Traditional free-weight exercises provide constant

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1 COMPARISON OF KINEMATICS AND MUSCLE ACTIVATION IN FREE-WEIGHT BACK SQUAT WITH AND WITHOUT ELASTIC BANDS ATLE H. SAETERBAKKEN, 1 VIDAR ANDERSEN, 1 AND ROLAND VAN DEN TILLAAR 2 1 Department of Teacher Education and Sports, Sogn and Fjordane University College, Sogndal, Norway; and 2 Department of Teacher Education, Nord Trøndelag University College, Levanger, Norway ABSTRACT Saeterbakken, AH, Andersen, V, and van den Tillaar, R. Comparison of kinematics and muscle activation in freeweight back squat with and without elastic bands. JStrength Cond Res 30(4): , 2016 The purpose of study was to compare kinematic muscle activation when performing 6 repetition maximum (6RM) squats using constant (free weights) or variable resistance (free weights + elastic bands). Twenty recreationally trained women were recruited with years of resistance training experience and a relative strength (6RM/body mass) of 1.1. After a familiarization session identifying 6RMloads,participants performed 6RM squats using constant and variable resistance in a randomized order. The total resistance in variable resistance group was similar to constant resistance in presticking region (98%), but greater in sticking region (105%) and poststicking region (113%). In addition, presticking barbell velocity was 21.0% greater using variable than constant resistance, but 22.8% lower in poststicking region. No significant differences in muscle electromyographic activity, time occurrence, and vertical displacement between squat modalities were observed, except for higher barbell displacement poststicking using variable resistance. It was concluded that, due to differences in total resistance in different regions performing variable compared with constant resistance, greater barbell velocity was observed in presticking region and lower resistance was observed in poststicking region. However, extra resistance in sticking and poststicking regions during variable resistance modality did not cause increased muscle activity. When performing squats with heavy resistance, authors recommend using variable resistance, but we Address correspondence to Atle H. Saeterbakken, atle.saeterbakken@ hisf.no. 30(4)/ Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association suggest increasing percentage resistance from elastic bands or using chains. KEY WORDS EMG, variable resistance, constant resistance, strength training, sticking region INTRODUCTION Traditional free-weight exercises provide constant resistance in range of movement (ROM). Because of this constant resistance, in multijoint exercises, such as bench press and squat, re are often regions in which subjects can produce less force than in or regions such as sticking region (11,29,31). The sticking region is referred to as region from initial maximum upward velocity to region associated with lowest concentric velocity of a barbell after which barbell velocity increases again (18) and has only been reported in near-maximal to maximal loads during upward movement in multijoint exercises (9,10,17,24,28), or when fatigued (12,26,30,31). In this region, failure often occurs during lifting (9,17,24,30); however, in barbell heights above sticking region (poststicking region), more force can be produced (28). It has been proposed that cause of sticking region in multijoint exercises is a poor mechanical force position in which maximal force generation declines because of reduced lengths and mechanical advantages of muscles involved (18,28,32). It has been demonstrated that some muscles ( pectoralis major, triceps, and deltoids during bench press) are responsible for getting loads through sticking region (28,33,34). For example, van den Tillaar et al. (28) demonstrated that, in bench press, some muscles are more active and can produce more force in poststicking region than in sticking region. Differences in muscle activation were recently reported in squats in sticking region (29). Thus, when performing free-weight exercises, re is a mismatch throughout ROM between torque created by weights and muscles ability to produce torque because of constant resistance. To maximize force and torque generation throughout ROM, variable resistance has been applied using cam-based machines, VOLUME 30 NUMBER 4 APRIL

