Identify & Fix Muscle Imbalances & Injury Risks
In a Nutshell
Asymmetries matter most when they're in the lower body, driven by morphology (muscle size/quality), or exceed 10-20% depending on the measure—anything over 20% is problematic across contexts. The biggest drivers are sport-specific movement patterns, injury compensations, and training imbalances, with unilateral plyometrics and targeted volume increases (extra reps/sets on the weaker side) being the most effective corrections. Small asymmetries (<5-10%) are normal and often harmless, but testing morphology via imaging provides more reliable data than functional tests, and addressing meaningful imbalances improves both performance and injury resilience.
These notes were generated by AI and may contain inaccuracies.
A single degree increase in ankle range of motion can improve best race time by 7-8 seconds. This improvement could help an elite male runner break under 24 minutes in an 8K or a female runner break under 20 minutes in a 6K. The key mechanism is reducing asymmetry between the two legs rather than simply increasing range of motion.
The Vitruvian Man by Leonardo da Vinci represents the romantic ideal of perfect human symmetry. In the early 1900s, scientists believed the most symmetrical athlete would be the most successful across all sports. This view was later disproven.
David Epstein's book The Sports Gene documents how the body type explosion revealed that some sports and activities actually benefit from asymmetry, including advantages from limbs of different lengths and bodies that don't function identically on all sides.
People with lower body asymmetries have up to a 35% increased injury risk. This creates confusion about whether asymmetries help or hurt performance. Handedness exists in approximately 98% of humans, with only about 2% being ambidextrous. The general rule of thumb states that differences within about 10% between sides are acceptable.
A recent review of over 6,000 athletes across 30 studies found conflicting results: one-third showed asymmetries led to more injury, one-third found asymmetries were not relevant, and one-third were inconclusive. A study of nearly 300 Jamaican children found that symmetry in wrist, ankle, and knee size predicted both current 100-meter dash performance and performance 14 years later.
Google searches for correcting asymmetries typically suggest two approaches that are incomplete: doing more repetitions or lifting more weight on one side, and performing corrective exercises with bodyweight or small elastic bands. Both approaches have merit but do not tell the complete story.
Morphology refers to size and shape differences. Top-to-bottom asymmetries, such as large upper bodies with small legs, lack compelling evidence of functional relevance except in aesthetic sports like bodybuilding. Front-to-back asymmetries, such as quadriceps-to-hamstring ratios, have documented relationships to knee and hip injury risk. Left-to-right asymmetries have clear functional relevance for both performance and injury prevention.
Quality is an emerging research area examining intracellular damage and fat infiltration within muscle cells. A single 1% increase in fat infiltration within skeletal muscle increases coronary dysfunction likelihood by 2% and heart event risk by 7%. This factor may be the most relevant of the three categories, particularly for aging and quality of life.
Functionality encompasses motor control, physical strength, endurance, range of motion, flexibility, and stability. Research exists across numerous functional measures including single-leg balance, pull-up shifting patterns, bench press elbow stability, running mechanics, cycling, and swimming asymmetries.
Skill dominance involves fine motor movements and precision tasks such as writing or performing surgery. Force dominance involves strength and the ability to produce maximum force. In badminton, the right hand and right leg are typically both skill-dominant and force-dominant. In soccer, the right leg may be skill-dominant for kicking while the left leg is force-dominant for bracing. The same pattern occurs in baseball pitching and golf, where the skill side and force side are often opposite.
The Van Valen model, developed approximately 60 years ago, categorizes asymmetries into three buckets, with a fourth added more recently.
Directional asymmetries are genetically driven and predictable across humans. Examples include the heart being positioned off-center to the left and the positions of the spleen and liver.
Anti-symmetries are genetically driven but unpredictable. Handedness cannot be predicted at birth, unlike organ placement.
Fluctuating asymmetries are environmentally driven, typically small random deviations resulting from external pressures. These are often associated with poor childhood development and include differences in nostril size or ear shape.
Sporting asymmetries are repeated movement-induced adaptations resulting from skill development. These represent the majority of asymmetries people seek to address, including differences in muscle size, motor control, and range of motion.
