In theory, core training is performed to improve performance, prevent injuries, and treat lower back injuries. For example, according to McGill (20), “The well-trained core is essential for optimal performance and injury prevention.” Statements such as this abound in the exercise science literature, popular media, and even product advertisements. Despite the frequency with which these statements occur, the evidence to support them is lacking, contradictory, or taken out of context. In this article, the underlying structural assumptions behind core training will be investigated and a review of the literature on core training and performance improvement, injury prevention, and treatment of lower back injuries will also be conducted.
STRUCTURE AND FUNCTION
A number of authors have presented progressive training programs for the core (6,7,32). These training recommendations for the core are often based on the structure of the spine proposed by Panjabi (25,26) in 2 articles in 1992 and by Hodges and Richardson (12,13) in 2 articles in 1997. According to Panjabi (25), the spinal system functions to allow movements between the limbs, carry loads, and protect the spine and nerves. He describes a spinal stabilizing system that consists of the following parts:
- A passive musculoskeletal subsystem including vertebrae, facet articulations, intervertebral discs, spinal ligaments, and joint capsules. The passive subsystem functions at the extremes of the range of motion.
- An active musculoskeletal subsystem, including muscles and tendons. The subsystem that generates force.
- Neural and feedback subsystem, receives information and instructs the active subsystem to achieve stability.
Panjabi (25) reports that the passive subsystem of the normal spine buckles at loads of 2-9 kg. These loads are exceeded in the activities of daily life and athletics. For example, according to Nachemson (21), normal loads for the lumbar vertebrae may reach 100-180 kg in sitting and 80-150 kg in standing.
The model of Panjabi and the reports of the loading capacity of the spine (i.e., 2-9 kg) need to be used with care. The majority of this is established through in vitro research, which either involves studying cadaver vertebrae or involves biomechanical modeling. This research is limited because it seeks to study tissue in isolation and is unable to take into account the many ligaments, muscles, bones, tendons, how they interact, and confounding factors such as fitness level and injury history.
Hodges and Richardson (12,13) demonstrated that the transversus abdominis is activated in an anticipatory manner before the muscles of the upper and lower extremity during upper and lower extremity movements. This leads to the assumption that “all movements either originate in or are coupled through the trunk” (9). It is not clear from the research of Hodges and Richardson, however, that this anticipatory activation exists in all tasks. It is also not clear that this is something that requires specialized training.
According to Willardson (37), “certain individuals have promoted core stability exercises for sports conditioning with little scientific evidence to support their claims.” Theoretically, the core impacts athletic performance in several ways. First, it stabilizes the trunk and pelvis, which could be beneficial to technique (9,18). Second, it is a vehicle to transfer force from the lower body to the upper body (during a shot put or a baseball pitch for example) (9,18). Second, it is a vehicle to transfer force from the lower body to the upper body (during a shot put or a baseball pitch for example) (9). Finally, the core's muscles may actively be recruited to perform an athletic movement (e.g., one involving rotation) (9).
Despite the performance enhancement claims, there has been little literature attempting to link core training with athletic performance. The research has produced mixed results. Note that Table 1 summarizes the core training programs for each of the studies that will be covered in this section.
Stanton et al. (31) investigated whether 6 weeks of stability ball training had an impact on running economy in high school athletes. Their subjects were divided into 2 groups, an experimental group, which performed 6 weeks of stability ball training in addition to basketball/touch football skills training and run-based conditioning, and a control group that did not perform the stability ball training but performed the skills and run-based conditioning.
After 6 weeks of training, the experimental group significantly improved their core stability (the control group did not change their core stability during that time). However, there was no relationship between improvements in core stability and maximal oxygen consumption, running economy (measured as the volume of oxygen consumed at 60, 70, 80, and 90 percent of maximal oxygen consumption), and mean trunk angle relative to vertical during the maximal oxygen consumption treadmill test. The authors of the study note that while core stability improved, it is possible that some combination of the participants' training status, loading parameters, or exercise selection might have contributed to the failure to observe an effect on running.
