Electromyographic activity and 15RM load during resistance exercises on stable and unstable surfaces.
In recent years, gyms have focused on physical fitness and training using unstable surface (US) for resistance exercises (RE), in which the traditional exercise machines for resistance training share space with US equipment, such as the Swiss ball, BOSU[R] ball, TRX[R], and balance disks. Regarding the US, the neuromuscular system in the back is believed to be more stimulated compared with traditional stable surface (SS) due to increased activation of trunk stabilizing muscles (7). However, studies comparing muscle strength on SS and US have reported conflicting results (6,12).
Bench press may be the most popular RE for the upper limbs among resistance training practitioners. Recent studies have investigated bench press on SS and US (2,6,11,14). Uribe et al. (14) reported similar electromyographic (EMG) activity in the anterior deltoid and pectoralis major during 3 repetitions at 80% of 1 maximum repetition (RM) on a bench and Swiss ball. Anderson and Behm (2) reported lower isometric strength on US (at ~60% of 1 RM). However, no difference in EMG activity was observed in the pectoralis major, anterior deltoid, triceps brachii, latissimus dorsi, or the rectus abdominis in the bench press exercise on a bench or Swiss ball. Saeterbakken and Fimland (11) observed lower strength in 6RM on US and similar EMG activity in the anterior deltoid, biceps brachii, and external oblique muscles. In contrast, in the triceps brachii and pectoralis major, the authors observed higher EMG activity on SS. In contrast, Goodman et al. (6) found no differences in 1RM strength during bench press performed on the bench or Swiss ball or in EMG activity in the outer portion of the pectoralis major, anterior deltoid, latissimus dorsi, external oblique, triceps brachii, and biceps brachii muscles.
The free squat is a primary resistance training exercise for the lower limbs (3,8,12). Saeterbakken and Fimland (12) investigated EMG activity in the rectus femoris, vastus lateralis, vastus medialis, biceps femoris, soleus, rectus abdominis, external oblique and erector spinae muscles during the free squat on the ground and different US (Power board[R], BOSU[R] and Balance Cone[R]). Only the rectus femoris exhibited higher EMG activity on a SS. In the other muscles, no differences were observed between the surfaces. McBride, Cormie, and Deane (8) reported greater EMG activity in the vastus lateralis and the vastus medialis during the isometric free squat on two balance disks with similar muscle activation for the biceps femoris and medial head of the gastrocnemius. Regarding the development and peak of isometric strength, these authors reported a lower value in the unstable condition compared with the stable condition. Corroborating these results, Behm et al. (3) observed lower isometric strength (70.5% and 20.2% for leg extension and plantar flexion, respectively) sitting on the Swiss ball US. EMG activity in the quadriceps and plantar flexors was lower on the US. In contrast, EMG activity in the hamstrings and tibialis anterior was higher on the US.
Most of these studies, except for Saeterbakken and Fimland (11), did not use strength tests on both surfaces. In addition, studies analyzing strength and EMG activity during 15RM are lacking. Such studies only report the maximal isometric force (2,8), 1RM dynamics (6), 6RM dynamics (11), or 4-repetition dynamics (13). Therefore, due to the wide use of resistance force in the dynamics of resistance training, a study analyzing 15RM strength and EMG activity in different muscle groups on SSs and USs is an important contribution to RT.
Thus, this study compared EMG activity and load during 15RM in the bench press and free squat on SSs and USs. We hypothesized that EMG activity would be higher and that strength would be lower during conditions of instability during 15RM.
Nineteen healthy men (age, 24.65 [+ or -] 3.48 yrs; height, 1.79 [+ or -] 0.08 m; weight, 80.61 [+ or -] 9.14 kg; and percent body fat, 11.86 [+ or -] 3.49%) with previous experience in resistance training (6 25 [+ or -] 4.61 yrs) participated in the study. The sample was selected in a non-probabilistic manner and allocated randomized. The inclusion criteria were: (a) men aged 19 to 30 yrs; (b) free from musculoskeletal injury (which could hinder the tests); (c) negative answers to all items in the Physical Activity Readiness Questionnaire/PAR-Q; and (d) familiarity with resistance training for >12 months.
Subjects taking medication, alcohol, and/or who smoked; who presented with a history of musculoskeletal disease and/or aggravation; or who had previous experience in resistance training with US were excluded. The research project was approved by the Human Research Ethics Committee of the Federal University of Juiz de Fora through Opinion No. 204.521/2013 in accordance with the regulations of resolution 196/96 of National Council on Ethics in Human Research and in accordance with the Declaration of Helsinki. All subjects signed an informed consent form according to National Health Council Resolution No. 196/96.
