Intermuscular and intramuscular fat are two distinct fat depots that occupy different physical spaces in the body and carry different health implications. Intermuscular fat (often abbreviated IMAT) sits between separate muscles, filling the connective-tissue planes that separate one muscle belly from the next. Intramuscular fat (sometimes called IntraMAT or IMF) lives inside the muscle itself, deposited among individual muscle fibers. Despite both being classified as “ectopic” fat, meaning fat stored outside the usual subcutaneous and visceral compartments, the two depots behave differently in metabolism, respond differently to exercise, and signal different things to clinicians reading a scan.
Where Each Fat Depot Actually Sits
Think of a chicken thigh. The visible white streaks running through the meat are intramuscular fat. The larger pads of fat you find when you pull two muscles apart from each other are intermuscular fat. The same spatial logic applies to human limbs. When you look at a cross-sectional MRI of someone’s thigh, the bright pixels scattered inside the muscle tissue represent intramuscular fat, while the bright regions nestled in the fascia between muscles represent intermuscular fat. MRI can distinguish between the two by plotting histogram thresholds that separate muscle pixels from fat pixels based on their signal intensity, though the exact cutoff values researchers use still vary across studies.
Both depots increase with age. A study examining thigh composition found a significant positive relationship between age and the percentage of intermuscular fat in the thigh, with fat steadily accumulating as people got older.1PubMed Central. Skeletal muscle fat infiltration: impact of age, inactivity, and exercise Intramuscular fat also increases with age and in certain disease states, with genetic analysis pointing to specific genes such as Lipin1, Lipin2, and Perilipin 2 as regulators of how intramuscular fat cells develop and accumulate lipid.2PubMed Central. Intermuscular and intramuscular adipose tissues: Bad vs. good adipose tissues But the two depots do not always increase at the same rate or in response to the same triggers, which is part of why researchers treat them as separate entities rather than lumping them together.
Metabolic Health and Disease Risk
Intermuscular fat has drawn particular attention for its metabolic associations. A systematic review examining both fat depots found that intermuscular fat was significantly associated with insulin resistance, type 2 diabetes, and metabolic syndrome across multiple studies. The odds ratios for type 2 diabetes ranged from about 1.2 to 3.3 depending on the study. Intramuscular fat was also linked to insulin resistance and metabolic syndrome, but its relationship with type 2 diabetes specifically was more mixed, with some studies showing a connection and others not.3PubMed. Association of intermuscular and intramuscular fat with insulin resistance, type 2 diabetes, and metabolic syndrome: A systematic review
The cardiovascular story follows a similar pattern for intermuscular fat. Research has found that intermuscular fat has independent associations with fasting blood glucose and post-meal blood glucose, rivaling the well-known associations seen with visceral belly fat.4PubMed Central. Intermuscular adipose tissue rivals visceral adipose tissue in independent associations with cardiovascular risk That word “independent” matters: the link held even after accounting for how much visceral fat someone carried, suggesting intermuscular fat is not just a proxy for overall obesity. A large community-based study (the CARDIA Study) found that people in the highest quarter of abdominal intermuscular fat volume had about 1.6 times the odds of having subclinical coronary artery calcification compared to those in the lowest quarter, and this association persisted after adjusting for BMI, visceral fat, and even pericardial fat.5PubMed Central. Intermuscular Adipose Tissue and Subclinical Coronary Artery Calcification in Midlife: The CARDIA Study
So intermuscular fat appears to be a fairly consistent metabolic troublemaker. Intramuscular fat is harder to pin down, and that ambiguity leads directly to one of the more interesting puzzles in exercise science.
The Athlete’s Paradox
Here is the twist that complicates the simple “fat in muscle equals bad” narrative. Endurance athletes carry more intramuscular fat than sedentary people, yet they are substantially more insulin sensitive, not less. Compared with untrained controls, athletes in one study had roughly 40% more intramuscular triglyceride stored in their muscles, along with a synthesis rate for that fat that was more than double the rate seen in controls. Yet those same athletes were about twice as insulin sensitive.6PubMed Central. Increased intramuscular lipid synthesis and low saturation relate to insulin sensitivity in endurance-trained athletes
The explanation appears to lie in how and where the fat is stored within the muscle cell, not simply how much is present. Research comparing lean, athletic, obese, and type 2 diabetic individuals found that insulin sensitivity followed a clear hierarchy: athletes were the most sensitive, then lean, then obese, then diabetic. Most intramuscular triglyceride species located near the cell membrane were elevated in obese and diabetic people compared with lean and athletic individuals. But triglycerides with only fully saturated fatty acid chains were elevated specifically in the diabetic group.7Diabetes. 230-OR: Intramuscular Triglyceride Subcellular Localization Is Related to Insulin Sensitivity in Humans Athletes also showed lower saturation of diacylglycerol, a lipid intermediate that, when saturated, tends to interfere with insulin signaling.