Sato and Mokha (29) also looked at core training and running performance. The study investigated the impact of a 6-week core training program on recreational runners and specifically looked at 5-km performance, kinetics, and lower extremity stability. They also had 2 groups, an experimental group that performed the core training 4 times per week for 6 weeks and a control group that maintained their training. At the conclusion of the training, the core training did not have a significant effect on ground reaction force or lower extremity stability. The core training group did improve their 5-km time by 47 seconds (compared with only 17 seconds with the control group), which was a statistically significant difference. The time results should be considered carefully, however, as the control group began the study noticeable faster than the experimental group (the control group ran 5 km in 26 minutes:30 seconds and the experimental group in 29 minutes:29 seconds at the pretests).
Abt et al. (1) investigated the relationship among core stability, cycling mechanics, and pedal force application in 15 competitive cyclists. Subjects completed 2 tests. In the first test, the subjects performed an incremental cycling test where they cycled at 25.8 km/h with an incline that increased by 1% every 3 minutes until exhaustion. In the second test, the subjects performed a 3-minute isokinetic rotational test, a 32-minute core circuit workout, a postworkout 3-minute isokinetic rotational test, and then the incremental cycling test. The 2 testing sessions were separated by a week.
The core circuit workout had a significant impact on kinetic measures, indicating that the workout did fatigue the core muscles. During the performance of the second cycling test, knee and ankle extraneous motion increased, although the kinetic measures of cycling performance did not change. The results suggest that subjects were selecting different movement strategies to maintain their power levels and compensate for fatigue. These altered movement patterns could contribute to abnormal wear and tear on the knee over time.
Tse et al. (34) examined the effect of 8 weeks of core training on the performance of 34 college-aged rowers. The rowers were divided into an experimental group, which performed 8 weeks of core training twice a week, and a control group, which did not perform the core training. The authors only describe their core training program in general terms, the specifics are not provided. Both the groups continued their rowing training during the study. The results in this study were mixed in that the core training group improved on one measure of core stability (the side bridge), the control group improved on one measure of core stability (back extensor), and neither group improved on the abdominal fatigue test. Neither group improved on any of the performance measures (vertical jump, broad jump, shuttle run, 40-m sprint, medicine ball throw, or 2000-m rowing test). When reviewing this study, it is difficult to draw conclusions about the effectiveness of core training on performance. The training intervention was not sufficient to improve the measures of the core stability. It is possible that given a more effective core training program, the results could have been different.
The above 4 studies largely look at core training and aerobic performance. Nesser et al. (24) examined the relationship between core stability and strength/power as measured by the vertical jump, shuttle run, 20/40-yd sprints, and 1-repetition maximum bench press/squat/power clean in Division I football players. According to the authors, the correlations “…ranged between weak and moderate, and they are not consistent.” The authors postulated that there could be 2 reasons for this weak and inconsistent relationship. First, the core tests measure endurance, whereas strength and power are different qualities. Second, core strength may only play a minor role in strength and power performance.
Results of studies looking at core training and performance would seem to suggest that if core training has a role in performance, it is very small. This suggests that if the goal is performance improvement, the strength and conditioning professional should not be putting a great deal of emphasis on core training. In fact, Willardson (37) states that improvements in core stability are likely skills specific, and healthy athletes who are performing “traditional” resistance exercises are probably receiving enough core training. This suggests that if the goal is performance improvement, strength and conditioning professionals can probably achieve this effect through the use of multijoint exercises such as cleans, snatches, jerks, pulls, deadlifts, and squats.
The author of this article, working with primarily collegiate track and field athletes, focuses on multijoint exercises to improve performance. With the exception of the outdoor season, training sessions for those athletes are geared around one of the following foci: maximal strength, power, and hypertrophy. The maximal strength sessions take a total body approach and include variations of the squat, deadlift, bench press, military press, and rows.