The experimental protocol was performed on SSs and USs using the bench press exercise on a bench (Righetto Fitness Equipment[R], Bench Press, SP, BR) and a Swiss ball (Mercur[R], Gym Ball, Rio Grande do Sul, Brazil). The subjects held a free bar while their shoulder blades and head rested on the surface with their feet planted securely on the floor.
The subjects performed the free squat on the ground and on two balance disks (Pretorian[R], Balance Cushion, SP, BR) on each foot (standing with the free bar supported by their back and held by their hands) and performed the motion until the knee formed a 90[degrees] angle with the ground between the thigh and leg.
In the first meeting, after measuring anthropometric variables, the subjects underwent a series of familiarization exercises on the US. The subjects performed three sets of 15 repetitions at approximately 60% of the maximum perceived exertion and received instruction regarding motor coordination and movement rhythm. In the 2nd, 3rd, 4th, and 5th sessions, the subjects were subjected specifically to tests and retests of 15RM, with an interval of 48 to 72 hrs between days. The surfaces were selected randomly. In the 6th and 7th sessions,
EMG activity was collected with a 24-hr interval between sessions. The surfaces were selected randomly. The bench press was performed first, then, the free squat was performed. For the bench press, the Swiss Ball (55, 65 or 75 cm) was used in accordance with the height of the subjects (1.55-1.69, 1.70-1.87, and 1.88-2.03 m, respectively). The ball was inflated to a pressure at which the footprint on the ground was the same as the diameter of the ball according to the manufacturer's specifications. For free squatting, two balance disks (one on each foot) were used and were inflated up to 6 cm for the performance of all tests according to the manufacturer's instructions.
The 15RM tests were performed on both platforms according to the preexisting protocol. A warm-up was performed with 15 repetitions at 40-60% of the perceived maximum load for 15RM. After a 1-min rest, the subjects performed 5 repetitions at 60-80% of the perceived maximum for 15RM. After a 1-min rest, the load test began. The subjects self-selected a load that they felt approximated their individual 15RM. In the load test, each individual performed a maximum of three attempts for each exercise, with a 5-min interval between attempts. After the subjects performed the first attempt, the load was increased or decreased until reaching 15RM. A range of motion limiter was used to determine the start and end positions of each exercise. After obtaining the load for the first exercise (bench press), the subjects rested for 10 min before the free squat tests.
To reduce the margin of error in the 15RM test, the following strategies were adopted: (a) an orientation before the test to inform the subjects of the data collection routine; (b) instructions on task performance and speed of the exercises (EMT-888 Tuner Metronome[R], SP, BR) with 1 sec to do the concentric phase and 2 sec to carry out the eccentric phase); (c) use of verbal stimuli; and (d) the weights were previously calibrated on a precision scale. The load used for determining 15RM was accomplished through the use of bars (Righetto Fitness Equipment[R], SP, BR). The bars weighed 2, 5, 10, 15, 20, and 25 kg.
The retest aimed to assess the reliability of the load. The greatest weight obtained on both days (test and retest) had a difference less than 5%. In cases of a greater difference, the subjects were required to perform the test again to calculate the difference between sessions.
To reduce possible interferences during EMG signal acquisition, the subjects underwent skin preparation with hair removal when needed and cleaning of the area with 70% alcohol prior to recording. This technique was designed to reduce the impedance of the skin to a value of less than 5k ohm (Q) as measured using a digital multimeter (Icel Manaus MD-5011[R], SP, BR).
Two electrodes (a channel) for each muscle were placed on the anterior deltoid and pectoralis major for the bench press and the vastus lateralis and biceps femoris for the free squat. Only the muscles on the right side were assessed. The monopolar reference electrode was placed on the medial epicondyle of the right elbow. Ag / AgCl bipolar electrodes (Double Electrodes Miotec[R], SP, BR) with 2 in of distance between the centers of the catchments were used. All electrodes were placed according to the location for standard positioning established by the European Recommendations for Surface Electromyography (SENIAN). Additionally, to prevent displacement during the procedures, the cables were attached with adhesive tape strips. The same researcher identified the anatomical points and placed the electrodes. If the measured impedance was greater than 5k [ohm], the electrodes were removed and the preparation procedures were performed again.
The determination of the electrode placement was performed only on the first day of testing. An outline of the electrodes was made on the skin of the volunteers with a high fixation pen (Faber Castel, Pilot 2.0 mm Az, Brazil) to ensure the same position in the subsequent test. The electrode cables were connected to the signal conditioner of the (EMG System Brazil[R], SP, BR) with a sampling frequency of 2000 Hz per channel, 14-bit resolution, and analog anti-aliasing band-pass filter with a cutoff frequency between 20 and 500 Hz. The signal conditioner was connected to an 11.1 V 2.2 mA/h Li-Ion battery. Microcomputers were also used with their batteries to avoid interference of power oscillation (60 Hz in BR) on the data.