2 Comparison of Squat With Variable and Constant Resistance variable resistance (chains and elastic bands) in free-weight squats, but authors only included eccentric vs. concentric phases in kinematics analyses. Therefore, aim of study was to compare kinematics and muscle activation in 2-legged free-weight back squats using constant or variable resistance with similar relative intensity to fatigue (6 repetition maximum [6RM] loads). It was hyposized that length of sticking region would increase due to increased resistance of elastic band. In addition, it was hyposized that muscle activity would be same in presticking region but would be increased in poststicking region using variable resistance. Figure 1. The placement of elastic bands performing squats with variable resistance. chains, or elastic rubber bands (5,19,22). These systems cause more force in regions in which muscles can produce more force (i.e., poststicking region). However, re is controversial evidence regarding effects of variable resistance. Comparably, increased muscle activation and force output have been demonstrated using variable instead of constant resistance throughout ROM (1,2,16,36). Increased resistance throughout ROM increased stress and neuromuscular activation in squats (15), which might enhance long-term training effects more than using constant resistance. However, to best of our knowledge, no studies have analyzed sticking region comparing effects of including elastic rubber bands, kinematics, and muscle activation in free-weight multijoint exercises. For example, Ebben and Jensen (8) reported similar muscle activity and ground reaction force comparing constant and METHODS Figure 2. Relationship between stretching length of elastic band and resistance provided by elastic bands. 946 Journal of Strength and Conditioning Research Experimental Approach to Problem To compare effect of elastic bands on kinematics and muscle activation in 6RM free-weight back squats, repeatedmeasures designing was conducted. After a familiarization session with 6RM loads, participants tested 6RM in back squat using constant resistance (free weights) and variable resistance (free weights + elastic bands) in a randomized order. The barbell velocity, barbell displacement, time spent in different lifting regions, and associated muscle activation performing 2 squat modalities were compared. Subjects Twenty healthy recreationally trained women (mean age = years, age range = years, mean stature = m, mean body mass = kg) who demonstrated a clear sticking region (29) in sixth repetition using both constant and variable resistance were included as participants in study. All participants had resistance training experience ( years) but were not competitive powerlifters or weightlifters. The participants were trained in back squats twice a week as part of ir training program. The participant s relative strength (6RM load/body mass) in squats was 1.1. Seventy-two hours before testing, participants were instructed to refrain from any additional resistance training. Before study, each subject was informed of testing procedures and possible risks, and written consent was obtained from each participant. The participants had to be free of any musculoskeletal pain, injury, or illness that might

3 Journal of Strength and Conditioning Research reduce ir maximal effort. None of participants experienced pain during test. Ethical approval for this research study was obtained from local research ethics committee (31359/3/SSA) and conformed to latest revision of Declaration of Helsinki. Procedures The free-weight back squat was performed in a power rack (Gym 2000, Modum, Norway) with an Olympic barbell (diameter = 2.8 cm, length = 1.92 m). The exercise started with fully extended knees and a natural sway in lower back, which was maintained throughout entire execution. Using a self-paced but controlled tempo, participants lowered mselves to 808 knee flexion (1808 fully extended knee) measured with a protractor (femur fibula). When participants had correct knee angle, a horizontal elastic band was adjusted (4,25). The participants had to touch band (midthigh) in every repetition before starting concentric phase. A test leader gave oral confirmation when participants touched band. If participants successfully lifted six repetitions, loads were increased until true 6RM in 2 squat modalities were achieved. The interclass coefficient between familiarization session and experimental session was using constant resistance (free weights) and using variable resistance (free weights + elastic bands). The procedures used in squats performed with variable resistance were identical to constant resistance condition with 1 exception: 2 elastic bands (R.O.P.E.S 3002 Bungee, Norway) were attached at bottom of power rack on both sides of barbell creating variable resistance (Figure 1). The external load provided by elastic bands decreased with decreasing knee angles and increased with increasing knee angles. The barbell and elastic bands did not provide sufficient 6RM loads alone. Therefore, weight plates were added to increase total resistance. The force provided from elastic bands with different lengths when stretched was measured using a force cell (Ergotest Technology AS, Langesund, Norway). The external forces provided by elastic bands as y stretched were close to having a linear relationship (Figure 2). Therefore, total resistance in variable resistance group during different concentric lifting phases was sum of weight of barbell, external loads, and force provided by stretch length of elastic bands (25). Before 6RM test, participants performed a 10- minute warm-up on a cycle ergometer or treadmill while talking. The participants n performed 3 warm-up sets of traditional back squats using ir self-estimated 