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Linearity and Reliability of the Mechanomyographic Amplitude Versus Concentric Dynamic Constant External Resistance Relationships for the Bench Press Exercise

Stock, Matt S; Beck, Travis W; DeFreitas, Jason M; Dillon, Michael A

Journal of Strength and Conditioning Research: March 2010 - Volume 24 - Issue 3 - p 785-795
doi: 10.1519/JSC.0b013e3181cc22f1
Original Research
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Stock, MS, Beck, TW, DeFreitas, JM, and Dillon, MA. Linearity and reliability of the mechanomyographic amplitude versus concentric dynamic constant external resistance relationships for the bench press exercise. J Strength Cond Res 24(3): 785-795, 2010-The purpose of the present study was to examine the linearity and reliability of the mechanomyographic (MMG) amplitude versus concentric dynamic constant external resistance (DCER) relationships for the bench press exercise. Twenty-one resistance-trained men (mean ± SD age = 23.5 ± 2.7 yr; 1 repetition maximum [1RM] bench press = 125.4 ± 18.4 kg) volunteered to perform submaximal bench press muscle actions as explosively as possible from 10% to 90% of the 1RM on 2 separate occasions. During each muscle action, surface MMG signals were detected from both the right and left pectoralis major and triceps brachii, and the concentric portion of the range of motion was selected for analysis. The coefficients of determination for the MMG amplitude versus concentric DCER relationships ranged from r2 = 0.010 to 0.980 for the right pectoralis major, r2 = 0.010 to 0.943 for the left pectoralis major, r2 = 0.010 to 0.920 for the right triceps brachii, and r2 = 0.020 to 0.915 for the left triceps brachii, thus indicating a wide range of linearity between subjects. The intraclass correlation coefficients (ICC) and corresponding standard error of measurements (SEM) for the linear slope coefficients for these relationships were 0.592 (39.3% of the mean value), 0.537 (41.9% of the mean value), 0.625 (42.0% of the mean value), and 0.460 (60.2% of the mean value) for the right pectoralis major, the left pectoralis major, the right triceps brachii, and the left triceps brachii, respectively. These data demonstrated that these relationships were neither linear nor reliable enough to be used for assessing issues such as the neural versus hypertrophic contributions to training-induced strength gains and the mechanisms underlying cross-education.

Department of Health and Exercise Science, University of Oklahoma, Norman, Oklahoma

Address correspondence to Matt S. Stock, mattstock@ou.edu.

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Introduction

Surface mechanomyography (MMG) records and quantifies the low-frequency lateral oscillations of contracting skeletal muscle fibers (25). Several studies have used MMG to examine muscle function during isokinetic muscle actions (8-11) and cycle ergometry (30,33), as well as to investigate muscle fatigue (19,28) and neuromuscular diseases (29). Of particular interest to the practitioner is the potential to use MMG for assessing the neuromuscular adaptations that occur with strength training. Using group mean data, many studies have reported a linear relationship between MMG amplitude and torque during isokinetic and dynamic constant external resistance (DCER) muscle actions (2,3,6,7,12,28). For example, Dalton and Stokes (12) reported that MMG amplitude for the biceps brachii increased linearly from 0 to 8.5 kg during concentric and eccentric muscle actions. In addition, Beck et al. (3) demonstrated that MMG amplitude closely followed torque during concentric isokinetic muscle actions of the biceps brachii. This linear relationship between MMG amplitude and torque implies that it could potentially be used to develop a mechanical analog to the “efficiency of electrical activity” (EEA) technique originally proposed by Fisher and Merhautova (16) and later examined directly by deVries (13). The EEA technique involves plotting electromyographic (EMG) amplitude as a function of force at different submaximal percentages of one's maximal strength for a given task. As the muscle fibers hypertrophy with chronic resistance training, each fiber becomes capable of producing more force, which allows fewer muscle fibers to be activated during any given submaximal task. Thus, a decrease in the linear slope coefficient for the EMG amplitude versus force relationship is thought to reflect a decreased amount of electrical activity required to produce any given submaximal force level (13). However, the use of this technique is dependent on a linear relationship between EMG amplitude and force. Furthermore, because the EEA technique is based on data analysis for individual subjects, this relationship must be highly linear on a subject-by-subject basis.

