Abdominal Bracing Increases Ground Reaction Forces During Landing

Research Review: Abdominal Bracing Increases Ground Reaction Forces During Landing

By Stefanie DiCarrado DPT, PT, MSCS, NASM CPT, CES, PES

Edited by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation: Campbell, A., Kemp-Smith, K., O'Sullivan, P., Straker, L. (2016). Abdominal bracing increases ground reaction forces and decreases knee and hip flexion during landing. Journal of Orthopaedic & Sports Physical Therapy, Prepublication - ABSTRACT

Jump landing test

Why is this relevant?: Abdominal bracing (AB) involves a co-contraction of all abdominal muscles - such as one would do if preparing to be punched in the stomach (1). As noted, and cited, by the authors, controversy among movement professionals exists as to the benefit of abdominal bracing for optimal spinal stabilization. In a previously reviewed article, Grenier & McGill (2007) proposed abdominal bracing improved spinal stability over abdominal hollowing, aka, targeted transverse abdominis (TVA)  activation via "drawing in" (1). On the opposing side of the discussion, previously reviewed research of Richardson et al (2002) involved an intrinsic system consisting of the TVA , multifidus (MF) , pelvic floor muscles (PF) and IO that increased sacroiliac joint stabilization and improved force transmission from the lower extremity to the trunk (2). Although these 2 articles assess and evaluate different things using different methods and cannot be directly compared, they both provide a small component of a larger of discussion on the two different methods of spinal stabilization (Bracing vs. Drawing-in). This study reviewed below, suggests that bracing may create excessive lumbopelvic hip complex rigidity and actually hinder proper force attenuation.

Study Summary

Caption: Illustration of the various layers of anterior abdominal muscles

Public Domain, https://commons.wikimedia.org/w/index.php?curid=789657

Review & Commentary:

This study is unique from previous studies cited, in that it evaluated the effects of abdominal bracing (AB) vs non-abdominal bracing (NAB) on vertical ground reaction forces (vGRF) during a two-footed drop landing. The authors provided ample citations explaining the link between spinal stability and decreased injury, namely, how abdominal activation techniques can improve spinal stability; therefore, having the potential to reduce the risk of injury. They provided citations and explanations pertaining to the two main schools of thought on abdominal activation: abdominal bracing and abdominal hollowing (TVA activation ). Additionally, the authors reviewed research to identify the appropriate percentage (30%) to implement in this study.

According to the literature review, abdominal bracing may result in excessive spinal compression; however, spinal compression may be detrimental during tasks like landing which also increase compressive forces on the spine. Further, the authors noted there is potential for overflow, in which abdominal co-contraction may lead to excessive lower extremity stiffness and reduced ability to attenuate vGRF. It is worth noting, that Campbell et al. did exceptional work describing the problem at hand, listing the potential solutions, and providing details to support the gap in current literature that this study aimed to fill.

The article presented clearly stated objectives - to gather data regarding the problem of high vGRF and lower extremity injuries. There were multiple objectives within this study: examine the effect of AB on vGRF and lumbar kinematics (primary), analyze if individuals can maintain AB while landing, if AB resulted in co-contraction of lumbar multifidus (MF) , and if AB reduces lumbar spine motion during landing. Despite a small sample size, the strong, standardized and clearly documented methodology enhances the impact of the results. Standardization allows for little, if any, variation between subject trials. Only young, healthy subjects, without recent injury participated, creating a homogeneous sample with less potential for confounding variables. Subjects acted as their own controls which limited variability during data comparison. Subjects received the same instruction, performed the same protocol, wore the same equipment with the same placement, were allowed the same practice time and performed the same number of trials. A physiotherapist provided the instruction on abdominal bracing taken from McGill & Karpowicz's book Exercises for Spine Stabilization.

