spine inward , increasing lordosis . When contracted jointly with the diaphragm and pelvic floor muscles , they provide increased spinal stability . Rectus abdominis is typically trained to cause trunk forward flexion , and the thinking that crunches and sit ups are the only way to get rock hard abs has led to an over dominance of this muscle in all core work . In addition to these are the connecting muscles that attach to ribs and thoracic spine , as well as the muscles of the hip and thigh which all play a role in influencing trunk function . Finally , the muscles that are often forgotten when trunk strengthening are the diaphragm and the pelvic floor . ‘ True spine stability is achieved with ‘ balanced stiffening from the entire musculature , including the rectus abdominis and the abdominal wall ; quadratus lumborum ; latissimus dorsi ; and the back extensors of longissimus , iliocostalis , and multifidus ’ ( McGill 2010 ).
Tranversus abdominis and hollowing
One of the common threads that has come through in all research is the need to maintain a normal lumbar curve in order to allow the trunk musculature to function effectively ( Reeve & Dilley 2009 ). Research shows that maintaining normal lumbar posture is important for general spine health and care should be taken with lumbar flattening or allowing posterior tilting in exercises when targeting improved lumbo-pelvic control . McGill believes it is impossible to train muscles such as transverse abdominis or multifidus in isolation – people cannot activate just these muscles ( 2009 ). Hence , all the research around the contraction of TrA is questionable .
Watanabe et al . ( 2015 ) found that seated with an anterior tilt and standing postures where TrA was contracted , created more cocontraction of multifidus than seated with neutral or posterior tilted postures . In contrast , Pinto et al . ( 2011 ) found that patients with no low back pain were able to achieve increased thickness in TrA when in flattened lumbar spine , compared to normal lumbar curve , and people with low back pain were not . However , muscle activity of accompanying muscles of the trunk was not monitored , and abdominal bracing combined with flattening of the lumbar spine may in fact actively promote the rectus abdominis muscle . A study by Jung-Seok Lee and colleagues ( 2015 ) compared the abdominal drawing-in exercise using biofeedback with curl ups , side bridges and bird dog exercises . They found little difference between the groups , although the TrA was found to be thicker after the intervention for the group focused on the abdominal drawing-in exercise . It has also been reported that drawing in the abdominals fails to target the major stabilisers of the spine and actually reduces stability ( Potvin and Brown 2005 ), yet many trainers continue to promote this action when teaching core training exercises . A study by Koumantakis et al . ( 2005 ) compared the addition of specific stabilisation exercises , including specific TrA contraction , to a general back and abdominal muscle exercise approach for patients with subacute or chronic nonspecific back pain . They found that adding the abdominal hollowing , through trying to add TrA activity , actually reduced effectiveness of the activity and provided no additional benefit to patients with subacute or chronic low back pain .
The value of contracting TrA for sacroiliac stability has also been questioned . Richardson et al ( 2002 ) suggested if TrA is contracted independently of the other trunk muscles it would decrease laxity of the sacroiliac joint , yet Gnat and colleagues found no stiffening of this joint when contracting the same muscle ( 2015 ). Ainscough- Potts et al . ( 2015 ) reported that when the subjects sat on a ball supported by both legs , there was no change in the transversus abdominis and internal oblique activity . However , when subjects sat on a ball supported by one leg , the activity of transverse abdominis increased . It appears that single leg activity creates greater demand on these muscles than two-leg seated or standing postures . Most researchers agree that the TrA has a role in stability and needs to be trained separately to other trunk muscles and taught how to contract tonically ( Hodges 1999 ). However , there remain questions over an individual ' s ability to contract TrA without the use of ultrasound .
Increased passive stiffness appears to be the end goal for many core training programs as it enhances load bearing ability , reduces painful vertebral micromovements , and enhances ballistic distal limb movement ( Lee et al . 2015 ). These researchers found that isometric stiffness training was far superior in improving trunk stiffness when compared with a dynamic training program over six weeks . The researchers suggest this was due to the increased time under tension of isometric exercises and the low loads allowing people to perform them daily .
The key reason why core stability training has become prominent in training and research is its role in low back and prevention of lower limb injury .
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