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Original Research

Effects of an Unstable Load on Force and Muscle Activation During a Parallel Back Squat

Lawrence, Michael A.; Carlson, Lara A.

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Journal of Strength and Conditioning Research: October 2015 - Volume 29 - Issue 10 - p 2949-2953
doi: 10.1519/JSC.0000000000000955
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Unstable surface training (UST) has been gaining popularity in the strength and conditioning field, with the assumption that performing resistance exercises in an unstable manner will increase activation of stabilizing muscles (specifically trunk musculature). Proponents of UST claim that these methods will improve balance, kinesthetic awareness, proprioception, and gradation of force (8). Data are available supporting greater trunk muscle activation using UST (3); however, several studies have shown decreased force production of the prime movers during UST (5,18), negating some of the strengthening benefits of the exercise.

Previous research measuring muscle activity during UST has produced mixed results. Exercises performed on unstable surfaces have been found to produce greater activity in primary and stabilizing muscles during a push-up (15,17) and an unstable leg extension (5), no change in any muscle activity during a chest press (12), and decreased trunk muscle activity during a deadlift (9). Unstable surface training has also been shown to decrease force output in the unstable condition during pressing (1), deadlifting (9), and dynamic leg extensions (5). A successful unstable training apparatus would increase both muscle activity and force output, hopefully leading to enhanced athletic performance. However, it was found that UST produced less performance improvements than those athletes training on a stable surface (10). Furthermore, the deficits in force output during UST are one of the major reasons UST is not recommended for use in athletic populations (6–7).

An alternative method to UST is unstable load training (ULT). What differentiates UST from ULT is where and how the instability is applied. In UST, the instability is between the body and an unstable training surface (i.e., squatting on a BOSU), whereas in ULT, the instability lies between an unstable load and the body. Recently, several modes have been used for ULT in the strength and conditioning community. Two common techniques to create the unstable load are suspending the load from elastic bands or using a flexible barbell such as the Tsunami Barbell (West Columbia, SC, USA). To the best of our knowledge, there are no published studies that have examined ULT.

Unstable load training may be more applicable than UST for field and court athletes who are routinely dealing with dynamic forces from either their own body or other players that must be countered by their trunk's stabilizing musculature. Unstable load training mimics this by applying the instability to the trunk instead of the bottom of the feet. The purpose of this investigation was to determine if an unstable load would increase activity of the stabilizing and prime muscles, while not sacrificing force output. We hypothesized that (a) squatting with an unstable load will increase muscle activity compared with squatting with a stable load and (b) vertical ground reaction forces (GRFs) will not be different between stable and unstable loads.


Experimental Approach to the Problem

Fifteen resistance-trained volunteers back squatted 60% of their single repetition maximum (1RM) parallel back squat under 2 loading conditions (stable and unstable). The 60% of 1RM is comparable with loads tested in studies where subjects squatted on unstable surfaces (16). Vertical GRFs of both limbs and muscle activity of the right lower limb and trunk were recorded.


Fifteen resistance-trained males (age: 24.2 ± 3.4 years, mass: 83.4 ± 18.7 kg, height: 1.7 ± 0.1 m, 8.1 ± 4.3 years lifting experience, and parallel back squat 1RM was 131.4 ± 21.4 kg) volunteered for this study. Individuals with current lower extremity injuries, injuries that prevented them from exercise in the past 6 months, or those who have had lower extremity surgeries were excluded from participation. Only 1 subject reported using ULT before participation in this investigation. This study was approved by the institutional review board, and all participants gave written informed consent (IRB 20130717LAWRM).


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 parallel back squat according to the National Strength and Conditioning Association's guidelines for maximal strength testing (4). To ensure proper squat depth, subjects were instructed to squat down to a box so the crease at the hip was below the top of the knee. Box height was set so each subject achieved a parallel squat. Subjects used a “touch-and-go” method for each squat attempt. The second testing session occurred at least 7 days after 1RM squat test.

A cluster of 3 reflective markers were placed over the subject's sacrum to determine the top and bottom position of each squat. The motion of the markers was tracked using 8 Oqus Series-3 cameras (Qualisys AB, Gothenburg, Sweden) set at 150 Hz. Eight 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 the muscle activation of the rectus femoris, vastus lateralis, vastus medialis, biceps femoris, soleus, rectus abdominis, external oblique, and erector spinae muscles on the right side of the body. Electromyography sensors were placed according to the SENIAM recommendations (11,14) and set to collect at 1,500 Hz. Each subject was then allowed to take their own preferred squat stance, with each foot on separate force plates (AMTI, Watertown, MA, USA), which were set at 1,500 Hz. Foot placement was marked to maintain consistency for each subject.

Subjects performed 2 warm-up sets of 10 repetitions of parallel back squat at 30 and 45% of 1RM. Sixty percent of the 1RM (16) was loaded for 2 randomized conditions (stable or unstable). Subjects performed 3 sets of 10 repetitions with each condition. Repetitions 3–8 of the third set were used for analysis. All squats were performed with a 1:1 cadence (1 second eccentric and 1 second concentric) followed by a 5-minute rest period. To ensure consistent squat depth, subjects were instructed to squat down to the box in a touch-and-go fashion. For the stable condition, weights were placed on the bar normally. In the unstable condition, weights were suspended from “mini” elastic resistance bands (EliteFTS, London, OH, USA). The bands were “quadruple looped” through the weights and hung on the bar (Figure 1). As a precautionary measure, the load on each band was limited to 50 lbs. If more than 50 lbs was needed to reach the 60% load, then additional bands were used until the correct load was achieved.

Figure 1:
Weights suspended by elastic bands.