To collect the electromyographic signal, a series of 15RMs (with the loads obtained in the strength tests) was performed for both exercises. The same execution speed, range of motion, and randomization of the strength tests were used. The subjects performed a maximal voluntary isometric action for 5 sec in both exercises, rested for 5 min and then performed a series of 15RMs. The sequence was first performed for the upper limbs and, then, for the lower limbs with a 5-min break between exercises.
The first heart sound was always excluded due to the possibility that unracking of bar and the movement amplitude adjustment might be captured by the surface electromyography. The second and third heart sounds were eliminated because the cadence was not appropriate in these repetitions. A violation of the cadence also occurred when individuals approached the fatigue state (the last sounds). Therefore, the sounds from the fourth heart sound to the thirteenth repetition were used. The root mean square (RMS) variable was calculated from all of the electromyographic signals related to 10 repetitions to ensure that the analyses were performed with repetitions involving the correct cadence and techniques. Then, the variable was normalized by the maximum RMS obtained during maximal voluntary isometric contraction on a SS each subject. The clippings and other signal processing were performed by specific routines developed in Matlab[R] (Mathworks, Natick, USA).
The Student's f-test was used to assess differences in EMG activity and 15RM load of the exercise protocols with SS and US. The data normality prerequisite was met according to the Shapiro-Wilk's test, with the Lilliefors significance correction. The reproducibility of the measurements was assessed with the intra-class correlation coefficient (ICC). The results are presented as the mean [+ or -] standard deviation. The effect size (ES) was calculated with Cohen's d. Analyses were performed using SPSS software, version 20.0 (IBM Corp., Armonk, NY, USA) with a significance level of 5% (P = 0.05).
No significant differences were observed in 15RM load for the bench press on SS and US (66 [+ or -] 10.15 kg vs. 64.2 [+ or -] 8.63 kg, P = 0.231, ES = 0.19). However, the free squat load was significantly higher on the SS compared with the US (83.9 [+ or -] 18.67 kg vs. 70.3 [+ or -] 10.07 kg, P = 0.001, ES = 0.94). High reproducibility coefficients were observed for the bench press on the SS (0.98) and the US (0.94) and the free squat on the SS (0.94) and the US (0.93).
Regarding the EMG variables, no significant differences were observed between the SS and US during either exercise.
The normalized values are presented in Table I.
While there were no significant differences observed in 15RM load for the bench press on SS and US, the free squat load was significantly higher on the SS compared to the US. For the normalized EMG activation values of the anterior deltoid, pectoralis major, biceps femoris, and vastus lateralis, no significant differences were observed between the stable and unstable surfaces. Standardized data during maximal voluntary isometric contraction on the SS were used because the non-normalized data did not indicate significant changes.
Saeterbakken and Fimland (11) reported greater load during 6RM for the bench press on the SS and higher EMG activation of the pectoralis major and triceps brachii muscles on the flat bench, which revealed that the increased instability on the Swiss ball did not require greater muscle activation during the exercise (but a higher total load was lifted by subjects on the SS). In contrast, in the present study, EMG activation in the anterior deltoids and pectoralis major was not greater, even when performed with higher loads on a SS. Only the anterior deltoid muscles had similar EMG activity in these two studies when comparing surfaces (as confirmed in previous research) (2,6,14). Corroborating these findings, two previous studies analyzed maximal isometric and dynamic contractions (1RM) and reported similar EMG activity in the pectoralis major, anterior deltoid, triceps brachii, and biceps brachii muscles (2,6).
Studies analyzing dynamic contractions have obtained different results compared with the findings observed here. Snar and Esco (13) analyzed EMG activity in the pectoralis major, anterior deltoid, and triceps brachii in 21 men during arm flexion on the floor and the TRX[R]. Four repetitions were performed with body weight, and increased muscle activation was observed in all muscles on the TRX[R]. Supporting this study, one research performed 10 fly repetitions on a SS (horizontal bench) and an US (Swiss ball) at 30% of the 1RM in both conditions. EMG activity in the pectoralis major, anterior deltoid and serratus anterior was significantly higher on the US (9). Limiting comparison factors in both studies included the use of body weight alone, other unstable conditions, fewer repetitions, and very low load percentage. The use of different surface types has indicated that the effect of instability on EMG activity differs for many muscle groups.
Resistance training performed on an US requires less strength and muscle activation of the primary muscles used for the movement analyzed. Kohler and colleagues (7) reported higher EMG activity in the triceps brachii during the exercise performance sitting on a flat bench compared with a Swiss ball. Behm and Anderson (3) demonstrated less EMG activation (44%) in the quadriceps during leg extension on an US. The mechanism for decreased activation in the primary muscles with instability can be theoretically confirmed by the increased stress associated with balance demand (5). The movement-stabilizing musculature may be more stimulated due to a greater requirement for exercise stabilization.
