The specific skeletal muscle adaptations elicited by resistance training have been previously related to the patterns of motor unit activation that occur during exercise (19). Improvements in muscle strength and force output have also been explained by changes in motor unit activation, such as greater motor unit recruitment (14,33) or increased firing rates (18,40). Motor unit activity can be measured through electromyography (EMG), which is commonly considered to reflect the neural drive to the muscle (13,37). It has been well documented that increasing force results in higher EMG amplitudes because of the greater motor unit recruitment necessary to increase contractile force (7,10,11,24,35). However, a number of studies have shown an increase in EMG amplitude over the duration of sustained or repeated muscle actions at a constant force level (6,15,23,25,27,29). Interestingly, firing rates of recruited motor units have been shown to decrease over prolonged exercise (16,27). Consequently, the rise in EMG amplitude has been attributed to increased motor unit recruitment as a compensatory mechanism for sustaining contractile force as fatigue accumulates (34,38,39). Accordingly, numerous investigations have demonstrated that EMG amplitude increases during dynamic exercise as the extent of the effort, or the number of repetitions performed, increases (17,30–32,34).
Yet the degree to which EMG amplitude increases when performing additional repetitions with a light resistance relative to using heavier external resistance is unclear. Pick and Beque (30) examined the EMG amplitude of the vastus lateralis (VL) in resistance-trained athletes and found that it was higher at the end of a maximal effort set of barbell squats with 85% 1 repetition maximum (1RM) than during 1RM testing. However, this finding was not consistent in the vastus medialis (VM), nor in untrained subjects examined. Previous studies have indicated that the magnitude of EMG amplitude increase during sustained isometric muscle actions is limited to a certain degree based on the target force level (6,13,15,27). Because of the extensive differences in EMG recordings between modes of muscle actions (5,8,26), it is difficult to extrapolate these findings to dynamic resistance exercise without further support from investigations on dynamic muscle actions.
Although the increase in EMG amplitude observed during repeated muscle actions has been explained by increased central drive necessary to sustain force as fatigue accumulates (34,38,39), it is inconclusive whether fatigue derived from exercise performed earlier induces greater EMG amplitude during subsequent exercise. Previous studies have shown that EMG amplitude diminishes after strenuous resistance exercise protocols (3,4). In contrast, Smilios et al. (34) demonstrated progressive increases in EMG amplitude over a series of 20-repetition sets with gradually decreasing resistance interspaced with 2-minute rest periods. Further uncertainly exists pertaining to consecutive maximal effort sets with progressively lighter resistance performed without allotted rest periods. This frequently incorporated training technique, commonly known as a “drop set,” has remained relatively uninvestigated. Our hypothesis was that if lighter weights could stimulate greater motor unit recruitment, it might be observed in such a workout sequence. Therefore, the assessment of drop sets may offer valuable insight regarding changes in EMG amplitude related to fatigue accumulation.
This investigation sought to examine several of the aforementioned issues related to the effects of repetition performance, external resistance, and neuromuscular fatigue on EMG amplitude in dynamic muscle actions. First, EMG amplitude would be significantly greater in light resistance exercise (50% 1RM) performed in rested conditions to a maximal number of repetitions than to a submaximal number of repetitions. Second, EMG amplitude would be significantly lower in maximal repetition sets performed in rested conditions with 50% 1RM resistance than with heavy resistance (90% 1RM). Third, EMG amplitude would be greater in maximal repetition 50% 1RM resistance sets performed in prefatigued conditions (no prior rest period) than in rested conditions. Ultimately, the findings of this investigation would provide critical information on understanding the changes in neuromuscular physiology during dynamic exercise related to variable levels of target repetitions, resistance, and fatigue.
Experimental Approach to the Problem
Proper familiarization of the protocol was used to eliminate learning effects and stabilize the responses to the workouts. This was aided by the use of resistance-trained subjects. This investigation used a randomized within-subject design consisting of 2 test visits: a drop-set day and a single-set day (Figure 1). Each visit was conducted at the same time of day and separated by a minimum of 72 hours. Subjects replicated their diet before each testing phase and were encouraged to hydrate themselves before testing. The experimental protocol was specifically designed to address the 3 primary research questions of this investigation. First, to compare 50% 1RM resistance sets performed in rested conditions to either a submaximal (D50Sub and S50Sub) or maximal (S50Max) number of repetitions. Second, to compare sets performed to a maximal number of repetitions in rested conditions with either 50% (S50Max) or 90% 1RM (D90Max) resistance. Third, to compare 50% 1RM resistance sets performed to a maximal number of repetitions in either rested (S50Max) or prefatigued (D50Max) conditions. Because a direct transition from light to heavy resistance would require a substantial increase in loading (45–75 kg per subject), submaximal sets of 70% 1RM × 7 repetitions (D70Sub and S70Sub) were performed after each 50Sub set. The 20% 1RM increase in loading between sets was equivalent to the maximum loading increase used in the 1RM test performed on the familiarization visit. The 70Sub sets would also provide additional context to the extent peak EMG amplitude increased during the 50Sub sets. Accordingly, the D70Max set was added for similar purposes and to mirror the 70Sub sets. Subjects were given 2 minutes of rest after each set with the exception of moving through the drop-set protocol from D90Max to the D50Max in which only the time necessary to reduce the resistance was provided (<5–10 seconds).
