Muscle Activation Strategies During Strength Training With Heavy Loading vs. Repetitions to Failure

Abstract

Sundstrup, E, Jakobsen, MD, Andersen, CH, Zebis, MK, Mortensen, OS, and Andersen, LL. Muscle activation strategies during strength training with heavy loading vs. repetitions to failure. J Strength Cond Res 26(7): 1897–1903, 2012—Going to failure, or not, has probably been one of the most debated issues during the history of strength training. However, few studies have directly compared the physiological effect of failure vs. nonfailure strength training. The purpose of this study was to evaluate muscle activation strategies with electromyography (EMG) during heavy repetitions vs. repetitions to failure with lighter resistance. Fifteen healthy untrained women performed a set with heavy loading (3 repetition maximum [RM]) and a set of repetitions to failure with lower resistance (∼15 RM) during lateral raise with elastic tubing. Electromyographic amplitude and median power frequency of specific shoulder and neck muscles were analyzed, and the BORG CR10 scale was used to rate perceived loading immediately after each set of exercise. During the failure set, normalized EMG was significantly lower during the first repetition and significantly higher during the latter repetitions compared with the heavy 3-RM set (p < 0.05). Normalized EMG for the examined muscles increased throughout the set to failure in a curvilinear fashion—e.g., for the trapezius from 86 to 124% maximal voluntary contraction (p < 0.001)—and reached a plateau during the final 3–5 repetitions before failure. Median power frequency for all examined muscles decreased throughout the set to failure in a linear fashion, indicating progressively increasing fatigue. In conclusion, going to complete failure during lateral raise is not necessary to recruit the entire motor unit pool in untrained women—i.e., muscle activity reached a plateau 3–5 repetitions from failure with an elastic resistance of approximately 15 RM. Furthermore, strengthening exercises performed with elastic tubing seem to be an efficient resistance exercise and a feasible and practical alternative to traditional resistance equipment.

Introduction

Going to failure—the state of momentary muscle fatigue—or not, has probably been one of the most controversial issues during the history of strength training. Anecdotally, bodybuilders swear to failure training, whereas lifters rarely perform such type of training (²⁹). Only few studies have directly compared the physiological effect of failure vs. nonfailure strength training. A randomized controlled trial among physically active men reported slightly better effects of nonfailure training compared with failure training on muscle strength and power (¹⁸). In contrast, a study in elite junior athletes reported better effects of failure training on bench press performance (⁹). Another study reported similar gains in muscular endurance in response to failure compared with nonfailure training with equated training volume (³⁰). Because all these studies used trained or physically active individuals, knowledge is lacking on adaptations in response to failure vs. nonfailure training among untrained individuals. Because going to failure can be an unpleasant experience, research should determine the necessity for such a training component among untrained individuals. Randomized controlled trials are considered the gold standard in the development of evidence-based guidelines, but they are expensive and time consuming. By contrast, studying muscle activation strategies with electromyography (EMG) is a more feasible method of estimating effectiveness of strength training exercises (^2,5).

Electromyography has been used extensively to evaluate muscle activation strategies during different tasks (^3,12,32). For instance, EMG can be used to estimate the level of muscle activation during strength training and rehabilitation exercises (^3,4). Furthermore, EMG amplitude and power spectral analysis are commonly used to study muscle fatigue (¹²). Recruitment of high-threshold motor units is considered essential to induce optimal strength gains (¹³). Fatiguing contractions increase recruitment of additional high-threshold motor units to preserve the desired force level (¹⁶). Thus, going close to failure may be a feasible method to recruit the entire motor unit pool without overloading the musculoskeletal system with very heavy loads. Accordingly, EMG can be used to study inter- and intramuscular recruitment strategies when going to failure.

Untrained individuals with a busy time schedule may find regular strength training in a fitness center challenging, and consequently, strength training coaches need an easy-to-use resistance training alternative to provide clients with home-based exercises. Comparably high levels of muscle activation have been observed during resistance exercises with dumbbells and elastic tubing (²). Thus, strengthening exercises performed with elastic resistance seem to be a feasible and practical alternative to heavy weights. However, most studies investigating the effects of elastic resistance are performed in a rehabilitation setting, which makes the results difficult to transfer to general strength and conditioning protocols. The practical relevance of an easy-to-use alternative to maintain or even increase strength and power is of great importance for both athletes and novice trainers with periodically restricted access to training facilities (e.g., during vacation).

