How Muscles Stabilize the Spine
The concepts of stability and stiffness are benchmarks for all human motion, including spine function. Without accounting for stability, almost all human activities could not be successfully performed. Three major tenets with regard to stiffness and stability exist for human function17
Sufficient stiffness allows for the body and spine to bear load.
Stiffness and stability are related through muscular mechanisms, creating a guy wire system for the spine.
Proximal stiffness may allow for distal mobility
In the context of the spine, Bergmark described the stability-stiffness continuum using a spring-mass system: the stability of a system (spine) is dependent on the stiffness of the springs that attach to it (the “core” muscles, i.e., rectus abdominis, quadratus lumborum, internal and external obliques).18
External perturbation (i.e., externally applied loads or movement) can potentially disrupt the equilibrium of the system. To maintain static equilibrium (ensuring the spine experiences no unwanted motion), the core muscles must generate sufficient stiffness. This is akin to how the rigging on a ship stabilizes its mast (Fig. 19.1
). The stiffness of the “guy wires” provides stability to the mast to ensure it stays upright against external forces such as the wind or rocking of the ship. Similarly, stiffness generated by the core muscles helps to “anchor” the spine, enhancing its ability to resist external perturbation. Insufficient stiffness of the guy wires may cause the mast to fall over or buckle if excessive perturbations are applied. Similarly, insufficient core stiffness will cause the system to mechanically buckle, resulting in an involuntary bending of the spine when trying to resist external perturbation (i.e., yielding during a heavy deadlift).
Figure 19.1 Rigging on a ship.
However, unlike guy wires, the core muscles have the ability to “fine-tune” the stiffness they generate. This is mitigated through muscular activation: when a muscle contracts it creates both force and stiffness. Though muscular stiffness always adds to joint stability,19
excessive force can contribute to destabilizing the system. Applied spinal forces will increase with muscular activation, but high levels of activation may prove harmful: increased compressive loads may take away from the spine’s work capacity and ultimately lead to injury. Because peak stiffness is not achieved at full activation of the muscle, in order to achieve optimal stability (maximizing stiffness with minimal force applied), the goal is to not activate the muscle as hard as possible but to finely tune the activation. So, what level of activation is required to achieve optimal stability? It has been demonstrated that stiffness values level off at approximately 25% core musculature maximum voluntary contraction (MVC).19,20
Based on this information, one goal of training is for the patient to learn how to tune core activation to create “sufficient stiffness”: maximizing spinal stability through maintaining just enough stiffness, while minimizing the amount of internal force generated from this contraction. According to Cholewicki and McGill, spine stability is greatly enhanced by cocontraction of antagonistic trunk muscles.21
Cocontractions increase spinal compressive load, as much as 12% to 18% or 440 N, but they increase spinal stability even more, by 36% to 64% or 2,925 N.22
Cocontractions have been shown to occur during most daily activities.23
This mechanism is present to such an extent that, without cocontractions, the spinal column is unstable even in an upright posture!6
Cocontractions are most obvious during reactions to unexpected or sudden loading.24,25
Stokes et al has described how there are basically two mechanisms by which this co-activation occurs26
: (a) a voluntary pre-contraction to stiffen and, thus, dampen the spinal column when faced with unexpected perturbations; and (b) an involuntary reflex contraction of the muscles quick enough to prevent excessive motion that would lead to buckling after either expected or unexpected perturbations.24,25,26,27,28,29
One mechanism of tissue injury is repetitive, end-range loading. A stable spine can avoid injurious, repetitive, end-range loading via the buttressing effect of agonist-antagonist co-activation in maintaining the integrity of the “neutral zone.” The neutral zone is the inner region of a joint’s range of motion (ROM), where minimal resistance to motion is encountered.30
This inner region’s mobility is restricted by passive ligamentous factors alone, and, when it is expanded (i.e., spinal flexion), creates joint instability, which places greater demands on the muscles to stabilize a joint. Thus, the most observable and measurable sign of instability is not joint hypermobility, but excessive agonist-antagonist muscular co-activation.31
Various studies have pointed out how important the motor control system is for preventing spinal injury. Ironically, when under load, the spine is best stabilized, but when “surprised” by trivial load at a vulnerable time, such as in the morning or after prolonged sitting, the spine stability system is most dysfunctional.32,33
Inappropriate muscle activation during seemingly trivial tasks, such as bending over to pick up a pencil after sitting for a prolonged period of time, can compromise spine stability and increase the likelihood of buckling of the passive ligamentous restraints.31
This motor control skill has also been shown to be more compromised under challenging aerobic circumstances.33
The basic science aspects of the spine stability system are presented in greater detail in Chapters 1
Motor Control Problems and Low Back Pain
Coordinated activation and control (motor control) of the core musculature is a major factor contributing to stabilizing the spine and preventing LBP. Stiffness created by activation of the core muscles arrests painful spinal micromovements, and buttresses the spine against unwanted motion when challenged by external perturbations and/or loads.17
However, dysfunctions in the timing, magnitude, and pattern of activation can contribute to pain and impaired spinal function.