Ten resistance-trained men (age, 23 ± 3 years; height, 187 ± 7 cm; body mass, 91.5 ± 6.9 kg; squat 1RM, 141 ± 28 kg; relative squat 1RM, 1.54 ± 0.24 kg·kg body mass−1) participated in this investigation. Each subject had been currently resistance training for a minimum of 1.5 years and has used the squats in their training programs. Each was able to successfully demonstrate proper squatting technique using a Smith machine and had experience with the equipment. All subjects filled a medical history questionnaire and were screened by a physician for any medications, dietary supplements, and any orthopedic, endocrine, or other medical problems that might confound the results of this investigation. To further ensure the safety of the subjects, all familiarization and testing procedures were supervised by a National Strength and Conditioning Association (NSCA) certified strength and conditioning specialists along with trained laboratory research assistants. After being briefed on the risks and benefits of the investigation, all participants provided written informed consent to participate. The study had been approved by the Institutional Review Board for use of human subjects at the University of Connecticut.
Before data collection, subjects attended familiarization visits, which involved exposure to experimental testing procedures, anthropometric measurements, and 1RM testing. The 1RM test sequence followed a standardized protocol outlined previously (21,22). Biomechanical markers for foot placement, parallel squat depth, and bar position were also established during the familiarization process and replicated during each subsequent visit to maintain identical testing conditions.
Dynamic Exercise Protocol
Each dynamic squatting exercise was performed on a standard Smith machine with the bar rested in a high bar position on the upper trapezius. To further emphasize the knee extensor muscles while limiting incorporation of hip extensors, subjects positioned their feet forward to allow for an upright spinal posture when reaching a parallel squat depth. Subjects were instructed to perform each nonfatiguing set to the specified number of repetitions and each fatiguing set to the point of volitional failure. Subjects were required to perform repetitions through full range of motion in a controlled nonballistic manner. Only repetitions that met these criteria were recorded for analysis. The number of successful repetitions performed through the full range of motion was recorded for each set with total repetitions calculated from the sum of the repetitions performed per day. Relative training volume was calculated by multiplying the resistance (%1RM) of each set by the repetitions performed.
Peak Isometric Force
Peak isometric force was measured during isometric squat maximum voluntary muscle actions (MVCs) recorded on a force plate (Advanced Mechanical Technology, Inc., Watertown, MA, USA; data analyzed with DartPower 2.0, Athletic Republic). For each MVC, subjects were positioned at parallel squat depth and instructed to maximally press upward for 3 seconds into the Smith machine bar, which was fixed in place. To compare subjects' maximum isometric force at the start of each day, 3 MVCs were performed with the highest value recorded for analysis (Pre-MVC). To identify any fatiguing effects of the 50Sub and 70Sub sets and detect any differences in protocol tolerance between days, an additional MVC was performed after the 70Sub set on each day (Mid-MVC). A final MVC was performed after the 50Max set on each day to assess any effects of accumulated fatigue from the maximal effort sets. Subjects were allotted 2 minutes of rest after each MVC.
