Throughout my life, I've probably seen five people who train their core correctly. Not because most people choose bad exercises, but because they take for granted three key concepts regarding this particular muscle group. Add a little critical thought to these concepts and you'll quickly realize what real ab training is all about.
Function of Abdominal Muscles
An exercise doesn't have to be considered functional to increase muscle size or strength. As a matter of fact, "functional" exercises prioritize proper movement integration and neuromuscular coordination, not size or strength. It is still important, however, to consider what the target muscle is actually in charge of doing before training it; and I see this disconnect more often in abdominal training than in any other form of exercise.
The major muscles of the midsection are great at translating and countering force. Their primary function is to maintain rigid stability–not to produce forceful movement. The force generated during a swing, for example, comes from the hips and is translated through the core by way of its rigid stability. Every movement we perform while standing or kneeling requires core stability, or else our spine would snap in half. Our core muscles provide this stability by compressing at each vertebral joint, most often in an effort to–get this–avoid flexion or extension at the spine from outside forces. Once we see the spine in this light (acting as a bunch of small joints rather than one large one), we can shift our focus to primary movements instead of incidental ones.
For example, when training abs with aesthetic-oriented movements, leg swings and arm movements should be thought of as byproducts of flexing or extending at the spine. Pelvic tilt should create leg movement, not vice versa. During functional core work, these limb movements are intended to create tension that the core then counters through compression and rigid stability. In either case, arm and leg movements are secondary to our focus of what's happening at the spine and pelvis.
Takeaway: Plan functional movements that challenge core stability by countering movements created by the limbs. If aesthetics are your goal, don't allow momentum from the limbs to control the reps. Instead, focus on movement at the spine and pelvis with any limb movement being the result of spinal flexion/extension or pelvic tilt. Try this core exercise to build functional strength.
Angle of Tension vs. Range of Motion
When doing ab exercises, consider that your center of gravity may gradually change as the rep progresses. This can be a good thing, allowing for a better tension curve throughout the rep, or a bad thing, allowing your abs to completely disengage at one point or another during the rep. Swiss Ball Crunches are a great exercise to demonstrate this concept. You can either use gravity to make the rep more effective–or neglect gravity and allow the rep to be rendered ineffective.
When moving in to the stretch position of a Swiss Ball Crunch, allow your back to conform to the ball. Then crunch up until you reach a point of full spinal flexion, but not beyond. This keeps constant tension on the rectus abdominus throughout its full range of motion. You will quickly notice how fast the burn sets in compared to sitting all the way up or not going all the way down. Also, notice that full spinal flexion happens much sooner than the point at which you would be sitting upright on the ball. If you were to sit up completely, you would shift your center of gravity and might no longer be working through the best angle of tension. Worst case scenario, and this happens all the time, trainees come completely upright on the ball, allowing their rectus abdominus to actually disengage.
Many people never reach the full stretch position because it's inherently the most difficult part of the rep. It's also where the angle of tension is most conducive to mechanically loading the abs during this particular exercise, since gravity's force is acting straight down.
You can also apply this difficult stretch position to oblique work on a roman chair. Many people never come close to the stretch position during side raises and hover around the top of the rep. Some even grab a weight when they could get better adaptations by moving further down into the stretch position without the weight. As always, notice how the most difficult positions are also the most effective.
Because we are normally fighting gravity during Crunches and Leg Raises, I'm a big fan of employing the adjustable decline bench. When fighting gravity, we can create more or less mechanical load by changing the slope of the bench (and therefore, the angle of tension.) Proper knee raises from a hanging position may be too difficult for some people, causing them to swing their legs to create momentum, which ruins the exercise more than helps it. On a decline bench, the angle relieves much of the load, allowing trainees to perform reps with more control. (Learn how to perform Decline Sit-Ups). A great progression for Leg Raises and Crunches is to start with a flat surface and gradually progress in difficulty by altering the angle of a bench.
Takeaway: Consider how range of motion affects the angle of tension and vice versa. Often, if you cross your center of gravity, or limit the range to the easiest part of the rep, you may allow your abdominals to disengage. Also, for aesthetic purposes, go slower for fewer reps and full range of motion (especially into the stretch position) before deciding to add weight.
Using Your Limbs as Resistance
Another great way to increase the load on the abs is to simply draw the legs or arms out away from the stomach. The further we extend our limbs, the longer the movement arm becomes, which increases the amount of stress on the abdominals.
This isn't without a drawback or two. First, be sure you perform plenty of mobility work and are properly warmed up so that no strain occurs at the lower back. Every degree of flexion at the spine increases the amount of stretch on the posterior chain. Also, be aware that the stress from extending your limbs may be felt in a specific area of your spine, so if you are predisposed to a herniation or tear, quickly remove spinal flexion exercises from your program. With that said, Side Raises, Leg Raises, Crunches, and even Hyperextensions can all be made more difficult (via increased load on the core muscles) by extending the limbs—as long as momentum does not take over the rep, of course.
Takeaway: A common cue I give trainees is: "drag your lower body (or upper torso) through the rep." This ensures that movement from the limbs doesn't contribute to the exercise. The limbs work as resistance, adding to the mechanical load on the core muscles.
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