Dominic Stone
The abdominal muscles are a complex system of muscles that work together to control the movement of the spine, pelvis, and rib cage. They are responsible for trunk movement, maintaining proper posture, and normalising intra-abdominal tension. During trunk rotation, the external oblique, rectus abdominis, and lumbar multifidus muscles act contralaterally, while the latissimus dorsi, internal oblique, and transversus abdominis muscles act ipsilaterally. The oblique abdominal muscles are major axial rotators of the trunk, with the obliquus externus abdominis active during contralateral trunk rotation and the obliquus internus abdominis active during ipsilateral trunk rotation.
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What You'll Learn
The role of transversus abdominis (TrA) in trunk rotation
The role of the transversus abdominis (TrA) in trunk rotation is not yet fully understood, and there is some controversy surrounding it. However, studies have shown that TrA is consistently active during trunk rotation, with the upper fascicles exhibiting opposite recruitment patterns to the middle and lower fascicles. During left rotation, the lower and middle regions of contralateral TrA exhibit greater activity, while the upper region of ipsilateral TrA and obliquus externus abdominis (OE) are predominantly active during right rotation. This indicates that TrA activity varies between different muscle regions.
TrA is one of the four abdominal muscles, along with obliquus internus abdominis (OI), OE, and rectus abdominis (RA). These muscles work together to facilitate trunk rotation, which is involved in various activities, from walking to sports. During ipsilateral trunk rotation at lumbar vertebral levels, the internal oblique and TrA muscles exhibit significantly increased activity compared to the 0-10% range of trunk rotation.
The oblique abdominal muscles, OE, and OI, are the major axial rotators of the trunk. OE is active during contralateral trunk rotation, while OI is active during ipsilateral trunk rotation. However, the role of TrA in trunk rotation is more complex. While TrA is consistently active during trunk rotation, the degree of activation varies between different muscle regions.
Some studies have reported both unilateral and bilateral recruitment of TrA during trunk rotation, with greater electromyographic (EMG) activity on the side to which the trunk rotates. Other studies have reported bilateral activity of TrA without a significant difference in activation between sides. However, further analysis of the data suggests a trend toward direction-specific activation.
The lower and middle fascicles of TrA may stabilize the anterior aponeuroses and linea alba against the superior, lateral pull of contralateral OE. Meanwhile, the upper fascicles may control the inferolateral pull of contralateral lower and middle OI. This creates a stable platform from which OE and OI can generate rotatory forces. Additionally, TrA may control the motion of the rib cage, lumbar spine, and sacroiliac joints through tensioning of its musculofascial attachments and/or generation of intra-abdominal pressure.
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The external oblique (EO) and internal oblique (IO) muscles
The EO and IO muscles are involved in trunk rotation, with the EO acting contralaterally and the IO acting ipsilaterally. During ipsilateral rotation at thoracic vertebral levels, the muscle activity of the EO is significantly increased compared to the activity in the 0-10% range of trunk rotation. During early ipsilateral rotation at lumbar vertebral levels, the muscle activity of the IO is also significantly increased compared to the 0-10% range of trunk rotation. During contralateral rotation at both thoracic and lumbar vertebral levels, the EO muscle activity is again significantly increased compared to the 0-10% range.
The EO and IO muscles exhibit a side asymmetry, with the IO showing greater and significant thickness changes from a resting to contracted state than the EO. This was observed in all participants of a study involving sprint hurdlers and sprinters. The IO side asymmetry was significantly different between the two groups. The internal and external oblique muscles are involved in generating torque and providing general core stability. The EO is a superficial core muscle, while the IO is a deep core muscle.
The EO and IO muscles' activities during forced expiration in a trunk-neutral posture are insufficient for muscle strengthening. The activities may be affected by trunk postures, which can change respiratory muscle activity. The EO muscle's activity during forced expiration was significantly higher in the right trunk flexion position than in the neutral trunk and right trunk rotation positions. The left EO muscle's activity was significantly higher in the right trunk rotation position than in the neutral trunk and right lateral trunk flexion positions. The bilateral IO muscles' activity during forced expiration did not significantly differ among the three postures.
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The lumbar multifidus (MF) muscle
The multifidus muscle is an important stabilizer of the lumbar spine and plays a crucial role in maintaining static and dynamic spinal stability. It spans three joint segments and works to stabilize these joints, increasing the efficiency of each vertebra and reducing degeneration of the joint structures caused by friction from normal physical activity. The stiffness and stability provided by the multifidus muscle help to prevent low back pain, and weakness or atrophy of this muscle has been associated with chronic low back pain.
The lumbar multifidus muscle is involved in various movements of the vertebral column. When contracting bilaterally, the multifidus muscles extend the spine. Unilateral contraction of the multifidus muscle aids in lateral flexion of the spine to the same side and rotation of the spine to the opposite side. During trunk rotation, the lumbar multifidus muscle acts contralaterally, while the internal oblique and transversus abdominis muscles act ipsilaterally.
To strengthen the lumbar multifidus muscle, individuals can perform exercises that involve tensing the pelvic floor muscles for a few seconds, similar to the action of stopping urination midstream. This helps to recruit and strengthen the lumbar multifidus, improving spinal stability and reducing the risk of low back pain.
