Superiority of Isometric Core Training

By Jen Weir, CSCS, CPT

Introduction

Of all the body segments, the core is ultimately the most important when it comes to achieving and maintaining a certain level of strength. It doesn’t matter if your client is concerned with sports performance or merely getting through the day pain free, conditioned core musculature is the place to start.

In sports and activities of daily living, the majority of actions require that power is generated in the hips and transmitted through the core to the upper body (1). If adequate core stability is lacking, energy is lost, along with any power that’s been generated, and the movement becomes less productive (1). This is detrimental to the baseball player swinging a bat as well as the mom juggling to get kids into car seats.

Science has proven time and again, how improved core stability has the ability to improve performance and reduce injury risk. Research by McGill et al. found that the core musculature assisted hip function in strongman competitors who lacked the hip strength to otherwise complete the task (5). Abt et al. reported that cyclists with improved core stability and endurance could maintain better alignment of the lower extremities, which may reduce the risk of injury (6).

In addition to impaired physical performance, clients who present with weak core musculature also likely display inefficient movement patterns as well as some level of back pain or discomfort (1). This is likely due to the fact that weak, impaired or imbalanced muscles can result in movement compensation strategies as well as damaged structural tissues (such as tendons and joints) that may ultimately lead to injury (2, 4). Back issues originate from inadequate spinal stability, adding increased stress to the spine itself.

Problems Observed in Conventional Core Training

Conventional core training programs are designed to include, or be made up entirely, of exercises that involve repeated bouts of spinal flexion as well as muscle isolation. This is both dangerous and ineffective. Repeatedly flexing the spine has the potential to increase injury risk, including intervertebral disc herniation (3). Isolating specific core muscles does not enhance stability but rather generally leads in an imbalance about the core, resulting in reduced stability (7).

Another problem seen in traditional core training is incorrect methods of working the musculature. It’s common for trainers to design programs where the core initiates strength and power. However, power does not originate in the core but instead from the lower extremities.

The core muscles primarily function to transmit power as well as control, prevent or resist movement, thereby acting as stabilizers not initiators (1).

Lastly, bracing the abdominal muscles should be encouraged in lieu of abdominal hollowing, which is a common training cue. Drawing the navel toward the spine reduces core stability, causing the spine to fail at lower applied loads (8, 9). In fact, abdominal hollowing resulted in 32 percent less stability than abdominal bracing (10). Abdominal bracing is also encouraged due to the fact that it creates intra-abdominal pressure, enhancing spinal stability.

Advantage of Isometric Core Training

Before we determine why isometric core training is superior to its dynamic counterpart, let’s take a minute to examine the elements of core training. Core strength is defined as spinal muscular control to maintain functional stability (11). Core stability, on the other hand, has been defined by Panjabi as “the capacity of the stabilizing systems to maintain the intervertebral neutral zones within physiological limits” (12).

Core stability can be broken down into three distinct variables: intra-abdominal pressure, spinal compressive forces, and hip and trunk muscle stiffness (13).

Isometric core exercise training has been shown to increase both strength and stability and has recently been shown to do so more effectively than conventional dynamic core training (14). Results by Lee and McGill suggest that the isometric exercise approach was superior in enhancing torso stiffness over a six-week training period to dynamic training. It was found that both exercise naïve and experienced subjects responded similarly to the training. Researchers believed the increases observed in stiffness and strength resulted from the increased time under tension compared to that implemented during dynamic movements.

Other research, performed by Olson, used electromyography, EMG, to compare the activation of deep core muscles of different Pilates and yoga exercises with that of conventional abdominal exercises, such as crunches. The isometric-style exercises produced EMG values in the deep core muscles 5% to 54% higher than the crunch (17).