This study evaluates muscle activation of agonistic and stabilizing neck and shoulder muscles with EMG when going to failure during dynamic shoulder abduction (lateral raise) performed with elastic resistance in untrained women. We hypothesized that a high level of muscle activity could be reached without the perceived discomfort of going to failure, thereby making strengthening exercises performed with elastic resistance an easy-to-use alternative to traditional training equipment “outside the gym.”

Methods

Experimental Approach to the Problem

To evaluate muscle activation strategies when going to failure, subjects performed a set with heavy loading (3 repetition maximum [RM]) and a set of repetitions to failure with lower resistance during the lateral raise with elastic tubing. Muscle activity of specific shoulder and neck muscles was measured, and perceived loading (BORG CR10) was rated immediately after each set of exercise. For each individual muscle, muscle activity of each of the dynamic muscle contraction was normalized to isometric maximal voluntary contractions (MVCs). All testing were conducted in April 2009 between 10 AM and 2 PM on weekdays.

Subjects

A group of healthy untrained women (N = 15, 42.2 ± 9.1 years; 167.7 ± 5.1 cm, 65.2 ± 11.1 kg) with no regular training background for at least 2 years before testing were recruited for the study. Exclusion criteria were subacromial impingement syndrome, a known history of disc prolapse, rheumatoid arthritis, and other serious musculoskeletal disorders. Participants were asked to consume a small meal with plenty of liquid 2 hours before testing. Furthermore, the participants were asked not to perform other physical exercise on the day of testing and the day before testing. All participants were informed about the purpose and content of the project and gave written informed consent to participate in the study that conformed to the Declaration of Helsinki and was approved by the local ethical committee (H-3-2010-062).

Study Design

Before the lateral raise exercise (described below), isometric MVCs were performed according to standardized procedures during neck extension, shoulder abduction, and shoulder external rotation to induce a maximal EMG response of the respective muscles (²⁰). Two isometric MVCs were performed for each muscle, and the trial with the highest EMG was used for the subsequent analyses. Subjects were instructed to gradually increase muscle contraction force toward maximum over a period of 2 seconds, sustain the MVC for 3 seconds, and then slowly release the force again. Verbal encouragement was given during all trials.

Five minutes after the isometric MVCs, a number of submaximal contractions of 3 repetitions, separated by rest periods of 1½ minutes to avoid fatigue, were performed to determine the voluntary 3 RM during lateral raise with Thera-Band elastic tubing (Hygenic Corporation, Akron, OH, USA). Resistances used were red, green, blue, black, combined blue + red, and silver tubing (²³). Thereafter, the fatiguing set was performed at 1 resistance level below the heavy 3 RM determined above. For example, if a subject performed the heavy set of 3 RM with the blue elastic tubing, then the green elastic tubing was used during repetitions to failure.

During the lateral raise exercise, the subject was standing erect while holding the tubing handles to the side and then abducting the shoulder joints until the upper arms were slightly above horizontal. The elbows were in a static slightly flexed position (∼5°) during the entire range of motion. The elastic tubing was stretched to slightly more than twice its resting length. The exercise was always performed in a slowly controlled manner, i.e., lifting (∼1½ seconds) and lowering (∼1½ seconds) without sudden jerk or acceleration, for consecutive repetitions to voluntary failure, i.e., to the point where the subject could not complete another repetition voluntarily through a full range of motion despite verbal encouragement. A physiotherapist surveyed all tests and discarded any repetition where the participant was not able to complete a repetition to at least 90° shoulder abduction (horizontal upper arm) with proper technique, i.e., with cheating or jerking. The lateral raise was performed bilaterally, but EMG was sampled only from the dominant side. Subjects were familiarized with the lateral raise exercise on a separate day before testing.