Because the application of the EEA technique involves assessing potential training-induced changes in the linear slope coefficient over time, it is vital that this variable demonstrates adequate reliability. As explained by Weir (34), the intraclass correlation coefficient (ICC) and standard error of measurement (SEM) collectively provide a comprehensive measure of reliability. Although these statistics are context specific to each study design (34), Weir et al. (35) investigated the linearity and reliability of the EMG amplitude versus isometric force relationship for both the biceps brachii and vastus lateralis. The authors reported that, for these relationships, an ICC of R = 0.86 (SEM = 31% of the mean value) for the biceps brachii reflected inadequate repeatability to be used for the EEA technique, but an ICC of R = 0.97 (SEM = 12% of the mean value) represented sufficient reliability.

Although amplitude and center frequency values during isometric muscle actions are not affected by issues such as changes in torque production, the number of active motor units, and muscle length, most athletes perform multijoint, dynamic muscle actions during their strength training programs (i.e., bench press, squat, deadlift). Whereas Dalton and Stokes (12) examined group mean data for the MMG amplitude versus torque relationship during submaximal concentric and eccentric DCER muscle actions, no study has investigated this relationship during a multijoint task. Therefore, the purpose of the present study was to examine the linearity and reliability of the MMG amplitude versus concentric DCER relationships for both the right and left pectoralis major and triceps brachii during the bench press exercise. If it is determined that these relationships are sufficiently linear with reliable slope coefficients, it may be possible to use them to examine issues such as the neural versus hypertrophic contributions to training-induced strength gains and the mechanisms underlying cross-education.

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Materials and Methods

Experimental Approach to the Problem

On the basis of previous findings (2,3,6,7,12,28), we hypothesized that when analyzed on a subject-by-subject basis, linear and reliable MMG amplitude versus concentric DCER relationships would be found for both the right and left pectoralis major and triceps brachii for the bench press exercise. To examine this hypothesis, a within-subjects design was used. After 1 repetition maximum (1RM) testing and familiarization, 21 resistance trained men performed submaximal muscle actions of the bench press exercise on 2 occasions separated by at least 48 hours. A recovery period of 48 hours was implemented between trials to minimize the effects of delayed-onset muscle soreness on performance. For each repetition, the subjects were instructed to maximize power output by pressing the barbell explosively, similar to what has been described as “speed reps” (22). During testing, surface MMG signals were detected from both the right and left pectoralis major and triceps brachii, and the concentric portion of the range of motion was selected for analysis. The linearity of the MMG amplitude versus concentric DCER relationship for each subject, muscle (right pectoralis major, left pectoralis major, right triceps brachii, left triceps brachii), and trial (trial 1 and trial 2) was determined using coefficients of determination (r2) and linear slope coefficients. Reliability of the linear slope coefficients for the MMG amplitude versus concentric DCER relationships, as well as the absolute MMG amplitude values, was assessed using ICCs (model 2,1), SEMs, and paired-samples t-tests.

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Subjects

Twenty-one healthy men (mean ± SD age = 23.5 ± 2.7 yr; body mass = 90.5 ± 14.6 kg) volunteered to participate in this investigation. All subjects were resistance trained (mean ± SD training experience = 6.5 ± 4.1 yr) and had experience with the free-weight bench press exercise (mean ± SD 1RM = 125.4 ± 18.4 kg). The 21 subjects in the study participated in 5.7 ± 2.7 hours of resistance exercise weekly. Each subject completed a pre-exercise health and exercise status questionnaire, which indicated no current or recent neuromuscular or musculoskeletal problems. The study was approved by the university institutional review board for human subjects, and all subjects signed an informed consent form before testing.

