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Original Research

Influence of “In Series” Elastic Resistance on Muscular Performance During a Biceps-curl Set on the Cable Machine

García-López, David1; Herrero, Azael J1,2; González-Calvo, Gustavo1; Rhea, Matthew R3; Marín, Pedro J1,2

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Journal of Strength and Conditioning Research: September 2010 - Volume 24 - Issue 9 - p 2449-2455
doi: 10.1519/JSC.0b013e3181e3482f
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Abstract

Introduction

Progressive resistance exercise can be performed in properly designed weight training programs using different modes of resistance. Although constant external loading is the most popular mode of resistance training (i.e., free weights and pulley systems), variable resistance has lately gained much attention and has become common in commercial gyms, high schools, and collegiate strength and conditioning programs (3,4,6). Variable resistance is designed to change the external resistance load throughout an exercise's range of motion (i.e., elastic bands, bands and/or chains attached to the ends of bars) (12). In this sense, in an attempt to merge the benefits of variable resistance with those of constant external loads, combinations of elastic resistances (ERs) with free-weight resistances (FWRs) have increased in popularity (15).

Usually, the ER is applied “in parallel,” being attached to the end of the barbell so that as the barbell ascends during the concentric phase (i.e., during a back squat or a bench press), an increasing load is applied as the ER lengthens. Different studies found improvements in strength and power during the back squat and bench press when using ER + FWR, in comparison with the same exercises performed only with FWR (1,15). The reason seems to be related to kinematics inherent to exercises combining ER + FWR. Explosive actions with FWR leads to an extended deceleration phase during the later stages of the concentric contraction (5). In combination with ER, the external resistance during an exercise should theoretically be more evenly distributed over the full range of motion than with FWR exercise. Load deceleration during FWR occurs as muscle contractions cease to slow to movement speed, which diminishes the overall exertion of the muscle during the exercise. Therefore, the combination of ER should decrease the need for dramatic deceleration during the movement and, as a result, an enhanced average muscle tension could be achieved throughout the range of motion, as has been hypothesized previously (1).

The application of ER “in parallel” to FWR is difficult in cable-based machines. In this line, an application of ER “in series” to load plates could take the advantage of such a combination in an inexpensive and readily available technique. A device is needed that could be linked between a cable attachment bar and a pulley cable (PC) (see Figure 1). To the best of our knowledge, no research has evaluated the purported advantages of in series ER, and this may represent a new training method. Therefore, the purpose of this study was to examine the acute effect of using ER + PC during a set of biceps-curl to volitional exhaustion (70% 1-repetition maximum [RM] load) on volume (number of repetitions achieved), kinematics (velocity and acceleration patterns), and perceived exertion. It was hypothesized that ER + PC would provide an additional stimulus for neuromuscular system, decreasing the inertia of the weight's mass, and subsequently generating a more uniformly distributed external resistance. This would change the total volume completed, and the kinematic pattern of the set. If demonstrated effectively, ER as applied in this manner would be a positive application in a muscular fitness training protocol.

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Figure 1:
Experimental setup.

Methods

Experimental Approach to the Problem

A randomized cross-over study was designed to compare biceps-curl performance during a set to failure with and without ER attached in series to a PC resistance device. Data collection took place over a period of 4 weeks with 1 testing session each week among 21 healthy college students. The first 2 sessions were used to familiarize subjects with testing procedures and to assess the subjects' 1RM. During each of the next 2 testing sessions, 1 set of the PC biceps-curl was performed at 70% of 1RM, leading to volitional exhaustion. During each 1 of such testing sessions, 1 of the 2 conditions was performed: traditional condition and ER + CP condition. A counterbalance procedure was used to determine the condition for each testing session. Data were analyzed to examine the effect of the ER on biceps-curl performance.