Radebold et al have shown that there is a predictable muscle response pattern to sudden trunk loading in individuals with LBP.34
This includes delayed initial activation, over-activation, and delayed subsequent relaxation of muscles. Researchers in Queensland, Australia, have found that delayed activation of the transverse abdominis during arm or leg movements distinguishes LBP patients from asymptomatic individuals.35,36
O’Sullivan found that an abdominal hollowing maneuver helped retrain perturbed motor patterns in deep abdominal muscles.7,8
This has motivated the use of hollowing exercises for use in rehabilitation programs for patients with segmental spinal instability. Unfortunately, others have misinterpreted these data to mean that hollowing maneuvers or minimizing rectus abdominis activity during core stability exercises directly enhances stability. In the context of the previous guy wire analogy, focusing on training a single muscle is like focusing on a single guy wire.37
Research from the University of Waterloo in Canada has found
that while certain muscles, such as multifidus and transverse abdominis, may have special relevance in distinguishing LBP subjects from asymptomatic individuals, these muscles are part of a much bigger orchestra responsible for spinal stability.37
This effect was explored further by Vera-Garcia et al, who showed that abdominal bracing performed better than abdominal hollowing for stabilizing the spine against rapid perturbations.38
Specifically, bracing actively stabilized the trunk and reduced the lumbar spine displacement after loading, but hollowing was ineffective for buttressing the spine against external perturbation. Brown and McGill also showed that coordinated activation of the abdominal wall (external oblique, internal oblique, transverse abdominis) had a synergistic effect in force and stiffness transmission.39
Though this study was performed in situ in rat specimens, the take-home message here is the coordinated activation of the core muscles (i.e., abdominal bracing) has a synergistic effect in creating core stiffness, rather than activating a single muscle (i.e., transverse abdominis through abdominal hollowing). Sufficient stability, according to Brown and McGill, is defined as the amount of muscle stiffness necessary for stability along with a safety margin.19
Cholewicki et al showed that modest levels of co-activation are necessary, but if a joint has lost its stiffness, greater amounts of co-activation are needed.40,41
Marras et al have reported that there is a different pattern of antagonist muscle co-activation in LBP individuals than in asymptomatic individuals’ torso kinematics (motion) while performing a functional lifting assessment.16
Patients were found to have greater spine load and less ability to generate normal trunk movement patterns during lifting tasks. Altered trunk movements were strongly related to spine load, being able to predict 87% of the variability in compression, 61% in anteroposterior shear, and 65% in lateral shear. The kinematic picture for the LBP individual showed excessive levels of antagonistic muscle co-activation, which reduced trunk motion, but increased spine loading.
Hodges and Moseley presented a proposed model for the interaction between pain and motor control.44
In this model, psychological factors related to fear, stress, and attention from pain/nociceptor stimulation interact with a complex pathway of various brain centers to affect motor control. The researchers stated it is unlikely that the simple inhibitory pathways (i.e., cortical inhibition, reflex inhibition, motoneuron inhibition) can mediate the complex changes in motor control of the trunk muscles, but the most likely candidates are changes in motor planning via a direct influence of pain on the motor centers, fear avoidance, or because of changes in the sensory system.
Although impaired motor control can lead to pain and dysfunction, it is possible to retrain these patterns using motor control exercise (MCE)—exercises that improve coordination and control of muscles that support the spine. A systematic review by Saragiotto et al revealed that MCE plays a large role in reducing pain and dysfunction, and improving quality of life.45
A detailed summary of the efficacy of exercise for spinal stability can be found in the following section.
Efficacy: Evidence of Effectiveness for Spine Stability Training
We can summarize that the ability to voluntarily control core musculature stiffness can provide relief and long-term benefit to an LBP patient via enhancing spinal stability. However, the “trainability” of core stiffness is an important question to answer: can we train to enhance core stiffness and if so, what is the most effective way possible?
Lee and McGill showed that it is possible to enhance core stiffness through a single session of isometric core exercise, and following a 6-week isometric
training protocol. A single bout of core exercise (5 sets of 10-second hold planks, side bridges, and bird dogs) showed significant increases in core stiffness, though how long this effect lasted is unknown.13
Both untrained individuals and individuals savvy to core exercise experienced increased measures of stiffness. In comparing long-term adaptations to isometric versus dynamic (repetition-based) core exercises, a 6-week isometric training protocol was also shown to be superior to a dynamic protocol for enhancing stiffness.12
It should be noted that both of these studies examined asymptomatic individuals, and may need to be modified to suit a pained population’s needs.
