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Original Research

Effect of an Unstable Load on Primary and Stabilizing Muscles During the Bench Press

Ostrowski, Stephanie J.1; Carlson, Lara A.2,3; Lawrence, Michael A.2

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Journal of Strength and Conditioning Research: February 2017 - Volume 31 - Issue 2 - p 430-434
doi: 10.1519/JSC.0000000000001497
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In recent years, it has become popular in many fitness facilities to train on an unstable surface or with an unstable load. It has been assumed that training with instability results in higher activation of the stabilizing muscles and is more beneficial to sport performance and everyday activities (11,14). However, previous research has provided mixed results. Exercises performed on unstable surfaces have been found to produce greater activity in primary and stabilizing muscles during a push-up (13) and an unstable leg extension (3); no change in any muscle activity during a chest press (8) and decreased trunk muscle activity during a deadlift (5). Furthermore, many studies have shown that peak force output is lower when training on an unstable surface (1,13). For example performing a chest press on an unstable surface resulted in a 59.6% decrease in maximal isometric force production compared with the stable condition (1). Deficits in force output while training on an unstable surface are one of the major reasons training on an unstable surface is not recommended in an athletic population (3,4).

Although there have been several studies on unstable surface training, fewer have been conducted on unstable load training. Kohler et al. (11) found that the participants' 10 repetition maximum (10RM) loads were lower with an unstable load, but the results did not support their hypothesis that stabilizer muscle activation would increase. This may be partially due to the fact that they used dumbbells as the unstable load, and the dumbbells may not have been an unstable enough stimulus to elicit the wanted response (12). Squatting with a barbell with elastic bands suspending the load has been shown to significantly increase activity of the stabilizing muscles (rectus abdominus, external obliques, and soleus) (12). Lawrence and Carlson (12) also found that squatting with the load suspended from elastic bands resulted in just a 3% decrease in force output when compared with using a stable load. However, there was no difference in muscle activation while bench pressing with the weight suspended from elastic bands verses bench pressing with a normally loaded barbell (6). The findings from Dunnick et al. (6) may be partially due to the 2-second cadence that was required on both the concentric and eccentric portions of the bench press, along with a slight pause at the bottom of the press. A slow cadence and a slight pause would mitigate some of the movement caused by the suspended weights, possibly making the press less challenging to control than an unrestricted cadence with a touch-and-go style of press.

To elicit the greatest muscular response during an unstable bench press, the load should be as unstable as safety allows. This includes allowing for a touch-and-go style of pressing and allowing participants to lower and press the weight as fast as possible. Instability could be further increased by using a specially designed flexible barbell as well as suspending the load with elastic bands. Furthermore, while Dunnick et al. (6) only suspended a portion of the load with elastic bands, suspending the entire load has the potential to increase the overall instability and requires more muscle activation of the stabilizing shoulder musculature.

More often than not, individuals have to manage unstable loads while on a stable surface (e.g., an opponent, carrying grocery bags, and moving furniture); however, the majority of research has focused on unstable surface training (11). Therefore, the purpose of this study is to examine the effects of an unstable load (as provided by a flexible barbell and a load suspended by elastic bands) on primary and stabilizing musculature while bench pressing. We hypothesize that bench pressing with an unstable load will increase the activity of stabilizing musculature (latissimus dorsi, lateral and posterior deltoid, biceps brachii, and upper trapezius) and not alter the activity of prime movers (pectoralis major, anterior deltoid, triceps).


Experimental Approach to the Problem

Fifteen resistance-trained volunteers bench pressed 60% of their 1RM bench press under the unstable condition and 75% of their 1RM under the stable condition. Owing to the increase in difficulty in the unstable condition, 1RM percentages were standardized. The 60% of 1RM is comparable to loads tested in studies where subjects squatted with unstable loads (12). The 75% load was used with the standard barbell as this is a typical load used for 5 repetitions. Through pilot testing, it was found that the 60% load during the unstable condition and the 75% load during the stable condition was the greatest load that could be used where the participants were still able to consistently complete 5 repetitions. Bar position and muscle activity of both upper limbs and trunk were recorded.


