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by Jonathon Sullivan MD, PhD, SSC

Preserving muscle tissue also helps preserve heart, brain, and bone tissue.

(This blog post was subsequently produced as a video, which you can view below.)

In emergency medicine, you often hear the phrases “time is brain,” “time is bowel,” or “time is muscle.” These aphorisms reflect the urgency of preserving critical tissues that are in danger of being lost without restoration of blood flow or relief of pressure. In the setting of stroke or cerebral ischemia, blood flow must be restored or the patient will lose brain tissue, which is every bit as bad as it sounds. In the setting of mesenteric ischemia, circulation to the gut is impaired, and without remediation the patient will end up with dead bowel, which carries an extremely high mortality. When ER docs say “time is muscle,” they mean heart muscle, which is being lost every minute that a coronary artery remains blocked. Other forms of tissue destruction are more gradual and apparently less urgent, but almost as devastating. Loss of pancreatic beta cells due to metabolic syndrome and “burnout” results in progressive and eventually irreversible Type 2 Diabetes. Loss of bone tissue culminates in full-blown osteoporosis and eventually pathological fractures that can cripple or kill. Loss of cartilage…well, I don’t have to tell you how much that sucks.

Tissue loss due to injury, illness, and chronic degeneration are part of aging, and not a good part. Loss of brain or heart tissue is potentially devastating, of course, and in my specialty they were a primary focus of medical practice. Entire critical-care teams stood ready to intervene if I identified a patient with a stroke or a heart attack. With one phone call, I could have a cardiac patient whisked away to the cath lab to open a coronary, or activate a stroke team to treat a cerebral infarction with clot-buster (thrombolytics). Dying gut was a surgical emergency and a quick trip to the interventional radiology suite or the OR. Delay in diagnosis and intervention was not acceptable. Time is tissue.

In emergency practice, the loss of bone and beta cell mass and cartilage due to slow degeneration was rather less urgent. Although at some level we recognized that such degeneration was serious, it wasn’t emergent. It was something for the patient’s primary physician to deal with. The problem, of course, is that all too often it wasn't dealt with. Modern medicine is fixated on medical interventions, with pharmaceuticals and procedures and, oh, all right then, eat-better-and-get-some-exercise.

Time is muscle and brain and gut. But time is also bone, cartilage, and beta cell mass.

And time is also skeletal muscle. When an emergency doctor says “time is muscle,” you can bank odds that he’s not talking about the patient’s hamstrings, hip abductors, and shoulders. He’s talking about heart muscle. And that’s appropriate right now. But if somebody (like, say, the patient or his primary doctor) had emphasized that time is also skeletal muscle, then we might have avoided altogether a scenario in which time is heart muscle.

We are now well past the stage where we understood skeletal muscle as only a motor organ—elaborate, miraculous, beautifully engineered, yes, but a motor organ nonetheless. It is most certainly that, but thanks to the groundbreaking work of Pederson and many others we now know that it is much more.


Muscle is now understood by biologists to be a secretory organ…a gland. Dozens of signaling molecules are released by muscle, and receptors for these signaling molecules are found in muscle itself, in pancreas, liver, fat tissue, bone, gut, immune tissues, brain, and heart. These myokine signaling molecules regulate metabolism, sleep, neuroplasticity, inflammation, fat accretion, appetite, cardiac and bone remodeling, and many other processes. So the effects of muscle as an endocrine organ are every bit as far-reaching and important for health as those of the thyroid or the adrenals.

This is both good and bad, because the myokine profile of trained, exercising muscle is different from that of untrained, atrophic, sedentary muscle. Untrained muscle therefore has direct and unhealthy signaling impacts on inflammation, fat storage, cardiac modeling, brain tissue modeling and retention, bone density, and metabolism.

Even if muscle were merely a motor organ, its retention and training would be mandatory for healthy aging. Strong muscle yanks on bones in a way that mechanically stimulates them to grow. Exercising muscle consumes calories and indirectly exercises the heart and lungs, to their benefit. Trained muscle tissue increases our strength, power, endurance, balance, body composition, and resistance to injury and illness. But we now know that trained muscle is also a trained gland, the most massive gland in the body, and that maintaining that gland in a healthy condition is critical for healthy aging in ways that transcend the mechanical effects of making our muscles bigger and stronger. Training our muscles preserves more than just strength and function. Training muscle preserves other tissues as well, by maintaining a myokine hormone profile that signals to the entire organism that it’s not even close to done living. In the catastrophic arena of emergency medicine, time (in minutes) is heart and brain. But in the arena of life, time (in years) is also skeletal muscle. If we take that time, now, to train muscle rather than lose it, then skeletal muscle is also heart and brain, and with luck we may never find ourselves in the situation where we are losing heart and brain tissue by the second.

Jonathon Sullivan MD, PhD, SSC, PBC is a retired emergency physician and research physiologist, and the owner and head coach of the Greysteel Strength and Conditioning Clinic in Farmington Hills Michigan, which specializes in training adults over 50. He is the author of The Barbell Prescription: Strength Training for Life After Forty, with Coach Andy Baker.

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