Comparison of the Pressure Biofeedback Unit and Rehabilitative Ultrasound Imaging as Biofeedback Tools to Train the Transversus Abdominis Muscle Activation in Healthy Subjects

1. Introduction Low back pain is defined as a pain in the lower back bad enough to limit your usual activities or change your daily routine for more than 1 day, that may or may not go down one or both legs and lasting at least a day. The pain must be comprised on the back, within the limits of the lower rib and the lower gluteal fold. Low back pain (LBP) can further be compartmentalized in "specific LBP", by definition a LBP that be attributable to a known cause, or in "non-specific LBP" (NSLBP), defined as a LBP that cannot be associated to a known cause. LBP is one of the leading causes of functional limitations and sick leave, carrying a heavy financial cost for both individuals and society. According the 2010 Global burden Disease study, LBP is classified the 6th in the overall burden when compared to 290 other pathologies. Global prevalence of LBP is between 11% and 84%, and would reach its peak value à 80 years old. Therefore, the impact of LBP would be more important in societies where the life expectancy is higher, such as Canada and the United States. Moreover, aging would be accompanied by an increase in the frequency of severe back pain, as well as an increase in the prevalence of pain and other symptoms and conditions associated with low back pain, occasioning higher social and functional limitations. It also is one of the main causes of physical therapy consultation, which uses therapeutic exercises, such as core muscle training to address it. Yet, it is unknown as today which method is to be privileged to teach how to contract those muscles in the elderly.
2. Literature review One of the favoured approaches in physical therapy to address LBP is the training of the core muscles, specifically the transversus abdominis (TrA). In a healthy subject, trunk muscles will maintain trunk stability by making a sequence of postural adjustment in advance of distal movement to prevent any loss of balance. This fine tuning adjusting happening before any kinesthetic or sensorial feedback may happen, it is considered as planned by the central nervous system (CNS). Yet, it has been shown that those postural adjustments are lacking in the patients with LBP, and would persist even after the resolution of the LBP episode, which might contribute to the recurrence of LBP. Furthermore, aging is also accompanied by a diminution of motor control of the trunk, constituting a part of the LBP in the elderly. Fortunately, these postural adjustments of the TrA have been shown to be trainable again.

Teaching to contract the TrA often gets to be quite a challenge, since its contraction creates no visible movement. It requires an understanding and/or a good feeling of the desired contraction from the patient. To help in the teaching of the TrA, physical therapists (PT) may use one of the few feedback tools that have been shown to be efficient in that regard. Biofeedback is a technique where the biological information otherwise unknown to the patient is transmitted in real time to him. That information goes over what is usually accessible for those patients. In physical therapy, biofeedback comes from the biomechanical measure of physiological measure of the human body. Adding biofeedback to a usual physical therapy intervention enables the patient to control physical adjustments that were thought to be controlled by the CNS, such as the postural adjustments. It also allows for a better tuning in the tasks and engages the patient in a more proactive therapy. In the literature, two non-invasive biofeedback tools used in clinics are documented for their efficiency to assist in the teaching of the TrA contraction and for their validity to measure its activation, the rehabilitative ultrasound imaging (RUSI) and the pressure biofeedback unit (PBU).

The use of RUSI by physiotherapists is growing. RUSI is a non-invasive technique to provide immediate visual feedback on changes in the thickness of the abdominal wall muscles, reflecting the contraction of these muscles, more specifically the TrA. This is done by transmitting, on a screen, an ultrasound image produced by a probe placed on the abdomen of the participant. The probe emits ultrasound waves which passes through the tissues and then return to the probe in the form of echoes. RUSI would detect recruitment of TrA at a contraction as low as 12% of the maximum voluntary contraction (MVC), and recruitment of the internal oblique (OI) when contracted to 22% a MVC. This visual feedback, coupled with the usual explanations for TrA recruitment, reduces the number of trials required for an effective recruitment of TrA in patients with and without low back pain immediately after training and in the short-term follow-up.

The PBU is a simple and inexpensive device that measures the change in pressure in a three-compartment bag filled with air placed under the patient's lumbar spine whilst lying on his back (supine position). The movement of the lumbar spine on the three-compartment bag produced by a false contraction of the TrA changes the intensity of the pressure and this change can be seen on an adjustable gauge, and therefore avoided. However, Lima et al. (2012) have shown a low association between the pressure measured with the PBU and the surface electromyography (EMG) (r = 0.2, p <0.2), with a sensitivity and a specificity of 60%. A systematic review by the same author concludes that inter- and intra-evaluative reliability ranges from medium to good (intra-class correlation (ICC) of 0.42 to 0.82 and 0.5 to 0.81, respectively) and a construct validity ranging from medium to excellent (ICC from 0.48 to 0.90). Nevertheless, it is a widely used apparatus for learning TRA in physiotherapy because of its low cost and ease of use.

Only three studies compared the efficacy of RUSI and PBU on the recruitment of TrA in both young and asymptomatic individuals and low back pain. Bajaj et al. (2010) report that low back pain patients trained with RUSI acquire more rapidly the ability to recruit TrA (significant reduction in the number of trials required to activate the TrA and the number of days needed to consolidate learning) than those performed with the PBU. Lee et al. (2016) randomized a sample of young and asymptomatic individuals into three groups according to the type of feedback used during training: tactile, RUSI or PBU. The primary measurement was the thickness of the TrA measured with an ultrasound. An increase in TrA thickness (reflecting a better TrA activation) was observed in all three groups. When compared to the tactile feedback group, the RUSI group had significantly higher TrA thickness. Finally, Wooldridge et al. (2014) compared the immediate effect of training with different types of feedback (PBU, RUSI and PBU + RUSI) in a population similar to that of Lee et al. (mean age = 22.2 years). The authors report no significant difference in the activation of TrA, measured with different parameters extracted from ultrasound images, pre and post-training and between groups.