Ever since news came out that the US military is ditching sit-ups and ab crunches for planks in their training programs, my clients have been asking me if they should be ditching them too. You may also be wondering if you should stop doing crunches. So, here’s a little Ab Strengthening 101 to help you sort fact from fiction and make sound decisions on how to strengthen your core safely.
When considering strength training for any muscle group, we have to ask ourselves what is the primary function of the muscle group because knowing this will tell us the most effective and safe way to train it. In this case, abdominal muscles are a set of muscles layered in the abdomen – rectus abdominis, transverse abdominis and internal and external obliques – that are themselves part of the group known as the core. The rest of the core, erector spinae and multifidi, are muscles that run along both sides of the spinal column. The primary purpose of the core is stabilization.
A strong core provides optimal stabilizing to absorb stresses so that the muscle groups that are designed for movement – such as the hips and shoulders – can function properly and without unnecessary strain. In the abdomen, the deepest muscles, the transverse abdominis, form a box of horizontal fibers that are most responsible for the bracing function. What that tells us is the core muscles as a whole, and the transverse abdominis specifically, are best (and safest) strengthened through a stabilizing function rather than through movement. This should bring to mind such exercises as planks, glute bridges, stability ball work and many yoga poses.
Secondarily, these muscles are also responsible for small movements that are generated at the torso: twisting and lateral flexion (bending sideways at the waist to get in or out of a car), flexing the abdomen (getting up from lying on your back) and extending the back (reaching for something overhead). Bicycle crunches, ab curls/modified crunches and supermans are strengthening exercises that mimic each of these functional movements, respectively.
But these are targeted strengthening movements. Unlike planks and other core stabilizing strengthening exercises that work all muscles of the core, bicycle crunches work the obliques in isolation, ab curls/crunches work the rectus abdominis in isolation and supermans work the erector spinae and multifidi in isolation. Working only one set of a muscle group in isolation creates an imbalance where the other muscles become weaker over time. If you want to avoid this imbalance, you must either do whole core exercises (such as planks) or do a series of exercises to target each of the core muscles in isolation.
Risk vs Reward
All exercise has risk. Some are riskier than others. And this is what the military took into consideration when deciding how best to build a strong core during training. Here are common abdominal strengthening exercises listed from most risky to least risky in terms of injury, particularly to the lower back and neck:
- Sit ups: repeatedly lifting entire torso from floor to seated position and lowering back to floor
- Full Abdominal Crunches: repeatedly lifting head, neck & upper back off floor and lowering back to floor (pictured above)
- Ab Curls/Modified Crunches: shoulders stay lifted off the floor by bracing the abs with repeated small pulses/crunches in this position (pictured below)
- Bicycle Crunches: assume same position as curls with torso twisting from side to side
- Planks: either on forearms or hands as well as side planks (forearm plank 2nd picture below)
And, of course, what is the point of exercise without reward? Exercise scientists have used monitoring devices to detect muscle engagement during abdominal exercises to measure which exercises engage the most muscle fibers and, therefore, strengthen the targeted muscles best. The following are the same common abdominal strengthening exercises listed from least effective to most effective, also listed are all the core muscles that are worked in each exercise:
- Sit Ups: mostly transverse abdominis
- Full Abdominal Crunches: mostly rectus abdominis
- Ab Curls/Modified Crunches: rectus abdominis and transverse abdominis
- Bicycle Crunches: obliques and transverse abdominis
- Planks: transverse abdominis, rectus abdominis, obliques, erector spinae and multifidi
And this explains why the military made the decision they did – planks strengthen all the muscles of the core equally, more efficiently and with less risk for back and neck injury than sit ups or crunches.
No one should do sit ups or full abdominal crunches. Neither has shown any real benefit to core strengthening but both are associated with a high risk of lower back and neck pain and injury.
I personally see a value in ab curls/modified crunches and bicycle crunches in that they do strengthen muscles that are responsible for functional trunk movement. But they must be done with perfect form and in conjunction with back strengthening exercises (such as supermans and birddog) as well as whole core exercises, like planks, in order to be safe and effective. If, however, you already have lower back or neck problems, I recommend using other exercises to strengthen the core that won’t put undo stress on the spine.
Keep in mind that planks done incorrectly can cause serious injury as well. As with all strength-training exercises, quality is more important than quantity. Perfect form is the best insurance against injury. Click here for visual and written instructions from ACE Fitness on performing the exercises listed in this blog as well as more suggestions for ab and core strengthening exercises.