Electromyographic Signal Sampling

Electromyographic signals were recorded from the midportion of the splenius capitis, upper trapezius, medial deltoid, and infraspinatus from the dominant side during the lateral raise. A bipolar surface EMG configuration (Neuroline 720 01-K; Medicotest A/S, Ølstykke, Denmark) and an interelectrode distance of 2 cm were used. Before affixing the electrodes, the skin of the respective area was prepared with scrubbing gel (Acqua gel; Meditec, Parma, Italy) to effectively lower the impedance to less than 10 kΩ. Electrode placement followed the SENIAM recommendations (²⁷). The EMG electrodes were connected directly to small preamplifiers located near the recording site. The raw EMG signals were lead through shielded wires to instrumental differentiation amplifiers, with a bandwidth of 10–500 Hz and a common mode rejection ratio better than 100 dB, sampled at 1,000 Hz using a 16-bit A/D converter (DAQ Card-Al-16XE-50; National Instruments, Austin, TX, USA) and recorded on a computer via a laboratory interface (CED 1401, Spike2 software; Cambridge Electronic Devices, Cambridge, United Kingdom).

Perceived Loading

Immediately after the set with heavy loading (3 RM) and the set of repetitions to failure with lower resistance, the participant rated perceived loading on the BORG CR10 scale (⁷). Before testing, the content of the scale was carefully explained to each subject. An increase in perceived loading, as a function of relative load, is previously found to show a strong linear trend (²⁴).

Electromyographic Signal Processing

During later off-line analysis, all EMG signals were digitally high pass filtered using a fourth-order zero-lag Butterworth filter (³¹) with a 5-Hz cutoff frequency and subsequently smoothed by a symmetrical moving root mean square (RMS) filter with a 500-ms time constant. Each muscle contraction's start time (START) and end time (END) point were located by the following routine: (a) locate the EMG peaks (MAX) separated by 1,000 ms and (b) locate the minimum EMG (MIN) before and between each MAX. START is now located as the first index (searching from MIN_i) >5%*(MAX_i − MIN_i) + MIN_i and END as the first index (searching from MAX_i) <5%*(MAX_i − MIN_{i + 1}) + MIN_{i + 1}.

For each individual muscle, peak RMS EMG of each of the dynamic muscle contraction was normalized to the maximal RMS EMG obtained during the isometric MVCs. Furthermore, the power spectral density of the EMG signals was calculated as the median power frequency (MPF) for each muscle contraction.

All EMG variables were averaged for the 3 repetitions of the heavy 3-RM set. For the set to failure, EMG variables were expressed on a normalized repetition scale with 10 data points from 10, 20, 30 … 100% of the number of total repetitions.

Statistical Analyses

All statistical analyses were performed in SAS version 9 (SAS Institute, Cary, NC, USA). A repeated-measures analysis of variance design was used with normalized EMG as the dependent variable and muscle and repetitions as the independent variables. Furthermore, we included a muscle by repetition interaction. Using the present procedure of surface EMG from the neck and shoulder muscles during evaluation of strength exercises, we have previously reported intraclass correlations (ICCs) above 0.94 for the test-retest reliability of normalized EMG between sets on the same testing day (³). Bonferroni-corrected post hoc tests were performed to locate relevant differences. An α level of 5% was accepted as statistically significant, and all values are reported as mean ± SE unless otherwise stated.

Results

Normalized Electromyography

Testing of main effects showed a significant effect of repetitions (p < 0.0001) and muscle (p < 0.0001) for normalized EMG but no significant repetitions by muscle interaction. Normalized EMG for all 4 muscles generally increased throughout the set to failure in a curvilinear fashion and tended to reach a plateau during the final repetitions. Normalized EMG averaged for all repetitions of the set to failure was highest in the trapezius > deltoid > splenius > infraspinatus.

Compared with normalized EMG during the heavy 3-RM set, normalized EMG during the set to failure was significantly lower during the first repetition and significantly higher during the latter repetitions (Figure 1).

Figure 1: Normalized electromyographic (EMG) activity (left) and median power frequency (right) for prime movers (deltoideus and trapezius) and stabilizing muscles (splenius and infraspinatus) during a set of repetitions to failure with lower resistance normalized to maximal repetitions (failure) compared with an average for 3 repetitions of a heavy 3–repetition maximum (RM) set (3 RM). *Significantly higher values than the 3-RM set (p < 0.05). §Significantly lower values than the 3-RM set (p < 0.05). MVC = maximal voluntary contraction; reps = repetitions.

Median Power Frequency

Testing of main effects showed a significant effect of repetitions (p < 0.0001), muscle (p < 0.0001), and repetitions by muscle (p < 0.0001) for MPF. Median power frequency for all 4 muscles generally decreased throughout the set to failure in a linear fashion, but the slope of the curve was significantly lower in the splenius compared with those of the other 3 muscles (p < 0.05). Median power frequency averaged for all repetitions of the set to failure was higher in the trapezius, deltoid, infraspinatus > splenius. Compared with MPF during the heavy 3-RM set, MPF during the set to failure was significantly lower during most repetitions, although, for the splenius muscle, this was only true for the last normalized repetition (Figure 1).

