From the rather disorganized article on Wikipedia:

“Muscle imbalance can be described as the respective equality (sic) between the antagonist and agonist, this balance is necessary for normal muscle movement and roles.[1] Muscular imbalance can also be explained in the scenario (sic) where the muscle performs outside of normal physiological muscle function.[2][3]

Muscle balance is considered to be the harmonious action where muscles that surround a joint work together with normal opposing force to keep the bones involved with the joint centered, thus accomplishing human movement[3] Muscles that have become imbalanced are usually (sic) result of either adaption (sic) or dysfunction, they can be classed as functional or pathological.”

Reference 2 doesn't actually support this definition, but that never stopped an “exercise scientist” from putting it in a reference list.

From the NASM Essentials of Corrective Exercise Training:

“Muscle imbalance is a condition in which there is a lack of balance between certain types of muscles.”

Well, okay. The problem with “muscle imbalances” isn't how poorly they are defined so much as how they are generally treated by Physical Therapists, Athletic Trainers (Certified), Chiropractors, Personal Trainers, Kinesiologists, Physiatrists, Biomechanists, Orthopedic Surgeons, Registered Nurses, PE Majors, and Corrective Exercise Specialists (CES). The Physical Therapy approach to injury rehab has been repurposed to deal with “muscle imbalances,” with similar levels of effectiveness: it doesn't work, because it can't work.

Google the term “Corrective Exercise” and look at the mess. “Muscle Imbalances” are corrected by the use of unilateral/contralateral movements applied either unloaded or using 3-pound dumbbells. Google the images and observe that awkward poses supervised by smiling young professionals in colorful clothes and short socks apparently correct the pain and embarrassment of Muscle Imbalances. It's not just Google Images that thinks so: the worldwide authority on strength imbalances and corrective exercise, the National Academy of Sports Medicine, publishes a profusely-illustrated textbook on the subject that shows exactly the same people, clothing, socks, small dumbbells, and unilateral/contralateral poses that are supposed to address these problems.

The assumption in these resources is that imbalances in muscular strength and function are caused by two things:

1. Repetitive motions at sub-maximal intensity, which cause disproportionate strengthening in the affected muscle groups while leaving the other muscles unaffected, and
2. The effects of staying in a position of joint flexion for extended periods of time, which apparently causes the shortening of the agonist muscles across the joint, e.g. sitting in a flexed hip position for long periods of time, sitting in thoracic flexion, or the wearing of heeled shoes.

It is undisputed that injuries and surgery can cause these problems, because trauma to a skeletal muscle obviously compromises its function. That does not seem to be the focus of the Corrective Exercise people.

Strength and Position

Strength is the ability to produce force against an external resistance. An increase in strength occurs when a stress is applied to a movement pattern that challenges the ability of the involved kinetic chain to generate enough force to execute the movement. If the effort required to execute the movement is within the range of the existing capacity – as it obviously is for motions that can be repeated all day – the strength already present in the kinetic chain is adequate, and the movement doesn't cause a strength increase. Something you're already strong enough to do over and over and over can't make you stronger because the stress does not approach the limit of your current force production capacity. Getting up out of the chair 40 times a day will not take your squat up over 405, and 100 pushups does not get your bench press up to 315.

Repetitive submaximal movements can irritate tendon origins and insertions, if the absolute strength of the kinetic chain is so low that the repeated movement is a significant percentage of it. This is how people get wrist tendonitis from working a mouse at the desk, or tennis elbow on the court – they're not very strong, and it never occurs to them that getting stronger might toughen up the injured tissues and make the repetitive motions easier. But this is just plain old irritation/inflammation, not a “muscle imbalance.”

And the idea that sitting at a desk shortens the hip flexors, or that resting in any flexed or extended joint position can have an effect on the muscle belly length, is very weird. By what mechanism does the muscle belly get shorter, i.e. reduced in length? Does the length of the belly of the antagonist on the other side of the joint also lengthen while the agonist shortens? How? Or is the explanation that the neurological resting length of the muscle bellies change in response to a prolonged exposure to the shortened or lengthened position? Why does this same mechanism not correct the problem during the night, lying in bed, when everything is back in roughly normal anatomical position? And how would a stretch that lasts for 20-30 seconds counter the devastating effects of sitting in hip flexion all day?