Australian researchers have reported that multifidus atrophy occurs with acute LBP.46
The atrophy does not spontaneously go away when the pain does.47
However, motor control training does restore the multifidus and reduces long-term recurrence rates of LBP.48
Tsao and Hodges showed that specific training of the transverse abdominis restored function of this muscle to resemble profiles of non-pained individuals in participants with LBP.49
This effect was observed with just a single training session and the magnitude of change correlated with the quality of training. Long-term adaptations were measured over a 4-week training period, and this change was retained even after a 6-month follow-up.50
O’Sullivan et al showed that specific spine stabilization exercises achieved superior outcomes to isotonic exercises in chronic patients with spondylolisthesis.7,8
In a large, randomized, controlled clinical trial, Timm showed that exercise was superior to passive care in treating failed back surgery in patients.51
In this study, a further comparison of exercise types showed that low-technology exercise (i.e., McKenzie and stabilization) was superior to high-technology exercise (i.e., isotonics and Cybex).
Yilmaz et al administered an 8-week stabilization program to postoperative lumbar microdiscectomy patients,52
and compared this program to home exercise and no exercise. At week 12, superior results were achieved in pain, function, mobility, and lifting ability for the stabilization group. Supervised stabilization training was superior to home exercises, which was superior to no exercise.
Stuge et al found that stabilizing exercises were superior to traditional physical therapy for pelvic girdle pain after pregnancy.53
The stabilization group had lower pain intensity and disability, higher quality of life and less impairments. The results persisted at the 1-year check, postpartum.
Safety: Is Spine Stability Training Safe?
Safe exercises for acute and subacute low back patients should have favorable biomechanical load profiles. The goal of these exercises is to create adequate core musculature challenge for the patient in order to strengthen tissues and enhance their load tolerance, while minimizing imposed spine loads. It is known that without muscles, the spine buckles at 90 N of compression (under its own weight!), yet, during routine activities of daily living (ADLs), compressive loads 20 times are routinely encountered.30
Thus, proper functioning muscles controlled by the central nervous system enable stability to be maintained. In fact, demanding ADLs involve compressive loads of approximately 6,000 N, and the National Institute for Occupational Safety and Health (NIOSH) work demand limit (a threshold value for elevated risk of back injury set by the NIOSH) is 6,400 N.6,54,55
Elite weight lifters manage, through highly skilled motor control strategies, to safely lift loads of nearly 20,000 N.56
McGill recommends that, for subacute exercise training, a safe limit is approximately 3,000 N.57 Table 19.1
lists exercises with varying safety levels based on loads imposed to the spine.
McGill’s recommendation helped influence the development of his “non-negotiable Big 3” core exercises (curl-up, side bridge, bird dog—see Stability Training section for descriptions of these exercises). Traditional core exercises involving motion of the spine (i.e., sit-ups, back extensions) provide a high degree of challenge to the core musculature but impose excessive spinal loads (>3,000 N).58
The “non-negotiable big 3” provide a much safer and effective alternative, imposing much lower spine loads while still creating a high degree of neuromuscular challenge.15
This is why they are considered non-negotiable—they provide a biomechanical onramp with low-load exercises for more sensitive patients. Selecting the wrong exercises will only worsen
their condition and adds to the myth that exercise is not well tolerated for individuals in pain. LBP patients can perform these exercises daily with minimal risk of overloading their spine, thus more effectively inducing stability and motor control adaptations to remove and prevent pain. Performed daily, these exercises allow patients build endurance of their core musculature allowing them to tolerate greater applied loads. Over time, these exercises can be progressed in terms of neuromuscular challenge to suit the patients’ functional needs.
Table 19.1 Exercise Safety Profiles
Quad single-leg raise—2,000-2,300 N
Side bridge on knees—less than 2,000 N
Sit-ups, bent knee—3,350 N
Sit-ups, straight knee—3,500 N
Curl-up on ball—4,000 N
Prone superman—4,300 N
From Gardner-Morse MG, Stokes IA. The effects of abdominal muscle coactivation on lumbar spine stability. Spine. 1998;23:86-92; McGill SM. Low Back Exercises: Prescription for the Healthy Back and When Recovering From Injury. Resources Manual for Guidelines for Exercise Testing and Prescription. 3rd ed. Indianapolis, IN: American College of Sports Medicine; 1998; McGill SM. The biomechanics of low back injury: implications on current practice in industry and the clinic. J Biomech. 1997;30:465-447; Panjabi MM. The stabilizing system of the spine. Part 1. Function, dysfunction, adaptation, and enhancement. J Spinal Disord. 1992;5:383-389; Stokes IA, Gardner-Morse M, Henry SM, Badger GJ. Decrease in trunk muscular response to perturbation with preactivation of lumbar spinal musculature. Spine. 2000;25:1957-1964.