In plain terms, athletes’ muscles are efficient at turning over intramuscular fat as fuel. The fat cycles in and out rapidly, stays relatively unsaturated, and is stored in locations within the cell that don’t gum up insulin signaling. In someone who is sedentary and metabolically unhealthy, the same type of fat sits in the wrong places, accumulates saturated species, and disrupts the cell’s ability to respond to insulin. The amount of intramuscular fat alone does not tell you whether it is harmful. Context, turnover rate, and molecular composition all matter.
How Doctors See and Measure Muscle Fat
Distinguishing between intermuscular and intramuscular fat in a living person requires imaging. CT scans, MRI, and ultrasound each offer different advantages. CT measures muscle attenuation, essentially how dense the tissue appears on the scan, which drops when fat infiltrates muscle. MRI can measure proton density fat fraction and use specialized sequences like the Dixon method to separate water-containing tissue from fat-containing tissue. Ultrasound has emerged more recently as a bedside option, using echo intensity and shear wave elastography to assess changes in muscle tissue composition.8PubMed Central. Myosteatosis: diagnostic significance and assessment by imaging approaches
MRI is generally considered the most precise tool for separating intermuscular from intramuscular fat, because it can produce detailed cross-sectional images where fat and muscle pixels have distinct signal intensities. However, the thresholds used to classify a pixel as “fat” versus “muscle” are not yet standardized. Different research groups use different cutoff methods, such as the Midpoint technique versus the Otsu technique, which can produce somewhat different estimates from the same scan.9PubMed Central. Quantification of intermuscular and intramuscular adipose tissue using magnetic resonance imaging after neurodegenerative disorders This lack of standardization is one reason published numbers on muscle fat prevalence vary between studies and should be compared cautiously.
When Fat Replaces Muscle in Disease
In neuromuscular diseases, intramuscular fat replacement takes on a dramatic and clinically significant form. In Duchenne muscular dystrophy and other dystrophies, progressive replacement of muscle fibers with fat and fibrous tissue leads to a phenomenon called pseudohypertrophy: the muscle looks larger but is actually weaker, because much of its volume is fat rather than contractile tissue. The same pattern occurs in denervation conditions such as spinal muscular atrophy, poliomyelitis, radiculopathy, and peripheral nerve injuries, where partial loss of nerve supply to a muscle triggers fat to fill in among the remaining fibers.10PubMed Central. Pseudohypertrophy of calf muscles associated with diabetic neuropathy
MRI has become an important diagnostic tool in these conditions precisely because different diseases produce characteristic patterns of which muscles get infiltrated and which are spared. By analyzing T1-weighted images of the pelvis, thigh, and leg, radiologists can often distinguish between different muscular dystrophies and inflammatory muscle diseases based on the specific muscles that show fat replacement.11PubMed. Morphologic imaging in muscular dystrophies and inflammatory myopathies A patient with limb-girdle dystrophy will show a different map of affected muscles than someone with facioscapulohumeral dystrophy, and this pattern recognition can narrow the diagnosis before genetic testing even comes back.
The relationship between fat and muscle during repair is not entirely adversarial, though. During muscle regeneration after an injury, a controlled amount of adipose tissue contributes metabolic support and signaling that helps the repair process along. The problem arises when that fat accumulation becomes excessive or dysregulated, leading to persistent functional impairment rather than recovery.
Can You Exercise Away Muscle Fat?
Given the metabolic associations, the natural question is whether exercise can reduce intermuscular or intramuscular fat. The answer is frustratingly inconsistent, especially for intermuscular fat. A systematic review of exercise interventions found that resistance and aerobic exercise combined with a calorie-restricted diet produced inconsistent results for reducing thigh intermuscular fat. When people exercised without cutting calories, there were consistently no significant changes in either intermuscular or intramuscular fat, regardless of the muscle group studied. Combining aerobic and resistance training simultaneously did not reliably reduce either depot with or without calorie restriction.12PubMed Central. Effects of exercise with or without a hypocaloric diet on intermuscular and intramuscular fat: a systematic review
This does not mean exercise is useless for muscle quality. Exercise clearly improves insulin sensitivity and metabolic health through multiple pathways. But the idea that you can “burn off” the visible fat between or within your muscles with a training program has limited support in the current evidence. The review also noted that very few studies have specifically tracked intramuscular fat changes in response to exercise, so the evidence base is thin. It may be that exercise changes the character of intramuscular fat (improving turnover and reducing saturated species, as seen in athletes) more readily than it reduces the total volume.
Aging, Mobility, and an Unresolved Debate
As mentioned earlier, intermuscular fat increases with age. But the assumption that this directly translates into worse physical function is not as settled as many clinicians assume. One large study following older adults in Iceland found no evidence that muscle fat infiltration was associated with new mobility disability or declining gait speed over the follow-up period, despite earlier work suggesting such a link.13The Journals of Gerontology: Series A. Muscle Quality and Muscle Fat Infiltration in Relation to Incident Mobility Disability and Gait Speed Decline: the Age, Gene/Environment Susceptibility-Reykjavik Study The researchers themselves raised the possibility that the relationship between muscle fat and physical function is less straightforward than previously believed.