Subjects

Twenty-one undergraduate students (17 men and 4 women) participated in the study. The subjects' mean ± SD age, height, body mass, and biceps-curl 1RM were 19.2 ± 1.1 years, 175.7 ± 8.6 cm, 70.1 ± 12.7 kg, and 33.0 ± 11.0 kg, respectively. Subjects were physically active, and each averaged at least 3 months' experience with FWR exercises and training leading to failure. Their normal workouts typically lasted just less than 90 minutes and entailed training of multiple body parts and exercises. However, at the time of the study and from 2 months before, none was engaged in any regular or organized resistance training program. Before data collection, subjects were informed of the requirements associated with participation and provided written informed consent. Moreover, subjects were encouraged to prevent changes in their sleeping, eating, and drinking habits throughout study participation. The research project was conducted according to the Declaration of Helsinki, and it was approved by the University Review Board for use of Human Subjects.

Procedures

Data collection took place over a period of 4 weeks with 1 testing session each week. In the first session, instructions regarding performance of the 1RM testing and proper form and biceps-curl technique for the PC machine (Telju, Toledo, Spain) were given to each participant. During the second experimental session, the assessment of the 1RM for the biceps curl was determined. During each of the next 2 testing sessions, 1 set of the biceps curl was performed to failure on the PC. During testing sessions, 1 of the 2 conditions designed was performed: traditional condition or ER + PC condition. A counterbalance procedure was used to determine the condition for each testing session. Thus, at the end of the experimental phase, all the subjects had been tested for the 2 conditions with the conditions being performed in a different order to prevent any learning effect or influence of fatigue. Testing sessions were carried out the same day of the week, in all cases at the same time of the day.

Maximal Strength Measurement

The 1RM biceps curl was estimated from a 1-3RM effort using the equation described by Wathan (16). Each subject carried out 3-5 attempts with progressively increasing weight to achieve a 1-3RM. Three minutes of resting period was allowed between attempts. It is known that direct 1RM testing is more reliable; however, in single-joint assistant exercises (i.e., biceps curl) the 1RM testing may not be safe (2). For biceps-curl repetitions, subjects lowered the bar until the elbow was completely extended. At that point, the extended arm was in line with the cable (see Figure 1). Hand spacing at the barbell was shoulder width. The elbows were flexed equally with the head and upper back remaining in contact with the wall throughout the lift. Feet spacing was also shoulder width, and 30° knee flexion was maintained during the exercise. No bouncing or arching of the back was allowed. Biceps-curl technique and settings were maintained throughout the whole experimental phase.

Biceps-Curl Sets to Failure

Each biceps-curl protocol consisted of performing 1 set to volitional exhaustion, with a load equivalent to the subject's 70% of 1RM. Both, in traditional condition and in the ER + CP condition, subjects began with a warm-up consisting of 5 minutes of low-resistance cycling on an ergometer (50 and 75 W for women and men, respectively), followed by 2 sets of free weight biceps curl comprising 15 repetitions at 6 kg and 1 set of 10 repetitions at the 40% of the 1RM, allowing 1 minute of resting between sets. One minute after the specific warm-up, individuals began the biceps-curl set in the PC machine at 70% of 1RM. Thus, they were asked to move the barbell as fast as possible during the concentric phase of each repetition, until volitional exhaustion. The elbow flexion range of motion was performed completely, starting from full extension to avoid compensation by the shoulders and trunk. Failure was defined, according to a previously established criterion (8), as the time point when the barbell ceased to move, if the subject paused more than 1 second when the arms were in the extended position, or if the subject was unable to reach the full flexion position of the arms. During the set, 1 examiner encouraged the subjects to execute the exercise properly, with verbal orientations to avoid alterations in speed and posture.

In the ER + CP condition, the ER device was set up between a cable attachment bar and the PC (see Figures 1 and 2). This device had different elastic bands (stretch limit of each band = 5 kg) with a stop cable. The number of elastic bands employed for each subjects was calculated according to individual's 70 1RM.

