although no significant differences were found in LBP occurrence between the 1998–1999 and 1999–2000 academic years, some interesting trends were observed. Males had a nonsignificant reduction in LBP occurrence after incorporation of the core-strengthening program. Specifically, LBP occurrence in male athletes decreased from 8 of 101 (8%) to 7 of 162 (4.3%). Although these present data are not sufficient to conclusively prove the role of the core strengthening in LBP reduction, they certainly demonstrate the importance of further studies to better understand the effects or advantages of the core-strengthening program in collegiate athletes. Interestingly, female incidence of LBP slightly increased after incorporation of the core-strengthening program. Specifically, female LBP occurrence increased from 5 of 63 (7.9%) to 7 of 74 (9.5%). It is conjectured that this increase may be an indication that the core-strengthening program may need modification to better accommodate female subjects. Once again, the small numbers of individuals who actually developed LBP makes any conclusions difficult.
Core strengthening and hip extensors.
Hip extensors (gluteus maximus) play a major role in stabilizing the pelvis during trunk rotation, or when the center of gravity is grossly shifted. Several studies support hip extensor involvement in individuals with LBP (18,22). Kankaanpaa et al. demonstrated increased fatigability of the gluteus maximus in individuals with chronic LBP (18). Leinonen et al. also demonstrated the gluteus maximus to be more easily fatigued in those with nonspecific chronic LBP, but noted improvement in the latency of firing in the gluteus maximus after rehabilitation (22). Although not fully understood, side-to-side strength differences of the hip extensor muscles have been attributed to both injury and/or specialized training/rehabilitation (27). Hewett et al. found that, in response to a jump training program, female subjects had a 44% increase in hamstring muscle power on the dominant side and only a 21% increase on the nondominant side (11). Our data for 2000–2001 demonstrated a significant increase in right hip extensor strength after incorporation of the core-strengthening program. This strength increase of the right hip extensors, the dominant side for 90% (192 of 213) of athletes in our study, is consistent with the results of Hewett et al.’s study. A similar response to training would be expected for both the hamstrings and gluteus maximus, as these two muscle groups work synergistically to extend the hip and stabilize the pelvis (17). Weakness of the right hip extensors in female athletes was previously noted in those with a history of LBP and in those who ultimately developed LBP (27,28). Therefore, the tendency for the core-strengthening program to increase right side extensor strength may be perceived as potentially beneficial. We are uncertain why hip extensor differences noted in 1999–2000 no longer influenced the development of LBP in female athletes (Fig. 5) as it did in 1998–1999 (Fig. 6). As shown in Figure 5, regardless of %ΔME, the probability that a subject would not develop LBP is high in 2000–2001. It is speculated that this occurrence may be evidence of some success of the core-strengthening program.
Male athletes demonstrated no significant relationship with %ΔME and LBP occurrence in 1998–1999 or 1999–2000. As mentioned previously, a statistically nonsignificant reduction in LBP occurrence after incorporation of the core-strengthening program was observed. This decline in LBP may also be a result of the positive effects of core strengthening on function of the hip extensors. Alternatively, abdominal muscle strength improvement may have affected LBP occurrence (12,32,33). More investigations are needed, however, because of the low incidence of LBP in this population.
Core strengthening and hip abductors.
Biomechanically, the hip extensors and abductors play a major role in all ambulatory activities, working synergistically to stabilize the pelvis and transfer forces from the lower extremities to the spine (17,23,31). Jaramillo et al. and Beckman and Buchanan noted strength and firing differences in the hip abductors in individuals with distal involvement of the lower extremities (1,14). Johnson observed that excessive hip slide, and sports that require high-speed rotation of the hip abductors, may predispose to low back injury (16). Nadler et al. noted that, for both male and female athletes (Fig. 7), there was no association between differences in side-to-side hip abduction strength and the likelihood of LBP occurrence in the ensuing 1998–1999 academic year (28). This outcome remained consistent for male athletes, after incorporation of core strengthening (1999–2000), but not for female athletes (28). As shown in Figure 4, female athletes with weaker left abductors were significantly more likely to develop LBP. Conversely, female athletes with stronger left abductors were significantly less likely to develop LBP. Lateral dominance within the lower extremities may help to explain this finding. Beling et al. found that the left leg is generally used for stance and posture, whereas the right leg is used for more coordinated function such as kicking and jumping (2). The hip abductor functions in midstance to stabilize the pelvis, preventing a downward inclination (Trendelenburg sign) during single leg stance. In the face of hip abduction weakness, increased muscular requirements of the lateral trunk stabilizers (i.e., quadratus lumborum) are necessary in order to better stabilize the pelvis. In light of issues of lateral dominance and an understanding of the kinesiology of pelvic and trunk musculature, increased abductor strength on the left side may theoretically help to prevent LBP occurrence.
Although the exact mechanism for these different findings are not known at this time, we speculate that the tendency of the core-strengthening program to primarily concentrate on extensor training (Fig. 4) may have contributed to the results. Isolated strengthening of the extensors may have resulted in some inhibition or neglect of the hip abductors, causing female athletes with weak left abductors, in particular, to be more prone to development of LBP. As noted previously, the hip abductor helps to maintain postural stability during midstance. Kollmitzer et al. demonstrated focused extensor training to result in decreased postural stability, in support of this concept (20). Because of the limitations of our data, in terms of study design and paucity of athletes with LBP occurrence, this discussion is speculative in nature. This theory would need to be clarified in a carefully modeled study. Overall, the need for both a more gender-specific strengthening program and the advantage of a more well-rounded and less-specific strengthening program may be implied by these data. Since abductor data were not collected during the 2000–2001 preparticipation physical, we are unable to identify whether the core-strengthening program had any influence on the average side-to-side differences in abductor strength that were previously observed.
There are several limitations noted in this pilot study. The number of athletes who required treatment for LBP was small, which could be secondary to poor record keeping by the athletic training staff, poor reporting by the athletes, better preseason conditioning, or as yet unknown factors. The changes that may have occurred in hip strength over the course of conditioning, as well as the effects of concomitant lower extremity injury during the competitive season, were not taken into account, which may have also influenced the results. Finally, other causes for LBP unrelated to hip strength could also have influenced the results, including concomitant medical conditions, lumbar disk injury, LBP secondary to factors outside of the sports season (i.e., work, school, and leisure activities), and previously unknown psychosocial issues. Overall, we feel the results of this study are noteworthy, but require further validation in a larger multicenter study.
Core strengthening has been advocated and utilized for conditioning athletes for years without any research to support an effect. A supervised core-strengthening program emphasizing the muscles of the trunk, spine, and hip extensors resulted in a statistically nonsignificant reduction in LBP in male athletes. Although the core program had no statistically significant effect on LBP occurrence, it may have been a factor in altering the dynamics of side-to-side hip strength in such a way as to have been advantageous in reducing risk for future LBP development. No reduction in LBP occurrence was observed in female athletes after inclusion of core strengthening, which may be related to the need for more isolated hip abduction strengthening in female athletes. The conclusions of the present study are limited secondary to the overall small numbers of subjects who developed LBP. Larger scale studies are needed both to validate the results of this study and to increase the overall understanding of the effects of core-strengthening programs.
Address for correspondence: Scott Nadler, D.O., Director of Sports Medicine, Department of Physical Medicine and Rehabilitation, UMDNJ-NJ Medical School, 90 Bergen Street, Suite 3100, Newark, NJ 07103; E-mail: firstname.lastname@example.org.
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Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
DYNAMOMETER; GENDER; GLUTEUS MAXIMUS; GLUTEUS MEDIUS