This is an area where the science is genuinely unsettled. Cross-sectional studies (snapshots at one point in time) consistently show that people with more muscle fat tend to be weaker and slower. But longitudinal studies (tracking people over years) have not always confirmed that gaining muscle fat predicts future decline. It could be that muscle fat is a marker of other processes, such as chronic inflammation, disuse, or metabolic dysfunction, rather than a direct cause of weakness. Or it could be that the imaging methods currently used are not precise enough to capture the depot-specific changes that matter most. Either way, treating intermuscular fat accumulation as an automatic death sentence for mobility is getting ahead of the evidence.
Intramuscular Injections and Why They Sometimes Miss
The word “intramuscular” also comes up in a completely different context: drug delivery. Intramuscular injections are designed to place medication deep into muscle tissue, which is more heavily supplied with blood vessels than the fat layer just beneath the skin. This richer blood supply means drugs are absorbed faster after an intramuscular injection, which is why certain vaccines, antibiotics, and psychiatric medications are given this way. The faster absorption also comes with trade-offs: intramuscular insulin, for instance, leads to larger swings in blood sugar, a higher risk of low blood sugar episodes, and more pain at the injection site compared with subcutaneous administration.14PubMed Central. Subcutaneous Injection of Drugs: Literature Review of Factors Influencing Pain Sensation at the Injection Site
A practical problem is that intended intramuscular injections frequently miss their target. Studies have found that the success rate of actually reaching muscle tissue with a gluteal injection ranges from roughly 32 to 52 percent, with the remainder depositing the drug into subcutaneous fat instead. The failure rate is even higher in populations with greater subcutaneous fat depth, including people who are obese and, for anatomical reasons, women more than men. This matters because a drug designed to absorb quickly from muscle tissue will absorb more slowly and unpredictably from fat, potentially affecting how well the medication works.
Intermuscular Versus Transmuscular in Surgery
Surgeons also use the intermuscular/intramuscular distinction when describing how they get to a joint. In hip replacement surgery, for example, an intermuscular approach means the surgeon navigates between muscles to reach the hip joint, ideally without cutting through any muscle belly. A transmuscular approach cuts through muscle tissue to gain access. The theory behind intermuscular approaches is that preserving the muscle should mean less damage, less pain, and faster recovery. In practice, the differences in outcomes are often more modest than patients expect. One comparison of an anterolateral intermuscular approach versus a lateral transmuscular approach for minimally invasive hip replacement found no significant differences in surgical time, blood loss, or hospital stay.
Marbling and Meat Quality
If the distinction between intermuscular and intramuscular fat sounds familiar from a non-medical context, it should. In meat science, these two fat types determine much of what makes a steak taste and feel the way it does. Intramuscular fat is marbling: the thin white streaks running through the lean meat that melt during cooking and contribute to flavor, juiciness, and tenderness. Intermuscular fat is the thicker seams of fat found between muscles in a primal cut, sometimes called seam fat. USDA beef grading relies heavily on marbling score as a quality indicator, and research dating back decades has shown that marbling combined with other measurements like external fat thickness and rib-eye area can account for a substantial portion of the variation in a carcass’s lean meat yield.15Oxford Academic (Journal of Animal Science). Marbling: Its Use in Predicting Beef Carcass Composition
Animal genetics researchers have found that the biological pathways governing intramuscular fat deposition are distinct from those governing fat deposited elsewhere in the animal. In chickens, intramuscular fat deposition in breast muscle is driven primarily by carbohydrate metabolism pathways, while abdominal fat deposition relies on fatty acid and glycerol metabolism.16PubMed Central. Differential regulation of intramuscular fat and abdominal fat deposition in chickens This means that breeding or feeding strategies aimed at increasing desirable marbling without also making the animal fatter overall require targeting specific metabolic pathways, not just total energy intake. The same biological independence between fat depots that makes human intermuscular and intramuscular fat behave differently in disease also makes them respond to different genetic and nutritional levers in livestock.
Force Transmission Between Muscles
The connective tissue planes where intermuscular fat accumulates are not inert packing material. Those fascial layers transmit mechanical force between adjacent muscles, which is why some researchers and clinicians talk about “myofascial chains” running along the limbs. Cadaver studies have measured this directly: when force was applied to the Achilles tendon, a well-correlated force could be measured in the plantar fascia of the foot, with almost half of the tendon force detectable in the fascia during simulated walking.17PubMed Central. Intermuscular force transmission along myofascial chains: a systematic review When fat infiltrates these connective tissue spaces, it could plausibly alter how force gets transferred from one structure to the next, though this specific connection has not been well studied in living humans. It is one more way in which intermuscular fat, by occupying a mechanical space between muscles rather than sitting quietly inside one, could influence function through pathways beyond pure metabolism.