Abdominal contractions and MF were compared to the subject's maximal voluntary isometric contraction (MVIC). Researchers used EMG biofeedback during practice and testing to ensure the targeted percentage of MVIC was met by each subject. Biofeedback provided a visual representation of the contraction for the subjects to assist them in maintaining the proper level of activation. Subjects practiced for an average of 20 minutes to ensure they could maintain the desired 25-30% MVIC abdominal contraction, and also practiced the drop test from a height of 40cm to land on both legs while maintaining AB by watching the biofeedback monitor. Electrode placement was consistent across subjects and well documented; the authors provided two citations to validate their placement. The authors went so far as to assess skin impedance, ensuring an acceptable value existed to maintain data integrity across subjects. Trials requiring AB began with a 20 second pre-activation to ensure the proper level of contraction.

Kinematic analysis involved the standardized placement of motion capture sensors for which the authors provided citations to previously used protocols in accordance with the International Society of Biomechanics. The areas of interest were the ankle, knee, hip and lumbar spine.Vertical ground reaction forces (vGRF), particularly the time from contact to peak vGRF, was measured by a force plate on which the subjects dropped off a step, landing on both legs. Subjects performed 3 trials of each condition. The condition tested first, AB vs NAB, was randomized to limit any carryover or training effect.

This study is not without limitations. First, although the methodology of this study was strong, the collection of EMG data varied greatly from others studies cited in the research review. Grenier & McGill (2007)  monitored the external obliques (EO) , rectus abdominis (RA) , and internal obliques (IO) since AB involves a co-contraction of all abdominal muscles (1). Richardson et al (2002) used EMG on EO and erector spinae (high activity = AB, low to no activity = TVA activation) along with real time ultrasound to monitor appropriate muscle contractions. Hodges & Richardson (1996) used fine wire electrodes to monitored TVA as it is a deep muscle and nearby muscle activity is likely picked up by surface electrodes (3). Campbell et al. should have considered including EO and/or RA as a focus for abdominal bracing rather than targeting the deep TVA and slightly more superficial IO ; however, due to the use of superficial electrodes, it is possible that data from EO was collected. This poses a potential limitation to the study as it is only assumed EO and RA were active sufficiently during the abdominal bracing. However, the authors did provide citations validating their placement according to established protocols.

Further conflicts exist between this study and another recent study published in the Journal of Sports & Physical Therapy (JOSPT) on vGRF which found an increase in knee flexion correlated to reduced vGRF. Campbell et al. found the opposite but both articles noted decreased hip flexion correlated with higher vGRF. Wernli et al. (2016) found a correlation between decreased ankle flexion and high vGRF but this article found no statistical difference (4). One reason could be the comparison of double leg vs single leg landing. A single leg landing may need greater LE excursion to attenuate forces. Unfortunately, this study did not monitor or track the sound of landing which would have been an interesting continuation of the aforementioned study.

This study used a small sample size which can be a limitation but provided statistically significant data. This may imply that it is worth the time and effort to conduct a similar study with a significantly larger sample size. Future studies could compare AB vs TVA activation, monitor landing sounds, and study lower limb and lumbar kinematics to gain further insight into optimal landing mechanics.

The authors do not provide information as to subject or tester blinding, and so it may be possible subjects altered landing mechanics if they understood the purpose of the testing. As with previous studies, it is unlikely the subjects were made fully aware of the study objectives and, therefore, this is a minimal risk. Despite the mentioned limitations, this study provides to us an important look into one particular lumbar stabilization technique and the related effects on vGRF attenuation during double leg landing.

The current study may indirectly support the findings of Richardson et al (2002) . Although the current study did investigate drawing-in versus AB, if bracing is associated with greater vGRF, than drawing-in may be the next best option until further research can be done. This study also found no co-activation of the multifidus (MF), whereas Richardson et al (2002)  described co-contraction MF with TVA activation. If MF contraction is important to lumbar stability, this may further support Richardson et al (2002) , as they hypothesized co-contraction of the larger global muscles of the trunk would not create the specific force closure needed to adequately stabilize the spine. Campbell et al. found no significant difference in lumbar spine motion, so previous studies on AB and decreased spinal motion may not directly apply to compressive forces or force attenuation during landing.

Why is this study important?

Landing tasks are common in sports, and less than optimal mechanics have been hypothesized to increase the risk of injury. This study does not offer a direct solution to that problem, but suggests abdominal bracing is not the type of abdominal activation technique appropriate for landing tasks. Further research is needed, to determine whether drawing-in, or perhaps a third method of abdominal stabilization is ideal for landing tasks.

