Stabilization exercise has become very popular in the treatment of low back pain (LBP). The goal of this type of exercise is to optimize muscle activation and coordination around the spine while minimizing the compressive load on the spinal tissues. Stabilization exercise in combination with daily activity modifications has been shown to be effective in the treatment of low back pain, although the exact mechanism of this benefit remains unclear.
The central issue of spinal stability is the method in which patients activate their abdominal muscles and the proper progression of the exercise that yields the maximum benefit. Previous research from the Queensland Australia group (Hides, Richardson, Jull etc.) showed that the transverse abdominis (TrA) is recruited later in patients with low back pain, leading to speculation that it was related to an unstable spine or unhealthy. This also led to the development of a specific rehabilitation protocol involving attempts to exclusively activate and rehabilitate TrA, primarily using a voiding technique. However, most of the existing evidence to support this concept has been indirect or qualitative. In fact, it has been shown that TrA can only be isolated at very low levels of activation (1-2% of maximal voluntary contraction or MVC).
At higher activation levels, such as during normal daily activities or athletic tasks, TrA has been shown to be an internal oblique (IO) synergist. In light of this, what the original research on TrA suggested is that there is a motor control deficit in patients with LBP, not necessarily that TrA is the most important factor in this relationship. Subsequent research has shown that patients with low back pain not only have delayed activation of TrA, but also other motor control deficits, including delays in other trunk muscles when the torso moves rapidly, inhibited knee extensors, activation patterns of disrupted buttocks when walking and inability to breathe deeply and maintain spinal stability. This indicates a global deficit in muscle coordination, rather than a specific deficit in one muscle.
As TrA research emerged in Australia, the University of Waterloo biomechanics laboratory (McGill and his PhD students, Grenier, etc.) developed an advanced and validated method for modeling the spine and calculating how muscles contribute movement and stability of the spine. What has evolved from this work is support for abdominal braces as the optimal method of activating the spinal musculature. They also developed the “Big Three” exercises for spinal stability rehabilitation: curl-up, front plank/side bridge, and cross crawl. These exercises maximize muscle activity, while minimizing spinal compression.
It is important to state here some definitions that apply to this study and to this topic of rehabilitation in general:
• Spinal stability: the ability of the spinal column to survive an applied disturbance (known as Euler’s spinal stability). If the input energy (disturbance) is greater than the potential energy of the spine (stored in discs, ligaments, muscles, and tendons), equilibrium will not be reached.
• Abdominal Cupping: An attempt to isolate the TrA by activating the lower abdominal wall while gently “retracting”.
• Abdominal curls: involves activating the abdominal muscles around the spine to a level that increases the rigidity of the torso.
This study, conducted at the University of Waterloo, aimed to determine which muscle activation strategy, abdominal emptying or abdominal bracing, is more effective for stabilizing the lumbar spine. Eight healthy male subjects aged 20 to 33 years participated in this laboratory study, which used EMG data and biomechanical model simulation to measure spinal stability during four loading conditions, performed with two stabilization strategies of bracing and casting. The four loading conditions were:
1) no load in hands (no lifting)
2) 10Kg in each hand (bilateral lift)
3) 10 kg only in the right hand and
4) 10Kg only in the left hand. EMG recordings were taken from surface electrodes placed on the following muscles: rectus abdominis, IO, EO, latissimus dorsi, erector spinae, erector spinae lumbar, and lumbar multifidus.
Since it has previously been established that TrA and IO are synergists, it was assumed that the IO log represented TrA activity (this assumption was previously validated). Bracing and casting were performed with ultrasound image guidance as per previous studies. Spinal kinematics were measured and modeled using the Isotrak 3D imaging system. The main outcome measures used were spinal stability index and spinal compression (each calculated via laboratory modeling techniques using the raw data).
Relevant results of this study include:
• Both simulation and in vivo data indicated that abdominal bracing was superior to abdominal tucking in terms of increased spinal stability with less compression
• Bracing increased spine stability by an additional 32% compared to casting, while only increasing compression by 15%
• in all subjects, selective activation of TrA was extremely difficult, if not impossible, as evidenced by the fact that all other abdominal muscles did not remain silent during abdominal hollowing; this suggests that abdominal hollowing seems to result in some degree of reinforcement
• all the simulations performed indicated that TrA had no effect on spinal stability
Conclusions and practical application:
The results of this study indicate that the abdominal brace is a superior strategy to increase spinal stability and avoid spinal loading compared to abdominal tucking. The authors were quick to point out that these results should not diminish the potential benefit of the void to retrain a TrA deficit from a motor control perspective, as it is still part of the abdominal wall. However, the common advice from therapists and exercise professionals to “retract” in an effort to increase stability seems to be misdirected. It may also be relevant that this study indicated that any attempt to cup seemed to recruit other abdominal muscles and thus represented low-level bracing. The authors speculate that during muscle activation, the layers of the abdominal wall may coalesce, resulting in increased spinal stiffness. If this is true, the TrA would still be important as a member of the “muscle orchestra” rather than as a solo contributor.
Returning to the original finding of delayed TrA activation in patients with back pain, the authors of this study cleverly suggest that this finding may be “statistically significant, but not mechanistically significant.” From a practical perspective, abdominal bracing is easier to achieve and appears to be a more effective strategy to implement with patients with low back pain. The exact role of TrA-specific training remains to be further studied.
It should be noted that this was a small study, conducted in a small group of healthy subjects. Further studies are required to quantify these relationships in older patients with back pain and other patient populations.