When we think of the brain and skeletal muscle, they seem entirely unrelated. They develop from different embryonic tissue, serve fundamentally different functions, and have historically been studied in isolation. Yet a growing body of research — accelerating rapidly since the mid-2000s — demonstrates that these two organ systems are in constant, bidirectional communication. Dr. Dvir Dori, a neurologist trained in sports medicine, neuromuscular disorders, and cognitive neurology, presented the latest evidence at the Baycrest Neuro Network Speaker Series. His message was clear: exercise is not a nice-to-have supplement to brain health — it is medicine, and its effects have been severely underestimated.

A Shifting Medical Paradigm

The medical community's understanding of exercise has undergone a dramatic transformation over the past seven decades. In the 1950s, the prevailing view held that physical activity offered little health benefit beyond cosmetic improvements. By the 1970s, physicians recognized some benefits but advised patients with chronic diseases to rest and conserve energy. As late as the 1980s, the exact mechanisms by which exercise helped remained unclear.

The cardiovascular connection came first — the understanding that aerobic activity reduces cholesterol deposits in arteries and strengthens the heart. For years, this vascular mechanism was assumed to be the only way exercise might affect the brain: if it clears arteries in the heart, it presumably clears the small vessels in the brain as well.

That view has now been superseded. Exercise benefits the brain through multiple independent pathways that go far beyond vascular health. And the volume of research reflects this shift — a search of the National Library of Medicine reveals a near-exponential rise in publications linking exercise to neurological conditions, with the growth curve still steepening.

The Effect Size Problem: Why Exercise Is Underestimated

While the scientific community now accepts that exercise benefits neurological health, the measured effect sizes in published studies are often characterized as small. Dr. Dori argued this is a measurement failure, not a biological reality, and outlined three reasons why.

No Standardized Definition of Exercise

Studies have historically lacked consistent definitions of what constitutes "exercise." One trial might count walking; another might involve supervised high-intensity intervals. Comparing effect sizes across such heterogeneous interventions inevitably dilutes the apparent impact.

Measurement Bias

Even within individual studies, exercise has been measured incorrectly. Dr. Dori cited a 2015 study published in Neurology that used a single VO2 max reading as a proxy for aerobic fitness. However, a single-point VO2 max measurement reflects genetic cardiopulmonary capacity more than it reflects training status. The researchers failed to track improvement over time — the metric that actually captures the effect of exercise.

Self-Report Bias

When participants are asked to recall how much they exercised over months or years, they consistently overestimate. If a subject reports exercising five hours per week but actually completed two, the calculated benefit-per-hour of exercise appears artificially small. Correcting for this inflation would dramatically increase the measured effect size.

What Mice Teach Us About Human Inactivity

One of the more provocative points in Dr. Dori's talk concerned the persistent gap between promising results in animal models and disappointing results in human trials. Many therapies that cure or slow Alzheimer's disease in mice fail to translate to humans. A commonly overlooked explanation: mice are dramatically more physically active than humans.

Mice are obligatory runners — they never walk, even in captivity. When researchers placed running wheels in the wild, free-living mice voluntarily used them extensively, debunking the theory that captive mice only run out of boredom. Scaled to human stride length, even a sedentary mouse's nightly activity would be equivalent to a person running 75 kilometres a day. The most active mice scale to 150 km. When drug trials succeed in animals running the equivalent of ultramarathons and fail in sedentary humans, baseline fitness may be the confounding variable.

Runner's High: It's Not the Endorphins

Mice enjoy running far more than most humans, in part because they possess a highly developed endocannabinoid system. The term derives from cannabis — these are naturally occurring substances in the body that produce effects similar to cannabis without the addictive downside.

The runner's high that many people associate with endorphins is actually produced by endocannabinoids. Endorphins, though released during exercise, are large molecules that cannot cross the blood-brain barrier. They act peripherally, primarily reducing pain perception — which is why an injured athlete can often continue playing and only feel the pain after the match. The euphoria, however, comes from endocannabinoids that do cross the blood-brain barrier and act directly on the brain.

Exercise as Medicine: The Pharmacological Framework

Beginning around 2013–2015, a new initiative emerged: treat exercise not merely as a lifestyle recommendation but as a prescribable medicine — investigated with the same rigor applied to pharmaceutical drugs. Dr. Dori described three pillars of this framework, drawn directly from pharmacology.

This pharmacological framing has practical implications. Just as a physician adjusts a medication dose based on a patient's body composition, genetics, and existing conditions, exercise prescriptions should be individualized. Two patients of different sexes, body types, and genetic backgrounds should not receive the same exercise instruction.

