- Re-activating and strengthening the gluteal muscles
- Shoulder instability and rotator cuff issues
- Training for power and speed
- Exercise groups in-depth
- Core training part I: Inner and outer unit
- Effective and safe supplements
- Planes of motion
- Core training part II: a functional approach
- Prevention and rehabilitation of hamstring injuries
- Knee flexion exercises - friend or foe?
Core training part II: a functional approach
By Bram Swinnen
In part I of this article I explained the two stabilising systems of the body and that the different structures of the body are biomechanically and anatomically connected. The core consists of local stabilisers (the inner unit) that provide segmental stabilisation and global stabilisers and movers (the outer unit) that generate and keep tight control over the movement. The inner and outer unit synergistically work together to stabilise the core and generate powerful movements of the extremities. Neither system in isolation can provide sufficient stability. A holistic core routine needs to address both systems.
In this article I want to address how to functionally train the core, focusing on movement patterns rather than muscles. A functional approach to core training means that the core is trained to do what it does. Regarding the role of the core I quote Shirley Sahrmann: “The most important aspect of abdominal muscle performance is obtaining the control that is necessary to (1) appropriately stabilize the spine, (2) maintain optimal alignment and movement relationships between the pelvis and spine, and (3) prevent excessive stress and compensatory motions of the pelvis during movements of the extremities.” [1, p69] In their book, Mechanical low back pain, Porterfield and DeRosa state: "Rather than considering the abdominals as flexors and rotators of the trunk- for which they certainly have the capacity- their function might be better viewed as antirotators and antilateral flexors of the trunk.” [2, p99]
The core plays an important role in transferring forces between the lower and upper extremities, stabilising the range of motion of the spine and maintaining optimal alignment of the spine and pelvis. The core muscles are stabilisers first and an important part of the core training should consist of preventing movement.
Core exercises can be categorized into 4 categories:
The main function of the core muscles is to co-contract and prevent movement. Crunches and sit-ups mainly activate the rectus abdominis with little activation of the obliques . Performing the crunches at a high rate further increases the activity of the rectus abdominis with no increase in activity in the oblique abdominals . The dominance of the rectus abdominis often compromises the participation of the oblique abdominals. The primary disadvantage of increased rectus abdominis activation and inhibited oblique muscles is that the rectus cannot produce or prevent rotation. Co-contraction is essential to maintain stability and prevent buckling of the spinal column . Rather than activating single muscles, a core routine should mainly comprise of exercises that enhance the co-contraction of the core muscles. An ab training protocol that consists solely of fast crunches has been shown to reduce the abdominal muscles’ stability capacity .
The resisted reverse crunch activates the external and internal obliques more compared to a regular crunch.
McGill also showed in a laboratory setting that repeated spinal flexion can lead to herniated discs . McGill concluded that each individual can perform a finite number of flexion cycles before spinal injury occurs. Based on the results of this study many fitness authorities and professionals jumped on the anti-crunch bandwagon. The methodological issues of this study however, do not allow us to draw firm conclusions on the safety of crunches and sit-ups. In his study McGill used cadaveric pig spines that were stripped of all muscles. Stripped cadaveric spines have altered biomechanics and do not have the ability to remodel like living spines. The study used thousands of non-stop flexions towards end-range and does not resemble a core or ab workout in which people perform a couple of sets of different ab exercises.
Although discs are avascular, they are able to recover from cellular damage. (However, larger damage like a rupture of the disc wall is replaced by inferior scar tissue and will leave the disc inferior in strength.) The alternating internal disc pressures during crunches increases the flow of nutrient-carrying fluids to the discs. This enables the disc to remodel and repair disc damage [8, 9]. Static and dynamic disc compression induce a different biological response from the intervertebral disc. A static compressive load suppresses collagen synthesis in the disc, while a dynamic load at an appropriate level benefits the synthetic activity and anabolic response of the disc [10, 11]. Repeated bouts of properly dosed spinal flexion likely strengthen disc tissue and ligaments . Proper dose progressions cause minor disc damage that through healing result in stronger tissue .
Ab crunches in which mainly the upper body is lifted from the floor and the main flexion occurs at the thoracic spine do not cause substantial intervertebral disc loading .
If you do not have a bulged or herniated disc there is no reason to completely avoid crunches. The fitness industry is subject to hype. While crunches and sit-ups were glorified back in the day, today they are reviled. The truth about crunches and sit-ups is more nuanced. Research points out that if the curl-up is performed at a slow controlled rate, there is a certain amount of carryover with enhanced core stability as outcome.
Static anti-extension exercises like the ab rollout, knee tuck, pike and plank activate the rectus abdominis more than a crunch and also require a strong co-contraction of the abdominal muscles to prevent the spine to hyperextend [14, 15, 16, 17, 18].
The knee tuck progression ranges in difficulty from beginner level to very challenging. The knee tuck and variations can be performed using a slide-board, TRX or stability ball. The more instability is added, the higher the demand on the core. By pulling the knees to the side, another dimension is added to the exercise. The concomitant torso rotation increases the functionality and intensity of the exercise.
The ab rollout strengthens the abs and entire core and also involves the hip flexors and upper body. This is a challenging exercise, but there are many variations possible that allow a progression.
Pressing or holding a weight overhead (from a standing position or to a lesser extent from a seated position without back support) requires a strong contraction of the anterior core to prevent lumbar hyperextension.
Combine the anti-extension exercises with overhead presses to develop a functional and versatile anterior core. Of course crunches can still make part of your core workout.
The outer unit system that contributes to the anti-extension stability of the spine is the anterior oblique system.