Electromyography Recording and Processing
Surface EMG signals were recorded from the VL and VM muscles using an IX-BIO4 Series 4 Channel Biopotential Recorder (iWorx Systems, Inc., Dover, NH, USA). The VL muscle was selected as it is commonly investigated (10,30,38) and since its motor units are recruited up to 95% MVC force (12). The VM muscle was selected because of its proximity and similarity in the recruitment pattern and function to the VL muscle. Before data collection during each visit, the subject's skin was shaved and cleaned with an alcohol swab before pairs of sensor electrodes were placed on the belly of each muscle approximately 5 cm apart in accordance with SENIAM (Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles) guidelines. The ground electrode was placed on the patella. All electrode positions were marked on the subjects during familiarization and replicated during each visit. Electromyography signals were sampled at 1,000 Hz and stored in the data collection computer for later off-line analysis. A high-pass filter of 20 Hz and a low-pass filter of 450 Hz were used with a gain of 25 mV per V. Interelectrode impedance, signal-to-noise ratio, and common-mode rejection ratio were <10,000 ohms, >20 dB, and 105 dB, respectively. The EMG signal was full-wave rectified and smoothed with a low-pass filter with a time constant of 25 milliseconds. The EMG signals were analyzed using iWorx LabScribe2 Data Recording and Analysis Software (iWorx Systems, Inc.). The peak EMG amplitude of each set was recorded as the highest value obtained above baseline. As the purpose of this investigation was to make within-subject comparisons of different sets on different days, normalization of the EMG signal was necessary to reduce intrasubject variability stemming from daily fluctuations in the EMG signal. Normalizing EMG measurements of dynamic muscle actions using peak EMG values obtained from isometric muscle actions has been previously advised against (8). In a recent review of EMG normalization procedures, Burden (5) similarly recommended normalization of dynamic muscle actions by EMGs recorded using the same muscle action. High reliability has been demonstrated when EMG signals are normalized to values obtained from submaximal muscle actions (9,41,42). Because the 50Sub sets on each day were the first set performed, volume-matched, and performed to an equivalent submaximal effort, they were used to normalize the peak EMG amplitude of each set. Because normalization to the peak EMG amplitude recorded during the 50Sub sets would exclude the 50Sub sets from statistical comparisons, the mean peak EMG amplitude of the 10 repetitions performed during the 50Sub sets was used for normalization.
Rating of Perceived Exertion
Immediately after the completion of each set, subjects were asked to rate their level of perceived exertion using the modified Borg 0–10 category-ratio Rating of Perceived Exertion (RPE) scale (28) with magnitude estimation above a rating of 10, which allowed a perceptual rating higher than 10 if desired (28). A rating of 0 indicated no exertion, whereas a rating of 10 indicated a maximum level of exertion akin to the most strenuous exercise the subject had ever performed. This scale was designed for anaerobic metabolism with an exponential rise in perceptual fatigue possible. All participants received verbal and written instruction on the use of the RPE scale at the start of each familiarization and testing visit (28).
All data are expressed as mean ± SD. A repeated-measures one-way analysis of variance was performed to detect significant mean differences. All data sets met the assumptions of linear statistics. When a significant repeated-measures effect for a dependent variable was found, a Fisher's least significant difference post hoc analysis was performed to determine the significance of differences between pairs of measures. Significance in this investigation was set at p ≤ 0.05.
Mean repetitions performed for each set are shown in Figure 2. As expected, subjects were able to perform all target repetitions for the 50Sub and 70Sub sets on each day. Mean repetitions for the 50Max set was significantly lower on the drop set than on the single-set day (p < 0.01). However, no significant differences were observed in total repetitions between days (p = 0.06). Notably, subjects performed significantly greater relative training volume during the fatiguing sets on the drop-set day (2770 ± 410%) than on the single-set day (2520 ± 430%) (p = 0.02).
Peak EMG Amplitude (Vastus Lateralis)
Peak EMG amplitude of the VL muscle for each set is shown in Figure 3. The D50Max and S50Max sets had significantly greater EMG amplitude than the D50Sub (p = 0.01 for each) and S50Sub sets (p = 0.01 and p < 0.01, respectively). D70Max was significantly greater than both D70Sub (p = 0.02) and S70Sub (p = 0.04) sets. Peak EMG amplitude was significantly greater in the D90Max set than each other set (p < 0.01 for each). No significant differences were detected between days in the 50Sub (p = 0.83), 70Sub (p = 0.65), and 50Max (p = 0.83) sets.
Peak EMG Amplitude (Vastus Medialis)
Peak EMG amplitude of the VM muscle for each set is shown in Figure 4. The D50Max and S50Max sets had significantly greater EMG amplitude than the D50Sub (p < 0.01 for each) and S50Sub sets (p = 0.03 and p < 0.01, respectively). D70Max was significantly greater than both 70Sub sets (p = 0.03 for each). Peak EMG amplitude was significantly greater in the D90Max set than each other set (p < 0.01 each). No significant differences were detected between days in the 50Sub (p = 0.20), 70Sub (p = 0.77), and 50Max (p = 0.58) sets.
Voluntary Muscle Action Isometric Force
Mean MVC isometric force per time point is shown in Figure 5. There were no significant differences between days at Pre-MVC (p = 0.40), Mid-MVC (p = 0.72), or Post-MVC (p = 0.69). However, on both days, Post-MVC was significantly lower than Pre-MVC (p < 0.01 for each) and Mid-MVC (p < 0.01 for each).