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Bench Press Strength Testing and Familiarization

During the first visit to the laboratory, the subjects were tested for 1RM strength on the free-weight bench press and familiarized with the testing procedures. The 1RM testing was performed on a standard free-weight bench with an Olympic bar according to previously described methods (23). Specifically, the subjects performed 8 to 10 repetitions with approximately 50% of their estimated 1RM, followed by another set of 3 to 5 repetitions with approximately 85% of their estimated 1RM. The Olympic bar was then loaded to a weight that was near the subject's estimated 1RM, and they performed 1 repetition with the weight. A repetition was considered successful if the subject was able to lower the Olympic bar to his chest, touch it briefly, and then press it upward by fully extending his forearms. The weight on the Olympic bar was progressively increased until the subject could no longer perform a repetition throughout the full range of motion and the 1RM had been established within 2.3 kg.

Five minutes after completion of the 1RM testing, all subjects performed submaximal sets of the bench press exercise with loads corresponding to each 10 percentile (10-90%) of their 1RM. To standardize the bar velocity between trial 1 and trial 2 for all subjects, each participant was instructed to press the barbell explosively during the concentric phase of all repetitions, similar to what has been described as “speed reps” (22). Most of the men in the study were more familiar with lifting at higher intensities (≥70% 1RM) during their training than lower intensities. Thus, special emphasis was placed on familiarizing the subjects with loads corresponding to 1% to 60% of their 1RM.

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Bench Press Power Testing

A minimum of 48 hours after establishing the 1RM and performing the familiarization, the subjects returned to the laboratory for trial 1 of the bench press power testing. Upon arrival, a goniometer was fixed to the right elbow to measure changes in elbow joint angle during all repetitions. After a brief warm-up with submaximal weights, the subjects performed 9 separate single repetitions with weights ranging from 10% to 90% 1RM in 10% increments. These repetitions were performed in sequential order (i.e., 10% 1RM, followed by 20% 1RM, etc.) and separated by 3 minutes of rest. For each repetition, the subjects were instructed to lower the barbell to the point where it was just above the chest, but not touching the skin. This ensured that the MMG signals from the pectoralis major muscles were not affected by movement artifact from the barbell touching the chest. After lowering the barbell, the subjects were instructed to press it upward as forcefully and as rapidly as possible to optimize power output. After the subjects had completed the 9 separate single repetitions, they were allowed to rest for at least 48 hours, after which they returned to the laboratory and performed trial 2 of the bench press power testing in the same manner as during trial 1.

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MMG Measurements

For all repetitions during the bench press power testing, MMG signals were detected from both the right and left pectoralis major and triceps brachii muscles with 4 separate accelerometers (PCB Piezotronics, Model 352A24, bandwidth 1.0 to 8000 Hz, dimensions: 0.19 × 0.48 × 0.28 in, mass 0.8 g, sensitivity 100 mV/g) placed over the belly of each muscleas shown in Figure 1. The skin over each muscle was prepared before testing by shaving and cleansing with rubbing alcohol. The accelerometers were fixed to the skin with double-sided foam tape. Upon completion of trial 1, the skin was marked with permanent marker to ensure that the accelerometers were placed in the same locations for trial 2.

Figure 1

Figure 1

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Signal Processing

All signal processing was performed using custom programs written with LabVIEW programming software (version 8.2, National Instruments, Austin, TX, USA). The raw MMG signals were digitized at 2,000 Hz and stored in a personal computer (NOBILIS, Model DCY30, Richardson, TX, USA) for subsequent analysis. All signals were bandpass filtered (zero-lag, fourth-order Butterworth) with a pass band of 10 to 100 Hz. The goniometer signal from the right elbow joint was used as a reference to ensure that the MMG signals from the concentric portion of each repetition were selected for analysis. The amplitude of the MMG signal in each data segment was calculated as the root mean square value.

