The ability to control the level of skeletal muscle activity in various muscle groups is a subject that interests researchers and practitioners from a number of fields. Physical therapists, for example, can reduce stress on joints by strengthening particular muscles. From an ergonomic point of view, manual laborers could benefit from an ability to reduce muscle activity in key muscles to avoid overuse injuries and to reduce job performance fatigue. Additionally, athletes engaged in resistance training may seek to target particular muscles for hypertrophy or increases in strength.
The most common ways to alter the activity of a selected muscle involve adjustments to grip (13,14,23), limb or body position (7,9,11,15,26), torso stability (1,17,18), range of motion (ROM) (4), and type of equipment (3,6,25). Less common, and far less researched, is the selective activation of a muscle or muscle group or the relaxation of a particular muscle group without the benefit of any changes in the execution of the exercise. Several groups of researchers, however, have shown it to be possible to train human subjects in this manner using biofeedback or verbal instruction. Palmerud et al. (19) showed that untrained subjects can reduce trapezius activity during low load isometric shoulder abduction using visual electromyographic (EMG) feedback and that activity in several other shoulder muscles is increased in a compensatory manner during the task (20), a phenomenon termed “voluntary force redistribution” (VFR). However, this research observed EMG activity during a 1-minute isometric action that is not applicable to resistance training activities, limiting the relevance of the results for athletes.
A number of multijoint resistance exercises involve multiple muscle groups and may be subject to altered muscle control. One example is the wide-grip front lat pull-down, which involves both shoulder adduction (using mainly the latissimus dorsi and the teres major/minor and to some extent the pectoralis major) and elbow flexion (controlled by the biceps brachii, brachialis, and brachioradialis). In theory, it should be possible for a submaximal action to be completed by selectively activating the shoulder adductors while reducing the contribution of the elbow flexors, in essence isolating the involved upper back muscles. This voluntary control over muscle activation patterns would have a variety of therapeutic and performance-related benefits.
No research has been carried out measuring the ability to selectively activate a given muscle group during dynamic multijoint resistance training activities. The purpose of this study was to observe electromyographic muscle activity in novice strength trainers performing the front lat pull-down exercise before and after specific technique instruction designed to increase the activity of the shoulder adductors while reducing the activity of the elbow flexors. It was hypothesized that instruction would result in increased activity in the latissimus dorsi and/or teres major, with a concurrent decrease in biceps brachii activity.
Experimental Approach to the Problem
The experimental design is outlined in Figure 1. The study was designed to determine whether an untrained subject performing a multijoint exercise could be taught to “isolate” a desired muscle group by emphasizing 1 muscle or muscle group while de-emphasizing another muscle or muscle group without changing any aspect of the exercise such as posture or limb position. Because the front lat pull-down involves multiple muscle groups on opposite sides of the primary joint (shoulder), it is an ideal exercise to test this hypothesis. In addition, the front lat pull-down, in which the bar is pulled in front of the head to the level of the clavicle, is generally considered a safer exercise than the standard lat pull-down, in which the bar is pulled to the base posterior aspect of the neck, and is recommended by the National Strength and Conditioning Association (NSCA) (2). The EMG activity of 3 key muscle groups-latissimus dorsi (LD), teres major (TM), and biceps brachii (BB)-at the 2 involved joints (elbow and shoulder) was observed before and after specific instructions on how to emphasize either the chest muscles or the triceps.
Three main questions were tested: (a) Was there a learning effect caused by the first exposure to the exercise? (b) Does specific instruction result in increase in of the activity of 1 muscle with a concurrent reduction in another during the exercise? (c) Was specific instruction effective in changing muscle activity on the first attempt, or was instructional reinforcement and a second attempt necessary to achieve the desired effect?
Eight women aged between 18 and 35 years (mean 22 years) with an average body weight of 62 kg participated in the study, having responded to a call for subjects via e-mail, bulletin board postings on campus, and word of mouth. Subjects with a history of low back pain, neuromuscular disorders, osteoporosis, recent muscular injury, or moderate exercise intolerance were disqualified from participation. At the time of the study, subjects had no previous experience with resistance training and little or no familiarity with the lat pull-down exercise. Participants signed a consent form stating the basic measurements to be obtained during the study but were not told the specific nature of the study because this knowledge could have altered the results. The research protocol was approved by the Schreiner University Undergraduate Research Ethics Committee.