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Figure 2:
Elastic resistance device connected to the pulley cable and to the bar (patent pending). The number of elastic bands attached was equivalent to the FWR load selected for each subject (70% of 1 repetition maximum).

Kinematic parameters of each repetition were monitored by linking a rotary encoder (Globus Real Power, Globus, Codogne, Italy) to the highest load plate (see Figure 1). The rotary encoder recorded the position of the load plate within an accuracy of 0.1 mm and time events with an accuracy of 0.001 seconds. Total repetitions performed and average and maximal velocity for each repetition, average and maximal acceleration for each repetition, and percentage of time in which the load plate was accelerated (during the concentric phase of each repetition) were analyzed. For comparison purposes, the number of repetitions was expressed as percentage of total number of repetitions (10, 20, 30,…,100%).

Just after the fifth repetition, the OMNI-Resistance Exercise Scale (OMNI-RES) perceived exertion scale (14) was verbally announced. OMNI-RES consists of 10 reporting options between 1 (extremely easy) to 10 (extremely hard). All subjects had previous experience with this OMNI-RES scale and its use in reporting perceived exertion during resistance training exercises. Even with this experience in previous laboratory sessions, a written copy of the OMNI-RES scale with instructions was given to the subject and administered consistently by the same investigator.

Statistical Analyses

Normality of the dependent variables was checked and subsequently confirmed using the Kolmogorov-Smirnov test. Comparisons of dependent variables between treatment conditions (i.e., ER + PC vs. traditional) were analyzed by Student's paired t-test. A repeated-measures analysis of variance was applied in the percentage of the total number of repetitions (10, 20, 30,…or 100%). When a significant F-value was achieved, pairwise comparisons were performed using a Bonferroni post hoc procedure to know the repetition in which the mean acceleration and mean velocity decayed significantly. The intraclass correlation coefficients were calculated for each dependent variable to determine test-retest reliability, obtaining values always greater than 0.91. Statistical significance was set at p ≤ 0.05. Effect sizes, d, were analyzed to determine the magnitude of an effect independent of sample size. Values are expressed as mean ± SD.

Results

Number of Repetitions and Perceived Exertion

The number of repetitions achieved was 12.4 ± 3.1 and 10.9 ± 2.3 in traditional and ER + PC conditions, respectively. A significant condition effect (12.3%; p < 0.01, d = 0.65) was observed regarding number of repetitions achieved. Perceived exertion (OMNI-RES value) at the fifth repetition was 7.9 ± 0.9 and 8.2 ± 1.3 in traditional and ER + PC conditions, respectively. Although the ER + PC mode was reported to fall slightly more difficult than the traditional mode (3.8%; p > 0.05, d = 0.23), no significant condition effect was observed concerning perceived exertion.

Accelerative Portion of the Concentric Phase

Accelerative profile of the concentric phase was similar in the traditional condition compared to the ER + PC condition. During both experimental conditions, the repetition with the highest accelerative portion (48.5 and 46.7% for traditional and ER + PC condition, respectively) corresponded to the first one. Figure 3 displays the acceleration-time curve observed during the fastest repetition in a representative subject, in both traditional and ER + PC conditions.

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Figure 3:
Acceleration-time curve during the fastest repetition exerted by a representative subject for traditional and elastic resistance + pulley-cable modes, respectively.

Mean Acceleration throughout the Set

Figure 4 displays the significant decrease (p < 0.05) of mean acceleration throughout the set in both traditional and ER + PC conditions. A significant condition effect (p < 0.01, d = 0.24) concerning acceleration pattern was observed. That is, the decrease in mean acceleration throughout the set was larger in the traditional condition in comparison to the ER + PC condition. In fact, the repetition at which a significant decrease in the mean acceleration occurred corresponded to 50% of the total number of repetitions achieved in the traditional mode. Conversely, in the ER + PC, the repetition at which a significant decrease in the mean acceleration occurred corresponded to 80% of the total number of repetitions achieved. Post hoc tests showed a significantly lower acceleration during the ER + PC for repetition corresponding to 10% of the total number of repetitions performed, in comparison to the corresponding traditional-mode repetition (Figure 4).

