Original ResearchSerratus anterior and trapezius muscle activity during knee push-up plus and knee-plus exercises performed on a stable, an unstable surface and during sling-suspension
Introduction
Shoulder pain affects one-third of adults during their life span (Chard et al., 1991, Walker-Bone et al., 2004, van der Heijden, 1999). Symptoms can be persistent and disabling, thereby increasing demands on health and thus increasing economic costs involved (Jeong et al., 2014, Miranda et al., 2008).
Shoulder impingement syndrome, shoulder tendonitis and glenohumeral instability are among the most common causes of shoulder pain and disabilities (Linaker & Walker-Bone, 2015), particularly in occupations or sports that require constant elevation of the arm above the head (Harkness et al., 2003, Miranda et al., 2008, Yanai et al., 2000). Safe and effective elevation of the arm via shoulder abduction and elevation, requires the coracoid process to move in a posterior and cranial direction while the inferior angle of the scapula moves anteriorly and caudally (Clarkson, 2005). This movement, termed scapular posterior tilt, is primarily controlled via the serratus anterior (SA) and the fibers of the lower (LT) and upper (UT) trapezius muscles (Norkin & Levangie, 1992). Impairments to the SA such as muscle imbalance or weakness often result in altered scapula positioning, scapulothoracic- and humeral motion, and muscle activation patterns (Ludewig & Reynolds, 2009), which increases the risk of developing shoulder impingement, glenohumeral instability, pain, or other such musculoskeletal disorders.
When clinicians identify impaired scapula kinematics, their primary goal (during rehabilitation) is the training of the scapular stabilizing muscles, in particular the SA and the LT (Ludewig and Reynolds, 2009, Pirauá et al., 2014). Typically, clinicians utilize the push-up, push-up plus and other push-up variations for retraining SA, LT and UT muscles. As measured via electromyography (EMG), these exercises facilitate motor recruitment within the scapulothoracic muscles (Herrington et al., 2015, Kim et al., 2014, Pirauá et al., 2014). Within these push-up variations, researchers demonstrated that SA activity is greater in push-up plus exercises compared to the regular push-up (Ludewig et al., 2004, Park and Yoo, 2011). It is theorized that the additional plus movement during the push-up plus, which involves posterior translation of the thorax while maintaining a relatively fixed position of the scapula, serves to further increase SA activity beyond that of the regular push-up (Hardwick, Beebe, McDonnell, & Lang, 2006).
To further increase muscle activity during exercising, clinicians commonly instruct their clients to perform the exercises on an unstable base of support. The current literature suggests that push-ups on an unstable base of support (e.g. foam mats, wobble-boards, Swiss ball based exercises or sling-suspension), requires greater activity of the proximal muscles to stabilize a joint and to maintain center of mass control (Maeo, Chou, Yamamoto, & Kanehisa, 2014). However, results reported in the literature, especially for the proximal shoulder stabilizers, are still conflicting and inconsistent as other researchers found no differences in muscle activities of scapula-stabilizing muscles when using an unstable base of support (Jeong et al., 2014, Kim et al., 2014, Lehman et al., 2008, Sandhu et al., 2008). One reason might be differences in methodology and muscle groups analyzed.
Push-ups and their variations are typically performed in a prone position, termed ‘prone bridging’, with the hands and feet acting as the base of support. This position requires a certain level of strength in the upper extremity in addition to well-developed trunk stability. Females, however, often have difficulty performing the push-ups correctly due to gender differences in strength and other muscle characteristics (Miller, MacDougall, Tarnopolsky, & Sale, 1993). To overcome this issue clinicians often recommend female clients to perform the push-up from a kneeling position to reduce the exercises' physical demand by decreasing the long moment arms (Baechle & Roger, 2008). Additionally, the physical demand can be further reduced by performing the plus movement independently from the push-up, termed plus or knee-plus exercise. For clinicians it is very important to understand differences in muscle activation patterns between these exercise variations. This enables the clinician to select the best-suited exercise variation tailored to the needs of their clients. Currently however, there is still a lack of literature that analyzes the muscle activity of the SA and trapezius during knee push-up plus and isolated knee-plus exercises. Furthermore, the current literature is limited to mostly male participants with conflicting results on the effects of unstable bases of support.
Therefore, the purpose of this study was to compare SA and trapezius muscle activity during knee push-up plus and knee-plus exercises during a stable and unstable base of support as well as during sling suspension in healthy female participants. In line with these goals, the following two hypotheses were developed. H1: The knee push-up plus, compared to the knee-plus, will not exhibit any differences in muscle activity for the SA. H2: An unstable base of support will elicit increased muscle activity during both exercise conditions for SA, LT, and UT.
This study will help extend the still controversial results from current research regarding the effect of push-up variations as well as different unstable bases of support or sling-suspension on muscle activity of the SA specifically for the female population. Acquired data will support clinicians as a rationale for developing exercise programs tailored to the needs of their clients.
Section snippets
Sample size estimation
Sample size estimation providing 80% power to detect a small to moderate effect size (partial η2 = 0.15) for a 2 × 3 repeated measures ANOVA and assuming no correlation among repeated measures (as a rather conservative situation with respect to necessary sample size) and assuming sphericity, revealed that 12 participants would be sufficient. Sample size was estimated using G*Power 3.1.
Participants
In total 19 healthy, physically active female participants volunteered to take part in this study (age: 23 ± 3
Results
The CV grand mean was 10.9% ± 2.1 for all participants and test conditions. Mean CVs per participant ranged from 6.5% to 14.0%.
No significant interaction (p < 0.05) between exercise type × base of support for SA, UT, and LT were found. For SA activity (Fig. 3), no additional main effects for exercise type and base of support were observed.
However, the UT (Fig. 4) showed a significant main effect of exercise type (F(1, 18) = 25.042, p = 0.000, η2 = 0.582). Muscle activity showed slightly greater
Discussion
The purpose of the present study was to compare EMG activity levels of the SA, UT, and LT activity in a young and healthy female population during knee push-up plus and knee-plus exercises performed on a stable and two unstable bases of support (foam mat and sling suspension). Our first hypothesis was that the knee push-up plus, compared to the knee-plus, would not exhibit any differences in muscle activity for the SA. The second hypothesis was that an unstable base of support would elicit
Conclusion
Consistent with our hypothesis, no differences in SA activity were observed between the push-up plus and the plus-movement from a kneeling position. Based on the results obtained in this study, it can be concluded that the plus movement in a kneeling position is as sufficient as the push-up plus exercise in activating the SA. Additionally, results from this study indicate that for the purpose of SA rehabilitation, the selected unstable base of support did not elicit differences in muscle
Conflict of interest
Authors declare that they have no conflict of interest.
Ethical statement
Authors declare that procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national). Participants provided oral and written informed consent and signed a university approved informed consent form prior to the study, to protect all associated rights.
Funding information
Authors received no funding for this study.
Acknowledgements
We gratefully would like to thank all participants who took part in this study as well as PD Dr. Tanja Stamm, PhD, MSc, MBA, Daniel Heinzl and Iris Ciganek for their great assistance in writing this manuscript.
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