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Trunk rotation and its impact on spinal range of motion
Trunk rotation is a movement that involves both the thoracic and lumbar vertebrae and is important for many daily activities, from walking to sports. However, it is also a key risk factor for low back pain. The external oblique, rectus abdominis, and lumbar multifidus muscles act contralaterally during trunk rotation, while the latissimus dorsi, internal oblique, and transversus abdominis muscles act ipsilaterally.
The oblique abdominal muscles are major axial rotators of the trunk. The obliquus externus abdominis (OE) is active during contralateral trunk rotation, while the obliquus internus abdominis (OI) is active during ipsilateral trunk rotation. The role of the transversus abdominis (TrA) in trunk rotation is controversial. Some studies have reported bilateral recruitment of TrA during trunk rotation, with greater activity on the side to which the trunk rotates. However, other studies have found no difference in activation between sides.
During ipsilateral rotation at thoracic vertebral levels, the muscle activity of the latissimus dorsi and external oblique is significantly increased compared to the 0-10% range of trunk rotation. During early ipsilateral rotation at lumbar vertebral levels, the internal oblique and transversus abdominis also show significantly increased activity. During contralateral rotation at both thoracic and lumbar vertebral levels, the external oblique is again the muscle with significantly increased activity.
The ratio of rotational angle for each spinal segment affects the local range of motion and muscle activities. Repeated trunk rotation can lead to changes in the local range of motion, which may be a risk factor for low back pain. The coordination between the LD and EO muscles during ipsilateral trunk rotation is important. The LD is responsible for shoulder extension and controls upper trunk motions.
Spinal posture and pelvic fixation also influence the ability to rotate the trunk. For example, forward trunk inclination at 45 degrees increases trunk rotation by 19% but decreases pelvic rotation by 40%. In addition, alterations in posture can impact the relative flexibility of the system, which may be beneficial for patients with dysfunction and pain. Investigations of lumbar extension have shown a reduction in axial rotation when spinal posture is altered.
In conclusion, trunk rotation has a significant impact on the spinal range of motion, with specific vertebral segments and their corresponding muscles demonstrating increased activity during different ranges of trunk rotation. The coordination between certain muscle groups and the ratio of rotational angles also play a role in the local range of motion and muscle activities. Finally, spinal posture and pelvic fixation influence the ability to rotate the trunk and can impact the risk of low back pain.
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The importance of abdominal muscle flexibility
The abdominal muscles are vital for stabilising the core, supporting the spine, and facilitating movement. They are also important in preventing excessive forward tilt of the pelvis and supporting the trunk in postures that may be considered compromising to the lower back.
Weak or imbalanced abdominal muscles can lead to a host of issues, including lower back pain, poor posture, and reduced mobility. For example, inadequate core stability shifts pressure onto the lumbar spine, which can cause low back pain. Poor posture can manifest as forward head posture, rounded shoulders, and pelvic misalignment. A weak core can also impair functional movement patterns such as bending, lifting, and twisting.
The abdominal region comprises four key muscle groups, each with a specific role in core stability and movement. The deepest layer, responsible for compressing the abdominal cavity and supporting the lower back, is formed by the transverse abdominis. This muscle is one of the main important core muscles that contribute to supporting lumbopelvic stability, and its deficit in function affects our back, causing low back pain. The abdominal muscles also include the rectus abdominis, external oblique muscles, and internal oblique muscles. The external oblique muscles allow the trunk to twist, but to the opposite side of whichever external oblique is contracting. For example, the right external oblique contracts to turn the body to the left. The internal oblique muscles operate in the opposite way to the external oblique muscles. For instance, twisting the trunk to the left requires the left side internal oblique and the right side external oblique to contract together.
The abdominal muscles are also important in maintaining proper muscle balance, where agonist and antagonist muscle pairs maintain appropriate ratios of strength, flexibility, and length to one another. This balance is important for avoiding musculoskeletal injury. Flexibility assessment and exercises are crucial for lengthening muscles that are too tight. To move body segments, the muscles opposite those performing the movement (antagonist muscles) must lengthen sufficiently. Tight muscles, tendons, and ligaments limit lengthening of the antagonist muscles and thus reduce the range of movement of body segments. Soreness or injury may result when tight muscles are subjected to strenuous physical activity.
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Frequently asked questions
The external oblique (EO), rectus abdominis (RA), and lumbar multifidus (MF) muscles act contralaterally during trunk rotation.
The EO, RA, and MF muscles are involved in various activities, from walking to sports. They are also key players in the respiratory system, assisting with forced exhalation.
During contralateral rotation at both thoracic and lumbar vertebral levels, the EO muscle activity is significantly increased compared to the 0-10% range of trunk rotation. The RA and MF muscles work together to control the movement of the spine, pelvis, and rib cage during gait.
Lateral abdominal wall exercises and stretches can help keep these muscles flexible and reduce the chance of strain or injury. Abdominal massages can also be beneficial, but they may require specific positioning for comfort and accessibility.