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Figure 4:
Evolution of mean acceleration throughout the set in traditional and elastic resistance + pulley-cable conditions, respectively. Number of repetitions is expressed as a percentage of total number of repetitions completed. Values are means ± SD. *Significantly different from first repetition (p < 0.05). #Significantly different from traditional-mode value at the same percentage of total repetitions achieved (p < 0.05).

Mean Velocity throughout the Set

Figure 5 displays the significant decrease (p < 0.05) of mean velocity throughout the set in both traditional and ER + PC conditions. The average velocity for the whole set was 0.157 ± 0.037 and 0.161 ± 0.041 m·s−1 in traditional and ER + PC conditions, respectively. No significant condition effect concerning velocity pattern was observed. That is, the velocity decay throughout the set was similar in the traditional condition in comparison to in the ER + PC condition. The repetition at which a significant decrease in the mean velocity occurred corresponded to 50% of the total number of repetitions achieved in both traditional and ER + PC modes.

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Figure 5:
Evolution of mean velocity throughout the set in traditional and elastic resistance + pulley-cable conditions, respectively. Number of repetitions is expressed as a percentage of total number of repetitions completed. Values are means ± SD. *Significantly different from first repetition (p < 0.05).

Discussion

The primary finding of the present study, as it relates to the research question, is that ER applied “in series” to a PC machine reduces the maximal number of repetitions and results in a smooth and consistent decline in mean acceleration throughout a biceps-curl set to failure, in comparison to the conventional PC mode. Although no significant differences were found concerning intrarepetition kinematics, the ER trended to reduce (18.6%) the peak acceleration of the load. With a more uniformly distributed external resistance, a greater average muscle tension could have been achieved throughout the range of movement, leading to greater fatigue that could explain the lower number of maximal repetitions achieved.

Kinematics associated with resistance exercises (e.g., velocity and acceleration) have been proposed as one of the most important stimuli for maximal strength and muscle power resistance training-induced adaptations (11). In this sense, traditional FWR exercises using low to moderate loads are characterized by an initial peak force phase after which a dramatic deceleration phase takes place during the later stages of the concentric contraction (5,8,13). Ballistic exercises (i.e., bench throws and squat jumps) have been prescribed to solve the deceleration problem in conventional FWR (13). However, ballistic training is not easily applicable to all the exercises commonly performed in resistance training facilities. A different possibility proposed to improve the kinematics associated with FWR exercises is the combination of ER with FWR (1,6,15). However, all these studies analyzed closed kinetic chain exercises (i.e., bench press or squat) applying the ER in parallel to FWR.

To the best of our knowledge, this is the first study analyzing the kinematics of “in series” combination of ER + PC during an open kinetic chain exercise performed in a PC machine. Our data indicate that during the fastest repetition of a biceps-curl set carried out in a PC machine with a load of 70% of 1RM, the load was accelerated during a 48.5% of the concentric movement time. This accelerative portion is lower than that reported in previous studies analyzing the bench press with loads equivalent to 45% of 1RM (8,13) or 60% of 1RM (8,9) in different populations. This difference could be explained through aspects related with the load used and mainly with the biomechanics of both exercise and joint. When ER was applied “in series” to the load, no differences were found concerning accelerative portion of the fastest repetition. That is, the load is accelerated during a similar portion of the concentric phase. However, the ER trended to reduce the peak acceleration of the load (see Figure 3), and therefore the peak force applied, which may not optimize strength training oriented to athletic performance. There is controversial information available regarding the effectiveness of ER applied “in parallel” to squat to improve peak force and peak power (6,15). These disagreements have been attributed to the different percentages of the overall load that is achieved from elastic bands in the different studies (15).

