Core training

Recently, resistance training while balancing on unstable surfaces such as exercise balls has become popular. Canadian researchers David Behm PhD and colleagues published a comprehensive review and position stand on the use of instability to train the core. Research has shown that exercises performed on unstable surfaces produce higher levels of muscle activation in both the core and extremity muscles compared to stable surfaces. However, force and power outputs are decreased while exercising on unstable surfaces, sometimes up to 70%. Interestingly, increasing levels of core muscle activation can also be achieved with free weight exercises such as squats and Olympic lifts without added instability.

In their article, the authors made several recommendations for both athletes and non-athletic conditioning based on their review of the literature. Dr. Behm et al. noted that athletes should emphasize “higher-intensity ground-based lifts” (such as Olympic lifts, squats and deadlifts) while including resistance exercises with unstable devices, as well as unilateral exercises that provide “transverse stress to the core musculature.” Furthermore, they stated that “unstable exercises should not be used when hypertrophy, absolute strength, or power is the primary training goal.”

Dr. David Behm

Similar recommendations were made for the general population, noting the benefits of both free weight and instability training on promoting spinal stability. It’s important to remember to decrease resistance loads on exercises performed on unstable surfaces.

During rehabilitation, unstable surfaces can be effective at improving muscle reaction time and co-contractions that protect joints. In addition, resistance training on unstable may provide localized muscle endurance training, beneficial for the high proportion of Type I “aerobic, slow-twitch” muscle fibers found in core muscles. Dr. Behm and colleagues recommend core endurance training exercises generally be performed at higher repetitions (greater than 15 per set), while athletes requiring more strength and power perform less than 6 repetitions per set. The authors further noted that unstable surfaces can provide musculoskeletal health benefits such as decreased injury risk and increased spinal stabilization as opposed to using free weights.

In summary, unstable exercise devices such as Thera-Band Exercise Balls and Stability Trainers should be included as part of a well-rounded conditioning program for athletes and non-athletes, but not for increasing primary strength and power. In addition, resistance exercises performed on an unstable surface should be performed at a reduced intensity level because of the reduction in force output.

REFERENCES:
Behm DG, et al. The use of instability to train the core musculature. Appl Physiol Nutr Metab. 2010 Feb;35(1):91-108.

Behm DG, et al. Canadian Society for Exercise Physiology position stand: The use of instability to train the core in athletic and nonathletic conditioning. Appl Physiol Nutr Metab. 2010 Feb;35(1):109-12.

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Dr. Colado has continued his scientific investigations on the efficacy of elastic resistance, this time comparing free weight exercise training to elastic tubing exercises using the Thera-Band Exercise Station. His results are published in the International Journal of Sports Medicine. 42 women were randomly assigned to either a free weight exercise group, a Thera-Band exercise group, or a non-exercising control group. The two exercise groups performed the same 15 upper and lower body exercises using the OMNI perceived exertion scale to standardize intensity between the groups. The intervention lasted eight weeks for two to four sessions per week. Subjects were tested for muscular strength before and after the training program. 

After the eight week program, both exercise groups reported up to 29% improvement in upper and lower body strength, while the control group did not.  There was no significant difference in improvements between the two exercise groups. The researchers concluded, “…the use of elastic tubing in strength training in young adults is effective and can yield results that are equivalent to those obtained with weight machines and free weights.”

One of the most important advantages of the Exercise Station is the fact that there is no need for multiple weights, barbells or dumbbells of varying resistance. The Exercise Station, combined with a Thera-Band® Pro Series SCP Exercise Ball provides all the resistance necessary for an effective strength training program.  It requires no more than a 6’x6’ space and can easily be placed under a bed or against a wall when not in use.

REFERENCE: Colado JC et al. A comparison of elastic tubing and isotonic resistance exercises. International Journal of Sports Medicine. Published Online 2010.

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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 in New Zealand set out to determine if the loss in force output while performing an overhead press exercise on an exercise ball was related to core muscle endurance. They were interested in the ‘specificity vs. generality” of core stability; in other words, is core stability related to overall physical performance, or simply a measure of core muscle endurance?

30 resistance-trained, healthy young males participated in the study. The subjects were tested for their predicted 1RM with an overhead dumbbell press while seated on a stable exercise bench with back support, and again on an exercise ball. Next, subjects were tested for muscular endurance of their core muscles in 3 positions in random order: supine curl-up, back extension on a roman chair, and a side-bridge.

The researchers calculated the percent difference in predicted 1RM strength between the stable and unstable overhead press, described as the “instability strength level” (ISL). 2 sub-groups were analyzed: a higher ISL (>90%) and a lower ISL (<85%). The analysis revealed no relationship between the ISL and core muscle endurance, with the exception of the supine curl-up. The authors concluded that the 3 core muscle tests were not able to predict core stability with an overhead press, noting that these tests were likely task-specific to only the core muscles. Therefore, total body exercises replicating functional movements may be necessary to assess and improve core stability.

This study raised several issues. First, the measures of “core stability” were actually measurements of static structural muscle endurance. It can be argued that the 3 tests are in fact not valid measures of functional stability. Functional stability relates to the ability to efficiently transfer forces through the spine (not just the “core”) and into the extremities for purposeful and coordinated movement. Static trunk muscle endurance arguably is not a pre-requisite of dynamic movement, particularly for an overhead press. True pelvic stability should be quantified by the ability of the pelvic stabilizing muscles to turn on and off in a coordinated manner as suggested by Dr. Stu McGill. It would be interesting to see results of a different exercise such as a bench press or lower body movement.

This study suggests that core training by simply increasing muscle endurance is not specific to “core stability” in relation to functional tasks. This may lead to the conclusion that isolated core strengthening exercise such as sit-ups and side bridges are not effective at providing true functional “core stability;” instead, whole-body functional movements that challenge reflexive core stabilization may be more appropriate.

In conclusion, the decreased force output that subjects experienced during the overhead press while sitting on an exercise ball was not accounted for by structural muscle endurance; it must be related to another structure or functional component. Obviously, more research is needed on this topic.

Keogh JW, et al. Can common measures of core stability distinguish performance in a shoulder pressing task under stable and unstable conditions? J Strength Cond Res. 2010 Feb;24(2):422-9.

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