The Plank, Disassembled

In the world of fitness , patterns in trends tell a story. Generations ago it was the sit-up that was the 'go-to' exercise for core strengthening. Years later came the rise of the abdominal crunch, with rumors that the sit-up had been 'bad for your back' all along. Today we live in the era of the plank. 

In the last 15 years, anyone who has had personal training, participated in organized sports, or enjoyed episodes of The Biggest Loser has bore witness to the plank.Depending on one's age, this may have been part of the gym class experience in high school. This exercise quickly became synonymous with the 'six pack.'  

Some of us were promised 'abs of steel' and miraculous weight loss. Others were simply trying to up their game on the field. Unlike gym culture, which moves at the speed of innovation, sports tend to follow tradition more than innovation. The plank has even become ubiquitous here. How does an exercise become so pervasive so quickly?

Is the plank the best way to strengthen the core? What does exercise research reveal about this popular exercise? My name is Dr. Allan Buccola, physical therapist and owner of Impetus Physical Therapy in Greensboro, NC. This post aims to shed light on this topic and highlight some of the latest available research. 

What Exactly is 'the core'? 

The upper and lower body are connected with a cylinder of muscles, imagine a canister if you will. It has multiple layers on all sides, front to back and top to bottom. These muscles dictate when the pelvis moves and when it remains still. When an arm is throwing a baseball, the core connects explosive energy and power from the foot to the ball. When a sprinter is coming out of the blocks with several hundred pounds of force, the core keeps the spine in place.  

Sprint

Anatomically, the core is most simply defined as all muscles that attach to the pelvis. This includes large leg muscles like the hamstring, which mechanically bend the knee, but is also an active rotator of the pelvis in exercises like the deadlift. Whether a muscle is functioning as part of the core at that moment depends on the task at hand. 

Physiologically, the core can be split into two groups: movers and stabilizers. The movers are often superficial, close to the skin and easily visualized in those lean and muscular. These muscles excel at producing short and powerful bursts of movement. These muscles could include the spine extensors, the rectus abdominis (six pack), and the lattissimus dorsi (lats). 

The core stabilizers are much deeper and are closer to the bones and joints. These muscles are considered 'postural,' optimal at low loads for prolonged periods of time. They excel at keeping the body stable, even when the rest of the body is in motion.

The deep core stabilizers play a critical role, but are not considered 'sexy' by any means, likely because they are often too deep to be seen. For this reason they are often forgotten at the gym. Muscles in this group include the lumbar multifidus, the diaphragm, and the transversus abdominis, as well as the pelvic floor muscles. 

In a healthy functional system, these two systems work together in intricate ways. They connect powerful movements with one part of the body with the other and to maintain the integrity and health of the spine during others. They also serve to stabilize the spine in the optimal loading position during quick movements or heavy lifting. 

The importance of function and balance in this system is critical for all people, not just athletes. Stabilization of the spine is necessary for everyday tasks like getting in and out of the bed, and just walking around the house. A strong functional core is critical during a hard sprint, a quick right hook, throwing a fast-ball, or landing a triple axel. A strong functional core is needed to lay a baby down for a nap, to bring in the groceries, and to lift your luggage at the airport. 

The Evidence for Core Training

Reasearch has produced data on core strength across a wide variety of people. Having a strong core isn't just about bikini season. Core strengthening has been applied to everyone from people with back pain to athletes seeking performance enhancement, the elderly and postpartum moms. The campaign for a stronger core applies to all, but may have been hijacked by the fitness world sometimes sending a confusing message.

Simple human movements are the product of a continuous chain of segments and moving parts that work together. Think about a tree swaying in the wind. Movement occurs from the trunk to main branches, and eventually to the tips of of the branches, and then the leaves. If a tree is to withstand hurricane force winds, strong roots needed more so than strong branches. The core can be equated to the roots of the tree. 

Consider very strong legs in a population that is constantly kicking and running. According to a recent systematic review of  7 studies measuring the effect of injury prevention programs in professional soccer players, programs that included core strengthening correlated with lower injury rates. The review crunched data of more than 2000 athletes performing programs that included planks and side planks. 

Another older study analyzed the body mechanics of a single leg squat in young adults in relation to measurement of core strength in several muscles. Weakness in certain core muscles correlated with hip/knee mechanics commonly associated with several different injuries of the knee including ACL rupture. When the core is unstable, the joints away from it also lose integrity in movement. 

 Weakness in lateral trunk flexors and hip abductors varied widely among participants, but were statistically different between males and females and associated with greater dynamic knee valgus. 

Weakness in lateral trunk flexors and hip abductors varied widely among participants, but were statistically different between males and females and associated with greater dynamic knee valgus. 

A more recent study from 2013 found similar findings. Study participants with greater weakness in the gluteus medius and gluteus maximus were found to compensate for a lack of force production during functional movements. A clear relationship was identified between weakness in these muscles and increased knee valgus or inward bowing. 

