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Effects of Conscious Abdominal Contraction on Periscapular Muscle Activity.

INTRODUCTION

Periscapular muscles play a role in the functional performance of upper limbs in rehabilitation, as well as clinical and sports contexts (9). Previous studies have highlighted the contribution of the anterior serratus and trapezius muscles given their motor and stabilizing contributions on the scapula (2,19). Considering that periscapular imbalances may be associated with kinematic disorders of the shoulder complex including scapular dyskinesis and impingement syndrome (7,12), shoulder rehabilitation has been implemented for restoring the function and timing of periscapular and scapulohumeral muscles, and especially the rotator cuff, anterior serratus and trapezius muscles (4,7,12).

Many studies have suggested that closed kinetic chain (CKC) exercises are an effective strategy to promote periscapular muscle activation, as well as for the maintenance of shoulder function (1,3,10,11,13,14,18,21).

According to the CKC concept, distal segment movements are influenced by the proximal segment stability through a connection in the control of body segments (8,10,14,20). In this line, trunk musculature can act in the stabilization, transfer and anticipation for the functional performance of the upper limbs because they are related to the scapulothoracic muscles (16,21).

The anatomical and functional relationship between the trunk and scapulothoracic muscles may be also explained by the Anatomical Trains Theory, which deals with the existence of a myofascial interaction in the same functional chain (16). For instance, Toro et al. (21) demonstrated that conscious activation of the abdominals induces an increase in the electromyographic activity (EMG) of the scapulothoracic muscles. Similarly, Maenhout et al. (13) identified that different positions of the lower limbs while executing push-ups provide improvements in the EMG activity of the periscapular muscles, and this seems to be at least partly explained by the transmission of forces from the extensor muscles of the trunk.

In this context, understanding the effects of conscious activation of the abdominal muscles during exercise in different positions is imperative, and the information obtained can aid in the selective activation of the periscapular muscles. Thus, the aims of this study were: 1) to analyze the effects of conscious abdominal contraction on the EMG of the periscapular muscles during two isometric exercise variations (bilateral and 3-point push-ups); and 2) to correlate the EMG of the abdominal and periscapular muscles in both exercise variations.

METHODS

Study design and Participants

This study is a crossover design. Eighteen (18) young men (aged between 18-28 years) with previous experience in resistance exercise (at least six months of structured classes), without self-reported pain, injury or surgery history on upper limbs and/or scapular dyskinesis participated in the study. Participants who failed to perform any of the proposed exercises or who had any interference in the quality of the EMG signal were excluded.

Data sources/Measurement

General characteristics and scapular dyskinesis screening: Anthropometric data and clinical assessment for identification of the scapular dyskinesis were performed. First, body weight (kg) and height (cm) were evaluated using a calibrated scale and stadiometer, respectively. Body mass index was calculated as body weight divided by height in metres (2). Participants were instructed to be barefoot and wearing few clothes. For clinical identification of scapular dyskinesis, each volunteer was in orthostatic position and instructed to perform the shoulder flexion movement in the scapular plane with a load equivalent to 5% of their body mass (kg). A video recording was performed simultaneously during the execution of the movement, and a trained-evaluator later performed an analysis of the files to identify the presence of scapular dyskinesis as described by Uhl et al. (24).

Electromyography assessment: An eight-channel Myosystem Br-1 system (Datahominis Tecnologia Ltda[R], Uberlandia, Brazil) was used to obtain the electromyographic records. The device has channels for simultaneous acquisition, bandpass filter 10 Hz to 5 kHz, three amplification stages, 10G[OMEGA] channel impedance in differential mode, 92 dB Common Mode Rejection Ratio, and a 16-bit analog-to-digital converter board.

The EMG signals from upper (UT), medium (MT) and lower trapezius (LT), upper (AS_5th) and lower portion (AS_7th) of the anterior serratus, external (EO) and internal oblique (IO) were obtained using simple active differential surface electrodes composed of two parallel rectangular bars of pure silver.