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Statistical Analyses

The linearity of the MMG amplitude (m/s2) versus concentric DCER relationship for each subject, muscle (right pectoralis major, left pectoralis major, right triceps brachii, left triceps brachii), and trial (trial 1 and trial 2) was determined using coefficients of determination (r2) and linear slope coefficients. The linear slope coefficients and y-intercepts for the MMG amplitude versus concentric DCER relationship for each subject and muscle were compared between the 2 trials using the procedures described by Pedhazur (26). As recommended, a type 1 error rate of 10% was used for the linear slope coefficient and y-intercept comparisons. A 2-way [trial (trials 1 and 2) × %1RM (10, 20, 30, 40, 50, 60, 70, 80, and 90 %1RM)] repeated measures analysis of variance (ANOVA) was used to analyze the mean MMG amplitude data for each muscle. When appropriate, follow-up analyses included 1-way repeated measures ANOVAs, paired-samples t-tests, and Bonferroni post hoc comparisons. The reliability of the linear slope coefficients for the MMG amplitude versus concentric DCER relationships was assessed using model 2,1 to calculate the ICC and SEM. According to Weir (34), model 2,1 is a 2-way random factor model that uses random and systematic error in the denominator of the ICC equation and, therefore, can be generalized to other laboratories and testers. The ICC and SEM were also used to determine the reliability of the MMG amplitude values for each muscle at each %1RM examined. Paired-samples t-tests were also performed for each muscle to determine whether there was a significant mean difference between the linear slope coefficients for the MMG amplitude versus concentric DCER relationships from the 2 trials. A type 1 error rate of 5% was used to determine statistical significance for all repeated measures ANOVAs, paired-samples t-tests, and Bonferroni post hoc comparisons. All SEMs were expressed as a percentage of the mean value. A post hoc statistical power analysis for MMG amplitude during DCER muscle actions with the 21 subjects used in this study indicated power values of 0.74, 0.75, 0.99, and 0.76 for the right pectoralis major, left pectoralis major, right triceps brachii, and left triceps brachii, respectively. SPSS (version 16.0; SPSS, Inc., Chicago, IL, USA) was used for all statistical analysis.

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Results

Tables 1 to 4 show the coefficients of determination (r2), linear slope coefficients, and p values for the MMG amplitude versus concentric DCER relationships for trials 1 and 2 for both the right and left pectoralis major and triceps brachii for each subject. For the right pectoralis major, the coefficients of determination ranged from r2 = 0.010 to 0.876 for trial 1 and 0.019 to 0.980 for trial 2, and in 71% (30 of 42) of the cases, bench press concentric DCER accounted for a statistically significant portion of the variance in MMG amplitude. For the left pectoralis major, the coefficients of determination ranged from r2 = 0.010 to 0.943 for trial 1 and 0.119 to 0.934 for trial 2, and in 67% (28 of 42) of the cases, bench press concentric DCER accounted for a statistically significant portion of the variance in MMG amplitude. For the right triceps brachii, the coefficients of determination ranged from r2 = 0.010 to 0.920 for trial 1 and 0.010 to 0.907 for trial 2, and in 57% (24 of 42) of the cases, bench press concentric DCER accounted for a statistically significant portion of the variance in MMG amplitude. For the left triceps brachii, the coefficients of determination ranged from r2 = 0.020 to 0.746 for trial 1 and 0.020 to 0.915 for trial 2, and in 62% (26 of 42) of the cases, bench press concentric DCER accounted for a statistically significant portion of the variance in MMG amplitude.