How does it affect practice?

Clinicians seeking to reduce vertical ground reaction forces (vGRF) during landing tasks should consider using means of lumbar stabilization other than abdominal bracing (AB). The increase in vGRF during landing with AB, may be due to excessive spinal compression and/or "overflow" into neighboring lumbopelvic hip muscles, creating excessive joint stiffness and decreased force attenuation.

How does it relate to Brookbush Institute Content?

The Brookbush Institute views abdominal bracing (AB) as a progression in muscle recruitment that depends on the task at hand. More aggressive activity, lifting or pushing heavier weight may require increased lumbar stability accomplished through AB.

From the article “Transverse Abdominis (TVA) “:

“In summary, neuromuscular reeducation of the intrinsic stabilization subsystem should be addressed first, followed by reeducation and conditioning of the global musculature (core subsystems), and finally integration into functional activities. All steps are necessary for a complete recovery from low back pain or attaining optimal performance. “Drawing in” should be sufficient for stabilization of the lumbar spine and pelvis during most daily activity – that is activity without an added external load or significant increase in velocity. “Bracing” is necessary during an individuals higher intensity activity – this could range from reaching over-head in an older individual, to loaded activities like carrying groceries in the middle aged and healthy, to weight lifting and sport in the athletic population. Cue the “drawing-in” maneuver during all activities, and “bracing” when appropriate. Even the valsalva maneuver may be considered a progression from “drawing in” and “bracing” for the upper limits of individuals capacity, i.e. 1 Rep Max.”

This research adds to our knowledge base, indicating abdominal bracing may create excessive rigidity in the spine and negatively impact vGRF attenuation during landing tasks. Spinal stabilization provided by the TVA is recognized and cued throughout the predictive models of Lower Leg Dysfunction , Lumbo Pelvic Hip Complex Dysfunction , Sacroiliac Joint Dysfunction , and Upper Body Dysfunction . TVA activation via drawing-in is used by the Brookbush Institute to pre-stabilize the spine - this was originally based on a classic study by Hodges & Richardson (1996) which showed that those without low back pain activated their TVA prior to moving the extremities, while those with low back pain exhibited a delay in TVA activation. This application of research findings has been supported by further research and continues to produce better than average results in a clinical and performance setting.

If an individual exhibits an inability to activate the TVA (draw-in), or losses the drawing-in maneuver during higher level activities (abdominal distension) it may be necessary to add TVA Activation techniques to their warm-up or corrective intervention. The following videos emphasize proper technique to activate the TVA and proper form during a jump down task.

Transverse Abdominis TVA Isolated Activation

TVA and Gluteus Maximus Activation and Progressions

Hardest Quadruped Progression Ever:

Dynamic Quadruped:

Hop Down to Stabilization

Hop Down to Single Leg Touchdown to Balance

Sources

1. Review: Grenier, S., McGill, S. (2007). Quantification of lumbar stability by using 2 different abdominal activation strategies. Archives of Physical Medicine & Rehabilitation. 88, 54-62
2. Review: Richardson, C., Snijders, C., Hides, J., Damen, L., Pas, M., Storm, J. (2002) The Relation Between the Transversus Abdominis Muscles, Sacroiliac Joint Mechanics, and Low Back Pain. Spine. 27 (4), 399-405
3. Review: Hodges, P., Richardson, C. (1996). Inefficient Muscular Stabilization of the Lumbar Spine Associated With Low Back Pain: A Motor Control Evaluation of Transverse Abdominis. Spine, 21(22), 2640-2650
4. Review Pending: Wernli, K., NG L., Phan, X., Davey, P., Grisbrook, T. (2016) Relationship between landing sound, ground reaction forces and lower limb kinematics, vertical ground reaction force, and kinematics of the lower limb during drop landing in healthy men. Journal of Orthopaedic & Sports Physical Therapy. 46(3) 194-199

© 2016 Brent Brookbush

Questions, comments, and criticisms are welcomed and encouraged -