Before placing the electrodes, local trichotomy with alcohol asepsis and slight abrasion were performed to reduce skin impedance. The positioning of the electrodes in the UT, MT and LT muscles followed previous recommendations (5). Placement of the electrodes on AS_5th and AS_7th was performed according to Park and Yoo (18) on the 5th and 7th ribs, respectively. The EO and IO muscles were in accordance with the recommendations proposed by Monfort-Panego et al. (15). Evaluation of all muscles was standardized on the dominant side of the participants, with the exception of the IO.

The EMG data analysis was performed using the Myosystem Br-1 program version 3.5.4 (Datahominis Tecnologia Ltda[R], Uberlandia, Brazil). The data were obtained at a sampling frequency of 2000 Hz with digital bandpass filters of 15-500 Hz, thereby acquiring the root mean square (RMS) values. The first and second seconds of each repetition were discarded from the analysis in order to avoid interference of the exercise adaptation. The average EMG activity values found during the three repetitions of each exercise variation were considered for analysis.

Experimental design: Surface electrodes were placed on the muscles prior to EMG data capture and the volunteers performed the maximum voluntary isometric contraction test (MVIC) to record the maximum value of the EMG amplitude for data normalization. Three MVIC with manual resistance were performed for each muscle following the previous study settings (6). Each volunteer performed three MVIC with a five-second duration and a one-minute recovery interval between trials. A 10-minute interval between exercises was additionally used to avoid residual fatigue.

The experimental set-up used in the study is displayed in Figure 1 (panel 1A and 1B). The exercise mode order (bilateral or 3-point push up) in the presence or absence of conscious abdominal contraction was performed in random order (Figure 1).

In order to perform the bilateral push-up, the volunteers remained with their hands and feet on the ground positioned in previously designated places, considering a distance between the hands equivalent to the measure obtained from the distance between the acromion, and the distance between the feet equivalent to 75% of the volunteer's height. Additionally, the 3-point push-up exercise was performed in the same manner, but with the contralateral upper limb positioned on the back.

Both exercises were performed with and without conscious contraction of the abdominal musculature, for which a one-minute interval between each exercise was adopted. Conscious contraction was encouraged through verbal feedback with an eight-second holding time, and always performed after the exercise without the command in order to prevent the volunteer from contracting the abdominal muscles without the examiner's instruction.

Study size

The sample size calculation was performed using GPower 3.1.7, considering an effect size for the F-test (repeated-measure ANOVA) = 0.35; [alpha] = 0.05; [beta] (power) = 0.90; number of measures = 4; correlation between the measures = 0.50 and sample loss of 10%; thus, the total sample was 18 participants.

Ethical considerations

The study was approved by the Research Ethics Committee of the University of Pernambuco - Brazil (CAAE 02617412.2.0000.5207). All volunteers received and signed the Informed Consent Form.

Statistical Analysis

Data were processed and analyzed using SPSS version 20.0 (IBM Corp. Armonk, NY). A descriptive analysis was performed and summarized participants' results using means and standard deviation (SD). The Shapiro-Wilk and Levene tests were applied to verify the distribution symmetry and variance of the data, respectively. Two-way ANOVA for repeated measures was performed to evaluate the effects of exercise type and conscious contraction of the abdominal muscles, as well as their interaction on the EMG variables. Effect size (ES) was calculated using the Cohen's d-test and classified as a small (values between 0.2 and 0.5), moderate (0.5 to 0.8) and high effect (> 0.80). Pearson correlation coefficient was used to verify the relationship between EMG activation between AS_5th, AS_7th, EO and IO muscles ([DELTA] change of EMG signal during exercise with - without conscious contraction of the abdomen). The level of significance was set at 5% for all analyzes.

RESULTS

Two out of the 20 recruited participants were excluded from the analyses because they had signal interference during EMG acquisition. Thus, the sample consisted of 18 participants (age 22 [+ or -] 2 years old; body mass 75 [+ or -] 10 kg; height 1.77 [+ or -] 0.06 cm; body mass index 23.69 [+ or -] 2.29 kg/m (2)).

