Spinal Muscle Thickness and Activation during Abdominal Hollowing and Bracing in CrossFit® Athletes

Abstract

Exercises that improve muscle activation are essential for maintaining spinal stability and preventing low back pain. The purpose of this study was to compare the effects of abdominal hollowing and bracing on the activation of the core muscles in CrossFit® participants using ultrasound and electromyography (EMG). Twenty-four healthy adults aged 21 to 42 years old with at least two years of CrossFit® experience performed three core stability exercises (plank, side plank, bridge) with abdominal hollowing and bracing. We measured the ultrasound relative thickness of the local core muscles (transversus abdominis, internal oblique, and lumbar multifidus), and the EMG percentage of maximal voluntary contraction (MVC) of the global core muscles (rectus abdominis, external oblique, and iliocostalis lumborum). Analysis of variance tests showed that the relative thickness of the local core muscles was greater (p = 0.016) during hollowing (range from 26.8 ± 5.33 to 88.4 ± 11.9% of rest) than bracing (range from 15.9 ± 3.54 to 61.2 ± 15.9% of rest), while the EMG of the global muscles was greater (p = 0.001) in bracing (range from 24.4 ± 7.30 to 72.5 ± 9.17% of MVC) than hollowing (range from 16.4 ± 3.70 to 56.6 ± 7.65% of MVC). These results indicate that the recruitment of spinal muscles during popular exercises is achieved with both hollowing and bracing. Nevertheless, it appears that hollowing tends to recruit more of the local muscles, whilst bracing recruits more of the global muscles. The grading of the exercises varied between muscles and varied between maneuvers, especially for the surface abdominals and lumbar muscles. CrossFit® practitioners can choose to use either hollowing or bracing to activate their core muscles more selectively or more appropriately, depending on the goal and purpose of the exercise.

A Comparison between Core Stability Exercises and Muscle Thickness Using Two Different Activation Maneuvers

“…Over the past few years, there has been a debate about the most effective motor control strategy to achieve the dynamic stabilization of the core [ 40 , 53 , 54 , 55 , 56 ]. This study’s findings confirm the third hypothesis as the average relative thickness of all three core muscles (TrA, IO, LM) was greater when using the hollowing compared to the bracing maneuver ( Figure 5 ), which is consistent with previous studies [ 15 , 32 ]. Some studies have suggested that deep abdominal muscles are activated to a greater extent by abdominal hollowing, while superficial muscles are activated to a greater extent by abdominal bracing [ 26 , 32 ].…”

“…This study’s findings confirm the third hypothesis as the average relative thickness of all three core muscles (TrA, IO, LM) was greater when using the hollowing compared to the bracing maneuver ( Figure 5 ), which is consistent with previous studies [ 15 , 32 ]. Some studies have suggested that deep abdominal muscles are activated to a greater extent by abdominal hollowing, while superficial muscles are activated to a greater extent by abdominal bracing [ 26 , 32 ]. Other studies, however, have reported exactly the opposite [ 14 , 27 , 57 ].…”

“…For example, several studies have examined individuals with LBP or untrained individuals [ 14 , 17 , 19 , 58 ]. As hollowing may be more difficult to understand than bracing [ 32 , 59 , 60 ], this alone could bias the results in favor of one technique.…”

“…The thickness of the LM was measured as the distance between the superior and inferior fascia of the muscle. The relative thickness during each exercise was calculated as a percentage of the resting thickness for each muscle and exercise using the formula [ 32 ]. …”