A more obvious use of the term “muscular imbalance” is as the description of a visible difference in size between two bilateral muscle bellies: pecs, bis, tris, glutes, quads, calves, delts, and abz. Visible asymmetries certainly do occur, as a result of either injuries, congenital defects, or asymmetrical workloads on the affected structures. If you were born with a missing pec, or if you ruptured one during training, you can't be a contest bodybuilder (an unfortunately very bigoted activity). But if you've had a job digging ditches for a plumber, are right-handed, and your left trapezius is bigger than your right, you have a visible asymmetry that can be corrected with exercise.

Barbell Training

But not with silly poses, silly stretches, 3-pound dumbbells, and light weights that cannot strengthen anything. It actually doesn't matter how the “imbalance” got that way, it can't be corrected by addressing the weakness in isolation from the rest of the musculoskeletal system in which it is anatomically embedded. In normal human movement, a quad doesn't function without the hamstring, and for normal human movement it cannot be strengthened without strengthening the hamstring at the same time. The rotator cuff muscles do not function without the delts, triceps, lats, and traps, the adductors don't function without the hamstrings, hip external rotators, and glutes, and the quads and hamstrings don't function without the gastrocs, soleus, and dorsiflexors. If one component of the system is weak, so are other components, and all the related pieces of the kinetic chain. And if a return to normal human movement is the objective, the superficial origin/insertion/action model of isolated muscle function rehab is of little value.

Any system of exercise that addresses the components of the kinetic chain of a movement pattern separately is incapable of strengthening the entire movement pattern, and in fact perpetuates the imbalance since none of the individual components actually function separately. Each muscle in the complex contributes to the production of force in the movement pattern according to its size and position within the kinetic chain, determined by the musculoskeletal anatomy and the leverage relationships this creates. The movement pattern using all the components of the kinetic chain must be strengthened – more weight must be lifted over the range of motion, over the course of time. When that happens, all the components are strengthened as a result, proportionate to their anatomically-determined function.

If a strength imbalance is the real issue, a strength increase must occur. If the imbalance itself is the real concern, it wouldn't surprise me at all if a Corrective Exercise Specialist advocated reducing strength in the healthy side. Easier, faster, don't have to learn how to coach barbells.

In fact, the entire discussion of “corrective exercise” becomes irrelevant when barbell training is available. Barbell training, when applied correctly, uses carefully designed bilaterally-symmetrical movement patterns normal to the human body using progressively heavier weights that cause a bilaterally-symmetrical strength adaptation – the precise answer to bilateral asymmetry. The key here is perfect form, and thus is dependent on coaching.

“Muscle imbalance” causes strength to be asymmetrical. If 1.) the barbell exercise movement patterns are applied with perfect form/bilateral symmetry over the longest effective range of motion, using the most muscle mass that can be recruited into the kinetic chain, 2.) the load is chosen for its ability to be handled with perfect bilaterally symmetrical form, i.e. light enough to the weak side to perform correctly, and 3.) the load increases incrementally, then the weak side is subjected to relatively higher escalating intensity than the strong side, and strength symmetry returns.

If the weak side (or weak component) is forced to perform its anatomically-symmetrical share of the work, it receives an adaptive stress commensurate with its current force-production capacity, while the strong side contributes to the symmetry of the whole movement. The weak side “catches up” by functioning within the same kinetic chain it fits into anatomically, while the strong side helps maintain movement symmetry. Function is therefore restored while performing the function.

This is apparently too simple and straightforward for some people to grasp.

Your bigger trap is stronger than your little trap – that's why it's bigger. So you start deadlifting with a weight you can pull with perfect form, say 185 x 5 reps. This set is more stressful for your little trap than the big trap, but since you pulled it with perfect symmetrical form (it's not a 5RM, it's just kinda heavy), the little trap worked a little harder in the same movement pattern you used for the big trap. In the event that the pull was asymmetrical, you started too heavy. But your approach to the 185 that first workout – you did 165 before 185, and it was light – indicated that 185 would be quite manageable with perfect form.

Next workout, you pull 195 x 5 for your work set. You've recovered from the previous workout, and are now stronger, so the 10-pound jump is both more weight but still quite manageable, and starts the process of forcing the weak side to catch up while performing its normal function as the biggest muscle of the superior posterior skeleton. That process will continue until the little trap has hypertrophied into equilibration with the big trap, because the bilateral stresses applied symmetrically across the structures have forced it to do so. Forcing the compromised structure to gradually return to normal function by progressively loading that normal function is the only way to ensure that it happens.

