Review of Literature
The core is a vital component in proper function of the kinetic chain. Athletic performance is most often produced by the kinetic chain to reach a desired athletic task (Kibler, B., Press, J., & Sciasca, A, 2006). The core is important for providing local strength and balance. Since the core is central to almost all the kinetic chain controlling balance and range of motion will maximize all kinetic chains of upper and lower extremity function. Therefore, this literature review will concentrate on: the structure of the core, upper extremity power, and lower extremity power. This review offers support for research examining the effect of core strength on upper extremity and lower extremity power.
Structure of the Core
Core Stability is generally defined by the ability to control the position and motion of the trunk over the pelvis and leg to allow optimum production, control of force and motion to the terminal segment in the kinetic chain activities (Kibler et al., 2006). It requires control of trunk motion in all three planes. There are many different structures in the core. The roof of the core is the diaphragm. This helps increase intra-abdominal pressure, providing more trunk support, helping decrease the load on the spine muscles.
The opposite end of the diaphragm is the core muscles of the pelvic floor (Kibler et al, 2006). These muscles are often ignored because of the difficulty of assessing them. Simultaneous contraction of the diaphragm, the pelvic floor muscles, and the abdominal muscles, is required to increase intra-abdominal pressure. This provides a more rigid cylinder for trunk support which will help decrease the load on the sine muscles and allows increased trunk stability. The diaphragm contributes to intro-abdominal pressure before the initiation of limb movements, thereby assisting in spine and trunk stability. This activation occurs independently of the respiratory actions.
The hips and pelvis are the base of support for the structure of the core (Prentice, 2015). The major muscle groups in this area are critical for the function of the hip and pelvis. These muscles have a large cross-sectional area and generate a great deal of force and power when performing activities. The hip and trunk contribute to about 50% of the kinetic energy and force in the entire throwing motion. The thoracolumbar fascia are important for connecting the upper and lower limps. This covers the deep muscles of the back and trunk. This fascia attaches to the internal oblique’s and transverse abdominal muscles providing support to the lumbar spine and helping the core stability.
The key lumbar spine muscles include the transverospinal group, erector spinae, quadratus lumborum, and latissimus dorsi (Prentice, 2015). The transverospinal group is a group of small muscles designed mainly for stabilization. This group of muscles is primary responsible for providing the central nervous system with proprioceptive information. The erector spinae muscles function to provide dynamic stabilization and eccentric deceleration during movements in the kinetic chain. The quadratus lumborum primary function is as a frontal plane stabilizer. Lastly the latissimus dorsi has the largest arm in all the back muscles. It is the bridge between the upper and lower extremity.
The key abdominal muscles include the rectus abdominis, external and external oblique, and the transverse abdominis (Prentice, 2015). The abdominals operate as an integrated functional unit. This helps maintain optimal spinal kinetics. The rectus abdominis eccentrically decelerates trunk extension and lateral flexion. The external oblique work to produce rotation and lateral flexion during functional movements. The internal oblique works to produce ipsilateral rotation and lateral flexion. Lastly, the transverse abdominis is one of the most important abdominal muscle. It functions to increase intra-abdomial pressure, produce dynamic stabilization and provide optional neuromuscular efficiency.
The key hip muscles include the gluteus maximus, gluteus medius, and psoas (Prentice, 2015). The gluteus maximus functions to accelerate hip extension and external rotation. It is a major dynamic stabilizer of the sacroiliac joint. It has the greatest capacity to provide compressive forces to the sacroiliac joint. The gluteus medius functions a primary frontal plane stabilizer of the pelvis and lower extremity during functional movements. The psoas helps with hip flexion and internal rotation. It creates a shear force at L4-L5.
Lastly, an effective core allows for maintenance of normal length-tension relationship of functional agonist and antagonist (Prentice, 2015). This also allows for maintenance of normal force-couple relationships in the lumbo pelvic hip complex. Maintaining these relationships helps with optimal arthokintices during functional movements. Optimal neuromuscular efficiency in the entire kinetic chain allows for, acceleration, deceleration, and dynamic stability. Overall, having an effective core can help with stability of the entire lower and upper extremity and its movements.
There are two types of core training: Static and dynamic training (Parkhouse and Ball, 2010). Static training involves the joint and muscle by working against an immovable force or being held in the static position while having a resistance force on it. Dynamic is the ability to exert muscle force concentrically or eccentrically repeatedly or continuously over time. Due to the body’s functional design, dynamic movement are more dependent on the core muscles. The surface the core exercise is performed on effect the force that is applied. Performing dynamic exercises on unstable surfaces are unable to reproduce the force and power found when performing that exercise on a stable surface.
Static training can involve different types of training. It involves anything that is stationery. In the core training program, bridges, planks, and side planks are all static exercises that are done. Dynamic training is anything moving. In the core training program there crunches, russian twist, leg lifts, bicycle crunch and leg lifts.
Injury Prevention of the Core
The importance of core stability for injury prevention and performance enhancement has been popularized lately (Bliven, K. & Anderson, B, 2013). The integration of core stabilization exercises into injury prevention programs, mainly more for lower extremity and back, is demonstrating decreased injury rates. There has been substantial evidence demonstrating core muscle recruitment alterations in low back pain patients compared with healthy controls.
