Let’s first start with a definition. Kibler et al., 2006 from the Journal of Sports Medicine, defines Core stability as “the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer and control of force and motion to the terminal segment in integrated athletic activities.”
Feedforward subconscious automatic trunk stabilization via core stability is crucial for efficient biomechanical sport specific movements in order to maximize force transmission and minimize joint loads. Core stability is defined most accurately as the “the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer, and control of force and motion to the terminal segment in integrated athletic activities” (8). High-level athletes must be able to transmit force across the body in the most efficient way possible in order to reduce the chance of peripheral muscle overuse and increased athletic performance with the least energy expenditure. In order to achieve this, the athlete must first focus on perfecting the correct core activation patterns while simultaneously achieving optimal breathing patterns demonstrated in last month’s article. Core activation and breathing are the foundation for core stability. We would never train an athlete to sprint before they could walk without faults. We must do the same when treating the core. The goal for core stability with all high-level athletes should be for them to attain subconscious feed forward trunk control and core stability with optimized breathing patterns under stress and fatigue. In this article, I will discuss how to best achieve this through evidence-based core anatomy and physiology, higher level core testing, and sport specific core training progressions.
Core musculature includes the muscles of the trunk and pelvis that are responsible for the maintenance and stability of the spine and pelvis (8). They help in the generation and transfer of energy from large to small body parts during many sports activities (8). These muscles include the scalenes, deep neck flexors, sternocleidomastoid, pectoralis, intercostals, diaphragm, rectus abdominis, transverse abdominis, internal and external oblique, pelvic floor, quadratus lumborum, and lumbar paraspinals as well as any muscles, which can affect these muscles. The core musculature is needed for force transmission across and through the body segments and in order to keep the trunk rigid when receiving or producing impact force. The diaphragm and pelvic floor, which work in synergy with the oblique’s and transverse abdominis, have also been postulated to be major contributing muscles needed in order to attain true trunk stability and control (6,7). They do this by helping to increase the pressure in the abdominal cavity, otherwise known as intraabdominal pressure, which leads to overall increased trunk stiffness (6,7,14,15). A common misconception is the training program that primarily focuses on specific single abdominal muscle hypertrophy or training these muscles as dynamic extremity prime movers, which can lead to muscle imbalances, fatigue, overuse, and injury. When we take a deeper look into the posterior superficial trunk muscle histology, especially the thoracic and lumbar erector spinae, we see muscle fiber characteristics that are predominantly large type I (slow twitch) fibers. This provides further evidence that these muscles are preset to perform best in postural isometric endurance and stability, aiding in force transmission and trunk stiffness (13).
Further, when we look at the deep spinal rotators and intertransversarii muscles, we see that these muscles have 4-7 times the muscle spindles as even the next most superficial multifidi muscles and that these muscles are most active with passive motion not active motion. This supports the hypothesis that this layer acts as potential vertebral position sensors or as a proprioceptive system not as prime movers or force generators (14). Due to this common practice of dynamic selective hypertrophy training as the best way for the abdominal muscles to gain true strength and stability it becomes important to shed light on the amount of actual muscle force needed to attain this sought after stability. To further reinforce this point, evidence has shown that muscular endurance is more protective of the spine than absolute strength and surprisingly in order to stiffen the spinal segments we only need up to 10-15% of maximal voluntary contraction for rigorous activity (12,14,15). If that is true then; Is core strength really about absolute strength gained through hypertrophy training? It seems that current evidence would point towards a training program that focuses on training muscular coordination and isometric core endurance. Then working that back into sport specific motions in order to achieve subconscious, feedforward, core stability and passive stiffness with optimized breathing patterns under sport specific stress and fatigue (5). With these principals our athlete can transmit forces at the correct time during the integrated athletic movement.