Fifteen resistance-trained men (age 24.2 ± 2.7 years, mass 84.1 ± 12.0 kg, height 1.77 ± 0.05 m, 9.9 ± 3.4 years' lifting experience, and bench press 1RM 107.5 ± 25.9 kg) volunteered for this study. Individuals with current upper extremity injuries; injuries that prevented them from exercise in the past 6 months; or those who have had upper extremity surgeries were excluded from participation. Seven subjects reported using unstable load training (ULT) before participation in this investigation. This study was approved by the Institutional Review Board at the University of New England, and all participants gave written informed consent (IRB #042815-014).


All subjects were asked to complete 2 testing sessions. To minimize the influence of fatigue, subjects were asked to abstain from exercise for 48 hours before testing. During the first testing session, subjects performed a 1RM bench press with a standard barbell and typical load according to the National Strength and Conditioning Association's guidelines for maximal strength testing (2). The second testing session occurred at least 7 days after the 1RM bench press test.

In both conditions, a reflective marker was placed at the center and each end of the barbells to determine the top and bottom positions of each press. The motion of the markers was tracked using 8 Oqus Series-3 cameras (Qualisys AB, Gothenburg, Sweden) set at 150 Hz. Sixteen wireless electromyography (EMG) sensors (Noraxon USA Inc., Scottsdale, AZ, USA) with disposable surface electrodes (2-cm interelectrode distance, 1-cm circular conductive area; Noraxon USA) were used to measure muscle activation of the pectoralis major, anterior deltoid, middle deltoid, posterior deltoid, biceps, triceps, upper trapezius, and latissimus dorsi bilaterally. Electromyography sensors were placed according to the Surface EMG for Non-Invasive Assessment of Muscles recommendations (7,10) and set to collect at 1,500 Hz. Each subject was then allowed to take their own preferred bench press position, including hand placement. Hand placement was marked to maintain consistency throughout all trials.

Subjects performed a warm-up of 1 set of 5 repetitions with an unloaded barbell, then 25%, then 50% of their 1RM with a 2-minute rest period after each warm-up set. Sixty percent of the 1RM (12) was loaded for the unstable condition and 75% of the 1RM was loaded for the stable condition. Pilot testing was done to determine the appropriate load for each condition. It was found that with a load >60% of 1RM was too challenging to consistently complete 5 repetitions. Similarly, it was found that with a load of 75% subjects were able to consistently complete 5 repetitions. The order in which the conditions were performed was randomized. Subjects performed 2 sets of 5 repetitions with each condition. Subjects were instructed to perform repetitions as fast as possible while maintaining control of the barbell. All repetitions of the second set were used for analysis. All presses were performed followed by a 5-minute rest period. For the stable condition, weights were placed on the barbell normally. In the unstable condition weights were suspended from “mini” elastic resistance bands (EliteFTS, London, USA). The bands were “quadruple looped” through the weights and hung on the barbell (Figure 1). As a precautionary measure, the load on each band was limited to 50 pounds. If >50 pounds was needed to reach the prescribed load, then additional bands were used until the correct load was achieved.

Figure 1.:
Load suspended by elastic bands.

All data analysis was completed using Visual 3D (C-Motion, Germantown, MD, USA). Vertical barbell motion was smoothed with a second-order low-pass Butterworth filter with a 6-Hz cutoff (9) and used to define each bench cycle. A second-order Butterworth band-pass (10- to 200-Hz) filter (14) was applied to the EMG signals which were then rectified; a linear envelope was created using a 6-Hz cutoff and integrated using the trapezoid rule.

Statistical Analyses

Data were compared using a multivariate analysis of variance (MANOVA; condition [stable vs. unstable] × phase [concentric vs. eccentric]) to determine differences in muscle activity. Statistical significance was set at the 2-tailed p ≤ 0.05 level of confidence. Statistical analyses were performed using the IBM SPSS Statistics version 21 (IBM Corporation, Somers, NY, USA) software package.


The left and right anterior deltoid, left and right pectoralis major, and the left triceps were significantly more active during the concentric (pressing up) phase. The right and left biceps were significantly more active during the eccentric (lowering down) phase. The right and left biceps and the left middle deltoid were significantly more active in the unstable condition (Table 1). In both conditions, the concentric phase was significantly shorter than the eccentric phase. Regardless of the phase, the stable condition was significantly faster than the unstable condition (Table 2). Intraclass correlations were also calculated (Table 3).