Asymmetrical training demands occur when programs are unbalanced. One personal example involved excessive pressing movements without adequate pulling movements, resulting in a torn labrum requiring surgery. The individual was unable to lift the right shoulder during an entire basketball season and had to throw underhand during an entire baseball season.
Injury compensations occur when movement patterns change to protect an injured area, potentially creating short-term or long-term asymmetries elsewhere in the kinetic chain.
Movement deficiencies and technique issues include mobility limitations, flexibility restrictions, and strength deficits. Stability issues are frequently misidentified as strength problems. Myofascial chains connect distant body parts, with research connecting ankle range of motion to head position, hamstrings to neck position, and the bottom of the foot to hamstrings.
The bilateral force deficit describes the phenomenon where combined limb force production is less than the sum of individual limb capacities. If someone can leg press 500 pounds with each leg independently, they should theoretically press 1,000 pounds bilaterally, but typically achieve only 800-900 pounds. This deficit occurs approximately twice as often in untrained or weaker individuals. Powerlifters often demonstrate the opposite effect, called bilateral facilitation, achieving 1,100-1,200 pounds when individual legs produce 500 pounds each.
Vocational demands create asymmetries through repeated unilateral movements, such as a plumber consistently drilling or screwing on one side. Repetitive sports like golf create asymmetries through thousands of identical swings per week. Evidence suggests athletes who specialize in sports early develop more asymmetries and experience more injuries both during and after their careers. Specialization before ages 14-15 carries elevated risk, with gymnastics noted as a potential exception.
Jumping asymmetries are normal and expected. Trained jumpers show approximately twice the asymmetry of untrained individuals, with high jumpers demonstrating substantially greater performance on their dominant jumping leg compared to their non-dominant leg.
Powerlifters who train and compete in squatting tend to jump equally on both legs because they have no asymmetrical training exposure. Everything they do involves both legs in the same position at the same time. High jumpers, by contrast, perform everything on one leg, jumping higher every time they execute their skill sport, which creates significant asymmetrical differences.
Bad data is worse than no data. If asymmetries cannot be collected reliably, it is better to skip testing altogether due to the high chance of false flags. When testing for asymmetries, first determine which of the three types is being measured: morphology (muscle size, quality, or functionality), as these are distinct and provide different information.
Key factors to consider include age, sex, and experience level. Highly trained athletes are more likely to exhibit larger and more serious asymmetries, while untrained individuals may perform differently. Tests should be conducted over normal range of motion and at normal speed. Deviating from what is normal produces different results. Studies have shown that highly trained bench pressers show no asymmetries in muscle activation at 50% of their one-rep max, but asymmetries appear dramatically at 90% of maximum. Testing at body weight or light loads can miss findings that emerge under heavy loads, fast speeds, or fatigue.
The dependent variable must be clearly defined: muscle activation, range of motion, a single muscle group, or a movement pattern. It is not possible to test everything, so select one to three relevant measures and develop standardized systems while recognizing the full picture is not being captured.
Morphology refers to muscle size and shape. While it provides no direct information on contraction, range of motion, or fatigue, it is easier to standardize than functionality measures. Muscle size remains relatively stable across conditions, and imaging allows simultaneous assessment of quality.
At home, a basic tape measure can be used with careful attention to standardized landmarks. DEXA scans offer an affordable option that provides right leg versus left leg comparisons, though they do not break results down by individual muscle. Ultrasound and MRI provide more detailed imaging, with costs decreasing and accessibility improving.
Tissue quality assessment is an emerging area, with particular focus on fat infiltration inside muscle cells (distinct from body fat and not detectable by DEXA). Recent research links neural health to muscle quality, with fat infiltration often indicating upstream neurological restrictions. Fat within muscle cells replaces contractile tissue (myosin and actin), directly compromising performance regardless of effort or overall muscle size.