Perceived Loading

Perceived loading (BORG CR10 scale) was rated significantly higher immediately after the set of repetitions to failure with lower loads compared with the heavy loading set (3 RM) (7.58 ± 2.02 and 4.54 ± 2.09, respectively).

Discussion

In the present study, we hypothesized that a high level of muscle activity could be reached with a medium load without going to complete failure. This hypothesis was supported by observed high level of muscle activity of agonistic and stabilizing neck and shoulder muscles during the lateral raise performed with elastic resistance. A plateau of high level of muscle activity was reached at 10–12 repetitions of a 15-RM load, showing that training to complete failure is not necessary to fully recruit the entire motor unit pool in untrained women.

The frequency spectrum, represented by the MPF, decreased for all muscles analyzed during the repetitions to failure, indicating local muscle fatigue development (⁶). The parallel increase in EMG amplitude and decrease in MPF can be interpreted in different ways. The shift of MPF toward lower frequencies with local muscle fatigue may be caused by decreased muscle fiber conduction velocity, diminished fast-twitch fiber recruitment or both (^5,21). However, because loadings were submaximal (∼15 RM), a derecruitment of fast-twitch fibers is unlikely, rather on the contrary because of increasing need for developing force as motor units fatigue. This downward shift of MPF may also reflect increased synchronization of motor units. Increased EMG amplitude suggests greater total muscle fiber recruitment for a fixed submaximal load (^10,22), increased firing rate and synchronization of motor unit recruitment or impaired excitation-contraction coupling (¹⁰). Although several of the mechanisms may act simultaneously, it is likely that increased recruitment of high-threshold motor units—to sustain force as fatigue sets in—along with increased motor unit synchronization caused the parallel increase in EMG amplitude and decrease in MPF. Thus, concurrent analyses of EMG amplitude and MPF can provide better understanding of muscle activation strategies during repetitions to failure and may provide a better understanding of adaptation mechanisms to this type of strength training. This may also explain the hypertrophy of fast-twitch fibers in response to strength training with submaximal loadings (²⁵).

In the present study, high EMG levels (>60% of MVC EMG) for the heavy 3-RM set were observed for deltoideus, trapezius, and splenius. However, the 15-RM set to failure induced higher muscle activity than the 3-RM set approximately halfway into the set (Figure 1). Furthermore, a plateau of high level of muscle activity was reached at approximately 10–12 repetitions of the 15 RM, indicating that a maximal level of EMG can be reached 3–5 repetitions before failure (Figure 1). Thus, training to complete failure is not necessary to fully recruit the entire motor unit pool in untrained women during the lateral raise with elastic resistance.

The regulation and manipulation of various resistance exercise variables could further influence the level of muscle activation. One study reported that a forced repetition exercise system for various under-extremity resistance exercises induced greater acute hormonal and neuromuscular responses than a traditional maximum repetition exercise system (¹). However, the exercise intensity was greater in the forced repetition group, possibly affecting the outcome measures. Furthermore, lifting velocity, which declines unintentionally as fatigue approaches, is another variable to address in this context. Izquierdo et al. (¹⁷) observed that relative average velocity decreased at a greater rate in bench press than in parallel squat performance. Thus, type of exercise and muscles used can significantly influence lifting velocity, hence affecting muscle coordination and activation. The manipulation of specific training variables (lifting velocity, going to failure or not, intensity, load, and rest period) will stress the muscles in different ways, and a specific relationship between training stimulus and adaptive response seems to exist (strength endurance continuum) (⁸). Failure training therefore could promote local muscle endurance and increased time to exhaustion, whereas nonfailure training could benefit strength and power gains.

