Biomechanical consequences of running with deep core muscle weakness
Introduction
In 2014, almost 19 million people completed a running road race in the USA, a 300% increase from 1990 (Running, 2015). Unfortunately healthy running habits are often interrupted by running injuries, with the annual running injury rate ranging from 24% to 65% (Macera et al., 1989, Marti et al., 1988, van Mechelen, 1992), and novice runners may be the most susceptible to developing these injuries (Tonoli et al., 2010). The prevalence of low back pain (LBP) in runners has been reported to be as high as 14% (Taunton et al., 2002, Taunton et al., 2003, Woolf et al., 2002). LBP is most often a chronic and recurrent condition that limits numerous activities of daily living (Hoy et al., 2010). Despite its high prevalence, the root cause of LBP remains unclear (Hoy et al., 2010). Improving core muscle strength and stability has shown potential to alleviate symptoms of LBP (Chang et al., 2015), however it remains unknown how one’s level of core strength may affect LBP risk.
The core musculature can be functionally separated into two groups, the superficial muscles and deep muscles (Willson et al., 2005). The superficial muscles (rectus abdominis (RA), external obliques (EO), internal obliques (IO), latissimus dorsi (LD), superficial fascicles of the erector spinae (ES)) primarily function to produce movement and transmit forces from the thoracic cage and pelvis to the extremities (Bergmark, 1989). The deep muscles (quadratus lumborum (QL), psoas major (PS), multifidus (MF), and the deep fascicles of the ES) attach directly to the lumbar vertebrae and are believed to function primarily to stabilize the lumbar spine (Fredericson and Moore, 2005). The deep core muscles have been shown to activate prior to voluntary movement of the lower extremities (Hodges and Richardson, 1997), suggesting they may act to stabilize the spine in preparation for the loads experienced during dynamic tasks like running (Fredericson and Moore, 2005). While a sufficient level of deep core muscle activation is needed to stabilize the spine, excessive or improper activation of these muscles could lead to abnormal spinal loading and consequently, LBP (Freeman et al., 2010, Hides et al., 2008). The transversus abdominis (TrA) is another deep core muscle which may contribute to spinal stability (Hodges, 1999), however it does not attach directly to the lumbar vertebrae and its mechanism for providing stability by increasing intra-abdominal pressure is different than the other deep core muscles (Hodges, 1999). The deep core muscles are often targeted by clinicians for treating musculoskeletal injuries primarily associated with the spine. The MF, QL, and TrA are the deep core muscles currently most often targeted (Martuscello et al., 2013), as research has associated dysfunction of these muscles with LBP (Freeman et al., 2010, Hides et al., 2008, Hodges, 1999). Dysfunction of the PS, especially during running, may affect back pain as it not only attaches to the spine but is also a primary hip flexor (Barker et al., 2004).
Most of the core strength and stability training exercises commonly performed by athletes emphasize the superficial rather than the deep musculature (Faries and Greenwood, 2007). Neglect or improper training of the deep core muscles will likely result in under-utilized and weak deep core muscles. Runners competing at elite levels, who have extensive training resources available, have been observed to have underdeveloped core musculature (Fredericson and Moore, 2005). Dysfunction of the deep core musculature during a repetitive, dynamic activity like running may lead to improper loading placed on the spine, poor muscular coordination, compensatory movement patterns, muscle strain, or injury to spinal structures (Fredericson and Moore, 2005, Hibbs et al., 2008). While there are many current opinion and review articles addressing the importance of core strength and stability in athletic function (Fredericson and Moore, 2005, Hibbs et al., 2008, Kibler et al., 2006, McGill, 2010, Willardson, 2007, Willson et al., 2005), very few high quality research studies exist that directly investigate this relationship in regards to running performance and injuries.
The deep core muscles directly influence spinal loading during dynamic tasks. Both elevated compressive and shear forces exerted on the spine have been associated with low back injuries like LBP (Marras et al., 2001). Compressive forces have higher magnitudes when compared to shear magnitudes, however spinal structures are much stronger in the axial direction than when loaded in shear (Gallagher and Marras, 2012). Therefore both increased compressive and shear loads may be detrimental to spinal structures, especially when this loading is repeated over thousands of gait cycles.
The purposes of this study were to identify potential strategies that could be used to compensate for weakness of the deep core musculature during running and to identify accompanying changes in spinal loading using simulations. We hypothesized that the superficial core musculature and other unaffected deep core muscles could compensate for muscle weakness by increasing their force production during running. Additionally, we hypothesized that the muscular compensations associated with deep core muscle weakness would result in increased loading on the lumbar spine.
Section snippets
Participants
Eight healthy participants (6F/2M, 1.71 ± 0.07 m, 66.00 ± 14.68 kg, 22.37 ± 3.93 y) provided IRB-approved informed consent. Participants were recruited from surrounding communities and were included if they had no history of musculoskeletal injury for at least 3 months and were between the ages of 18 and 55. Exclusion criteria included a history of recurrent back pain, previous abdominal or lower extremity surgery, and a BMI greater than 30. Participants had minimal to no athletic experience,
Muscle compensations
Only the superficial and deep trunk muscles were investigated as potential compensators for deep core muscle weakness. As muscles were progressively weakened, the level of required compensations increased. Table 1 shows all significant muscle compensations required to maintain original running kinematics for individual and all deep core muscle weakness. The MF and deep ES required the largest number of muscles to compensate when each of these muscles was weakened individually, while the PS
Discussion
Using kinematically-driven simulations, this study estimated that deep core muscle weakness increased compressive and shear loading on the lumbar spine. The superficial trunk muscles were found to be the primary compensators for many conditions of muscle weakness, indicating these muscles may be at highest risk of injury or fatigue when the deep core muscles are weak. As LBP symptoms can present from both structural damage to the spine due to excessive loading and from a muscular injury (Deyo
Conclusions
In conclusion, the large, superficial trunk muscles were the primary compensators for deep core muscle weakness. For individual muscle weakness, the largest compensations were required when the deep ES was weakened, which also led to the largest increases in compression of the upper lumbar vertebrae, decreases in compression of the lower lumbar vertebrae, and increases in shear loading on all lumbar vertebrae. Sufficient strength of the deep core musculature as a whole, and especially the deep
Acknowledgements
The authors would like to acknowledge support from National Institute of Arthritis and Musculoskeletal and Skin Diseases Grant R03-AR065215.
Conflict of interest
None.
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