An excerpt from the VeloPress book Crosstraining for Endurance Athletes: Building Stability, Balance, and Strength by Raul Giusado

DEVELOPING CORE STRENGTH
Stability of the trunk is dependent on core strength. Popular exercises such as crunches are just one of many ways to strengthen the abdominal and back musculature. However, such exercises only recruit very specific musculature. It's imperative to go one step further by performing movements that simultaneously strengthen multiple core stabilizers in order to maximize trunk strength and stability.

We want to strengthen the muscle groups that stabilize our skeletal structure. For example, the muscles in the thoracic area determine your posture in each sport and help connect your upper and lower body. Trunk stabilizers don't have the ability to drive you forward, but they are the base from which your arms and legs work. We need this platform in order to maintain proper running and cycling posture. In developed swimming movements, especially, it can be even more important. Your torso can actually contribute force through rotation as the direction of force from arms and legs oppose each other. Consider how a fish moves when it swims. Its fins and tail don't move independently from the rest of its body. The force that a fish creates is through longitudinal flexion of its whole body. Similarly, a swimmer with a strong core will be more efficient and able to produce more force.

As we develop core strength and stability, we can increase body awareness of individual and small groups of muscles. This awareness is the first step in improving imbalances in posture and correcting form issues. For instance, as we discussed earlier, many of us have poor posture that can negatively impact our endurance sport performance. The first step to improving your posture is being able to contract individual back muscles effectively by performing specific exercises to strengthen them. As you develop greater body awareness and strength, you will be better able to perform movements that require recruitment of multiple trunk stabilizers as you work postural muscles. In other words, you'll train your body to use these muscles simultaneously in sport-specific movements while maximizing your time and energy by fatiguing primary movers and stabilizers at the same time.

Forward propulsion in any endurance sport is limited by your weakest muscular link. For example, even if you have the legs of a power lifter, you must have the strength in your upper body to control the force these muscles can produce. When you ride a bike, gravity dictates that all down force generated is limited to your body weight and the opposing action of pulling up on the opposite crank arm. However, you can increase down force by pulling up on the handlebar, thus opposing the tendency for your body to rise as you extend your legs. But, since your legs are attached at your hips, the stable platform your arms create must be extended to your hips and legs through your trunk. There are similar benefits of having a strong and stable trunk in the sports of running and swimming.

When you become fatigued running, swimming, or cycling, your form falls apart. It's not just because of tired legs but also tired arms and a tired back. Having a strong torso helps hold your form together in the latter stages of any endurance effort.

A variety of techniques have developed to strengthen the core and develop the movements that will enhance performance in endurance sports such as running, cycling, and swimming. The basic concept is to apply a factor of instability to exercises we already do, as well as adding specific exercises to develop trunk strength and stability.

For instance, instead of sitting on a bench and performing a dumbbell arm curl, perform the same exercise while standing on one leg. Now your trunk musculature is forced to stabilize your pelvis and spine and help keep the weight centered from side to side and front to back. This is the most basic approach. More advanced core training involves standing, kneeling, or sitting on unstable surfaces such as a stability ball, balance board, or foam roller.

If you've ever seen those big colored balls at the gym, those are stability balls, and hundreds of exercises involve their use. Consider a normal dumbbell bench press on a standard weight lifting bench. Now, replace the bench with a stability ball, with the ball resting between your shoulder blades. You perform the normal bench-pressing movement, but now your core muscles are acting to stabilize and balance your body to keep the weight in the correct center as you perform the exercise.

TRANSVERSE ABDOMINIS
For the majority of people, the transverse abdominis (TVA) is the most neglected and detrained of the trunk musculature. As the inner abdominal wall, it can also be described as the body's "natural weight belt." This muscle connects to the thoracolumbar fascia of the back to form a ring around the core at the navel. Unlike all of the other muscles described in this book, this muscle doesn't attach to any bones. As a result, it's not designed to bring two bones together as your biceps brings your lower arm closer to your upper arm. In other words, the TVA is purely a stabilizer and not responsible for movement.

Because your center of mass is below and behind your navel, this muscle plays a huge role in all aspects of stability. The human torso is capable of so much movement in this area, and endurance athletes need to take advantage of all the stability help they can get. The more we can control spinal forward flexion, extension, lateral flexion, and rotation during movement, the better we can control our mass. When we control our mass better, we allow primary movers to spend more energy producing force and less energy stabilizing. The end result is-you guessed it-an increase in efficiency and performance.

Merely learning how to activate the transverse abdominis can improve trunk stability and balance immediately. The goal is that, eventually, the body will become accustomed to contracting this "natural weight belt" prior to performing any movement. As a result, you will be better able to stabilize your lower back, protecting it from injury and also providing a stronger platform for your arms and legs.

Pulling the belly button in toward the spine can activate the transverse abdominis. This is not to be confused with sucking in your gut. Activation of the TVA should result in a narrowing around the entire waist and a downward slope from the sternum or breastbone to the navel. Through training and awareness, everyone can learn to coordinate the contraction of the TVA with every movement. At first, drawing the navel in before performing an exercise might feel awkward and difficult, but soon it will become both comfortable and habitual. It's possible to continue to breathe normally and deeply in spite of drawing the navel inward. However, deep breaths while activating the TVA require more expansion of the midthoracic region of the chest. This exercise takes a little practice in order to keep from elevating your shoulders when you take a full breath, but it will feel increasingly more natural.

LEARNING HOW TO ACTIVATE THE TVA
One of the easiest ways to learn how to activate the TVA is through the recruitment of the lower abdominal muscles. While lying faceup, with knees bent and feet flat on the floor, place a pair of folded socks or something similar under the small of your back. Try to relax your entire body, especially your legs and shoulders. Now draw in your navel, keeping one hand around your waist and the other on your belly. You should feel your midsection narrow and a downward slope from your chest to your hips. Alternate lifting each bent leg off the ground, without losing pressure on the pair of socks. Be sure not to move your pelvis or affect the downward slope and narrowed waist.

If your lower abdominals or TVA are very weak, you'll find it difficult to keep your pelvis from moving when you lift your foot.

If this exercise seems or becomes very easy, try it with a straight leg or with both legs bent simultaneously. Stellar lower abdominal strength and transverse activation would be to lower both straight legs at the same time from ninety degrees to the floor and up again without allowing your lower back to arch.

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