Researchers at the University of Kentucky evaluated electromyographic (EMG) levels of 4 exercises in total hip patients between 7 and 32 weeks post-operative. Dr. Cale Jacobs and colleagues were interested to see if there was a progression of muscle activation to guide clinical decisions about hip abduction exercise prescription. They published their findings in the Journal of Arthroplasty. The 4 exercises included 2 non-weight bearing and 2 weight-bearing exercises:

• Standing hip abduction with cuff weight at 1% of bodyweight, lifted to 30 degrees
• Sidelying hip abduction with cuff weight at 0.5% of bodyweight, lifted to 50% of leg length
• Standing hip abduction with uninvolved leg to 30 degrees
• Lateral walk with Thera-Band resistance band attached with extremity straps around ankle

Researchers used a level of Thera-Band resistance that created the same torque as the standing hip abduction exercise (1% body weight) with stretch on the band at 50% elongation. The researchers used this chart of Thera-Band elastic resistance pull forces to prescribe the appropriate color of band.

Both the sidelying abduction and weight-bearing standing abduction with the other leg produced an average of 67% maximum voluntary isometric contraction (MVIC).  The Thera-Band resisted lateral walk exercise produced an average of 63% MVIC and the non-weight bearing standing abduction produced 58%. There was no significant difference in activation between exercises, indicating no clear order of exercise progression.  The researchers suggested these exercises can be used interchangeably, particularly when balance impairment or postural position may be an issue.

In conclusion, non-weight bearing exercises involving open-chain hip abduction provide no additional benefit of gluteus medius activation compared to more functional closed-chain exercises. Thera-Band loop resisted walking provides moderate (>60% MVIC) of gluteus medius activation in post-operative total hip replacement patients.

REFERENCE: Jacobs CA, et al. Electromyographic analysis of hip abductor exercises performed by a sample of total hip arthroplasty patients. J Arthroplasty. 2009 Oct;24(7):1130-6. Epub 2008 Aug 30.

Disclosure: Thera-Band Academy provided the bands used in this study, but did not provide monetary funding.

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Roll Out

Physical therapy researchers quantified the electromyographic (EMG) activity of the abdominals, latissimus dorsi, lower back, and quadriceps muscles during eight “core” exercises on the exercise ball in 18 healthy subjects. They reported their findings in the Journal of Orthopedic and Sports Physical Therapy.

They found that the upper and lower rectus abdominus muscle were most activated during the roll-out (63% and 53% of maximum, respectively), and pike exercises (47% and 55%), while the internal and external obliques were most active during the pike (84% and 56% respectively) and skier exercises (73% and 47%). Not surprisingly, the lumbar paravertebral muscles, latissimus dorsi, and rectus femoris only produced low- to-moderate activity (less than 40% maximal activation) in all exercises.

Pike

Skier

The authors concluded that the roll-out and pike exercises on a Thera-Band exercise ball were the most effective exercises in activating the abdominals while minimizing low back and rectus femoris activation. In addition, these exercises produced more activation of the core muscles than a traditional crunch or sit-up.

REFERENCE: Escamilla R et al. Core muscle activation during swiss ball and traditional abdominal exercises. J Orthop Sports Phys Ther. 2010 May;40(5):265-76.

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As the exercise ball increased in popularity, trainers and therapists began performing traditional resistance training exercises such as the bench press with dumbbells on an exercise ball. Theoretically, the unstable surface would increase activation of both trunk and shoulder muscles. In addition to the increased risk of injuries associated with traditional resistance training on an exercise ball, researchers began to show that using an exercise ball for extremity exercises was not as effective as once thought.

Canadian researchers (Andersen & Behm 2002) were among the first to show that performing a chest press on an exercise ball significantly decreased force output by 60%. Interestingly, the muscle activation levels remained the same between surface types despite a decrease in force output, suggesting resistance training on an unstable surface may produce less efficient muscle contractions. Subsequently, other researchers (Marshall & Murphy 2006) reported an increase in EMG levels of the deltoid and abdominals during a chest press on an exercise ball compared to a flat bench.  These conflicting results may be due to the fact that these 2 studies used 75% 1RM and 60% 1RM resistances, respectively.

Researchers at Cal State Fullerton wanted to investigate the EMG levels of the arms and abdominals while performing a chest press and shoulder press on an exercise ball at 80% 1RM. They recorded EMG levels of the anterior deltoid, pectoralis major, and rectus abdominus during 4 conditions: chest press on and off an exercise ball, and overhead shoulder press on and off an exercise ball. Subjects performed 3 repetitions at 80% of their previously determined maximal strength using dumbbells.  The resistance levels were the same between the stable and unstable conditions for each type of lift.  Muscle activation was normalized to their maximal EMG levels during the maximal strength testing. The researchers found no significant difference between the conditions: both the exercise ball and stable surfaces produced similar muscle activation levels at the same resistance levels.