The anti-rotation function of the core stabilises the trunk in the transverse plane of motion. Recent research points to the importance of rotator and anti-rotator function of the core. In diagnosis and treatment of movement impairment syndromes Sahrmann states: "During most daily activities, the primary role of the abdominal muscles is to provide isometric support and limit the degree of rotation of the trunk which, as discussed, is limited in the lumbar spine." A large percentage of low back problems occur because the abdominal muscles are not maintaining tight control over the rotation between the pelvis and the spine at the L5-S1 segment.[1, p70]
The overall range of lumbar spine rotation is about 13 degrees. The rotation between each segment from T10 to L5 (thoracic 10th vertebrae - lumbar 5th vertebrae) is 2 degrees. The greatest rotational range is between L5 and S1, which is 5 degrees [1, 19]. Increased rotational angles are possible in a seated position . Rotational ranges of 3 ½ between segments have been shown to cause damage to the discs . This means that repeated rotation in a seated position increases the vulnerability of the discs.
Vertical compression locks the facet joints and makes the spine more resistant to rotation.
This also has its’ implications for stretching. Back stretches that combine lumbar flexion with rotation should better be eliminated from our repertoire of exercises. Dynamic spine mobility drills in which the range of motion is actively controlled by the muscles are preferable over passive stretches in which the supportive tissues are relaxed. An overall range of lumbar spine rotation of 20 to 25 degrees is abnormal and unstable . Most people do not need to improve their lumbar rotational range of motion. The thoracic spine should be the site of greatest amount of rotation of the trunk. A solid range of motion in the hip is also important to avoid too much rotation at the lumbar spine. This is supported by a considerable amount of studies that show a correlation between a hip rotation deficit and low back pain [22, 23, 24, 25, 26]. If the hips stiffen up, excessive low back movement will occur. Especially in sports where the feet are planted during trunk rotation, like golf, a hip internal rotation deficit contributes to excessive lumbar spine rotation and lower back pain .
Solid range of motion in the hip prevents lower back pain.
Rotational or anti-rotator spine stability is a big leap forward in sports training because lumbar spine rotation has been proven to be so damaging and yet most human movement is rotational in nature. Think of the counter-rotation of the pelvis and trunk during gait. At a running speed of 14,4 km/h an average lumbar spine angular rotation of 13,3° was reported . A study by Browning even showed that 73% of the runners evaluated had higher spinal rotational angles when running barefoot compared to running with proper running shoes . Through evolution lumbar spine rotation always formed part of human movement. Banning all lumbar spine rotation from our workouts is like throwing out the baby with the bathwater. Controlling and stabilising the lumbar rotation is key to performance enhancement and back health and exercises in which the core is forced to stabilise against rotary forces should form part of most workouts.
Two outer unit systems that play an important role in generating and stabilising trunk rotation are the posterior oblique system and the anterior oblique system.
The barbell torque trains the anti-rotator and anti-lateral flexion function of the core. Producing a large arc increases the demand on the core to stabilise against the rotational torque. The core has to stabilise and maintain proper alignment of the spine during movements of the arms. The core is trained to prevent movement in a standing position. It does not get more functional than this. Many back health specialists consider the ability to resist rotational forces as more important than the ability to create rotation.
The horizontal wood chop is an exercise in which the core muscles have to generate rotation during the concentric part of the movement and actively control the range of motion during the eccentric part. Next to core strengthening this exercise also mobilises the thoracic spine and the hips. The majority of the spinal rotation occurs at the thoracic spine and is combined with internal rotation over the lead hip. Spinal rotation in a standing position (upright position combined with neutral alignment of the spine) is the safest form of rotation and has more carryover to sports. Vertical compression locks the facet assembly of the spine and makes it more resistant to torsion and less susceptible to disc injury (Mel Siff).
Anti-lateral flexion is the ability to stabilise the body in the frontal plane of motion. The pelvic stabilisers work in concert with the muscles of the core to resist a side-bending motion.
The lateral system plays an important role in anti-lateral flexion stability. Deficiency of the lateral system has implications all the way down the kinetic chain and is associated with increased incidence of ACL injuries, anterior knee pain and ankle sprains [29, 30, 31, 32].
These drills may start with a basic side bridge and progress to exercises performed in a standing position that require more coordination and balance.
Side/oblique bridge: The side bridge is a beginner exercise to train the anti-lateral flexion function. Through various progressions the intensity can be increased. The side bridge is a static exercise, while the oblique bridge is the dynamic version.
The Single-leg box squat is a challenging exercise that places a high demand on the core muscles and the gluteal muscles of the supporting leg. The movement takes place in one plane of motion (sagittal) and requires stabilisation in the other two planes (frontal/transverse).
The split stance core press allows you to train the anti-lateral flexion and anti-rotaror stability of the core in a standing position.
Anti-flexion stability of the core resists bending through the spine and counteracts the forces that tend to flex the trunk forward.
Traditional strength training exercises like the squat and deadlift are great to develop anti-flexion stability. During a squat and a deadlift the erector spinae and multifidus muscles are highly activated. Some of the strength guys state that squatting and deadlifting is enough for core conditioning, but this is untrue. These exercises are great to develop the posterior core, but do little for the anterior core muscles. Research shows that during squats and deadlifts the external and internal obliques and rectus abdominis are only moderately engaged [33, 34].
The deep longitudinal system and the gluteus maximus play an important role in pelvic and spinal anti-flexion. A contraction of the gluteus maximus and hamstring muscles stabilises the sacroiliac joint (force-closure) and will generate tension in the erector spinae muscles, providing stiffness to the spinal column [35, 36, 37]. Strong gluteus and hamstring muscles take pressure of the low back when lifting [38,39].
The Romanian deadlift strengthens the entire posterior chain. The gluteus muscles, hamstrings and adductor magnus are strengthened dynamically while synergistically working together to extend the hips. The lower back extensors function as stabilisers and are strengthened isometrically.
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