Sit-Ups, Crunches, Planks, Knee-Ups: The Evolution of Core Fitness Testing


Core strength testing has come a long way from the straight-legged, twisting sit-ups used before World War II. We've moved on to bent-knee sit-ups, to crunches and now to the plank pose or hanging knee-ups (depending on your military branch).

From the first full twisting sit-ups, you tested more than just stomach muscles. Everything from the hips and hamstrings to the neck and shoulders were tested.

Why was this important? What we call the "core" actually is a system that transfers power and force to the arms and legs in any movement from throws to punches. We test the core because it needs to be strong, flexible and stable to generate any movement with power and speed through the body to the arms and legs.

The core is not just your abs or stomach muscles. It's actually a system of muscles that make all power movements possible. Here is a list of most of the core muscles you work in a large variety of movements by the spine and extremities (depending upon the movement, you may engage other muscle groups -- off the shoulder girdle, neck and hip/legs -- that are still part of the "core" system):

Moving muscles: Rectus abdominis, external obliques, erector spinae, latissimus dorsi, hamstrings, hip adductors, hip abductors

Stabilizing muscles: Transverse abdominis, internal obliques, lumbar multifidus, pelvic floor muscles, diaphragm, transversospinalis

When thinking about the torso, hips and upper leg test, you have to ask what are we testing and why. The following is a historical description of the variety of ways to test "core stability and muscle stamina."

Straight-legged sit-ups (pre-WWII)

The sit-ups of old tested a significantly larger number of muscles in movement as troops had to lift their entire upper body off the floor (with hands locked behind their head) and then forward bend at the waist, with legs straight, all the way to touch their elbow to their knee. More lower back and hamstring flexibility was needed in the first sit-up tests done by the military.

It evolved into a bent-knee sit-up that required less lower-back flexibility and mobility. Though the target muscle of the sit-ups is the rectus abdominis, there are many other muscles that get tested (and stretched) during the sit-up: iliopsoas, tensor fasciae latae, rectus femoris, sartorius and obliques (diagrams from

The sit-up does not just work the abs, and it is not an abs test. It is more a dynamic test of hip strength and endurance and an isometric flex of the abdominal muscles, as there is a steady flex of the rectus abdominis during the sit-up.

Crunches evolved to make sit-ups safer over the years

In some military branches, the technique evolved: Placing the hands over the chest, versus locking them behind the head, helped prevent people from stressing their necks, but the movement of crunches and sit-ups stayed the same.

The crunch exercise took out much of the strain on back muscles and solely focused on the rectus abdominis and the obliques. You move the torso barely off the floor so the elbows touch the thighs. The crunch was the easier, safer version of the sit-up exercise of old.

This test does not require one to produce a lot of the movements that the older sit-up version did, but it was here to stay for nearly 30 years in some branches of service. The crunch is pretty much a pure abs endurance test with a small range of motion.

Evolution into plank pose

After decades of testing, the crunches are beginning to evolve into the plank pose -- a completely different exercise. So far, the Navy and the Marine Corps have swapped out crunches for planks. Taking out all motion whatsoever, will it be considered safer? Will it test anything useful? Perhaps.

The plank pose is a classic example of an isometric flex of multiple muscle groups of the core system. An isometric exercise is a contraction of a muscle or group of muscles. The muscles do not move or change in length. The sit-up and crunch have their versions of an isometric flex of the rectus abdominis, while the movement of the hip joint requires a steady flex/rest to lift the upper body off the floor.

In the sit-up and crunch, the muscles of the rectus abdominis may be in an isometric state while the supporting muscles of the hips, spine and legs create the movements of the sit-up or crunch.

However, in the plank, all muscles are still with no movement of any joints. The plank will work many of the same muscles groups (and more) and test muscle stamina of an isometric hold, but it does not test the movement of any joints like the previous core exercises of the military.

However, if practiced regularly and your capability increases to a few minutes, stability is built and safely tested. Once again, stability of the core is essential to the production of powerful and speedy movements. This is not a better or worse exercise -- just different.

Muscles used in the plank: rectus abdominis, obliques, iliopsoas, tensor fasciae latae, quadriceps, sartorius, pectoralis major, serratus anterior. Other stabilizers: erector spinae, trapezius middle, trapezius lower, rhomboids

Evolution to hanging knee-ups (Army)

The Army's knee-up, done by hanging from a pull-up bar, not only tests the ability to move the hips, core strength and lower-back flexibility, but also tests grip, a highly important element of tactical fitness. This test, in my opinion, is superior to any of the previously tested sit-ups, crunches and plank pose as it engages the same groups of muscles, requires movement with hip and spine mobility, and engages forearm and grip muscles that are needed for tactical skills (stretcher carry, buddy drags, obstacle climbing, etc.).

Muscles used in the hanging knee-up: rectus abdominis, iliopsoas, tensor fasciae latae, pectineus, sartorius, adductor longus, adductor brevis, obliques, brachialis, brachioradialis, and more grip muscles of the forearm. Other stabilizers: rectus femoris,

Closing remarks

As the military evolves with fitness testing, finding tests that are useful for military member development is the ultimate goal. Having a test that regularly enforces that development is needed to meet certain fitness, and wellness standards is one of the "whys" discussed.

By ensuring the core system is strong and stable but also flexible, these latest tests can be a way of keeping military members ready to engage job-related movements and to lift equipment and people without injury. You still should be moving, lifting and doing these types of core activities regularly. Do not just take 2-3 weeks to prep for a test and avoid the gym the rest of the year.

Stew Smith is a former Navy SEAL and fitness author certified as a Strength and Conditioning Specialist (CSCS) with the National Strength and Conditioning Association. Visit his Fitness eBook store if you’re looking to start a workout program to create a healthy lifestyle. Send your fitness questions to

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