Your left knee is caving into valgus when you squat. Your barbell coach has ruled out a leg length discrepancy, and has advised a 20% deload, or whatever deload is necessary to allow for absolute attention being paid to keeping your knees symmetrical. In this case, holding the left knee in symmetry with the right knee using your brain forces the external rotators and abductors of the hip to do their job. You don't think about the gluteus medius or the piriformis or the superior gemellus or the TFL, because you can't. You cannot identify the individual components of a complex kinetic chain and force them to “fire” because your brain is not connected to the little pieces – it is connected to the big pieces that the muscles move. You think about the knee, about keeping it out, and in doing so you force the compromised components of the kinetic chain to do their job, under gradually increasing weight over time.

In contrast, what would your Corrective Exercise Specialist (CES) have programmed? For your trap discrepancy, my guess would be a 20-pound one-hand dumbbell shrug and 20-pound dumbbell rows for the weak side, if not just bodyweight unilateral shrugs on a swiss ball. This superficial approach ignores the anatomy and the anatomically-determined function of the structures involved in this asymmetry: the trap's normal function is not concentric elevation of the shoulder, it is isometric stabilization of the scapulae, clavicles, and cervical spine. For the valgus knee, it would probably be cable abductions, or bands around the knees while you walk, or maybe just standing there holding your leg out, despite the fact that you were strong enough to do this when you were 8. It cannot possibly work, and it's not hard to see why: the back and the hips are a system of muscles and skeletal levers, all working dependently and synergistically, and systemic asymmetry cannot be corrected by addressing any single component of it asymmetrically. It doesn't function that way, and it can't be fixed that way.

Injuries

An injured muscle can't be effectively rehabbed this way either. Most Physical Therapists don't seem to appreciate the fact that injuries heal the way they are moved – if you move normally to the extent possible, the injury returns to full function, and if you don't, it won't. It has to be this way: 30,000 years ago, if you tore a hamstring and waited on it to heal, the hyenas consumed your lazy ass. So you walked on it if you wanted to live. Your body has evolved the ability to heal an injury while using the injured component, because it was necessary. Keep in mind the fact that ribs are the fastest-healing bones in the human body, because they never stop moving while they heal. Movement heals, and normal movement heals to normal function.

For example, you've injured a lateral quad doing something stupid, like falling off your bike. Rehab for this injury starts as soon as the muscle tear stops bleeding, maybe a couple of days. When the muscle is at full function, it is a component of the knee extensors, one of 4 muscles that accomplish this task. This particular component is injured, thus compromising knee extension. We have a machine for that, right?

Except that we don't, because in normal human movement the knee extensors never function in the absence of their antagonists in the posterior chain: the hamstrings, glutes, and adductors, in addition to the trunk musculature, the calves, and the feet. Actual functional rehab must take place in the context of the movement patterns to which the injured muscle must return. The squat is the most normal movement pattern the hips and legs can execute over their full range of motion, so we have to squat. If we squat, all the other muscles in the kinetic chain work in their anatomically-determined roles, and the injured muscle performs at the level it can.

The weight will be light, the reps might be higher in number, it will hurt. But the injured muscle responds to stress the same way a healthy muscle does – it adapts and gets stronger. Like the “muscle imbalance” scenario, we carefully work to the limit of the compromised component using perfect movement symmetry, with the other components of the kinetic chain taking their share of the stress, and as the injury heals it returns to full function because it has to. We've given it no other choice. Rehabbed this way, a belly tear can return to nearly complete function in 2-3 weeks, versus never with a less-aggressive less-logical approach. I have personally rehabbed dozens of athletes, including myself, over the last 35 years using this method, and I know that it works far better than the standard Physical Therapy approach.

The best question you could now ask is: if this is so obvious, why isn't it the default method used in rehab? I don't know. They never asked me about it. They don't teach it in PT school, and they seem content to keep using the origin/insertion/action method of isolated muscle function rehab. If you talk to sentient Physical Therapy patients, they will all tell you the same thing – it didn't work, and it was time-consuming and expensive. They will also tell you that when insurance runs out, they were given a different, cheaper rate card for the therapy than the one used to bill insurance, so there's that, too. The only logical approach to muscle imbalances, “muscle imbalances,” and actual injuries is the method detailed above. The sooner it is widely implemented, the better.

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