Science of Muscle Growth, Increasing Strength & Muscular Recovery

This episode is the fourth and final one in a month of episodes on skills and athletic performance. Earlier episodes covered neuroscience-based tools for faster fat loss and skill learning, including motor skills such as playing the piano. The host, who introduces himself as Andrew Huberman, a professor of neurobiology and ophthalmology at Stanford School of Medicine, says this one is about muscle. He presents muscle as important for athletic performance, for daily life and for longevity. He says hypertrophy will come up, but the main subject is how muscle relates to the nervous system.

the whole reason why you have a brain is so that you can move.

The host lists several jobs muscle does. The brain can drive movements of different speeds and durations. Training can make muscles lift heavier loads or carry us further (endurance), and it can make movement smoother, faster and more supple. Muscle also matters for metabolism: the host says that having more muscle, and better-quality muscle, raises metabolism and goes with better health. He also says that the ability to jump, and to stand up or get off the floor quickly, is "one of the most predictive markers of aging and biological aging," and he ties this to the connection between brain and muscle. He states this claim without citing a specific study.

Posture gets special attention as something people "don't talk about" enough. The host links posture to breathing and even to how alert or sleepy a person feels. He says metabolism, movement, posture and appearance are all connected. When posture changes, appearance and movement change too. Better muscle quality, with or without bigger muscles, is said to affect the immune system and other organs as well as the nervous and muscular systems.

The planned structure is about ten minutes on mechanism: how neurons control muscle and how muscle uses energy. After that come protocols for strength, hypertrophy, endurance, posture and movement. He also previews nutrition themes, including an amino acid that he says can help build and improve muscle when it is in the bloodstream often enough and at high enough levels, along with exercise regimes and supplementation. Recovery is framed as the period when improvement actually happens: "None of that actually happens during training, it happens after training." He notes that people are often confused about how to optimize recovery and how to measure it.

Before the main topic, the host recaps the previous episode. It covered shiver-induced fat loss, NEAT (non-exercise activity thermogenesis) and using cold to boost fat loss. The protocol described there was to get into some form of cold (an ice bath, cold shower, river or ocean) until shivering starts, then get out without crossing the arms or huddling, let the cold evaporate off the body while shivering continues, and then go back into the cold. He says an illustrated version is free at thecoldplunge.com under a protocols tab. He also asks listeners to subscribe to the YouTube channel as a no-cost way to support the podcast.

The host argues that muscle is involved in nearly everything: speaking, sitting, standing, lifting, breathing, walking and skills of every kind. So the real subject is what controls muscle, which is the nervous system. He describes three main nodes of control:

Upper motor neurons in the motor cortex, which drive deliberate movements, such as choosing to pick up a pen and put it down. Lower motor neurons in the spinal cord, which send axons to muscles and make them contract by releasing acetylcholine onto them. In the neuromuscular system, the host calls this release "the way, the only way, that muscles can contract." Central pattern generators (CPGs) in the spinal cord, which produce rhythmic, reflexive movements such as walking. When a movement becomes deliberate, top-down control from the upper motor neurons takes over.

The host notes that acetylcholine also has a role in the brain, where he says it is important for focus and can gate neuroplasticity. From this simple system he draws a strong conclusion about hypertrophy: muscle growth is controlled by the nervous system, not by processes inside the muscle alone. He says the same holds for endurance, flexibility, suppleness and explosiveness. The practical question then becomes which part of the system to target: the upper motor neurons, the CPGs, the lower motor neurons, or all three.

So you can forget the idea that the muscles have memory or that muscles grow in response to something that's just happening within the muscle; it's the nerve-to-muscle connection that actually creates hypertrophy.

For historical context, the host cites the Nobel laureate Sherrington, who called movement "the final common path." He suggests that movement is one reason, "perhaps the predominant reason," the human brain is so large. He argues that most of the brain's "real estate" goes to vision and movement rather than to thinking or creativity. Other animals have neuromuscular junctions and central pattern generators, he says, but lack upper motor neurons that can direct muscles so precisely. These comparative claims are stated without specific evidence in this section.

Any movement, whether a lift, a yoga pose or picking up a pen, involves flexors and extensors. The biceps (a flexor) and triceps (an extensor) are antagonistic muscles that move a limb in opposite directions. Bringing the wrist toward the shoulder is flexion. Moving it away is extension. The host says the wiring between nerves and muscle means that when acetylcholine activates the biceps, the triceps is inhibited, and the reverse is also true. He adds that "There are ways to bypass this, but that's the typical mode of action."

The same pattern applies to the trunk and legs. The abdominal muscles, among others, flex the spine and bring the chin toward the waist. The lower-back extensors tilt the chin back toward the ceiling. The hamstrings bring the heel up toward the glutes, and the quadriceps are their extensor opposite. Stabilizing muscles also exist, but the host describes this reciprocal arrangement, also called mutual inhibition, as typical of most limb movements. He offers it as the simplest form of neuromuscular physiology before turning to metabolism.

The fat-loss episode ended with ATP, which the host calls "the bottleneck and final common path" for energy production in the body. Muscles, he says, do not normally run on fat. They run on glycolysis, the breakdown of glycogen and glucose. He describes glucose stored in muscle as having "six carbons and six waters," which is split into two three-carbon pyruvate molecules. This step yields only a little ATP.

Oxygen is the key variable. With enough oxygen, pyruvate goes into the mitochondria, where the electron transport chain and citric acid cycle produce "28 to 30 ATP, which is a lot of ATP." From this the host concludes that muscle is metabolically expensive. Compared with fat, bone and almost every other tissue except the brain, muscle is the most metabolically demanding. That, he says, is why people with more muscle relative to fat can eat more and act like "more of a furnace."

Next the host takes on a common belief about the burning feeling in hard-working muscles, the kind felt when carrying heavy groceries or exercising hard. He means low oxygen in the muscle, not in the environment. When a muscle works too hard or too long and lacks oxygen, pyruvate takes on hydrogen and becomes lactate.

So believe it or not, humans don't make lactic acid. That's another species; we make lactate.

He rejects the idea that lactate causes the burn or limits performance ("Guess what? That's not true at all.") and says lactate has three functions. First, it buffers acidity: the burn is acidity, and lactate is there "to suppress the burn, to suppress acidity," the opposite of the popular view. Second, it is a fuel: lactate is shuttled to working areas of the muscle, so contractions can continue without oxygen. Third, covered next, it acts as a hormonal signal.

The main practical recommendation in this part applies to any kind of exercise, including weight training, running, cycling and swimming. About 10% of the time, train hard enough to feel the burn. The stated reason is that lactate arriving at the site of the burn acts as a hormonal signal to other organs. The host gives the general definition of a hormone, a chemical released in one place that travels and affects other organs, and says lactate can send positive signals to the heart, the liver and the brain. He says these effects last "not just in those moments but in the period of time that follows."