Rating of Perceived Exertion
Mean RPE for each set is shown in Figure 6. Although the S50Sub (4.1 ± 1.1) and D50Sub (4.0 ± 1.0) sets were not significantly different (p = 0.90), both had significantly lower RPE than all other sets (p < 0.01 for each). Similarly, the S70Sub (6.0 ± 1.5) and D70Sub sets (6.3 ± 1.0) did not significantly differ (p = 0.42) but were each significantly lower than all max sets (p < 0.01 for each). No significant differences were detected in RPE between the S50Max set (9.7 ± 0.5) and the D90Max (9.6 ± 0.7, p = 0.68), D70Max (9.6 ± 0.7, p = 0.59), and D50Max (9.9 ± 0.3, p = 0.17) sets. Furthermore, RPE was not significantly different between the D90Max and D70Max (p = 0.17), D90Max and D50Max (p = 0.08), or D70Max and D50Max sets (p = 0.34).
The primary finding of this investigation was that motor unit activity was highest with heavy loading and even with fatigue at the end of a drop set, motor unit activation at 50% of the 1RM was lower than the heavy load. In addition, perceptual ratings at failure were similar. Thus, perceptual feelings about a workout may be misconstrued as similar when the amount of activated tissue differs substantially.
The peak EMG amplitude was significantly greater during the S50Max set than both the S50Sub and D50Sub sets. This is not surprising considering the large magnitude of difference between repetitions performed in the S50Max set compared with the S50Sub and D50Sub sets. This finding is consistent with previous research demonstrating higher EMG amplitude when additional repetitions were performed (30,34). Further support comes from the absence of significant differences in peak EMG amplitude of the VL between the S50Max set and the 70Sub sets. Although both 70Sub sets were significantly greater than either 50Sub set, the increased level of effort expressed during the S50Max set likely increased motor unit activity to the extent in which peak EMG amplitude was not significantly different from the 70Sub sets.
Although EMG amplitude was higher during light resistance sets performed maximally (versus sub-maximally) in rested conditions, it did not reach maximum values. Additionally, peak EMG amplitude was significantly greater during the D90Max set than the S50Max set in both VL and VM muscles, confirming one hypothesis of this investigation. This finding is consistent with the previously mentioned isometric studies, which demonstrated that rises in EMG amplitude during low-force muscle actions performed to failure are constrained from maximal values (6,13,15,27). Ultimately, it may not be possible to reach maximal motor unit activity with light resistance exercise even if the set is performed to failure.
Although the absence of significant differences between the D50Max and S50Max sets in peak EMG amplitude was the opposite of what was predicted by the third hypothesis, both the D50Max and D70Max sets had significantly greater peak VL and VM EMG amplitude than during the submaximal sets with the same resistance. Similar to conclusions made by previous studies (1,2,34), the increase in EMG amplitude likely resulted from greater motor unit recruitment to compensate for declining motor unit firing rates as a result of fatigue. Additionally, subjects performed more repetitions during the D50Max and D70Max sets than during the respective submaximal effort sets. Thus, the relatively greater peak EMG amplitude could have also been attributed to the higher work performed.
Although subjects were unable to produce EMG amplitude during the sets with lighter resistance comparable to the 90Max set, it is important to note that both the VL and VM require relatively high resistance to produce maximal motor unit recruitment (12). Exercises targeting smaller muscles involved with postural or fine motor skills may be capable of eliciting EMG amplitudes closer to maximal values when relatively light resistances are performed to failure. Consequently, it remains imperative for strength and conditioning professionals to be knowledgeable of the physiological differences that exist between muscles when selecting optimal methods to achieve training objectives.
An unexpected observation of this investigation was the similarity in total repetitions performed between days despite the greater resistances used on the drop-set day. It is important to note that the subjects assessed in this investigation followed resistance training programs with traditional resistance and repetition ranges recommended for strength and hypertrophy gains. Greater disparity in total repetitions between days might have been observed in populations of predominantly aerobically trained athletes with less heavy resistance exercise experience.
In this study, ratings of perceived exertion did not differentiate the intensity of the exercise when performed to failure. However, lower ratings were observed with lighter loads when performed with submaximal volumes. Ratings of perceived exertion when sets are done to failure typically reflect the size of the muscle being trained with the exception that bicep curls or calf raises have similar high ratings when lifted to a failure (20). Similarly, the order of exercise in a resistance training protocol does not seem influential, again suggesting that the performance end point of failure does not allow differentiation (36). Being careful not to lift to failure may be one way of allowing better perceptual monitoring during a workout.
The results of this investigation indicate using higher external resistance is a more effective means of maximizing muscle activation when compared to increasing the number of repetitions. Accordingly, previous recommendations for the use of heavier loads during resistance training programs emphasizing strength and hypertrophy are further supported. Given the significantly greater training volume performed on the drop-set day, drop sets may offer considerable utility as a means of augmenting training volume without substantially increasing training time. However, strength and conditioning professionals should be cautious when incorporating drop sets into training programs, particularly in inexperienced athletes because of the accumulated stress of consecutive maximal sets.
The authors have no conflicts of interest to declare. The study was supported by internal laboratory grant funds. The authors thank the subjects for their support of this project.
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