So Much More Than a Six-Pack

Many equate the core with the six-pack and the plank as an exercise that is for building 'six-pack abs.' The truth is much more complicated. The plank is a full body exercise that involves 4 limbs and the middle body segment that connects them. To accomplish this task, numerous core muscles must work together.   

At the level of the core, there are two basic activities going on during the plank: trunk flexion and hip flexion. Trunk flexion is the approximation of the rib cage and the pelvis. Hip flexion is the approximation of the end of the thigh to the upper body.

During a standard front plank, both of these muscle groups are active. In the most simplified count, 8 major muscle pairs are performing this task: the rectus abdominis ("six-pack") is only accounting for 12.5% of these. There are 4 abdominal muscles involved in trunk flexion and 4 muscles that contribute to hip flexion (see below.)

 Electromyography recorded during a supine straight leg raise shows muscles activated in the first 30 degrees of hip flexion from neutral. Taken from Hu et al. 2011.  

Electromyography recorded during a supine straight leg raise shows muscles activated in the first 30 degrees of hip flexion from neutral. Taken from Hu et al. 2011.  

In standard plank, this two groups might not work equally. Weakness in the hip flexor group (see above) might result with a butt higher in the air, whereas weakness in the abdominal group might result in greater arching of the low back. Whether these segments are engaged equally is highly variable, which means that different people will struggle with the plank in different ways. 

 Weakness in hip flexors may result in posturing the body out of alignment. The body is unable to come to a neutral posture.

Weakness in hip flexors may result in posturing the body out of alignment. The body is unable to come to a neutral posture.

 Sagging or overarching of the lower back can result from insufficient abdominal muscle strength, teaching the body to stabilize in less than optimal postures. The body sags below the line of neutral alignment. 

Sagging or overarching of the lower back can result from insufficient abdominal muscle strength, teaching the body to stabilize in less than optimal postures. The body sags below the line of neutral alignment. 

Comparatively, data published from a study by Juker et al. in 1998 shows that the push-up activates the psoas up to 25% of maximal effort. This is taken from a normal, injury free and active population. These levels of activation may be more difficult to predict across all people. Unfortunately though, this level of activation is not sufficient for strengthening in the hip flexors, likely making the plank inadequate for core strengthening for some people. 

Although activation levels will vary, the plank still may be a better way to activate multiple abdominal muscles at once. According to one study from 1997 by Andersson et al, the plank is superior in this regard when compared to other classic abdominal exercises.

According to their data, the sit-up tends to more heavily bias the hip flexors, abdominal crunches tend to bias the upper abdominals, and the curl-up seems to target the lower abdominal. This may make the plank better suited for maintenance work at the gym, or multi-segment engagement, rather than targeted strengthening. 

Making the Plank More Effective (Harder)

If the exercise can be made more intense, then it should be more effective for isolated strengthening. Some have tried to make the plank more difficult by adding in single limb movements or altering positions of the limbs. Scientists studying these variations have found some useful results. 

A 2016 study by Kim et al. shows data that demonstrate how a lateral instability force of one leg using an elastic band can increase activation of the abdominal muscles. Although this finding is helpful, the exercises are perhaps not practical for most people. They may require equipment or difficult set-up unfamiliar to many people. There are, however, simpler ways to progress the plank.

 Taken from Kim et al 2016. 

Taken from Kim et al 2016. 

Another study in 2014 by Schoenfeld et al. assessed changes in pelvic rotation and arm position commonly thought to make the plank more difficult. Data from this study reveal significant increase in abdominal activation when the elbows were moved further away from the knees. Although data were not collected, I'll guess that this these would also increase in activation to stabilize the shoulder and shoulder blade.  

In this particular study, changes to pelvic tilt (arching and rounding of the lower back) did little to change activation. This is a common recommendation from trainers in a movement that somewhat appears to combine the plank with an abdominal crunch.  According to this data, it may not be an effective way to progress this exercise. 

 Taken from Schonfeld et al. 2014. 

Taken from Schonfeld et al. 2014. 

Even as variations of the plank have emerged and improved effectiveness for strengthening, it's still incomplete. Commonly targeted muscles are exclusive to the front of the body, excluding core muscles on the sides and back. Can the plank effectively engage the core as a whole?

Total Core Engagement

The popularity of the plank has lead to associated with muscles at the belly, those easily seen looking forward into a mirror. Remember though that the core is a cylinder and includes muscle groups that laterally flex the trunk, abduct limbs, rotate limbs, and extend the hips and trunk. To the extent that the plank is able to engage all of these in a meaningful way is unclear.