Fat infiltration is not solely driven by body composition and frequently results from previous injury. An example involves Doug Larson, who discovered one hip muscle was 45% smaller on the right side, with 25% fat infiltration on the right versus less than 5% on the left. This traced back to a hip dislocation approximately 10 years earlier during judo or wrestling. The muscle had not fully reattached, received no neural activation for a decade, filled with fat, and atrophied, despite surrounding muscles compensating and no ongoing pain.
Baseline scans of healthy athletes are increasingly used to detect such hidden dysfunctions during rehabilitation. Other quality factors include fascial length, muscle geometry, and pennation angle (the orientation of muscle fibers relative to bone). Parallel orientation (0°) optimizes for velocity, while perpendicular orientation (90°) optimizes for strength. Fat infiltration and asymmetries can alter pennation angles, and restoring normative angles can improve force or speed output.
Low-cost or widely available tools for muscle quality are limited. Handheld ultrasound devices are becoming more affordable and enable field research, though professional units remain expensive. Muscle biopsies are traditional but inaccessible for most. Ultrasound and MRI remain primary options. The Springbok scan (a 45-minute MRI) creates 3D models of 140 muscles, revealing asymmetries, size, and fat infiltration. Results are normalized for age, sex, sport, and position. This approach has been used with NFL, NBA, and English Premier League athletes.
Functionality testing offers unlimited options. Basic mobility can be assessed with a goniometer (approximately $25) to measure joint flexion or extension degrees. The Functional Movement Screen (FMS) evaluates whole-body movement patterns rather than isolated muscles. Examples include quadruped contralateral reach tests (checking stability differences between sides) and overhead squats (observing knee depth asymmetry).
Force plates provide sensitive measurements across multiple variables and have become more affordable (previously $35,000, now available for a few hundred dollars). Brands like VALD offer devices for grip, legs, and upper body. Isokinetic dynamometers are more common in research settings. EMG measures muscle activation via surface electrodes. Motion capture systems (such as those used at Titleist Performance Institute) track joint markers during movement to analyze force production, timing, and range of motion.
The Y-balance test or star excursion balance test (SEBT) involves single-leg balance with reaches in multiple directions (anterior, posterior, lateral), with differences exceeding four centimeters considered notable. Endless low-, moderate-, and high-cost options exist for functionality assessment.
Interpretation is the critical challenge. Asymmetries should be evaluated through two pillars: performance (does the asymmetry hinder or enhance output?) and injury risk (does it increase, decrease, or provide no insight into risk?). Asymmetry is only one of many injury variables, alongside training, nutrition, hydration, sleep, prior injury history, and age. No single asymmetry can fully predict injury outcomes.
When a muscle size asymmetry is identified, first confirm it is real. Apparent differences in trapezius muscles may result from postural compensation due to injury rather than true morphological change. Shoulder shrugging to protect a neck injury can make one trap appear larger without actual size difference. Before implementing unilateral volume increases, verify the asymmetry is not postural or injury-driven.
Anatomical factors also create normal asymmetries. When individuals become very lean, differences appear in lats and especially abs. Nearly no one has a perfectly symmetrical six-pack because bones are not symmetrical, so muscle origins and insertions vary naturally.
In general population and aging research, lower limb muscle mass asymmetries exceeding 3.5% correlate with significantly increased injury rates, with one study showing up to 35% higher injury risk. A 1% increase in asymmetry was associated with up to 83% increased injury risk in women. These small percentage differences produce clinically meaningful effects.
Aging research highlights that muscle size, fat infiltration, and connective tissue quality predict functional outcomes including balance, walking speed, flexibility, and strength. These measures are receiving increased scientific attention for understanding aging muscle and its impact on daily movement.
Morphology stacks very differently than functionality. While functionality is wildly context dependent, morphology might actually be a better place to pay attention. The math is different depending on standardization, whether tests are fast or slow or strong, and what specific test is being performed, making standardization almost impractical. The approach has shifted toward morphology because it is much easier to standardize. Bad data is worse than no data, so the focus has turned to at least getting quality data even though it is limited, using muscle size and quality imaging.
When seeing a muscle morphology, symmetry, or quality asymmetry, the threshold for action goes much lower. There is evidence that a three and a half percent difference in strength does not matter at all, but from a muscle size perspective that same difference might matter.