6RM loads to calculate warm-up resistance: 20 repetitions at 25%, 10 repetitions at 50%, and 8 repetitions at 70% of 6RM load. The testing order was randomized. The load in 6RM test was increased to eir a load that resulted in failure to complete final repetition or to a load where participants and test leaders agreed that it was true TABLE 1. Kinematics of sixth repetition for variable and constant resistance modalities in free-weight back squats. Lifting regions Vertical displacement (cm) Velocity (m$s 21 ) Time (s) Total resistance (N) Squat modality Constant Variable Constant Variable Constant Variable Constant Variable Presticking * Sticking * Poststicking * * * *Significant differences between modalities on a p # 0.05 level. VOLUME 30 NUMBER 4 APRIL

4 Comparison of Squat With Variable and Constant Resistance Figure 3. The barbell kinematics in sixth repetition in squat using constant or variable resistance. *Significant difference (p # 0.05) between 2 squat modalities in barbell speed in different lifting regions. 6RM load. The 6RM was achieved within 1 4 attempts with 4 5 minutes of rest between each attempt (13). Half of participants started with constant resistance, while or half started with variable resistance 6RM attempts. After shifting from first squat modality to or, participants executed 2 or 3 nonfatigue habitation sets (4 8 repetitions at 50% of familiarization 6RM loads with 3 minutes of rest between each set). The surface electromyographic (EMG) bipolar electrodes (contact diameter = 11 mm and center-to-center distance = 20 mm) were positioned on preferred foot (4) on vastus medialis (80% of distal distance between anterior spina iliaca superior and joint space in front of anterior border of medial ligament), vastus lateralis (2/3 of distal distance between anterior spina iliaca superior and lateral side of patella), and biceps femoris (50% of distance between ischial tuberosity and lateral epicondyle of tibia) according to SENIAMs recommendations (14). Before placement of gelcoated self-adhesive electrodes (Dri-Stick Silver Circular semg Electrodes AE-131; NeuroDyne Medical, Cambridge, MA, USA), skin was shaved, washed with alcohol, and abraded, as recommended in a previous study (14). A commercial EMG recording system was used to measure EMG activation (MuscleLab 4020e, Ergotest Technology AS). To minimize noise induced from external sources through signal cables, EMG raw signal was amplified and filtered using a preamplifier located as close as possible to pickup point. The preamplifier had a common mode rejection ratio of 100 db. The EMG raw signal was n bandpass-filtered (fourth-order Butterworth filter) with cut-off frequencies of 8 and 600 Hz. The bandpassed EMG signals were converted to root mean square (RMS) signals using a hardware circuit network (frequency response = khz, average constant = 100 milliseconds, total error = 6 0.5%). Finally, RMS-converted signal was sampled at 100 Hz using a 16-bit A/D converter (AD637). A commercial software program (MuscleLab V8.13; Ergotest Technology AS) was used to analyze stored EMG data. Only final repetition of 6RM lift was included in analyses because sticking region occurs only during upward phase at near-maximal load or fatigue (9,17,29,31). A linear encoder (sampling frequency of 100 Hz, ET-Enc-02; Ergotest Technology AS) was attached to barbell during squats to measure barbell velocity, lifting time, and vertical displacement. The linear encoder synchronized EMG measurements using MuscleLab 4020e (Ergotest Technology AS) and used to identify different lifting regions using same approaches as previous studies (29,34). The following regions were identified and used to calculate EMG activity: (a) presticking region from lowest barbell position until first barbell peak velocity (V max1 ), (b) sticking region from first barbell peak velocity until lowest barbell velocity (V min ), and (c) poststicking region from first located lowest barbell velocity until second barbell peak velocity (V max2 )for sixth repetition (9,17). The mean muscle activity (RMS) of 3 regions was calculated and used for furr analysis. The 948 Journal of Strength and Conditioning Research

5 Journal of Strength and Conditioning Research RMS values were not normalized as aim of study was to compare muscle activity between 2 squat modalities and relative muscle activation values from normalization would not provide any furr information (20). Figure 4. A C) The muscle activity (mean 6 SD) in sixth repetition in squat using constant or variable resistance for biceps femoris (A), vastus medialis (B), and vastus lateralis (C). Statistical Analyses To assess differences in EMG activity between constant and variable resistance, a 2-way (lifting phase: presticking and poststicking phase 3 squat modality: constant vs. variable) analysis of variance (ANOVA) with repeated measurements was used. If differences were detected by ANOVA, paired t post hoc tests with Bonferroni post hoc corrections were used to determine identity of differences. To assess differences in barbell