A standard lat pull machine was outfitted with a strain gauge (Takei Scientific Instruments, Ltd., Japan) attached between the pulley and the bar handle. The chain length was adjusted so that with the upper body in an erect position, the subject could grasp the handle with shoulders abducted to 90 degrees and elbows flexed to 90 degrees. In this position, each subject was asked to perform a voluntary maximal isometric (MVIC1) while force and peak isometric EMG activity were recorded. This effort was repeated after a 2-minute rest period (MVIC2), and additional efforts were performed if it was determined that maximal effort was not given. The best maximal effort in kilograms was converted into pounds and the weight stack was adjusted to reflect 30% of maximal force at the 90/90 degree position. The 30% value was chosen because it was felt that an effort closer to maximum would most likely require a higher minimum biceps activity and restrain subjects from redistributing muscle force to the back muscles. In addition, pilot work indicated that untrained subjects were uncomfortable with substantially higher loads.
After removal of the strain gauge, the subjects were given basic instructions on how to perform the exercise but were not given any specific technique instruction. Subjects were told to begin with the bar in full elbow extension at the top of the movement, then to move the bar in front of the face with a slight hyperextension of the neck and complete the movement by bringing the bar down to the level of the clavicles. They then performed 2 sets of 3 consecutive dynamic repetitions (DYN S1 and DYN S2) with a 2-minute rest between sets, at a pace of approximately 2 seconds each for concentric and eccentric phases. A 3-repetition set was chosen because it provided ample data to address the main hypothesis and was less likely to result in fatigue. Each subject was then given specific technique instruction by a NSCA Certified Strength and Conditioning Specialist using 3 specific elements: (a) palpating the latissimus dorsi, (b) informing the subject that this muscle should be the targeted muscle for the exercise, and (c) explaining to the subject to pull with their back, instead of with their arms, by adducting the scapulae and concentrating on the tension in the back musculature. Subjects were also advised not to change their posture, pace, hand position, or any other aspect of the exercise from the first 2 sets, and the examiners closely monitored each repetition to ensure that these instructions were followed. Speed of the repetition was paid special attention and was monitored by observing the time signature on the real-time data collection software. Data from incorrectly performed repetitions was not used in the final analysis. Once specific technique instruction had been given, 2 additional sets were performed as before, with a technique reminder between sets.
EMG Measurements and Analysis
Before testing, the skin surface at each site was rubbed with light abrasive paper and cleaned with an alcohol pad to optimize the strength of the EMG signal. Pregelled surface electrodes of 1-cm diameter were placed 2 to 3 cm apart on the skin above the LD and 2 to 3 cm apart on the skin above the midsection of the TM and BB muscles. The surface electrodes were connected to a Biopac MP35 data collection system (Biopac Systems, Inc., Goleta, California, U.S.A.), which amplified and converted the analog signal to digital, which was then displayed on a laptop computer. All raw EMG signals were converted to root mean square (rms) and normalized by dividing by the peak isometric rmsEMG activity. Peak isometric rmsEMG was determined by obtaining the average amplitude of a 1-second window of the EMG activity during the best maximal isometric effort. The rmsEMG during dynamic efforts was determined by measuring the peak rmsEMG amplitude (which consistently occurred at the same point in the ROM, as determined by a digital marker) during the full range of motion for each repetition and taking the average of the 3 repetitions. EMG activity was then expressed as a percentage of the peak isometric electrical activity.
Two-tailed paired t-tests were used to detect differences in EMG activity between separate attempts for each muscle, with the experiment-wide level of statistical significance set to p ≤ 0.05. A Bonferroni adjustment was made to account for additional type I error generated by multiple comparisons (3 for a given muscle), with the level of significance for each muscle set to p ≤ 0.016.
Two sets (DYN S1 and DYN S2) were performed before the instruction to determine any possible learning effect resulting from the first exposure to the exercise. The comparison between the 2 preinstruction sets revealed no differences in the LD, TM, or BB (Figure 2).