The Muscle-Brain Axis: Your Muscles Are an Endocrine Organ

Perhaps the most important scientific development Dr. Dori described is the discovery that the muscle-brain connection is not one-directional. For decades, neuroscience understood the pathway from brain to muscle: the motor cortex issues a command, the signal travels through upper and lower motor neurons, and the muscle contracts. This was assumed to be a one-way street.

We now know muscle communicates back to the brain — and it does so not only through neural retrograde transport but by functioning as an endocrine organ. During exercise, particularly during resistance training, contracting muscle cells release proteins that act as hormones, enter the bloodstream, and cross the blood-brain barrier.

Irisin

Irisin is a myokine released primarily during strength training. After crossing the blood-brain barrier, it acts on the hippocampus — the brain's critical memory centre. There, it promotes neurogenesis (the creation of new neurons) and inhibits the rate at which existing neurons degrade. In essence, irisin both builds and preserves the brain's memory infrastructure.

Cathepsin B

Cathepsin B is released during combined aerobic and resistance exercise and has potent anti-inflammatory properties that target the brain specifically rather than acting systemically. This specificity is significant because neuroinflammation is a central driver of Alzheimer's and other neurodegenerative diseases. The mechanism has an intuitive logic: when exercise causes micro-tears in muscle fibres (the foundation of strength training), rebuilding requires a low-inflammation environment, so the muscle signals the body to suppress inflammation — and the brain benefits directly.

Measuring What Matters

A recurring theme in Dr. Dori's research is the importance of precise measurement. At the Kimmel Family Center for Brain Health and Wellness at Baycrest, his team uses DEXA scanning — typically used for bone density — to measure body composition, distinguishing fat tissue from muscle tissue. Combined with strength measurements from dynamometers and Cybex machines, they can calculate a metric that accounts for body size: how much force a given quantity of muscle can produce.

This approach allows fair comparisons across sexes and body types and may eventually identify individuals whose muscles are unusually efficient — producing disproportionate strength relative to muscle mass. Studying the genetic and protein profiles of these individuals could reveal new drug targets for cognitive protection.

Beyond Brain and Muscle: Exercise Across Neurological Disease

The benefits of exercise extend across virtually every category of neurological disease. For stroke, exercise serves as primary prevention through vascular health. For Parkinson's disease — a neurodegenerative condition where pathological brain changes begin decades before clinical symptoms — exercise is one of the most effective early interventions, with structured dance (learning systematic choreography) showing particular promise. Exercise also reduces seizure frequency in epilepsy, slows progression in ALS and neuromuscular disease, and helps prevent chronic headaches.

Current Exercise Recommendations for Brain Health

Dr. Dori emphasized that these recommendations should be individualized. Aerobic intensity is relative: for someone with limited mobility or chronic heart disease, walking at a pace that makes conversation difficult is aerobic exercise. People who use wheelchairs can achieve aerobic training through hand cycles. The threshold is personal — the talking test applies universally because it calibrates to the individual's capacity.

The One-Third Rule: Why Dieting Alone Fails

An important practical takeaway concerned weight management. When individuals lose weight through caloric restriction alone — without exercise — approximately one-third of the weight lost is muscle mass, not fat. For some people, this ratio rises to one-half. Since muscle tissue is itself protective of brain health (through the myokines it releases), losing muscle undermines the very system that supports cognitive function.

The countermeasure is twofold: adequate protein intake and resistance training. Moreover, without exercise, calorie-restricted diets tend to plateau after about two weeks as the body's metabolic systems adapt to conserve energy — a survival mechanism inherited from our evolutionary ancestors who sometimes subsisted on as few as 500 calories per day. Sustained weight management requires the metabolic stimulus of physical activity.

The Road Ahead

Dr. Dori's research at Baycrest's Kimmel Family Center is part of a broader effort to move exercise science from population-level recommendations to precision prescriptions. By combining detailed body composition analysis, muscle-quality metrics, genetic profiling, and longitudinal cognitive tracking, his team aims to identify the specific mechanisms by which different types and intensities of exercise protect the brain — and ultimately to translate those mechanisms into targeted therapies.

The challenge is also an institutional one. Pharmaceutical companies have little commercial incentive to fund exercise research. Government agencies sometimes decline to fund it because exercise operates through too many simultaneous mechanisms to isolate a single causal pathway. The specialized measurement tools at the Kimmel Center are designed precisely to overcome this objection — to isolate specific aspects of muscle function and physical activity and connect them to measurable cognitive and biomarker outcomes.

As Dr. Dori noted, the world may not be able to match the exercise habits of laboratory mice. But the gap between what we currently do and what the evidence suggests we should do remains enormous — and closing even a fraction of it may be among the most powerful interventions available against cognitive decline.