Table 1

Table 1

Table 2

Table 2

Table 3

Table 3

Table 4

Table 4

The linear MMG amplitude versus concentric DCER slope coefficient for trial 1 was significantly different from that for trial 2 for 3 (1 was greater, 2 were smaller) of the 21 subjects for the right pectoralis major (Table 1). Of the remaining 18 subjects, 7 exhibited a significantly different y-intercept for trial 1 versus trial 2 (5 were greater, 2 were smaller). For the left pectoralis major (Table 2), the linear MMG amplitude versus concentric DCER slope coefficient for trial 1 was significantly less than that for trial 2 for only 1 of the 21 subjects. Of the remaining 20 subjects, 9 exhibited a significantly different y-intercept for trial 1 versus trial 2 (4 were greater, 5 were smaller). For the right triceps brachii (Table 3), the linear slope coefficients for the MMG amplitude versus concentric DCER slope coefficient for trial 1 was significantly less than that for trial 2 for only 1 of the 21 subjects. Of the remaining 20 subjects, 7 exhibited a significantly different y-intercept for trial 1 versus trial 2 (3 were greater, 4 were smaller). For the left triceps brachii (Table 4), the linear MMG amplitude versus concentric DCER slope coefficient for trial 1 was significantly less than that for trial 2 for 2 of the 21 subjects. Of the remaining 19 subjects, 7 exhibited a significantly different y-intercept for trial 1 versus trial 2 (4 were greater, 3 were smaller).

Table 5 shows the ICCs, SEMs, and p values for the linear MMG amplitude versus concentric DCER slope coefficients for trials 1 and 2 for both the right and left pectoralis major and triceps brachii. The ICCs, SEMs, and significance levels for the absolute MMG amplitude values for trials 1 and 2 at each %1RM examined are shown in Table 6. Figures 2 to 5 show the mean ± SEM absolute MMG amplitude values at each %1RM examined for trials 1 and 2 for the right pectoralis major, left pectoralis major, right triceps brachii, and left triceps brachii, respectively. The results from each of the 2-way repeated measures ANOVAs indicate that there were no significant trial × %1RM interactions and no main effects for trial, but there were significant main effects for %1RM (see figures for marginal mean differences).

Table 5

Table 5

Table 6

Table 6

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

Figure 5

Figure 5

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Discussion

The results from the present study showed that for trial 1, the coefficients of determination for the MMG amplitude versus concentric DCER relationship ranged from r2 = 0.010 to 0.876 for the right pectoralis major (Table 1), r2 = 0.010 to 0.943 for the left pectoralis major (Table 2), r2 = 0.010 to 0.920 for the right triceps brachii (Table 3), and r2 = 0.020 to 0.746 for the left triceps brachii (Table 4). The corresponding values for trial 2 ranged from r2 = 0.019 to 0.980 for the right pectoralis major, r2 = 0.119 to 0.934 for the left pectoralis major, r2 = 0.010 to 0.907 for the right triceps brachii, and r2 = 0.020 to 0.915 for the left triceps brachii. For the 42 total trials, these relationships were statistically significant in 71%, 67%, 57%, and 62% of the cases for the right pectoralis major, left pectoralis major, right triceps brachii, and left triceps brachii, respectively. These findings demonstrated that for each of the muscles tested, there was a large range of linearity for the MMG amplitude versus concentric DCER relationship. Furthermore, these patterns were generally more linear for the pectoralis major than the triceps brachii.