velocity, barbell displacement, and time spent in different lifting phases between constant and variable resistance, paired sample t-tests were used. To compare total resistance in 3 lifting phases between squat modalities, a 1-way AN- OVA with Bonferroni post hoc corrections was used. Where sphericity assumptions were violated, Greenhouse-Geisser adjustment of p-values was reported. The criterion level for significance was set at p # Effect size was evaluated with h 2 (eta partial squared), where 0.01, h 2, 0.06 constitutes a small effect, 0.06, h 2, 0.14 constitutes a medium effect, and h constitutes a large effect (7). Statistical analysis was performed in SPSS, version 21.0 (SPSS, Inc., Chicago, IL, USA) and differences with p # 0.05 were considered statistically significant. All results are presented as mean 6 SD values. VOLUME 30 NUMBER 4 APRIL

6 Comparison of Squat With Variable and Constant Resistance RESULTS The average 6RM lifting weight with constant resistance was kg and kg with variable resistance weight (loads: kg + mean resistance from elastic band: kg). A 1-way ANOVA for repeated measures indicated a significant effect for resistance during different lifting phases with constant resistance (F = 87.0; p, 0.01; h 2 = 0.82). The post hoc comparison showed that constant resistance load (712 N) was comparable with variable resistance load at presticking region (697; p = 0.78), while variable resistance was significantly higher at sticking (105%; p = 0.02) and poststicking regions (113%; p, 0.01) when compared with constant resistance load (Table 1). The first peak barbell velocity (V max1 ) was significantly higher when using variable resistance than constant resistance (21.0%), whereas opposite was found for second barbell peak velocity (222.8%). No significant differences in velocity were observed at V min (p = 0.42; Table 1; Figure 3). When comparing vertical displacement and time occurrence of different regions, only a significantly higher barbell displacement at V max2 was found for variable resistance compared with constant resistance (Table 1). No or significant differences in measured kinematics were found (Table 1). A 2-way ANOVA for repeated measures performed on EMG and squat modality of different muscles indicated significant main effects for lifting phase (F $ 5.505; p # 0.008; h 2 $ 0.23) in biceps femoris, vastus medialis, and vastus lateralis. However, no main effects for squat modality (F # 1.545; p $ 0.229; h 2 = 0.08) or interaction (F # 2.819; p $ 0.072; h 2 # 0.13) for any of muscles were found. Post hoc comparisons revealed that, for biceps femoris, activity significantly increased from presticking to sticking phase (constant; 50.0%, variable 69.5%) with no significant change from sticking to poststicking phase (Figure 4A). The EMG activity decreased significantly from sticking to poststicking region in vastus medialis (constant 212.1%; variable 28.8%) and vastus lateralis (constant 212.6%; variable 29.9%), while no significant changes were observed from presticking to sticking region (Figures 4B, C). DISCUSSION The purpose of study was to compare kinematics and muscle activation in 2-legged free-weight back squats using constant or variable resistance with a similar relative intensity to fatigue (6RM loads). The main findings were that length of sticking region was same for both squat modalities and that first and second peak barbell velocities were results of variable resistance, which caused a higher resistance in poststicking region. In addition, no significant differences in EMG activity were observed between modalities. As hyposized, a clear sticking region was observed in both squat modalities in sixth repetition. The results are in line with previous studies examining bench press at near-maximal effort (9,17,24,31) and, most recently, in squats (29). Furrmore, variable resistance of elastic bands influenced kinematics of barbell; first barbell peak velocity (V max1 ) was greater and second barbell peak velocity (V max2 ) lower than constant resistance of freeweight 6RM squats. The results are in line with hyposes. Using variable resistance in squats, force needed to move barbell upward increased with greater knee angles as elastic bands provide increasing resistance with greater length of elastic bands (Figure 2) (25). This resulted in a significantly greater total resistance using variable resistance in sticking and poststicking regions compared with constant resistance (Table 1). This explains decreased barbell velocity at V max2 (11). The increased V max1 with variable resistance squat modality was not expected, because resistance at this lifting height was approximately same between 2 (Table 1). Possible explanations for se findings can be potentiation, active state of muscles, and stability during lifting with variable resistance modality. Because of nature of elastic bands, participants are pulled down more at beginning of downward movement. This might cause a higher active state and more potentiation, and/or store more elastic energy at bottom of lift, which can