Aside from the strength and stiffness improvements recorded with isometric training, it is also a safer alternative for your clients than repeated bouts of spinal flexion. Isometric core exercises allow clients to develop core stiffness attributes, such as the ability of the spine to bear greater loads (15) and express greater athleticism (16), while minimizing the imposed loads on the spine (14). Essentially, your clients will experience greater gains with lower risks.

Problems Observed in Conventional Core Training

Implementing isometric core training into your client’s existing program is relatively simple and can be achieved using one or more of the following techniques: core-specific exercises, heavy resistance exercise, unilateral training and unstable training. There are:

Core-Specific Exercises

Core exercises are those specifically intended to target the core muscles with the intent of enhancing spinal stability, the transfer of torque, and angular velocity from the lower to upper extremities (2). In the dynamic sense, we think of sit-ups, while planks and similar exercises are used in an isometric fashion. In an effort to prevent imbalances about the core, a minimum of a one to one ratio should be performed between the four basic trunk movement patterns (see examples in photos 1 through 4), unless an obvious weakness exists (2).

Pictured are example exercises that target each of the four trunk movements.

McGill recommends each isometric muscle action be held for a maximum of 10 seconds rather than the 30 to 60-second holds popular in current programs in order to build endurance without muscles cramping (1). Because the core musculature has a high percentage of slow-twitch muscle fibers,

it responds well to multiple sets of high repetitions (2). Begin with three to five sets of at least 10 repetitions, basing your exercise prescription on your client’s needs, abilities and fitness level.

Numerous isometric core exercises exist; the ones you choose to use should be decided on an individual basis with each client while also taking into consideration the equipment you have available. Regardless of the isometric core exercises prescribed to your client, training of core muscles should include progressive changes in the volume, intensity and variety of program over time. Photos 5 and 6 show example progressions of the plank and side plank exercises.

image 4-6 Gradual progression of exercise intensity is critical in achieving enhanced strength and stability in the core. Pictured above are progressions of the plank and side- plank; once your client has the basic position mastered, add a more challenging concept to the exercise. As seen in the plank position, move from the floor to a stability ball. From there, have him or her reduce the number of contact points by lifting a foot or increase the challenge by performing a “stir the pot” movement with the elbows on the ball.

Unilateral Training

While core-specific exercises are an effective means for improving core conditioning, research has shown

that traditional resistance training exercises performed with heavy weights to be an excellent training option for developing core strength and stability as well as maximizing overall strength (18, 19). Heavy weight training has displayed significant core muscle activity; squats and deadlifts using 80% of one-repetition maximum produced 34% to 70% greater activity in the back muscles than unstable core-specific exercises (20).

Heavy resistance training can be used by clients who present no contraindications to such intense training. Using this method to improve core strength and stability requires no special programming alterations; conventional periodization should be used to keep weight, volume and intensity applicable to each individual client’s needs, progressing when appropriate. To ensure maximal spinal stability

Unstable Training

Whether your client prefers bilateral or unilateral exercises, the core can be further challenged with the use of unstable surfaces such as BOSU and stability balls. Increasing the level of instability when lifting weights causes an increase in core muscle activity in order to maintain technique and balance (21). Behm et al. found instability generated 27.9% greater activation of the lower-abdominal stabilizer musculature with six different trunk exercises and all of the trunk stabilizer muscles displayed 37.7% to 54.3% during an unstable chest press (18).

It is important to note that unstable training may not be the most productive method for every client. While it does allow for high trunk muscle activity without the use of heavy weights (22), Wahl and Behm demonstrated that highly resistance trained athletes did not experience significantly greater muscle activation when exercises were performed on unstable devices (23). Therefore, unstable training may be more suited for the sedentary or moderately trained clients who require a lower training stimulus.

Conclusion

Isometric core training has been shown to be more effective for improving core strength and stability, generating greater muscular activity, and being more spine-friendly than conventional dynamic core exercises. This method of training is appropriate for all levels clients, the specific core-training technique(s) used for each client should depend upon his or her individual needs, goals, fitness level and ability.


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