The results of the EMG activity of the periscapular and abdominal muscles during the isometric exercise in the different variations are described in Table 1.

The conscious abdominal contraction did not present a significant effect on the EMG activity of the UT (F = 0.174, p = 0.682, ES = 0.10) or MT (F = 0.012, p = 0.913, ES = 0.03). The type of exercise moderately influenced the EMG activity of UT (F = 7.511, p = 0.014, ES = 0.66) and MT (F = 5.309, p = 0.034, ES = 0.56). A moderate interaction effect of the factors was observed for UT (F = 4.530, p = 0.049, ES = 0.50) and MT (F = 4.532, p = 0.048, ES = 0.52). In the presence of conscious abdominal contraction, no significant differences were observed in the EMG activity of the UT (p = 0.08) or MT (p = 0.76) muscles between the exercises. On the other hand, higher EMG activity of both muscles during the 3-point task without conscious abdominal contraction was observed (p < 0.02).

There were no significant effects of conscious abdominal contraction (F = 2.506. p = 0.132. ES = 0.38) or type of exercise (F = 2.014. p = 0.174. ES = 0.33) for the LT. A high magnitude interaction effect was observed for LT (F = 12.354, p = 0.003, ES = 0.85). The presence of conscious abdominal contraction provided greater EMG activity during the bilateral exercise (p = 0.026). Conversely, the absence of conscious abdominal contraction caused higher EMG activity during the 3-point push-up (p = 0.002).

It was observed that the conscious contraction of the abdominal muscles caused an increase in the EMG activity of AS_5th (F = 7.788, p = 0.013, ES = 0.67) and AS_7th (F = 8.676, p = 0.009, ES = 0.71). The type of exercise also significantly influenced AS_5th (F = 9.964, p = 0.006, ES = 0.76) and AS_7th (F = 11.424; p = 0.004; ES = 0.82), with EMG activity being highest during the 3-point exercise. No interaction effects were observed for both portions (p> 0.171).

Conscious abdominal contraction provided a significant increase in the EMG activity of the EO (F = 10.254, p = 0.005, ES = 0.78) and IO (F = 14.963, p = 0.001, ES = 0.93). The 3-point exercise increased EMG activity of the EO (F = 9.046, p = 0.008, ES = 0.73), but no significant effect was observed for IO (F = 1.027, p = 0.325, ES = 0.23). No interaction effect was observed for either muscle (p > 0.349).

The relationship between the EMG activity of the abdominal and periscapular muscles in both exercise variations (with and without conscious abdominal contraction) is displayed in Table 2.

Positive and significant correlations were observed between EMG activity of the EO muscle with AS_5th (bilateral: r = 0.43; p = 0.030; 3-Point: r =0.64; p= 0.002) and AS_7th (r = 0.80; p = 0.001; r =0.82; p = 0.001) in both exercise variations. On the other hand, the IO only showed positive and significant correlation with the two portions of the AS (AS_5th: r = 0.43; p = 0.037; AS_7th: r = 0.62; p = 0.003) muscle in the 3-point push-up.

DISCUSSION

The key findings of the study were that: 1) the presence of conscious abdominal contraction promoted greater EMG activation in all portions of the serratus anterior, EO and IO; 2) the three portions of trapezius muscle were influenced by task configuration, with EMG activity being greater in unilateral exercise. However, although it has no isolated effect, conscious abdominal contraction appears to have been effective in balancing the differences in EMG activity between tasks (unilateral x bilateral); 3) a positive relationship was observed between the EMG activity of the oblique muscles with the anterior serratus (AS_5th and AS_7th) during both exercises (except for the IO, where the positive relationship was observed only in the 3-point push-up).

Studies have shown that CKC exercises promote significant muscular activation of the periscapular muscles (17,18,22). However, depending on the form of the push-up exercise, the EMG activity may change between muscle groups (13,23). In addition, another important factor refers to the role of conscious abdominal contraction, since studies have shown that an increase in the EMG activation of the abdominal muscles can potentiate the muscular activation of proximal regions (21).

