Allison GT, Morris SL, Lay B. Feedforward Responses of Transversus Abdominus Are Directionally Specific and Act Asymmetrically: Implications for Core Stability Theories. J Orthop Sports PT. 2008 38: 5-237.
This research study investigated the timing, intensity, and symmetry of muscle responses during unilateral arm raising. The authors designed the study to validate previous research that proposed that the Transversus Abdominus muscle (TrA) served a stabilizing function based on symmetrical activation during arm raining. The previous research papers were the genesis of isolation TrA training protocols that have been adopted all over the world. These protocols involve a conscious activation of the TrA while attempting to keep other muscles from contracting.
This study, as well as many others, looked at the timing of activation of trunk and leg muscles relative to the activation of the deltoid muscle to raise the arm. It is well documented that muscles of the trunk and leg fire before the deltoid. This activation before the deltoid has been referred to as Anticipatory Postural Adjustments indicating a “feedforward” process.
Certain findings of this study conflict with those of previous research. Specifically, that the activation of the TrA is not symmetrical, but instead is asymmetrical dictated by which arm is being raised. Changing the arm creates an “opposite” mirror response in most of the muscles. The TrA, along with other trunk and leg muscles do fire before the deltoid, but the asymmetrical activation of most muscles suggests that this is more likely occurs to provide a stable base for the for the raising of the arm.
The study concluded that “activation of the TrA is directionally specific and that symmetrical bilateral preactivation is not a normal activation pattern during unilateral ballistic action.” Their findings “suggest that the underlying rationale that the training program … via the isolated role of bilateral activation of the TrA in the feedforward window is not supported.”
For more than 20 years, Dr. Gary Gray has proposed that the isolation muscle activation protocol created by a conscious contraction in a horizontal position makes little sense because it is so inconsistent with human function. During function, muscles are never activated via a conscious contraction. The majority of abdominal muscle function occurs during upright function with movement in all three planes at the same time. Muscles do not function in isolation, but rather as part of a muscle synergy created for a specific task. A quick look at the Functional Movement Spectrum reminds us how far from function the muscle isolation activation programs are. (Click here to access Gray Institute’s Functional Movement Spectrum)
If the pattern of TrA muscle activation is different in patients with low back pain, then the goal of training programs is to utilize functional movements to create the activation. Dr. Gray would say that if the TrA is not turning on properly, find the “switch” that is dysfunctional. Lack of transverse plane motion in the hips or thoracic spine would be the first suspects. It is the motion between the thorax and the pelvis that lengthens and activates all of the abdominal muscles. Pain from an injury can cause patients / clients to restrict motion. Pain and fear of movement may alter the function of the muscle spindles via sympathetic neural connects.
Based on Applied Functional Science® strategies (that emanate from the truth of human movement), a functional program would use task-specific movements performed in an upright posture. The program would avoid painful movements while building on the patient’s / client’s existing movement success. Programs would be designed using the drivers of movement in this and other studies: arms and legs. Restoration of joint motion and control of that motion would allow for the progression of a program of global movements designed to lengthen and load the abdominals in the sagittal plane, frontal plane, and the all-important transverse plane.