Lower Back Pain
The transverse abdominus and multifidus display change in recruitment that limit their ability to effectively stabilize the spine and provide accurate proprioceptive information (Bliven, K. & Anderson, B, 2013). After examining core muscle recruitment patterns during lower and upper extremity movements in low back pain patients to healthy patients the transverse abdominus was the first muscle recruited, followed by the multifudus, obliques, and rectus abdominus. All local stabilizers and global mobilizer core muscles were recruited before any extremity movement. This showed that the core muscles provide proximal stability for distal mobility. In the lower back pain patients, transverse abdominus recruitment was delayed in upper and lower extremity movements in all directions. The gluteus maximus activation was delayed, suggesting an inability to compress and stabilize the sacroiliac joint and pelvis with lower extremity movement. This shows alterations in muscle recruitment, suggesting that deficiencies in core stabilization and load transfer muscles may be related to the lower extremity function and injuries.
When athletes performed the functional movement screening, female collegiate athletes who scored 14 or less out of 21 were four times more likely to sustain injury. (Bliven, K. & Anderson, B, 2013). The functional movement screen does not isolate core stability it provides insight into the relationship of core stability to the injury risk. If they have a lower score it can be related to have a weak core and not being able to stabilize to perform all the exercises.
Males athlete generally demonstrate greater core stability measures then females (Leetun, L., Ireland., Wilson, J. et al., 2004). There is significant differences between male and females for hip abduction, external rotation and side bridges. These are all things used by the core. The knee was primary complaint of injuries and the foot and ankle were most frequently injuired body region. After the athlete experienced an injury over the course of the season there core stability lowered. Working on the core over the season helps fix these problems.
Effects of Core Training
The ability of core training to help with upper and lower extremity power is still questionable. Researchers have shown many different things. Studies in this area have been investigated and shown to have a positive impact. The results of the medicine ball throw have been positively correlated to the power output of the trunk muscles (Strockbrugger & Haennel, 2001). Researchers have found that peak power and work for trunk extensors are positively correlated with throw distance. The trunk and upper-body muscles may contribute more significantly to overall performance during a total-body activity instead of upper or lower body activity. The use of the medicine ball will increase physical demand on the upper extremity and trunk. This may facilitate a higher neural activation of the area. According to Manchado, an increase in the level of core muscle strength results in an improvement in kinetic chain, this can also result in an increase of improvement in the medicine ball throw.
Research also shows that males score higher on core stability tests, on average when compared to females (Sharrock et al, 2011). Men have different postural differences and bone structured compared to females, this can have an effect on the core. Having subtle changes in the angle of pull of the core musculature of the pelvis may result in decreased ability to control the trunk.
There are also other researchers have said that there is no effect on vertical jump or medicine ball after performing core training. According to Parkhouse et al, the results provided no support for the proposal of a more enhanced and efficient transfer of energy. It states that they needed a much larger sample size to get better results. They also stated that the core training my of not helped with performance, but it helped improve kinematic and kinetic measures.
In conclusion, the effect of core training for upper and lower extremity power is still up for subjective interpretation. Some research favors the use of a core training program in helping produce more upper and lower extremity power in the vertical jump and medicine ball throw while others do not. Having a stronger core can benefit the body and performance in sports in many different ways, but from the research reviewed, there is no concise evidence that it is helping with power in either way.
Research has found that vertical jump can be used to show lower extremity power. This is a device that many athletes need explosive lower extremity power in order to perform certain movements in their sport (Sharrock et al, 2011). Getting off the ground to reach a maximum jump height is one of them. The increase in vertical takeoff velocity after core strengthening exercises shows that the need for the core strenghting exercise program in games that include vertical jump can be a part of the winning characteristics (Sharma et al, 2012). The Vertex Jump tester is used to test this. Research has shown that after a core training program is performed the vertical jump increases.
The vertical jump is measured using Vertec machine. The tester will adjust the height of the stack of movable color coded horizontal plastic vanes to be within the athletes standing reach. (Haff & Triplett, 2016). The highest vane that can be reached and pushed forward with the dominant hand while standing flat-footed determine the standing touch height. The vane stack is then raised according to the measured distance so the athlete can get an accurate jump. Without a initial or stutter step, the athlete will perform a countermovement by quickly flexing the knees and hips, moving the trunk forward and downward, and swinging the arms backward. When jumping the dominant arms moves upward. The athlete then taps the highest possible vane with the fingers of dominant hand. The score is the distance between the heights of the highest vane taped when tapped during standing reach and the one tapped during the jump. The best of three trails is recorded.
Medicine Ball Throw
There limited amount of research on the use of the medicine ball. It is more of newly used training. The medicine ball is starting to grow in use in sports training because the practitioners see the wide range of skills that can be used for (Stockbrugegar & Hannel, 2001). The medicine ball throw can be used for showing upper extremity power. Research performed on the effects core training has on medicine ball throw are positive (Manchado et al, 2017). This is a very easy test to administer and perform. The subject stands at a line with the feet side by side and slightly apart, and facing the direction to which the ball is to be thrown. The ball is held with the hands on the side and slightly behind the center. The throwing action is similar to that used for a soccer ball throw-in. The ball is brought back behind the head, then thrown vigorously forward as far as possible. The subject is permitted to step forward over the line after the ball is released. Three attempts are allowed. The distance from where the ball was thrown and then landed is measured by a tape measure to get the measurement.