All data analysis was completed with Visual 3D (C-Motion, Germantown, MD, USA). Vertical pelvis motion was smoothed with a second-order low-pass Butterworth filter with a 6 Hz cutoff (13) and used to define each squat cycle. A second-order Butterworth band pass (10–200 Hz) filter (19) was applied to the EMG signals, which were then rectified, smoothed with a moving root mean square filter (100 milliseconds) (9), and integrated using the trapezoid rule. Peak vertical GRFs were found for the concentric portion of each squat.

Statistical Analyses

Data were compared using a multivariate analysis of variance (MANOVA; condition [stable vs. unstable] × repetition [6 repetitions] × contraction [concentric vs. eccentric]) to determine differences in muscle activity and an MANOVA (condition [stable vs. unstable] × repetition [6 repetitions] × limb [right vs. left]) to determine differences in peak vertical GRFs. Statistical significance was set at the 2-tailed p ≤ 0.05 level of confidence. Statistical analyses were performed using the SPSS IBM SPSS Statistics 21 (IBM Corp., Somers, NY, USA) software package.


The unstable condition produced significantly greater integrated EMG in the external oblique (p < 0.01), rectus abdominis (p = 0.05), and soleus (p < 0.01) muscles (Table 1). The concentric portion of the squat produced significantly greater (p ≤ 0.05) integrated EMG for every muscle except the external oblique. There were no differences between trials or any interaction effects.

Table 1:
Integrated muscle activity (mV·s), mean ± SD.*

There were no limb differences in peak vertical GRF (p = 0.75). Peak vertical GRF was significantly (p = 0.03) greater when squatting with the normally loaded barbell; however, the decrease in force during the unstable condition was only 3.9% (Table 2).

Table 2:
Peak vertical ground reaction forces (N), mean ± SD.*


The objective of this study was to determine if using an unstable load during a parallel back squat could increase muscle activation and force production vs. squatting with a stable load. Minimal research to date has provided information regarding the efficacy of ULT. We hypothesized that (a) squatting with an unstable load would result in greater muscle activity than squatting with a stable load and (b) the unstable load would increase peak vertical GRF. We found squatting with an unstable load significantly increased muscle activity in the rectus abdominis, external obliques, and soleus muscles. However, squatting with an unstable load also resulted in a significant decrease in peak vertical GRF during the concentric portion of the lift. Further analysis showed no differences in either peak vertical GRF or muscle activation between each of the 6 trials.

The present findings from our study showed that ULT increased muscle activation in the trunk (rectus abdominis [85.6% increase], external oblique [13.1% increase], and soleus [72.2% increase]) muscles. Although ULT has not previously been investigated, our findings are consistent with some unstable training methods. Wahl and Behm reported an increase in only lower abdominal and soleus muscle activity during an isometric squat on a Swiss ball and wobble board (20). Anderson and Behm also found increases in abdominal stabilizers (18.6%) and soleus muscle (58.5%) activity while squatting on an unstable surface with light loads (unloaded, an empty bar, and 60% of bodyweight (3)). However, there was also a decrease in vastus lateralis activity on the unstable surface when compared with squatting on a stable surface (3). In this study, squatting with an unstable load did not result in decreased activation of limb musculature. This difference in limb muscle activation could be because the loads were so different (60% of bodyweight vs. 60% of 1RM back squat), the source of the instability (unstable surface vs. unstable load), or a combination of the 2. Further analysis showed no differences in muscle activation across the 6 repetitions or any interaction effects between repetitions and condition. This suggests that squatting with an unstable load can provide muscular stimulus with the same consistency as squatting with a stable load when performing 6 or fewer repetitions. Although muscle activation was greater in the concentric phase than the eccentric phase of the squat (except for no difference in the external oblique), there was no phase by condition (stable vs. unstable) interaction. Based on the muscle activity results, squatting with an unstable load may be more advantageous to increase trunk stabilizer strength than UST.

As it is possible for force output to decrease when antagonistic muscle activity increases, muscle activity alone should not be used to determine the effectiveness of an unstable training program (2). As is common with UST (1,5,9), we observed a decrease in vertical GRF production when using an unstable load. However, there was only a 3.9% decrease, which is much lower than previously found with UST. With UST, decreases of 45.6% have been found when squatting on a Bosu ball (18); deadlifting on a Bosu ball and T-Bow have also produced larger reductions of vertical GRF (34.19 and 8.80%, respectively) (9). Further analysis showed that peak vertical GRF did not significantly vary between repetitions and limbs. Although the force reduction is significant, a reduction under 5% may be more acceptable to athletes and coaches than the much larger force reductions found with UST, especially if the goal of that particular training session is to increase trunk muscular strength and not to generate the greatest force output possible.

This is the first investigation to compare force production and muscle recruitment during ULT with a lower extremity exercise. Several factors such as load, elasticity of the bands suspending the load, tempo of the movement, or rigidity/flexibility of the bar may have not been ideal to produce the best results from the unstable load. Also, the squat box may have provided some proprioceptive feedback to assist with stabilization. Another limitation is that only 1 participant in this study had any previous experience with ULT. Therefore, the results of this study should only be applied to those newly introduced to ULT. Future investigations should endeavor to have a familiarization period, although it is unknown how long it will take for subjects to become familiarized with an unstable load. There is anecdotal evidence for the effectiveness of ULT; however, research is needed to confirm the effectiveness of ULT as a performance-enhancing or rehabilitation tool.

Practical Applications

We found that squatting with an unstable load increased muscle activation of the rectus abdominis, external obliques, and soleus muscles without causing a decrease in the activation of the limb musculature. We also found that squatting with an unstable load caused a small (3.9%) decrease in peak vertical GRF. Our findings suggest that suspending the weights from the bar with elastic bands may increase trunk stabilizing muscle strength more than free weights but may not be ideal when training to produce large forces.


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ground reaction force; resistance training; lower extremity

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