Your strength comes from a chain of systems working together: your brain sending signals to your muscles, your muscle fibers contracting in response, your tendons transmitting that force to your bones, and your bones acting as levers to move a load. No single factor determines how strong you are. Strength is the product of your nervous system, your muscle tissue, your body’s geometry, and even your genetics, all firing in coordination.
Your Brain Controls How Much Muscle You Use
Strength starts in the nervous system, not in the muscle itself. Your brain generates a signal that travels down motor neurons to activate muscle fibers. Each motor neuron controls a bundle of fibers called a motor unit, and your brain controls force output in two ways: recruiting more motor units and increasing the rate at which those units fire electrical signals.
At low effort levels, recruitment does most of the work. Your brain activates just enough motor units to match the demand. But at moderate to high forces, the firing rate of each motor unit becomes the dominant factor. When all 120 or so motor units in a typical muscle fire at their minimum rate (about 8 pulses per second), the result is only about 25% of that muscle’s maximum force. Ramping the firing rate up to 25 to 35 pulses per second accounts for the remaining 75%. In other words, how fast your neurons fire matters more than how many fibers you activate once you’re past light effort.
This is why strength training improves performance before muscles visibly grow. Early gains in a new lifting program are largely neural: your brain learns to recruit motor units more efficiently and fire them at higher rates.
Your Brain Also Holds You Back
Your nervous system doesn’t just produce force. It also limits it. A theory known as the central governor model proposes that your subconscious brain regulates power output by controlling how many motor units it allows to activate. The purpose is self-preservation: matching energy consumption to energy production so your muscles never reach a state of catastrophic failure.
This is why people in extreme emergencies sometimes display strength far beyond what they can produce under normal conditions. The brain’s safety margin gets overridden by a flood of adrenaline and acute stress signals. It also explains why experienced lifters can access more of their muscle’s theoretical capacity than beginners. Training teaches your nervous system that higher force levels are safe, gradually loosening the governor.
Muscle Fiber Types Set Your Ceiling
Your muscles contain two broad categories of fibers: slow-twitch (Type I) and fast-twitch (Type II, further divided into IIA and IIX). Slow-twitch fibers resist fatigue well but produce relatively modest force. Fast-twitch fibers generate force quickly and powerfully but tire out fast.
The difference is substantial. In direct comparisons, people with a higher proportion of fast-twitch fibers produced 43% more maximal voluntary torque, 24% more force from high-frequency stimulation, and 21% more force from a single twitch compared to those dominated by slow-twitch fibers. They also developed force faster and relaxed faster between contractions. Your ratio of slow-twitch to fast-twitch fibers is largely determined by genetics, which is one reason some people are naturally more explosive while others excel at endurance.
Genetics and the ACTN3 Gene
One of the most studied genes related to strength is ACTN3, which codes for a protein called alpha-actinin-3. This protein is found exclusively in fast-twitch muscle fibers, where it acts as a structural crosslink inside the fiber’s contractile machinery. It helps transmit and absorb force during rapid, powerful contractions.
About 18% of the global population carries two copies of a variant that prevents them from producing alpha-actinin-3 at all. These individuals aren’t weak, but they tend to be underrepresented among elite sprinters and power athletes. The presence of the protein appears to give fast-twitch fibers a greater capacity for high-velocity force production, which is why researchers have found it at unusually high frequency in Olympic-level sprint and power competitors.
Muscle Size Matters, but Not as Much as You’d Think
Bigger muscles generally produce more force. There’s a statistically significant positive correlation between a muscle’s cross-sectional area and its strength in both men and women. But the relationship is far from perfect. Researchers have found such wide variation in the ratio of strength to muscle size that knowing someone’s muscle cross-sectional area isn’t a reliable way to predict how strong they actually are.
This is because size is only one ingredient. Two people with identically sized quadriceps can have very different strength levels depending on their fiber type distribution, neural drive, tendon properties, and the internal architecture of the muscle itself. The angle at which muscle fibers attach to their tendon (called pennation angle) changes how much of each fiber’s force gets transmitted along the tendon’s line of pull. A muscle with fibers angled more steeply can pack more fibers into a given volume, increasing its total force-generating cross-section, though the effect only becomes significant at angles above about 23 degrees.
Tendons and Bones as Force Multipliers
Muscles produce force, but tendons transmit it to your skeleton, and where a tendon attaches to a bone relative to the joint determines your mechanical advantage. A tendon that inserts farther from the joint creates a longer lever arm, producing more torque (rotational force) for the same amount of muscle contraction. A tendon that inserts closer to the joint produces less torque but allows faster movement and greater range of motion.
These attachment points are set by your anatomy and can’t be changed through training. Two people with identical muscle size and neural drive can produce different amounts of usable force at the same joint simply because one has a slightly longer lever arm. Your posture during a lift also changes your effective mechanical advantage: a deep crouch reduces the leverage of your leg extensors, which is why the bottom of a squat is the hardest part even though your muscles haven’t changed.
Tendon stiffness plays a role too. A stiffer tendon transmits force to the bone more quickly, contributing to how fast you can develop force, not just how much total force you produce. Tendon stiffness is influenced by collagen density and adapts to heavy loading over time, though more slowly than muscle tissue does.
Where the Energy Comes From
Every muscle contraction requires a molecule called ATP as its immediate fuel. For maximal efforts lasting up to about 5 or 6 seconds, your muscles rely almost entirely on the phosphagen system: a reserve of creatine phosphate stored directly in muscle tissue that can regenerate ATP almost instantly. Resting muscle stores about 26 millimoles of creatine phosphate per kilogram, enough to fuel a few seconds of all-out effort.
During a 10-second maximal sprint, the energy breakdown shifts to roughly 53% from the phosphagen system, 44% from the breakdown of glucose without oxygen (glycolysis), and just 3% from aerobic metabolism. This is why true maximal strength efforts, like a one-rep max deadlift, feel completely different from sustained cardio. They draw on a different fuel system entirely, one that’s powerful but extremely short-lived.
Why Strength Declines With Age
After about age 50, strength begins to decline, and it accelerates over time. By age 75, men lose strength at a rate of 3 to 4% per year, while women lose 2.5 to 3% per year. Critically, strength drops two to five times faster than muscle mass does. This means the loss isn’t just about shrinking muscles. It reflects deterioration in neural drive, motor unit number, tendon stiffness, and the selective loss of fast-twitch fibers, which are the first to atrophy with disuse and aging.
Resistance training remains the most effective intervention at every age. It addresses nearly every link in the chain: it improves neural recruitment, maintains fast-twitch fiber size, increases tendon stiffness, and preserves the coordination between systems that produces usable strength in daily life.