Table 1.:
Muscle activity (μV), mean ± SD, N = 15.*
Table 2.:
Pressing time (s), mean ± SD (intraclass coefficient), N = 15.*
Table 3.:
Intraclass coefficients for muscle activity.*


Training the upper body muscles with an unstable load may be beneficial to patients undergoing physical therapy and individuals interested in strength and conditioning by activating the muscles similar to how they would work in everyday life, such as carrying heavy grocery bags. We hypothesized that bench pressing with an unstable load would increase the activity of stabilizing muscles and not alter the activity of prime movers. To induce as much instability as possible, a flexible barbell was used and the entire load was suspended by elastic bands. Although the stabilizing muscles were not more active during the unstable press, we found that the left middle deltoid and both biceps were more active in the unstable condition.

The ability of individuals to press 75% of 1RM with an unstable load for 5 repetitions was found to be inconsistent during pilot testing; however, we found pressing 60% of 1RM more reliable yet still challenging. We understand that it is not common to press only 60% of 1RM for 5 repetitions with a standard barbell; therefore, we used 75% for the stable load, as we found that this load could be consistently used for sets of 5 repetitions. Previous research has shown that when using an unstable load, 10RM overhead shoulder press (11) and 1RM during the squat (14) were lower. So, different percentages of 1RM with a standard barbell were used to better mimic what an individual would do on his or her own for a workout. Regardless of using a greater load with the stable condition as compared with the unstable condition, it is important to note that the unstable condition produced muscle activation levels that were greater to or no different than the stable condition. This is imperative to consider as previous studies used identical loads but found no differences in muscle activity (5,6,8,16).

In both of our conditions, the concentric phase of the press was significantly faster than the eccentric phase, which is typical for a bench press (15). Pressing time during both concentric and eccentric phases was longer with the unstable load. These findings differ from Goodman et al. (8) who reported no significant difference in cadence or muscle activity while performing the bench press on a stable versus an unstable surface. Furthermore, Dunnick et al. (6) used a slow cadence (2:2) while pressing an unstable load, which may have made the bar easier to control and required less activation of the stabilizing musculature. Our subjects pressed the unstable load at almost twice the speed as the subjects in Dunnick et al.'s investigation, which most likely made the load even more unstable due to the swaying of the load suspended by the elastic bands. However, the unstable load took longer to press resulting in more time under tension.

Only 2 other studies have investigated the effects of an unstable load suspended with elastic bands on muscle activation of primary and stabilizing musculature (6,12). Lawrence and Carlson (12) compared a stable and unstable squat with a load of 60% of 1RM and found that the unstable load increased muscle activation levels in the torso. It should be noted that Lawrence and Carlson had subjects perform 10 repetitions with 60% in both the stable and unstable conditions; however, pilot testing determined that bench pressing with an unstable load of >60% 1RM for 5 repetitions was too difficult for subjects to complete. This suggests that loading schemes for unstable loads may vary between different exercises, and generalizations should not be made across different exercises. The findings of the present study differ from those previously reported findings of no differences between stable and unstable loads (6). This may be due to some factors in the previous study such as suspending only a portion of the load with elastic bands, using a slow (2:2) cadence, and pausing the weight at the bottom and top of each lift to ensure control (6). The present study attempted to replicate a more realistic movement by having subjects press the weight as fast as possible while maintaining control and not enforcing a pause at the top and bottom of each repetition. The present study also used a flexible barbell instead of a standard metal barbell to increase instability. We believe that these differences in methodology allowed a generation of greater biceps and left middle deltoid activation in the unstable load, although the unstable load was significantly lighter.

Practical Applications

The stabilizing muscles were either just as active or more active with a lighter unstable load than a heavier stable load. The unstable load also took longer to press (increased duration of the concentric and eccentric phases) than the stable load. The results of this study suggest that using an unstable load can increase the activation of the stabilizing muscles more so than using a heavier stable load. However, this study also suggests that an appropriate setup (flexible barbell, all weights suspended) and a quick pressing cadence are crucial to activate stabilizing musculature. An unstable load will also increase the time under tension, which may lead to different training effects. In summary, unstable load training may be an effective accessory exercise to increase the time under tension and maintain a higher level of muscle activation across primary and stabilizing muscles with a lighter load.


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resistance training; upper extremity; power lifting

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