Using group mean data, several studies have reported a linear relationship between MMG amplitude and torque during concentric isokinetic (2,3,6,7) and DCER (12,28) muscle actions. Dalton and Stokes (12) were the first to examine the MMG amplitude versus torque relationship using DCER muscle actions. In their study, the subjects lifted weights ranging from 0 to 8.5 kg while a condenser microphone recorded MMG signals from the biceps brachii muscle. The present study, however, was the first to examine the linearity of the MMG amplitude versus concentric DCER relationship during a multijoint, free-weight movement, and the numerous discrepancies between the methods of the present investigation and others (2,3,6,7,12,28) complicates the comparison of results. First, whereas the present investigation tested loads from 10 to 90% of the pretested 1RM in 10% increments, Dalton and Stokes (12) had all subjects lift the same loads (0-8.5 kg) during concentric muscle actions of the biceps brachii. It is therefore plausible to speculate that the differences between the relationship found by Dalton and Stokes (12) versus the results of the present study may be at least in part influenced by methodologic differences in the relative intensities lifted by the subjects. Furthermore, to date, no study has tested MMG responses of the pectoralis major and triceps brachii muscles. Both of these large muscles are considered to be of mixed fiber type distribution, and there is no apparent difference in the fiber type proportions of the sternal versus clavicular head of the pectoralis major muscle (20). An additional difference between the present investigation and those reporting a linear relationship between MMG amplitude and concentric torque (2,3,6,7,12,28) is the use of group mean analysis as opposed to analysis on a subject-by-subject basis. In using group mean data, interindividual variability among the MMG amplitude versus force or torque relationship may be eliminated. Finally, it is worthy to note that in the cases of high linearity found in the present study (i.e., Table 1, Subject 18; Table 2, Subject 15; Table 3, Subject 5), these results are in contrast with studies finding velocity-related dissociations between MMG amplitude and peak torque (PT); that is, as velocity increased, MMG amplitude also increased, but PT decreased (14,31,32). Cases in which MMG amplitude increased with concentric DCER in the present study would indicate that these relationships follow the size principle in which low-threshold motor unit action potentials are recruited first, and demands for larger forces are met by recruitment of increasingly forceful high-threshold motor unit action potentials (17). Collectively, the results of the present study demonstrated that although many subjects exhibited highly linear MMG amplitude versus concentric DCER relationships, in general, these patterns were not consistent enough to be used on a subject-by-subject basis.

The ICCs and SEMs for the MMG amplitude versus concentric DCER linear slope coefficients in the present study indicated insufficient reliability and consistency to be of practical use. Specifically, the ICCs and SEMs were .592 (SEM = 39.3% of the mean value) for the right pectoralis major, .537 (SEM= 41.9% of the mean value) for the left pectoralis major, .625 (SEM = 42.0% of the mean value) for the right triceps brachii, and .460 (SEM = 60.2% of the mean value) for the left triceps brachii (Table 5). Although an ICC of 0 indicates no reliability, and an ICC of 1.0 indicates perfect reliability, as explained by Weir (34), evaluating such results is complicated by factors such as the version of the ICC used and the amount of between-subjects variability in the dependent variable. Despite the fact that the ICC is context specific (34), Weir et al. (35) provided a frame of reference by concluding that an ICC of .86 for the EMG amplitude versus isometric force linear slope coefficient for the biceps brachii was unsatisfactory. By comparison, the linear slope coefficient with the highest ICC in the present study was .625 (right triceps brachii). Finally, the SEMs for each of the linear slope coefficients were less precise than the 31% of the mean value for the biceps brachii considered insufficient by Weir et al. (35). Therefore, future studies should investigate potential methods of improving the reliability of the linear slope coefficient for the MMG amplitude versus concentric DCER relationship so that this technique may be used to monitor the mechanisms underlying upper-body strength changes during resistance training.

An additional purpose of the present study was to determine the reliability of the MMG amplitude values at each of the intensities tested (10-90% 1RM in 10% increments). Previous studies have investigated the reliability of the MMG amplitude and mean frequency values during isometric muscle actions (1,18), as well as concentric isokinetic muscle actions at varying velocities (11,14). However, only the study by Herda et al. (18) assessed reliability at each submaximal percentage of the isometric maximal voluntary contraction. The authors reported that the ICCs ranged from .39 to .89 and .36 and .80 for the MMG amplitude and mean frequency values, respectively. The results of the present study indicated that for each of the muscles tested, the ICCs and SEMs for each of the intensities tested were unsatisfactory when compared with the findings of Herda et al. (18). Although exceptions existed, the ICCs for each of the muscles tested followed a similar trend; that is, they decreased from 10% 1RM to 20% 1RM, increased from 30% 1RM to 40% 1RM, and were highest at 80% 1RM and 90% 1RM. The reasons for these patterns are not known. Herda et al. (18) speculated that because fewer motor units are recruited at lower isometric force levels, lower between-subject variability caused suppression of the ICCs. However, in the present study, this hypothesis would not explain why higher ICCs were found at 10% 1RM than 20% 1RM and 30% 1RM for the right pectoralis major, the right triceps brachii, and the left triceps brachii. Overall, these data demonstrated that the MMG amplitude values at each of the intensities tested were highly variable. However, for each of the muscles tested, the highest ICCs and lowest SEMs were found at 80% 1RM or 90% 1RM.