Our results indicated that the conscious abdominal contraction did not increase the EMG activity of the three portions of the trapezius, and its association with the two types of exercises influenced the behavior of these muscles differently. In the bilateral push-up, the use of conscious abdominal contraction was shown to be a resource to increase the EMG activity of the UT, MT and LT, which did not occur in the 3-point push-up. Therefore, the association between the 3-point exercise with conscious abdominal activation does not appear to be an interesting strategy for activating the trapezius muscles.

In line with this, Toro et al. (21) demonstrated that conscious abdominal contraction increased the EMG activity of the trapezius during dynamic exercises (Wall Press, Wall Slide and Knee Push-up). However, no significant difference was found when conscious contraction was used in the isometric exercises. In another study, Maenhout et al. (13) found that the knee push-up plus (scapular protraction) with bilateral and unilateral support did not alter the EMG activity of UT and MT. These differences may be related to the execution of the exercises. In the Maenhout et al. (13) study, the push-up was performed with the feet and knees on the ground, while our study performed it only with supported feet. Considering that the overload promoted by the upper limb is superior in exercise without knee support, the increase in EMG activity in the UT and MT can be at least partly explained by the greater demand for stabilization during the task.

We observed that both the exercises (bilateral and 3-point) and contraction type did not promote changes in EMG activation of the LT muscle. Previous studies have suggested that the position of the lower limb may contribute to the activation of this muscular group. For instance, Maenhout et al. (13) found an increase in LT EMG activity during knee push-up with contralateral leg extension. The authors suggested that the extension of the contralateral leg promoted a tension that propagated through the thoracolumbar fascia towards the opposite scapula, facilitating the greater recruitment of the LT muscle.

Regarding the AS, EO and IO muscles, both exercises associated with conscious contraction of the abdominal muscles increased the EMG activity in the different muscular portions, which agrees with the results observed by Toro et al. (21). These findings can be explained based on the Anatomical Trains Theory, which supports the existence of an anatomical-functional relationship between the abdominal muscles and the AS (16). According to this theory, these muscle groups are interconnected by means of fascia so that the tension generated by conscious abdominal contraction would be transferred to AS due to the existing myofascial continuity (16). In this sense, we analyzed whether the EMG activation of EO and IO muscles are related to the muscle activation of AS. The results showed the presence of positive correlation of EO with the two portions of the muscle in both performed exercises, whereas this correlation for IO was only observed in the 3-point exercise. These findings may be justified because of the greater anatomical proximity established between EO and AS muscles.

This study has limitations that should be highlighted. For instance, the sample only included male volunteers with previous experience in resistance exercise, and no previously diagnosed or self-reported osteomioarticular problem. Thus, our findings cannot be extrapolated to other subgroups of volunteers (males without experience in resistance exercises, participants with chronic pain, scapular dyskinesis or other self-reported or diagnosed problem), as well as exercise mode (i.e. push-ups with unstable surfaces).

In summary, conscious abdominal contraction caused an increase in EMG activity of the AS while performing the push-up both with unilateral and bilateral support, showing up as an effective strategy for the activation of this muscle. Regarding the trapezius muscles, conscious abdominal contraction is effective when combined with a bilateral push-up exercise, but not with the 3-point push-up, reinforcing the importance of the type of exercise on muscular behavior.

Clinical Implications

In cases that are indicated the application of isometric exercises to upper limb and trunk in rehabilitation or training context, conscious abdominal contraction seems to be a strategy to increase the muscular activity of the anterior serratus for both exercises. However, for the activation of trapezius, the exercises differ on muscular behavior, where only the use of bilateral push-up seems to activate the trapezius when realized with conscious abdominal contraction.

Conflicts of interest: The authors declare no conflicts of interest.