It’s important to remember that this study used higher loads than previous studies. The authors hypothesized that EMG levels would be similar between stable and unstable conditions because of the stabilization of the ball caused by its deformity as a result of the subject’s body weight and resistance load. This suggests the exercise ball surface may have become more stable, comparable to a bench surface. Because the researchers did not control for the deformity of the exercise ball, more research is needed to determine if this was a reason for a lack of difference between conditions.

Based on the results of this study, the use of an exercise ball does not increase or decrease EMG of the shoulder or trunk muscles during the chest or overhead press at high loads.

Reference: Uribe BP, et al. Muscle activation when performing the chest press and shoulder press on a stable bench vs. a Swiss ball. J Strength Cond Res. 2010 Apr;24(4):1028-33.

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Researchers at the Mayo clinic in Rochester Minnesota performed a descriptive electromyographic (EMG) study to quantify the activation of the gluteus medius during different closed-chain weight-bearing exercises. Young, healthy subjects performed 5 exercises: bilateral stance, unilateral stance, unilateral stance on foam, single leg squat, and single leg squat on foam.

The researchers found that single leg stance exercises produced significantly greater EMG activity compared to bilateral stance. In addition, single leg squats produced significantly more EMG activity than single leg stance. While not statistically significant, performing single-leg exercises on a foam pad did produce more EMG activity of the gluteus medius than a stable surface.

Below are the estimated peak muscle activation levels for each exercise. Unfortunately, the authors did not provide the actual % maximal contraction values (MVICs) of each exercise, instead providing only a graphical summary.

• Double limb stance ~ 4% MVIC
• Single leg stance ~ 20% MVIC
• Single leg stance on foam ~ 26% MVIC
• Single leg squat ~ 50% MVIC
• Single leg squat on foam ~ 60%

In conclusion, the authors suggested that in order to challenge the gluteus medius, weight bearing exercises should be performed unilaterally. In addition, single leg stance on foam pads such as Thera-Band Stability Trainers may provide additional demand on the gluteus medius muscle. This paper also outlined a clinical progression of activation of the gluteus medius during closed-chain exercises. Further research on patients with weakness or inhibition of the gluteus should validate this progression in the clinical setting.

Reference: Krause DA, et al. Electromyographic analysis of the gluteus medius in five weight-bearing exercises. J Strength Cond Res. 2009 Dec;23(9):2689-94.

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Canadian researchers Anderson and Behm have shown that instability training can provide similar muscle activation levels at lower resistance levels compared to training on stable surfaces. Unfortunately, however, unstable surface training also results in a decrease force output of prime movers.
California State University researchers set out to evaluate the electromyographic (EMG) activity of prime movers and core stabilizers during a seated overhead press exercise. 30 healthy, resistance-trained subjects performed an overhead press with dumbbells and barbells while sitting on a stable surface and on an exercise ball. The investigators were interested in looking for differences in EMG levels between the stable load (barbell) and unstable load (dumbbell), on both stable (bench) and unstable surfaces (exercise ball).  Subjects used a 10RM resistance (maximum amount of weight lifted for 10 repetitions) for each of the 4 conditions; thus, each condition had a slightly different absolute load.

As expected, the loads used in each condition decreased with decreasing stability. Most resistance was used with the barbell on the bench (stable load on stable surface), followed by the barbell on the exercise ball (stable on unstable), utilizing 89% of the stable on stable condition. The dumbbell on bench (unstable on stable) utilized 86%, while the dumbbell on the ball (unstable on unstable) used 79% of the stable conditions. The researchers suggested that exercising on unstable surfaces reduces force output approximately 15%.

The upper extremity muscle EMG also decreased as the stability decreased, while the erector spinae muscles had great activation on the exercise ball compared to the stable bench. Interestingly, exercises performed on a stable surface required more abdominal activation, indicating that the abdominals require greater activation with greater loads during an overhead lift.

The authors concluded that exercise balls should not be used during the overhead press exercise when the goal is to increase muscle activation of the arms or abdominals, even with reduced loads. However, performing barbell overhead press on an exercise ball does result in increased activation of the low back muscles.

These results are relatively consistent with other studies that suggest unstable surfaces such as exercise balls should not be used to increase strength of extremities using traditional resistance training exercises. While other studies have shown increased activation of the abdominals with resistance training, these authors did not evaluate the seated overhead press exercise; therefore, preferential activation of certain core muscles may be specific to the exercise performed on the exercise ball.  The Thera-Band exercise ball remains a valuable tool for stabilization training, although its role in core strengthening during traditional resistance exercise remains questionable.

REFERENCE: Kohler JM, Flanagan SP, Whiting WC. Muscle activation patterns while lifting stable and unstable loads on stable and unstable surfaces. J Strength Cond Res. 2010 Feb;24(2):313-21.

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