A comprehensive review and meta analysis published in February examined upper body asymmetry across nearly 90 studies. This research stood out because most asymmetry research focuses on the lower body. The study provided charts showing asymmetry levels by joints and movements.
The shoulder showed 15 to 20 percent differences in strength between right and left sides as very common. This included internal to external rotation, adduction and abduction, flexion and extension, and many different movement patterns. The elbow and wrist showed much lower asymmetries, typically around 5 percent. The forearm, wrist, and fingers should be really close to symmetrical.
Thresholds for action should be joint by joint and movement by movement. A 10 percent asymmetry in the shoulder might be acceptable to let go, but the same 10 percent in the elbow would warrant attention because normal asymmetry is much lower there.
Answers differ between athlete groups and non-athletes. Movement-based considerations include the classic push versus pull imbalance. Pushing exercises move something away from the body, such as push-ups or bench press. Pulling exercises bring something closer, such as bent rows, biceps curls, Romanian deadlifts, or pull-ups.
Anterior injuries often result from doing all pressing work and neglecting muscles that cannot be seen in the mirror, which form the posterior chain. Good coaches often recommend a minimum 1:1 push-to-pull ratio, with many suggesting 2:1 as a coaching tool. There is no real evidence supporting the 2:1 number specifically, but it serves as an effective coaching tactic because most people skip pulling exercises when short on time or tired.
It is almost never heard of someone developing a shoulder or knee injury from doing too much pulling. Imbalances between pressing and pulling strength are not a problem at all and can be explained almost exclusively by limb lengths.
The Vitruvian Man shows that arm span fingertip to fingertip equals height in perfect symmetry. Being a negative 2, meaning arms are 2 inches shorter than height, creates a mechanical advantage in pressing because the weight does not have to move as far. This makes pressing exercises relatively easier compared to pulling.
NBA and WNBA players average between 6 to 10 inches plus, with some seven-foot players having eight and a half foot wingspans. Longer reach provides huge advantages in pulling movements like deadlifts because the butt does not have to drop as low. Good distance runners tend to have short torsos and higher hips relative to their height. Good swimmers have shorter legs but long torsos. Limb length ratios including femur to shank to waist determine pressing versus pulling balance and should not be overcorrected.
The Functional Movement Screen research is not incredibly compelling but can be helpful when limitations are understood. The FMS involves five, six, or eight exercises performed without maximal strength, endurance, speed, fatigue, or force components. The goal is assessing quality of movement, such as whether stepping forward over a rope looks the same on left and right sides.
Scoring a D or lower indicates much higher likelihood of injury due to movement quality. Moving from a B to an A does not meaningfully change injury risk. Untrained individuals tend to show less symmetry while highly trained athletes tend to perform better. A C or lower on movement screens warrants addressing because it indicates really bad movement that increases injury or dysfunction risk.
David makes protein bars with 28 grams of protein, only 150 calories, and zero grams of sugar. Seventy-five percent of calories come from protein, which is 50 percent higher than the next closest protein bar. The gold bar contains 28 grams of protein and the bronze bar contains 20 grams of protein, both at 150 calories. One gram of protein per pound of body weight supports muscle health, recovery, lean body mass, and satiety, but most people find this challenging to achieve.
Research on asymmetrical function or performance and injury risk falls into two categories: strength and force tests, and power or functional jumping tests. Strength tests include leg extension and grip strength. Power tests include single leg hops and counter movement jumps.
Among athletes, eight of ten high-quality studies found positive associations between asymmetry in strength or power and injury risk. Six studies examined unilateral jumping specifically, and five of those six found positive associations with injury. Athletes in symmetrical sports should not show large right-to-left asymmetries in jumping.
In non-athletic populations, research focuses primarily on aging, typically 50 or 60 years old and above. One compelling study found that 60 percent of older adults who fell had more than 10 percent asymmetry in lower body power.