Another factor that can influence study results is the air pressure within some US devices, the body weight of the subjects and the load lifted must be considered (given that both modify the deformity level). Furthermore, the positioning of the upper limbs on the Swiss ball is an important parameter to control because the individual supporting the head and dorsal region gains greater stability (2,6).
Regarding EMG activity during the free squat, the results were similar in both conditions because the load during 15RM decreased significantly during the US. This indicates that relatively greater muscle activation is required to sustain a smaller load, which may be explained by the degree of difficulty and instability of the subjects when performing the exercise on the two balance disks.
In accordance with the free squat results observed in the present study, Saeterbakken and Fimland (12) reported lower values of maximal isometric strength on the US compared with the SS. Although EMG activity was similar, several studies (4,10,14) have demonstrated higher EMG activity on the US. But, it is important to mention that the earlier studies used the same absolute load value on both platforms and did not analyze a high number of repetitions, which differs from the current study.
Researchers observed similar EMG activity in the vastus lateralis and biceps femoris muscles, but with increased muscle activation in the soleus during free squatting on two balance disks. Anderson and Behm (1) used the same absolute load on both surfaces. A survey analyzed muscle activation during isometric free squatting on the floor using Dyna disks[R], the Swiss ball, BOSU[R] and the Wobble Board[R]. Similar EMG activation was observed in the rectus femoris, biceps femoris, and erector spinae. However, EMG activity in the soleus was higher, and EMG activity in the rectus abdominis was lower when performed on the Wobble Board[R] and Swiss ball compared with the other conditions. Nevertheless, this study was limited by only using body weight (15). The prescription of resistance training is commonly performed with maximum repetition (i.e., 10RM or 15RM) or by a percentage of 1RM. Comparing relatively different loads becomes important for athletes and practitioners of resistance training.
Limitations of this Study
This study has some limitations. Only subjects experienced in resistance training with no experience on USs were recruited. Thus, the results cannot be generalized to all populations. EMG activity in the stabilizing muscles was not monitored. Even if stabilizing muscles had been analyzed in previous studies, these studies did not evaluate 15RM resistance training.
The increased instability in the bench press and free squat did not cause greater EMG activation in the muscles analyzed. However, a considerable decrease in 15RM load was observed for the free squat. This study has important applications for prescribing resistance programs and exercise for recreational practitioners. The free squat exercise on two balance disks can be performed with a low load to obtain the same EMG activation compared with a 15RM load. Therefore, using a lower load on an US can produce effects similar to higher loads on SSs.
The authors would like to thank the Coordination of Undergraduate Improvement (Higher Education Person Improvement Coordination, CAPES), a government office of the Ministry of Education, for providing a scholarship. The authors also thank the participants for volunteering for the study.
Address for correspondence: Liliane Cunha Aranda, MS, Motor Evaluation Laboratory, Federal University of Juiz de Fora, Juiz de Fora, Minas Gerais, Brazil, zip-code 36033-180, Email: firstname.lastname@example.org.
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Liliane Cunha Aranda , Marcelly Mancini , Francisco Zacaron Werneck , Jefferson Da Silva Novaes , Marzo Edir Da Silva-Grigoletto , Jeferson Macedo Vianna 
 Motor Evaluation Laboratory, Federal University of Juiz de Fora, Juiz de Fora, Brazil,  Sports Centre, Federal University of Ouro Preto, Ouro Preto, Brazil, department of Gymnastics, Physical Education Graduate Program, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil,  Physical Education Graduate Program, Federal University of Sergipe, Sao Cristovao, Brazil,  Department of Sports, Physical Education Graduate Program, Federal University of Juiz de Fora, Juiz de Fora, Brazil
Table 1. Mean and Standard Deviation of the EMG Activity Root Mean Square (RMS) in Resistance Exercises Performed on Stable and Unstable Surfaces (n = 19). RMS--Stable (un.) RMS--Unstable (un.) Anterior Deltoid 0.52 [+ or -] 0.13 0.65 [+ or -] 0.42 Pectoralis Major 0.40 [+ or -] 0.44 0.30 [+ or -] 0.21 Biceps Femoris 0.09 [+ or -] 0.08 0.08 [+ or -] 0.06 Vastus Lateralis 0.34 [+ or -] 0.21 0.34 [+ or -] 0.22 P / ES Anterior Deltoid 0.06 / 0.47 Pectoralis Major 0.14 / 0.31 Biceps Femoris 0.50 / 0.14 Vastus Lateralis 0.76 / 0.00 * Effect size (ES); Significantly different (P [less than or equal to] 0.05).
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|Author:||Aranda, Liliane Cunha; Mancini, Marcelly; Werneck, Francisco Zacaron; Novaes, Jefferson Da Silva; Da|
|Publication:||Journal of Exercise Physiology Online|
|Date:||Feb 1, 2016|
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