The power sessions focus on variations of the clean, snatch, pull, and jerk. Hypertrophy sessions focus on upper- and lower-body exercises, primarily multijoint exercises, to encourage anatomical adaptations. The number of sessions, exercises, nature of exercises, load, and volume vary according to the athlete's development and the time of year. The author is also conscious that this age group wants to perform core training for the sake of vanity. To accommodate that, the author alternates among 3 types of core training that are used primarily during the warm-up.
The first takes a circuit approach and includes exercises that are variations of the crunch, sit-up, and leg raise. The second type also takes a circuit approach and focuses on stabilization exercises such as the stability ball and the various holds (prone hold, side hold, supine hold, etc). The final type uses medicine balls for throws, and this also allows the author to combine the core training with the injury prevention work (e.g., chest passes on 1 leg to strengthen ankles and shins).
Statements linking core training and the prevention of injuries fail to identify what specific injuries core training is meant to prevent. Although this invariably refers to lower back injuries, there are a number of challenges in preventing lower back injuries. First, in the literature, lower back pain (LBP) is divided into 2 types: specific and nonspecific (35). Specific LBP (SLBP) refers to the symptoms caused by a specific pathophysiologic mechanism. For example, a disc herniation. SLBP affects an estimated 10% of patients with LBP (35). Nonspecific LBP (NSLBP) refers to the symptoms without a clear specific cause (35). This is an important distinction because it is possible that the 2 types of LBP have different causes and respond to exercise differently.
Second, it is unclear what causes NSLBP. The literature is often conflicting. For example, various authors have found a relationship among age, body mass index, sports participation, heavy lifting at work, and NSLBP (3,10,30). Other authors have found no relationship among range of motion, sitting at work, sleeping, playing sports, prolonged standing/walking, and exercise (2,3).
Bakker et al. (2) in a literature review note that there is conflicting information about the role of a number of activities traditionally thought to contribute to LBP and the occurrence of NSLBP. This may be complicated further by the fact that the “cause” of NSLBP may not even have originated around the onset of symptoms, and it may have been in response to something that occurred earlier in life than the manifestations of the pain (28).
SLBP results from traumatic loading or degenerative disease (38). McGill (20) describes a gradual process of degeneration leading to injury. Other authors do not describe this gradual process, referring instead to an “incident” that causes the SLBP (30,35,38).
It is theorized that an insufficiency of the trunk muscles may increase the risks of SLBP and NSLBP (33,38). If true, this would mean that fatigue negatively impacts the control and coordination of the muscles that protect the lumbar spine, increasing injury risks (27).
The fact that the cause of NSLBP is unclear, combined with the fact that SLBP may be caused by an “incident,” suggests that the core training may not be able to prevent injuries to the lower back. In fact, although some authors state that exercise reduces the risk of many ailments including LBP (15), the results in the literature are conflicting. Table 2 provides an overview of the core training programs used in the literature that will be described below.
Nadler et al. (22) when looking at the effect of a periodized, 30-45 minutes per day, and 2-5 times per week core strengthening on LBP in Division I athletes found conflicting results. Male athletes had a statistically insignificant reduction in the occurrence of LBP, whereas female athletes had a statistically insignificant increase in the occurrence of LBP. Durall et al. (5) investigated the relationship between a 10-week, 2 times per week core training program on LBP in Division III female gymnasts. The study found that the gymnasts did not suffer a new episode of LBP during the season. However, it is difficult for Durall et al. to prove the cause and effect in this study as other confounding factors could have contributed to the lack of LBP episodes.
If the core training prevents LBP, then there should be a statistical relationship between the performance of the core and incidents of LBP. Balague et al. (3) did not find a relationship between trunk muscle performance (as measured by maximal isometric/isokinetic flexion, extension, lateral flexion, and rotation strength tests) with LBP.