Hand grip asymmetry has the most research in this field, with more than 25 studies in the last five years alone examining health consequences. Around half of people tested show hand grip asymmetry of more than 10 percent, and this climbs to upwards of 75 percent in aging populations. This asymmetry is independent of being generally strong or generally weak.
Hand grip asymmetry shows two to threefold increased risk of sarcopenia. Associated outcomes include falls, cognitive disorders, functional limitations, walking speed, all-cause mortality, cardiovascular disease, respiratory disease, cancers, COPD, multimorbidities, and likelihood of being on five or more medications.
One paper found that for every 0.1 increase in hand grip asymmetry ratio, older adults had a 26 percent increased likelihood of falling. A study examining plasma neural cell adhesions, an indicator of neurological issues, found levels 20 percent higher in people with greater than 10 percent asymmetry.
Hand grip dynamometers are inexpensive, require no warm-up, and are minimally affected by hydration status or other variables that complicate other functional tests.
Whether asymmetries affect physical performance depends on sport type. Symmetrical sports like running and cycling likely need more symmetry to maximize performance. Change of direction sports show less impact from muscle asymmetries because movement patterns vary. When the same motion is repeated over and over in identical patterns with equal force on both sides, symmetry matters more.
Symmetrical sports require a lower threshold for determining when asymmetries warrant action because lower asymmetry reduces injury risk and enhances performance. Research examining performance, function, morphology, kinetics, and kinematics shows conflicting data often due to differences in exercise style tested or athlete type.
A review of studies examining performance, function, morphology, kinetics, and kinematics found that 30 papers showed no significant relationship between asymmetry and sport performance. Sixteen studies found asymmetry negatively impacted performance, and one study found it was beneficial. Of the 47 total studies, 46 found asymmetry either had no effect or hurt performance.
Greater than 5% asymmetry creates measurable performance detriment. Runners with asymmetry exceeding 5% run slower times. A 10% difference in step time or ground contact time results in a 4-8% increase in metabolic cost. Studies on force production show a 10% asymmetry leads to a 2-4% increase in metabolic power. Combined effects can raise metabolic cost between 2-14%. Research shows an R value of approximately 0.8 correlation.
A 1% increase in ground contact time asymmetry equals approximately 0.354 calories per kilogram of body weight per kilometer. This translates to roughly a 2-3 calorie increase per kilometer. At the 10% threshold, this becomes approximately 30 extra calories per kilometer, or about 300 extra calories per 10K. A marathon may require an additional 1,200-1,500 calories due to this inefficiency. One degree of difference in ankle dorsiflexion led to performance improvements of about 7.6% in men's 8K and women's 6K events.
The 10% rule of asymmetry is considered an okay heuristic but scientifically shaky. For muscle size and quality, less than 5% asymmetry is considered green with no action needed. The 5-10% range is yellow, where action is taken because it likely reduces performance even if injury risk increase is uncertain. Above 10% is red, becoming a top-tier training priority though not necessarily disqualifying an athlete from competition.
For muscle function, red begins at 20% rather than 10%. Yellow is 10-20%, and green is less than 10%. Context matters: throwing a baseball may naturally be highly asymmetrical, but a 20% difference in leg extension testing would be considered too high. Athletes should minimize asymmetry unless they have a specific reason to be asymmetrical. Asymmetries appear twice as often in the lower body compared to the upper body.
Asymmetries matter more in active people than inactive people. Asymmetries do not significantly increase injury risk in inactive individuals, but once activity begins—whether as an athlete or exerciser—the asymmetry receives more exposure and becomes problematic. The threshold for peak performance is lower than the threshold for minimal injury risk. An 8% asymmetry in a performance test may not indicate injury concern but signals inefficiency that should be addressed for optimal performance.
Regardless of athlete status, morphology versus functionality, skill or force measures, anything over 20% asymmetry is considered problematic and should be addressed. No high-quality randomized control trials exist showing that correcting a specific percentage of asymmetry reliably prevents or reduces pain or injury.