result in a higher first peak barbell velocity that has been demonstrated in explosive movements (35). Still, prolonged transition period between eccentric and concentric phases would result in loss of any stretch shortening cycling enhancements (23,27,28) and, reby, not be explanation of increased V max1 using variable resistance. Anor possibility is lifting movement caused by elastic bands. These bands are connected from barbell to ground, and participants are pulled down to attachment point of ground. It could refore be speculated that lifting pattern was more vertical with less hip flexion (decreased horizontal movement) than with free weights and, reby, could increase V max1 in variable squat modality. However, 3D analyses were not conducted to support speculation. It was hyposized that length of sticking region would increase because of increased resistance of elastic band. However, this was not found in this study: lifting phases occurred with similar vertical displacement of barbell and time. This surprising result could be cause of low variable resistance differences. However, loads performing variable resistance were 98, 105, and 113% of constant resistance in presticking and poststicking regions. The differences in loads between modalities were perhaps not large enough to change sticking region. Furrmore, results indicated that sticking region was caused by a poor mechanical force position at specific joint angles, as previous studies have suggested for 950 Journal of Strength and Conditioning Research

7 Journal of Strength and Conditioning Research bench press (9,18). This is true even when greater peak barbell velocity (V max1 ) was observed with variable resistance, which, oretically, should lead to a greater vertical displacement in presticking region. However, because of combination of a small time interval ( seconds), low maximal barbell velocity ( m$s 21 ), and increasing resistance (with variable resistance) in presticking region, no significant differences in displacement were observed. Because only final repetition in 6RM in each squat modality was studied, participants were close to fatigue and near-maximal effort (28,29,33). This resulted in a sticking region for both squat modalities, which is in line with findings of earlier studies using multijoint exercises as bench press (30,31) and back squats (29). No difference in EMG activity was observed between variable and constant resistance, which was not hyposized, because resistance increased significantly in last region with more than 100 N. Normally, this would result in higher EMG activity of prime movers (3,21). However, low differences in resistance between 2 modalities and possible different lifting movement in variable resistance condition could explain that EMG activity is not different between 2. The results are supported by Ebben and Jensen (8) who reported similar quadriceps and hamstring activation comparing free-weight squats and squats with variable resistance (elastic bands and chains). Unfortunately, Ebben and Jensen (8) only analyzed lifting movement in eccentric and concentric phases and results are refore not comparable with those of this study. The EMG activity of biceps femoris and vastus lateralis and medialis showed same muscle activation pattern as found by van den Tillaar et al. (17), in which biceps femoris increased EMG activity from presticking to sticking region, whereas vastus muscles decreased ir activity in poststicking region. However, both this study and study by van den Tillaar et al. (17) were limited by not including glutei muscles. The gluteus muscles are mainly responsible for hip extension in multijoint exercises as squat (6), and it can be speculated that EMG activity can be changed with variable resistance. The majority of participants refused to participate if EMG measurement of gluteus maximus was included. Therefore, it was not possible to include EMG measurement of gluteus maximus in this study. Future studies should include EMG measurement of glutei muscles, use 3D analyses, and test 1RM to examine what limitations are during squat lifting. In addition, to improve knowledge of what happens in muscles and kinematics using variable resistance, authors suggest increasing percentage resistance from elastic bands or using chains in total resistance group. PRACTICAL APPLICATIONS Heavy resistance training (i.e., squats) has been shown to be effective for improving maximal strength and jump heights. However, when training with heavy resistance, maximal effort is only required at beginning of concentric lifting phase: neuromuscular stress decreases throughout concentric phase. Theoretically, variable resistance may provide near-maximal neuromuscular stress throughout whole ROM because of increasing load in concentric phase. However, this study did not demonstrate that variable resistance training had an increased effect on muscle activation in sticking and poststicking regions because of increased resistance when compared with constant resistance. 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