No measurable difference was seen between the 2 postinstruction sets (DYN S3 and DYN S4) for any of the tested muscles; therefore, the average of the 2 sets pre and post was determined to be the most accurate way to compare pre- and postinstruction muscle activation levels. Mean LD activity increased significantly (p = 0.005) after instruction from 71.1% to 83.59% of peak isometric activity, a 17.6% change. Mean TM activity was slightly higher but not significantly (p = 0.09) measuring 42.69% of peak preinstruction compared to 50.79% postinstruction. BB was 30.45% of peak pre- and 31.36% postinstruction, which was not significantly different (Figure 3).
The latissimus dorsi muscle is an important back muscle involved in shoulder adduction, extension, internal humeral rotation, trunk rotation, and general shoulder stabilization. It helps in accomplishing a number of movement tasks, both in daily life and during athletic activities; thus, strengthening exercises for this muscle should be included in all resistance training routines. The lat pull-down exercise can be performed either by pulling the bar in front of or behind the head, and, although both are generally considered back exercises, the front lat pull-down has been shown to strongly activate both the LD and the BB, even in weight-trained subjects (10) and does not pose an injury risk. In the current study, we sought to discover to what extent the latissimus dorsi could be specifically activated during this exercise.
The most important finding of the study is that specific instructions for the lat pull-down exercise, including palpation of the latissimus dorsi and a verbal request to attempt to increase the participation of back muscles while reducing arm activity, result in a change in the pattern of activity in certain involved muscles. Specifically, subjects were able to increase LD activation after instruction. Subjects were not, however, able to reduce the involvement of the biceps brachii muscles, as measured by EMG activity, and thus the change in activity pattern cannot be characterized as selective activation, or “isolation,” of the back muscles. The current study's protocol distinguishes itself from most attempts to selectively activate a desired muscle by doing so without any change in posture, range of motion, grip or stance width, bar placement, or any other exercise variation. To our knowledge only a few other studies have been performed to date using this type of procedure, and ours is the first to investigate a multijoint dynamic training activity.
Palmerud et al. (20) explored activity of shoulder muscles during an isometric abduction exercise. Using EMG biofeedback, subjects were able to reduce the activity of the upper trapezius to 67% of the value at the onset of the 1-minute contraction. During this time, rhomboids major and minor and the transverse trapezius muscles increased their activity from 175 to 232%, suggesting ability by subjects to voluntarily redistribute muscle force with the help of visual feedback.
Several other studies have utilized verbal instructions (5,12,21) or visual or auditory (16,24) feedback from EMG or mechanomyography (8) in an attempt to selectively reduce muscle activity during various activities. Cowling et al. (5) investigated the effectiveness of verbal instructions on sport performance and injury prevention by collecting kinematic and electromyographic data from trained athletes during a complex landing task. Simple verbal instructions resulted in increased knee bend during landing but did not alter the timing of muscle activation as expected. Nonetheless, the authors concluded that verbal instructions could alter task performance in a manner that could result in decreased injuries.
Karst and Willett (12) observed EMG activity of the rectus abdominus and external oblique during a standard trunk curl, followed by specific instructions to emphasize either muscle, and found that subjects could voluntarily increase external oblique activity while also decreasing rectus abdominus activity when asked to do so. However, the instructions for emphasizing the rectus abdominus muscle did not result in changes in EMG activity in either muscle. The authors suggested that this was a result of the natural dominance of the rectus abdominus during the trunk curl.
The current study found an increase in LD activity following specific technique instruction. TM activity, although moderately increased, did not reach statistical significance. Biceps activity was not decreased, despite instructions to de-emphasize this muscle during the second set of exercises. Although unexpected, this latter finding is not unique because other studies reveal incomplete control over voluntary recruitment of muscles. For example, Palmerud et al. (19) observed activity of 5 muscles during various degrees of shoulder abduction. The EMG of each muscle was displayed 1 at a time on a display unit and subjects were asked to spend 1 minute attempting to reduce the activity of each while holding the arm in position. Only the trapezius showed a significant reduction, whereas the supraspinatus, infraspinatus, and anterior and medial deltoids did not respond to attempts to elicit relaxation. The authors concluded that it is possible that only certain muscles can be individually deactivated to any degree without a concurrent change in the performance of the task.