As noted by Farina (15), because of the many factors that may influence the EMG amplitude versus force or torque relationship (many of which apply to MMG as well), there is no reason to expect a specific relationship to have general validity. There are a number of factors that can influence the MMG signal during dynamic muscle actions (2). These factors include changes in torque production during the movement, muscle length, tissue thickness between the muscle, and the MMG sensor used and are generally not significant contributors to the MMG signal during isometric muscle actions. Unfortunately, it is not possible to quantify the relative contributions of each of these factors within the context of this study. During both isokinetic and DCER muscle actions, there are acceleration and deceleration phases during the range of motion. Newton et al. (24) demonstrated that when resistance trained men performed the bench press exercise explosively at varying submaximal intensities, peak bar velocity occurred at approximately 50% of the range of motion. However, unlike concentric isokinetic muscle actions, there was no constant velocity phase of the movement. Furthermore, force production during the bench press is highly dependent on use of the stretch-shortening cycle (24). That is, during the bench press exercise, concentric strength is enhanced because of recovery of stored elastic energy and increased agonist muscle activation as a result of the stretch reflex (5,21). Furthermore, we cannot exclude the possibility that other muscles such as the pectoralis minor and the anterior deltoid contributed to force production during each repetition. We may therefore speculate that each of these factors affected the number of active motor units and their firing rates throughout the concentric range of motion and that variability in these factors influenced the linearity and reliability of the MMG amplitude versus concentric DCER relationships in the present study.

In summary, the results from the present study showed that, in some cases, the MMG amplitude versus concentric DCER relationships were highly linear for both the right and left pectoralis major and triceps brachii, which was similar to the findings of Dalton and Stokes (12) for the MMG amplitude versus torque relationship during submaximal concentric and eccentric DCER muscle actions. There was, however, a large amount of between-subjects variance for both trial 1 and trial 2. Furthermore, the ICCs and SEMs for the linear slope coefficients for the MMG amplitude versus concentric DCER relationships were not indicative of acceptable reliability. Collectively, these data demonstrated that within the context of the bench press protocol used in the present study, the MMG amplitude versus concentric DCER relationships were neither linear nor reliable enough to be used for assessing issues such as the neural versus hypertrophic contributions to training-induced strength gains and the mechanisms underlying cross-education. Future studies should consider investigating the linearity and reliability of the MMG amplitude versus concentric DCER relationship during single-joint DCER muscle actions.

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Practical Applications

Using isometric muscle actions, deVries (13) examined the “efficiency of electrical activity” technique, which involves plotting EMG amplitude as a function of force at different submaximal percentages of one's maximal strength for a given task. The use of this technique is dependent on a linear EMG amplitude versus force relationship with a reliable slope coefficient. Using group mean data, many recent studies have reported a linear relationship between MMG amplitude and torque during isokinetic and DCER muscle actions (2,3,6,7,12,28), and it has been proposed that MMG amplitude may be used to develop a mechanical analog of the EEA technique (2). The results of the present study demonstrated that when resistance trained subjects performed the bench press exercise with loads corresponding to 10% to 90% of the 1RM, the MMG amplitude versus concentric DCER relationships were neither linear nor reliable. On the basis of these findings, it is recommended that coaches and trainers refrain from using the MMG versus concentric DCER relationships for the pectoralis major and triceps brachii muscles during the bench press exercise.

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Keywords:

efficiency of electrical activity; mechanomyogram; intraclass correlation coefficient

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