Previous studies showed that adults can gain strength and power without going to the perceived discomfort and acute physical effort associated with failure training (^9,26). By contrast, other studies observed greater strength gains by repetitions performed to failure and relate it to recruitment of additional motor units (^9,26) and high mechanical stress with its associated gene expression and damage and repair muscle process (¹⁵). In our study, maximal level of muscle activity for the deltoid, trapezius, and splenius was obtained at approximately 3–5 repetitions before failure with a 15-RM loading, followed by a plateau region where the EMG failed to increase any further. Thus, very high levels of muscle activity—expressing recruitment of additional fast-twitch fibers—can be reached without going to the state of momentary muscle fatigue. Supported by the literature, our study suggests that training to failure may not be necessary for power and strength gains but could instead be incorporated periodically as a progression variable and might allow advanced lifters to break through training plateaus (²⁹). However, it is important to note that failure training can lead to enhanced joint compression, more Valsalva maneuvers, and, if performed regularly, cause overreaching. In line with this, Izquierdo et al. (¹⁸) demonstrated a potential beneficial stimulus of nonfailure resistance training for improving strength and power, whereas failure training seemed more beneficial for enhancing upper-body local muscle endurance. However, the subjects included in that study was elite male Basque ball players, so caution should be taken when projecting or transferring those results to untrained women.

We have previously shown that resistance exercises with dumbbells and elastic tubing yield comparable high levels of muscle activation during the lateral raise (²). Our results elaborate on these findings by showing that elastic tubing of medium resistance promotes a high level of muscle activity without going to complete failure. Untrained individuals with a busy time schedule may find regular strength training in a fitness center challenging, and consequently, strength training coaches need an easy-to-use resistance training alternative to provide clients with home-based exercises. Furthermore, many athletes are struggling in increasing or even maintaining muscular adaptations when on vacation or otherwise prohibited access to their regular training facilities. Thus, incorporating exercises performed with elastic tubing seems to be a feasible and practical tool both inside and outside the gym, making it an appropriate training tool for untrained individuals and athletes.

There are some limitations of our study. The generalizability of our study is limited to the neck and shoulder muscles of untrained women. Because training adaptations, among other factors, are highly dependable of training state, caution should be taken when incorporating and transferring the results of the present study to other population groups than untrained women. Furthermore, neck and shoulder muscles are generally dominated by type I muscle fibers, and it is plausible that the EMG-repetition relationship is different in type II dominant muscles. Future studies should determine the EMG-repetition relationship in other muscle groups, in trained individuals, and also during more common exercises, such as bench press and squat.

Many factors influence EMG amplitude during muscle fatigue. In the present study, the observed stagnated EMG amplitude as fatigue approach could also be subscribed to factors that enhance amplitude cancellation. It is possible that a decrease in muscle fiber conduction velocity that results in an increase in the duration of motor unit potentials during sustained contractions (²⁸), leading to greater overlap between potentials (¹⁴), and an increase in amplitude cancellation could be responsible. Furthermore, sustained activation preferentially decreases the amplitude and area of the largest motor unit potentials (¹¹), resulting in a more narrow range of amplitudes for the motor unit potentials. Nonetheless, the normalization of the surface EMG amplitude to the values obtained with maximal activation (MVCs) increases the reliability of the measurement (¹⁹).

In conclusion, going to complete failure during lateral raise is not necessary to recruit the entire motor unit pool in untrained women—i.e., muscle activity reached a plateau 3–5 repetitions from failure with an elastic resistance of approximately 15 RM. Furthermore, strengthening exercises performed with elastic tubing seem to be an efficient resistance exercise and a feasible and practical alternative to traditional resistance equipment.

Practical Applications

Strength training coaches working with untrained individuals can use the present knowledge to better guide clients regarding repetitions necessary to induce an efficient training stimulus. Our results show that full activation of agonistic and stabilizing neck and shoulder muscles can be reached 3–5 repetitions before failure with a 15-RM load during the lateral raise in untrained individuals. Thus, training to complete failure is not necessary to effectively recruit the involved muscles, indicating that strength and power can be reached without the unpleasant experience of going to failure. Furthermore, many untrained individuals and athletes with a busy time schedule may find regular strength training in a fitness center challenging or experience situations with restricted access to training facilities and could therefore benefit from an effective, practical, and easy-to-use alternative to traditional training equipment. Resistance exercise performed with elastic tubing seems to fulfill these demands, making it possible to maintain or even gain strength and power both inside and outside the gym.

Acknowledgments

L. L. Andersen received a grant from the Danish Working Environment Research Fund (grant 20100020189/4) and the Danish Rheumatism Association (grant R68_A993) for this study. Warm thanks to the students from the Metropolitan University College for practical help during the project.