After a comprehensive breathing and core activation examination is completed the core musculature should be further evaluated under load and trained based on sport specificity. This can be completed in many ways, but I will focus on static testing which correlates to current evidence and trunk tissue histology and function. One very popular extensor chain muscle endurance test is the Sorensen extension test; due to the original studies’ large population, in depth pre and post-testing criteria, as well as the studies’ 1-year follow-up results. This study showed that low back extension holding times in 30-60 year old males of less than 176 seconds were predictive of low back pain during the next year, whereas a holding time greater than 198 seconds predicted an absence of low back pain (3). Other studies have shown that hold times of less than 58s was associated with a three-fold increase in the risk of low back pain, when compared to a hold time of greater than 104 seconds (12). This test has since been shown in various studies from 2001-2017 to have clinical significance regardless of sex, age, and physical activity level. Due to the difficulty in order to set up and perform the Sorensen test in the clinic and for a home exercise program, another test which has been useful in my practice to test low back and posterior chain static endurance is a static bent over weighted hold. This is done at 45 degrees of hip flexion and slight knee flexion. This static posterior chain test is similar to that of the Sorensen test but in a closed chain isometric standing position, with the trunk at 45-degree angle from the ground, using hip flexion. The best part is the evaluation becomes the treatment and is already setting the athlete up to brace and hold in a functional position. To make up for the lost torque on the trunk muscles and posterior chain due to this test’s 45 degrees hold position vs parallel to the ground in the Sorensen test the patient needs to hold 55.34% their body weight for men and 54.54% their body weight for females, which are standardized trunk to lower extremity anthropomorphic ratios per sex split between two hands. This does mean that grip strength will also play a role in this test but this can be bypassed via wrist straps or other aids. Now I understand there are many more components at play with this version of a closed chain posterior muscular endurance test so feel free to try it and if it is not applicable for your patient or your find other obstacles with this form of testing, fortunately, there are many other tests to use. Please refer the video attachment for how to perform and progress this modified evaluation from static testing into a dynamic treatment, as I am currently in the process of obtaining an IRB in order to prove the correlation of the two evaluation tests.
For the athletic population, I suggest using Mcgill’s static trunk endurance normative values as they combine the sit up, side plank, and Sorensen extension test to come up with normative cut off values as well as muscle endurance ratios with a younger athletic population (14,15). Other tests supported by sport-specific function or return to sport evidence that should be considered involving the lower extremity with trunk control would be the lower quarter Y-balance test or for the upper extremity the upper quarter Y- balance test (4). Another test with considerable athletic evidence that incorporates trunk control and upper extremity speed and ballistic motion would be the Closed Kinetic Upper Extremity Test (CKUET) (17,18). One part of higher level core evaluation that is consistently missing in current practice is evaluating the core in these positions or with these tests while the athlete is under systemic or sport specific fatigue then comparing those to normative values or the athletes non-fatigued values. Mcgill describes one version of this with the running population, where the athlete would sprint until fatigued and then hold side plank as long as possible to evaluate and eventually treat fatigue related core impairments (14,15). This concept should be translated into all trunk and body evaluative testing as you would not send a patient with a tibiofemoral pathology back to sport without seeing the quality of their movement when they are both fresh and fatigued for objective proof of complete resolution of contributing movement faults. With fatigue, we see a decrease in position sense/ proprioception of sport-specific central and peripheral joints leading to increased tissue overload and damage. (11,19). Therefore, this is of upmost importance because each high-level athlete must be able to maintain automatic feed-forward core stability and trunk control with optimized breathing patterns while under stress and fatigue.
The concept of training for sport specific core muscle symmetry and co-contraction are pertinent when thinking of functional trunk exercises. As previously stated, when athletes are training trunk stabilizing muscles as prime movers, in isolation for the majority of the workout, or in ways that overemphasize certain overdeveloped trunk muscle groups we run into huge problems. This type of training can lead to suboptimal movement patterns and compromised central stability. Current evidence and muscular histological studies show that static/ isometric trunk stability exercises/ training programs will both increase cross sectional area as well as increase passive trunk stiffness of the core musculature which will then aid in force transmission, increased trunk stability and control, with reduced pain. (9,10).