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Michelle R. Santos, Bruno R. Cavalcante, Fernanda L.S. Ferreira, Vinicius Y.S. Nascimento, Valeria M.A. Oliveira, Francis T. Souza, Ana C.R. Pitangui, and Rodrigo C. de Araujo

University of Pernambuco, Petrolina, Pernambuco, Brazil

AUTHOR CORRESPONDENCE:

Valeria Mayaly Alves de Oliveira

Helio Falcao Avenue, 623; 16 floor

Boa Viagem, Recife, Brazil

Post Code: 51.021-070

Contact number: +55 87 99952-4987

E-mail: valeria.mayaly@gmail.com
Table 1. Results of periscapular muscle EMG activity during isometric
exercise with and without conscious abdominal contraction.

                     With conscious contraction
Muscle        Bilateral push-up            3-Point push-up

UT (*)    4.61 [+ or -] 3.03           7.50 [+ or -] 3.91
MT (*)   11.06 [+ or -] 16.02         12.56 [+ or -] 8.3
LT (*)   26.61 [+ or -] 17.58 (b)     16.78 [+ or -] 12.25 (b)
AS_5th   66.55 [+ or -] 40.17 (b,c)  105.33 [+ or -] 75.68 (b,d)
AS_7th   56.67 [+ or -] 35.89 (b,c)   72.94 [+ or -] 41.09 (b,d)
EO       59.22 [+ or -] 29.17 (b,c)   93.88 [+ or -] 83.84 (b,d)
IO      118.94 [+ or -] 104.02 (c)   129.50 [+ or -] 117.47 (d)

                     Without conscious contraction
Muscle        Bilateral push-up           3-Point push-up

UT (*)   3.39 [+ or -] 2.63 (a)      9.72 [+ or -] 8.64 (a)
MT (*)   6.28 [+ or -] 11.35 (a)    16.83 [+ or -] 16.16 (a)
LT (*)   9.95 [+ or -] 6.49 (a)     25.67 [+ or -] 18.43 (a)
AS_5th  49.72 [+ or -] 37.96 (a,c)  83.06 [+ or -] 59.59 (a,d)
AS_7th  33.83 [+ or -] 20.94 (a,c)  57.22 [+ or -] 39.11 (a,d)
EO      30.72 [+ or -] 19.03 (a,c)  64.77 [+ or -] 45.95 (a,d)
IO      22.83 [+ or -] 15.31 (c)    53.16 [+ or -] 64.16 (d)

Note: upper (UT), medium (MT) and lower trapezius (LT), upper (AS_5th)
and lower portion (AS_7th) of the anterior serratus, external oblique
(EO) and internal oblique (IO).
(*) - Interaction between factors
(a) - difference between exercises (bilateral and 3-Point) performed
without conscious abdominal contraction;
(b) - difference between exercises (bilateral and 3-Point) performed
with conscious abdominal contraction;
(c) - difference between bilateral push-up performed without conscious
abdominal contraction;
(d) - difference between 3-Point push-up performed without conscious
abdominal contraction.

Table 2. Analysis of the correlation of the EO and IO muscles with the
two portions of serratus anterior during push up and 3-Point push up,
considering the difference ([DELTA]) between the situations with and
without command for conscious abdominal contraction.

Muscles        Bilateral push-up              3-Point push-up
             EO               IO           EO               IO
           r     p          r     p      r     p          r     p

AS_5th   0.43  0.030 (*)  0.36  0.07   0.64  0.002 (*)  0.43  0.037 (*)
AS_7th   0.80  0.001 (*)  0.12  0.322  0.82  0.001 (*)  0.62  0.003 (*)

Note: upper (AS_5tth) and lower portion (AS_7th) of the anterior
serratus, external (EO) and internal oblique (IO); r - Pearson
coefficient correlation;
(*) p < 0.05.
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Author:Santos, Michelle R.; Cavalcante, Bruno R.; Ferreira, Fernanda L.S.; Nascimento, Vinicius Y.S.; Olive
Publication:Clinical Kinesiology: Journal of the American Kinesiotherapy Association
Geographic Code:3BRAZ
Date:Dec 22, 2018
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