The results of the current study seem to confirm this line of reasoning, suggesting that the biceps cannot be individually controlled during the lat pull-down exercise in untrained individuals. It is also possible that the lat pull-down requires a minimum level of biceps activity for completion; however, the use of a submaximal intensity (30% maximum isometric force) and the fact that even the elevated LD activity reached only 72% of its maximum (at the 90/90 position) suggest that other muscles most likely had the capacity to take on the load of the biceps. Unfortunately, no EMG data were collected from other potentially involved muscles such as the pectoralis major and the brachioradialis. The brachioradialis, in particular, may have presented reduced activity in response to the exercise because the instructions only asked subjects to reduce “arm activity” in favor of upper back muscles. At least 1 study has shown that even small variations in the wording of the instructions can be important in the performance of a given motor task (22). It may also have been that subjects were fearful that a reduction of arm muscle activity might result in an inability to complete the task or an inadvertent release of the bar and possible injury to themselves.
The use of untrained subjects may have also played a role in the level of muscle control observed. Untrained subjects were originally chosen with the reasoning that trained subjects may have already made the desired changes in muscular activity, emphasizing the LD naturally or having been taught to do so over several years of training. It is also possible, however, that trained subjects (especially athletes) may possess more muscular control and therefore be more likely to be able to voluntarily alter muscle activity on cue, although even trained subjects do not exhibit complete muscle control during complex movement activities (5).
That the increased LD activity without a concurrent decrease in BB activity did not result in any observable change in the speed of the movement is puzzling because greater overall activity of the agonist muscles would seem likely to produce greater total torque around the involved joints. It is possible that subjects simply increased the activity level of the LD as if they were “flexing” it, avoiding a change in the movement pattern by recruiting an antagonist muscle such as the deltoid or trapezius to counteract the additional force created by the LD. Future studies should collect data from antagonist muscles to prove or disprove this theory.
As far as the long-term effects of muscle force redistribution or isolation of desired muscles, the question arises as to whether subjects who successfully do so in a lab can sustain this ability outside of the lab for any period of time and successfully increase the training effect on a desired muscle. Several studies have examined retention of specific motor skills and have mixed results (12,24), but it seems likely that regular reinforcement and practice could result in a habitual recruitment pattern or a retained ability to redistribute muscle forces at will.
The present study shows that untrained subjects can modify muscle activity during a lat pull-down exercise without any changes in posture, position, or grip. LD activity was increased, indicating an added degree of voluntary control over the muscle during dynamic activity, but a concurrent decrease in biceps activity, and thus a functional isolation of the back muscle, was not observed. Whether the ability to increase LD activity has functional implications needs further study, but there are several interesting possibilities. Emphasis or isolation of certain muscles has long been advocated by trainers as a method for increasing the size or strength of desired muscles. The current study utilized a load equal to 30% of the maximum voluntary isometric contraction (MVIC), and it is likely that greater loads would be necessary for voluntary force redistribution to increase the hypertrophic potential of a particular lift. However, at some as yet undetermined %MVIC, it is possible that the redistribution of force among involved muscles will be difficult, if not impossible, because the minimum muscle activation level needed to complete the movement becomes higher. Another possible implication is the effect of verbal instruction or biofeedback on skill performance. It should be possible for coaches to monitor muscle activity; identify problems; and then reduce potential injuries or even improve performance of stereotyped activities, such as throwing a baseball or executing a golf swing, by helping the athlete to make small alterations to muscle activity. EMG technology is more available than ever and can prove valuable in such efforts. Last, the effects of voluntary force redistribution on muscle fatigue during endurance activities could prove interesting. For example, if endurance athletes could shift external loads away from tiring muscles, it could present a new way to combat fatigue during endurance events. Evetovich et al. (8) found that EMG and mechanomyographic biofeedback could cause relaxation of the biceps during arm curls to exhaustion but did not increase time to fatigue. This study only involved 1 joint and 1 muscle group, however, and data on multijoint activities may yield different results.
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