Proving that exercise prevents LBP is difficult. It is logical that it could prevent NSLBP if the idea that fatigue impacts control and coordination is accepted, although the literature is hardly conclusive. It is more difficult to demonstrate that exercise prevents SLBP. McGill (20) suggests that even SLBP has a gradual onset, a series of degenerations that eventually result in an incident. Other authors do not agree with that approach. If the model of McGill is correct, then exercise could contribute to preventing SLBP because it might prevent the gradual deterioration that leads to the injury.
To the author's knowledge, the literature does not demonstrate that one type of core training is more effective than others in terms of prevention. The studies discussed above employed multijoint strength training exercises (e.g., squats), stability exercises (e.g., side bridges), and more traditional abdominal exercises such as sit-ups. This is an area that needs further research.
Exercise is used as a treatment for LBP; in fact, some authors consider exercise and NSAIDs to be the most effective treatments for LBP (38). Exercise is often used as a treatment for LBP under the rationale that individuals experiencing LBP have different muscle activation patterns than those who are asymptomatic (14). For example, Nelson-Wong and Callaghan (23) found that those individuals who developed LBP as a result of a 2-hour standing task had different muscle activation patterns of the gluteus medius and the trunk flexors/extensors.
Is exercise an effective treatment for LBP? Several studies have found a positive effect of exercise on LBP, and others have real concerns with how that research is conducted and with its significance.
Some studies show that exercise can be effective in the treatment of LBP. Table 3 provides an overview of the core training programs used in these studies. Taken together, these studies indicate that core training is effective in preventing recurrences of LBP (11), reduces disability from LBP (8), and improves daily life performance measures (19). The challenge with these studies is that in several cases, the core training program is not described in detail, which makes providing the strength and conditioning coach with a recommendation impossible.
Stabilization and lumbar strengthening exercises are not the only types of exercises to be effective with patients with LBP (35). Kell and Asmundson (16) studied 27 subjects with NSLBP. Subjects were divided into a control group, a resistance training group, and an aerobic training group. The resistance training group participated in a 3 times per week, 16-week long periodized resistance training program focusing on 11 exercises (barbell, dumbbell, machine, and bodyweight) that focused on the entire body. The aerobic training group also performed 3 sessions per week for 20-35 minutes each session. The control group did nothing. At the conclusion of 16 weeks of training, the aerobic training group made significantly greater improvements on measures of pain, disability, and quality of life compared with the control group. The resistance training group, however, made significantly greater improvements on measures of pain, disability, and quality of life compared with both the control and the aerobic training groups.
Taken together, these studies seem to indicate that exercise has an effective role in the treatment of LBP. Not all authors agree with this, however. There are several criticisms of exercise studies and their results. First, they have small effect sizes, which impact the significance of the results (4,17). Second, there is little long-term follow-up (17). Third, some authors feel that the treatments for acute LBP are just as effective as placebo (4). Weiner (36) goes so far as to state that “degenerative disc disease is the biostatistical norm in humans.” In other words, LBP is the price of walking upright. In his editorial, he essentially suggests that treating degenerative disc disease is akin to having cosmetic surgery performed that is often unnecessary.
The studies above have some limitations. There are different injuries or diseases that cause SLBP; none of the above studies take these into account. This is an area that needs further research because different injuries and different diseases could certainly modify the treatment that is provided.
Exercise science literature, popular media, and even product commercials extol the virtue of core training for the improvement of performance, prevention of injuries to the lower back, and treatment of lower back issues. Despite these claims, the literature is hardly conclusive about the benefits of core training. Taken together, there is not enough evidence for the benefits of core training and performance to warrant this mode of exercise making up a significant part of a strength and conditioning program. With regard to the prevention of injuries, the information is conflicting, and there is a real need to look not only at NSLBP versus SLBP but also at which types of exercise are more effective than others. In terms of treatment, exercise seems to be effective (although this is controversial), but there is a need for greater detail of those exercise programs in the literature.
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