It is necessary at this point to remember the differences existing between the ER applied “in parallel” (i.e., in bench press or squat) and the ER applied “in series.” When applied in parallel, the trainee should overcome the load provided by FWR and ER together, although ER increases throughout the concentric phase, that is, the overall load increases as the mechanical advantage increases. On the other hand, the device proposed in the present study offers isolated ER during the early phase of the concentric phase. That is, in the starting phase of the lift, the trainee only has to overcome the ER. Only when elastic bands are totally stretched (at that point the ER is equal to the FWR load) do the weight plates start to move. This sequential release of load leads to a more uniformly distributed external resistance, reducing largely the high impact experienced while starting and stopping the load in powerful exercises performed in PC machines.

The perception of effort did not change in the ER + PC approach, in comparison with the conventional PC mode. Previous studies observed that subjects perceive a greater effort during Olympic lifting exercises when combining variable resistance (chains) to FWR in comparison to conventional approach (3,4). Differences related to type of variable resistance “in series” ER vs. chains attached to the barbell, the exercise analyzed (biceps curl on PC vs. Olympic clean) and the tool selected to measure perceptual response (OMNI-RES vs. open-answer questionnaire) could explain this disparity of results concerning perceived exertion.

Given that multiple-repetition sets are generally performed in a typical strength training session, it is important to study kinematics not only through single-repetition but also through multiple-repetitions experimental approaches. In this sense, it has been shown that over a set of repetitions leading to failure the speed of the repetitions slows naturally with this decrease being significant when the number of repetitions was over 50% of the total number of repetitions performed (7-10), both in bench press and biceps curl (performed in the Preach Bench). The present data indicate a similar evolution of mean velocity throughout a biceps-curl set to failure performed in a PC machine, with a significant decrease at 50% of maximal number of repetitions achieved. In this regard, the application of “in series” ER did not change the decay pattern of mean velocity. On the contrary, the evolution of mean acceleration was significantly different in the ER + PC approach in comparison to the conventional approach. That is, the application of “in series” ER resulted in a smoother decay of mean acceleration. A more uniformly distributed external resistance would allow for greater loading at positions of greater leverage. Thus, greater average muscle tension could be achieved throughout the range of movement, which could explain the lower number of repetitions achieved in ER + PC in comparison to the conventional PC approach. This could be interesting in training approaches in which the maintenance of a homogeneous average muscle tension can be advantageous for goal adaptations (i.e., muscle hypertrophy).

In summary, the present study shows that ER applied “in series” to a PC machine reduces the maximal number of repetitions and makes the natural decay of mean acceleration throughout a biceps-curl set to failure more consistent and smooth, in comparison to the conventional PC mode. Although no significant differences were found concerning intrarepetition kinematics, the ER trended to reduce the peak acceleration of the load. With a more uniformly distributed external resistance a greater average muscle tension could have been achieved throughout the range of movement, leading in a greater fatigue that could explain the lower number of maximal repetitions achieved.

Practical Applications

The findings of this investigation demonstrate that applying ER in series with resistance provided by a cable-based resistance machine may enhance the overall stimulus applied to the muscle. The application of force in a smooth, consistent fashion during each repetition of an exercise, while avoiding active deceleration, is expected to enhance the benefits of the resistance exercise. In particular, those seeking greater increases in muscular hypertrophy may benefit the greatest from this form of training as the enhanced muscular contraction and pattern may result in greater muscle overload and adaptation. The device used in this study represents a novel and inexpensive supplement to cable-based resistance machines. Because the subjects in this study were only recreationally trained, extrapolating these findings to work with athletes is somewhat limited; however, the concept identified may be something of consideration to sports conditioning efforts. Methods to reduce active deceleration may be of value to athletes in preparation for competitive environments where such deceleration is not desirable.

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Keywords:

elastic tension; accommodate resistance; weight training; kinematics; muscle tension

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