A 2014 study by Snarr and Esco produced data regarding activation of the spine extensors as plank exercises were performed on unstable surfaces. These surfaces would create instability to the system in three dimensions rather than one. These data indicate that these plank variations could produce engagement of the core as a whole, but unfortunately not great enough for meaning strengthening of all groups. 

 Adding unstable surfaces to the plank can create core stabilization in 3 planes and greater activation of all muscle groups. 

Adding unstable surfaces to the plank can create core stabilization in 3 planes and greater activation of all muscle groups. 

In 2014, Byrne et al produced data on muscle activation during plank exercises that involved body suspension systems, limbs supported in straps. These data reveal that when an element of instability is imposed during the plank (instability now in three planes of movement), muscles must work at a higher rate. This included abdominals, hip flexors, and scapular stabilizers. 

As plank varieties get more complex, the evidence shows that they better activate the entire core, not just the front. This is good news as humans don't simply move in one direction. Bodies move in every direction and the core must be able to stabilize and function in all of them. In the absence of focal core strength deficits, exercises that engage as much of the core as possible might make more sense functionally. This makes sense when approaching exercise from a weight loss perspective too. 

The Take Home Message

All people can benefit from core strengthening: not just athletes and models. Benefits of core strengthening can include reduction in back pain, reduction in injury risk, and improvement in overall health. In some circles, the stigma associated with 'core strengthening' and 'abs' has misled many to believe that the primary benefit is physique change. In fact, the most important results are the functional ones. 

Another challenge is helping everyone understand that the core is a cylinder, with a front, sides, and a back. The washboard analogy leaves much to be desired. The core has multiple layers of power AND endurance muscle fibers. A strong washboard stomach alone will not get the job done. There must be balance in the system.

With many older traditional exercises like the sit-up and abdominal crunch focusing heavily on the core muscles ability to move the trunk/pelvis, the plank is different in that it targets the core's ability to stabilize the spine. It is for this reason that it is a great starting place in terms of stability training, but only when performed correctly. 

Where to Begin

The standard plank is probably the best place to start. For some this will be challenge enough, but be sure that you are meticulous about form. The whole purpose of stabilization is to maintain the body in the optimal posture for loading, so be sure to keep the body in line from the ankles to the shoulders. If you are unsure, use a mirror or a friend to get it lined up just right.  

 When planking in correct posture the body should be in a straight line from the ankle, passing behind the knee, through the center of the hip, through the shoulder, and just behind the ear. 

When planking in correct posture the body should be in a straight line from the ankle, passing behind the knee, through the center of the hip, through the shoulder, and just behind the ear. 

How long Should I hold it?

Although commonly seen performed for marathon performances, back pain expert and researcher Stuart McGill was quoted in one article saying that there is likely no benefit to be gained by holding the plank for greater than 10 seconds. He recommended that strength be achieved and progressed by incrementally adding reps of 10 seconds or less. 

If one is able to hold a plank for a time that is substantially longer than this, it is likely that the exercise is not difficult enough to elicit a true strengthening adaptation. For these individuals, a more challenging variation may be of more benefit. 

What If You Can't DO a Plank?

If you are unable to assume the plank position for even just a second or two, try starting with an easier variation. By performing a plank in a push-up position, the ankle of the trunk is decreased, and therefore there is less resistance. If a plank is performed on knees instead of feet, the lever of the body is reduced which will also decrease the load. Just be sure to align the trunk properly. 

 Performing a plank on hands rather than elbows can be an easier starting point for some. 

Performing a plank on hands rather than elbows can be an easier starting point for some. 

 Performing a plank on knees, rather than feet, can decrease the level of effort required and may be a good starting point for those unable to perform a standard plank. 

Performing a plank on knees, rather than feet, can decrease the level of effort required and may be a good starting point for those unable to perform a standard plank. 

Individuals unable to perform these exercises due to weakness/pain and those with pre-existing injury may require some further modification of these activities. This is a good situation in which to consult a physical therapist, personal trainer, or other qualified human movement professional. 

Many other movement specialists from personal trainers to yoga teachers and so on, are also able to find ways to modify their programs to better suit clients. Be sure as you are working with these professionals that they are fully aware of any previous injuries or pain, and that you feel as though your program is being fully catered to your capabilities.

The Plank Cannot Stand Alone

Even after a solid progression of this exercise, even after the most creative modifications, the plank only begins to engage the rest of the core: the sides and back. From studies cited above, these modification still fall short of true strengthening for other parts of the core. Additional exercises are almost always need to be included in any well-rounded core strengthening program. 

Don't be overwhelmed. Know that a commitment to strengthening your core doesn't require hours upon hours each week of dozens of exercises. No two people will require the exact same exercise program, but understanding which one is right for you can largely be figured out by listening to your body. Professional expertise can help with the rest if needed.  

For more detail on core strengthening beyond the plank, be sure to check out part two. Until next time, don't stop moving! 

-Allan

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