Stabilization exercise has become very popular in the treatment of low back pain (LBP). The goal of this type of exercise is to optimize the muscle activation and coordination around the spine while minimizing compressive load to spinal tissues. Stabilization exercise in combination with daily activity modifications have been shown to be effective in the treatment of LBP, although the exact mechanism of this benefit has not been clarified.
Central to the theme of spinal stability is the method in which patients activate their abdominal muscles, and the appropriate exercise progression that gleans the maximum benefit. Previous research from the Queensland Australia group (Hides, Richardson, Jull etc.) showed that the transverse abdominus (TrA) is recruited later in LBP patients, which led to speculation that it was related to an unstable or unhealthy spine. This also led to the development of a specific rehabilitation protocol involving attempts to exclusively activate and rehabilitate the TrA, using primarily an abdominal hollowing technique. However, most existing evidence to support this concept has been indirect or qualitative. In fact, it has been shown that the TrA can only be isolated at very low levels of activation (1-2% maximum voluntary contraction or MVC).
At higher activation levels, such as during normal daily activities or athletic tasks, the TrA has been shown to be a synergist to the internal oblique (IO). In light of this, what the original research on TrA did suggest is that a motor control deficit exists in LBP patients, not necessarily that the TrA is the most important factor in this relationship. Subsequent research has demonstrated that LBP patients not only have delayed TrA activation, but other motor control deficits as well, including delays in other trunk muscles when the torso is moved quickly, inhibited knee extensors, perturbed gluteal firing patterns during walking, and an inability to breathe heavily and maintain spinal stability. This indicates a global deficit in muscle coordination, rather than a specific deficit in one muscle.
As the research on TrA was emerging from Australia, the biomechanics lab at the University of Waterloo (McGill and his PhD students – Grenier etc.) developed an advanced and validated method of modeling the spine and calculating how muscles contribute to spinal motion and stability. What has evolved from this work is support for abdominal bracing as the optimal method for activating spinal musculature. They also developed the “Big Three” exercises for spinal stability rehab – the curl-up, front plank/side bridge, and cross-crawl. These exercises maximize muscle activity, while minimizing spinal compression.
It is important here to establish some definitions as they apply to this study, and this rehabilitation topic in general:
• Spinal Stability- The ability of the spinal column to survive an applied perturbation (known as Euler column stability). If the input energy (perturbation) is greater than the potential energy of the column (stored in discs, ligaments, muscles and tendons), equilibrium will not be attained.
• Abdominal Hollowing- Attempting to isolate the TrA by activating the lower abdominal wall while gently “drawing in”.
• Abdominal Bracing- Involves activation of the abdominal muscles all around the spinal column to a level that increases torso stiffness.
This study, conducted at the University of Waterloo, aimed to determine which muscle activation strategy – abdominal hollowing or abdominal bracing – is more effective for lumbar spine stabilization. Eight healthy male subjects aged 20-33 participated in this laboratory study, which utilized EMG data and biomechanical model simulation to measure spinal stability during four loading conditions, performed with two stabilization strategies of bracing and hollowing. The four loading conditions were:
1) no load in the hands (no lift)
2) 10Kg in each hand (bilateral lift)
3)10Kg in the right hand only and
4) 10Kg in the left hand only. EMG recordings were taken from surface electrodes placed over the following muscles: rectus abdominus, IO, EO, latissumus dorsi, thoracic erector spinae, lumbar erector spinae, and lumbar multifidus.
Since it has been previously established that TrA and IO are synergists, the IO recording was presumed to represent TrA activity (this presumption has been previously validated). Bracing and hollowing were both conducted with ultrasound imaging guidance as per previous studies. Spine kinematics were measured and modeled utilizing the 3D imaging Isotrak system. The main outcome measures utilized were the spinal stability index and spinal compression (each calculated through laboratory modeling techniques using the raw data).
Pertinent results of this study include:
• simulation and in vivo data both indicated that abdominal bracing was superior to abdominal hollowing in terms or increasing spinal stability with lower compression
• bracing increased spinal stability an additional 32% compared to hollowing, while only increasing compression 15%
• in all subjects, selectively activating the TrA proved extremely difficult, if not impossible, as evidenced by all other abdominal muscles not being silent during abdominal hollowing – this suggests that abdominal hollowing seems to result in some degree of bracing
• all simulations conducted indicated that TrA had no effect on spinal stability
Conclusions & Practical Application:
The results of this study indicate that abdominal bracing is a superior strategy for increasing spinal stability and sparing spinal load compared to abdominal hollowing. The authors were quick to point out that these results should not diminish the potential benefit of hollowing for retraining 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 “draw in” in an effort to increase stability seems to be misdirected. It may also be relevant that this study indicated that any attempt to hollow seemed to recruit other abdominal muscles, and thus represented a low-level brace. The authors speculate that during muscle activation, the layers of the abdominal wall may bind together, resulting in increased stiffness of the spinal column. If this is true, the TrA would still be important as a member of the “muscle orchestra” rather than a solo contributor.
Going back to the original finding of delayed TrA activation in back pain patients, the authors of this study adeptly suggest that this finding may be “statistically significant, but not mechanically significant”. From a practical perspective, abdominal bracing is easier to achieve, and appears to be a more effective strategy to implement with LBP patients. The exact role of TrA-specific training still needs to be further studied.
It should be noted that this was a small study, performed on a small group of healthy subjects. Further studies are required to quantify these relationships in older back pain patients, and other patient populations.