From a physique or bodybuilding perspective, the solution is typically more volume. From a physical therapy lens, the approach involves corrective exercise, slow movement, correct patterns, and bodyweight or light loads. Research in this area is thin overall. Plyometrics is one of the most studied interventions, with a review of eight randomized control trials involving about 150 participants showing unilateral plyometrics are generally more effective than bilateral plyometrics at reducing asymmetry.
Unilateral plyometrics significantly decrease asymmetry by creating neurological demand through fast movement and landing with load and force. The brain must rapidly mobilize more motor units, enhancing neuromuscular coordination and control. These movements require bilateral control through the torso, engaging hip, shoulder, neck, and posture control. This creates a balance component and ankle stability that traditional leg extension exercises do not provide.
Step 1: Pre-exercise work. Begin with soft tissue work or mobility/flexibility using a foam roller or equivalent to ensure joints can move symmetrically. Follow immediately with pre-activation on the weaker side to turn on the muscle and prepare it to fire.
Step 2: Exercise selection. Hedge programming toward unilateral exercises. Replace barbell movements with dumbbells to allow independent arm or leg work. Modify exercises to allow unilateral or isolating loading. Individualize exercises rather than using preset selections. Find variations that produce strong contraction in the target muscle even if unconventional.
Step 3: Exercise technique. Practice mindfulness during execution—remove music, podcasts, and distractions. Focus on quality of execution and verbal cues such as "squeeze" or naming the target side. Use slow tempos or isometrics to expose asymmetries and compensations that may be missed in faster movement. Work through a large range of motion, especially in the stretched position.
Step 4: Programming order. Start workouts with complex bilateral compound movements, then finish with unilateral work. Begin unilateral exercises on the weaker side. If doing isolation work, place it at the very end. Think about symmetry across the entire week rather than every single workout. Aim to train everything at least once per week.
Step 5: Volume manipulation. Add 2-5 repetitions on the weaker side per set. Perform one additional set on the weaker side. Accumulate 20% more total volume on the lagging body part throughout the week. Add one exercise per workout dedicated entirely to the lagging side. Finish with one set to failure of 15-30 repetitions on the lagging part. Perform mobility work during rest intervals rather than in separate dedicated sessions.
Recommended resources include Greg Rose from Titleist Performance Institute, Eric Cressy's Assess and Correct series with Mike Robertson, Dr. Charlie Weing's educational materials, Dr. Kelly Starret's The Ready State for mobility and movement, Aaron Alexander's alignment method, and Dr. Ryan McGrath's research at North Dakota.
Dr. Ryan McGrath at North Dakota State University runs the Hand Lab and has conducted extensive research on hand grip asymmetry and related topics.
Muscle imbalances and asymmetry concerns apply to everyone, not only athletes. These issues can significantly affect how people look, feel, and perform both currently and throughout their lives, regardless of past or future sports participation.
Most available data on asymmetries are observational and low quality, with few interventional or mechanistic studies existing. Despite this limitation, action should still be taken. The recommended approach is to collect good data and interpret findings with proper context rather than applying blanket rules such as "as long as it's less than 10% it's okay."
Skill-driven asymmetries are likely beneficial for sports performance. A clear example is throwing harder with the dominant hand when playing baseball. These skill-driven asymmetries occur mostly in the upper body and are probably less detrimental overall.
Lower body asymmetries are more likely to be problematic. In sports requiring foot skill, some lower body asymmetry may appear, but most of the time lower body imbalances warrant further analysis.
Upper body asymmetries that are skill-driven are generally acceptable. Lower body asymmetries require more scrutiny and are less likely to be harmless.
Movements intended to be symmetrical, including gait-related activities such as running, squatting, and pushing, should demonstrate even performance. When asymmetries appear in these movements, action should be considered.
Muscle size and muscle morphology asymmetries represent a bigger concern than functional or performance-based asymmetries.
Listeners are encouraged to perform asymmetry testing on themselves. Small imbalances and differences are normal and not necessarily harmful. Testing may also reveal opportunities to improve performance and set new personal records.
If you have a body, you are an athlete.
— Bill Bowerman
Keep Andy Galpin in your library
Save the videos and channels worth coming back to, and find them again in one place.