Some exercises to consider would be lumbar extension or side plank isometric holds on a roman chair at the gym with the hip pad positioned at different vertebral levels. Then we would progress this exercise by adding in a secondary component related to the sport such as the holds while catching a football, dribbling, shooting. Another exercise that is beneficial for trunk rotational faults would be side step outs without allowing trunk rotation while holding a weighted pulley or band with both upper extremities static at the navel level. You could also trial the static 45 degree hip flexion holds with a theraband around the thighs to promote posterior chain contribution and then add increased weight and lower extremity motions in all planes with the trunk held stable as shown in the accompanying video.
Side plank variations with the lower or upper extremity open chain limbs performing sports specific motions are also great functional exercises to perform that have been shown to maximize trunk activation at minimal spine load. Due to the high demand of some sports it may become important for the athlete to work on core stability with ipsilateral or contralateral lower and upper extremity open and closed chain functional patterns. If they are related to sport specific movements trial neutral spine association or dissociation with crawling, supine frozen bear rolling, bird dog, or dead bug type exercises with possible cable machine, free weight, or theraband resistance attached to upper or lower extremities.
Most of these exercises can be progressed with them being performed on unstable surfaces or with external perturbation training but keep in mind as we increase the demand on the trunk we increase the muscular demand and co-contraction and thus can increase the spine load which can be important when treating those athletes with concurrent spinal irritability, so progress slowly and with purpose. Also, keep in mind that it has been shown that there is no single core exercise that challenges every abdominal muscle at the same time, therefore we must pick a multitude of exercises that are most sport specific each session in order to fully optimize core stability and coordination (2). During the same training session, after priming the trunk with specific activation, strength, and coordination exercises, incorporate sports related functional movements with the athlete focusing on conscious timing of the trunk muscles. This can be done prior to and during limb movement and then progress them towards attaining an automatic subconscious activation pattern of proximal stability prior to and during these movements. Also spend time on correct sport specific movement patterning, paying close attention to shoulder, hip, and trunk association or disassociation while still having a baseline of passive automatic feedforward trunk stability and control. Next try all of these under local muscle or cardiovascular fatigue in order to test core stability and coordination with increased diaphragmatic demand.
If we put it all together, one possible version of a graded progressive treatment protocol for the trunk would focus on breathing with core activation and timing while accepting the weight of lower and upper extremities in supine, sitting, and standing. Then core isometric training, working towards evidence-based normative values progressed with sport specific motions (14). Concurrently or after this phase focusing on conscious to unconscious automatic timing and activation of the core musculature with corrected sport-specific movements. Finally, work on all of these concepts under fatigue, on unstable surfaces, or with perturbations in order to achieve the most beneficial sport specific stability possible.
I hope this article shed light on the importance of automatic proximal stability before and during distal mobility through evidence-based static and functional core training for all integrated athletic movements and that from these tests and sample treatment examples we can further progress each athlete to stabilize and efficiently transmit force in order to reduce proximal or distal tissue injury and fully optimize each athlete.
EVALUATION REFERENCE SHEET
Isometric Trunk Tests:
Side Plank Right: >=83 sec
Side Plank Left: >=86 sec
Side Plank Right/ Left Endurance Ratio: <.05 seconds Difference
Flexor Endurance Test: >=134 sec
Extensor Endurance Test: >=173 sec
Flexion/ Extension Endurance Ratio: <1.0 seconds Difference
Side Plank/ Extension Endurance Ratio: <.75 seconds Difference
Upper Quarter Y-Balance Test:
High school male and female baseball/ softball players: Use for R to L differences, No difference between throwing and non-throwing arm
Lower Quarter Y-Balance Test:
> 4cm anterior – 2.5x greater risk
Composite score < 94% - 6.5x greater risk
Collegiate Male Baseball players: Normative mean value is 30 touches
Collegiate Football players: Cut off value of less than 21 touches could identify athletes at risk for future injury
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